Dynorphin
Encyclopedia
Dynorphins are a class of opioid peptides that arise from the precursor protein
prodynorphin
. When prodynorphin is cleaved during processing by proprotein convertase 2
(PC2), multiple active peptides are released: dynorphin A
, dynorphin B
, and α
/β-neo-endorphin
. Depolarization of a neuron
containing prodynorphin stimulates PC2 processing, which occurs within synaptic vesicles in the presynaptic terminal. Occasionally, prodynorphin is not fully processed, leading to the release of “big dynorphin.” This 32-amino acid molecule consists of both dynorphin A and dynorphin B.
Dynorphin A, dynorphin B, and big dynorphin
all contain a high proportion of basic amino acid residues, in particular lysine and arginine (29.4%, 23.1%, and 31.2% basic residues, respectively), as well as many hydrophobic residues (41.2%, 30.8%, and 34.4% hydrophobic residues, respectively). Although dynorphins are found widely distributed in the CNS, they have the highest concentrations in the hypothalamus
, medulla
, pons
, midbrain, and spinal cord
. Dynorphins are stored in large (80-120 nm diameter) dense-core vesicles
that are considerably larger than vesicles storing neurotransmitters. These large dense-core vesicles differ from small synaptic vesicles in that a more intense and prolonged stimulus is needed to cause the large vesicles to release their contents into the synaptic cleft. Dense-core vesicle storage is characteristic of opioid peptides storage.
The first clues to the functionality of dynorphins came from Goldstein et al. in their work with opioid peptides. The group discovered an endogenous opioid peptide in the porcine pituitary that proved difficult to isolate. By sequencing the first 13 amino acids of the peptide, they created a synthetic version of the peptide with a similar potency to the natural peptide. Goldstein et al. applied the synthetic peptide to the guinea ileum longitudinal muscle and found it to be an extraordinarily potent opioid peptide. The peptide was called dynorphin (from the Greek dynamis=power) to describe its potency.
Dynorphins exert their effects primarily through the κ-opioid receptor (KOR), a G-protein-coupled receptor. Two subtypes of KORs have been identified: K1 and K2. Although KOR is the primary receptor for all dynorphins, the peptides do have some affinity for the μ-opioid receptor (MOR), δ-opioid receptor (DOR), N-methyl-D-aspartic acid (NMDA
)-type glutamate receptor Different dynorphins show different receptor selectivities and potencies at receptors. Big dynorphin and dynorphin A have the same selectivity for human KOR, but dynorphin A is more selective for KOR over MOR and DOR than is big dynorphin. Big dynorphin is more potent at KORs than is dynorphin A. Both big dynorphin and dynorphin A are more potent and more selective than dynorphin B.
, the hippocampus
and the spinal cord
, and has many different physiological actions, depending upon its site of production.
Han and Xie found dynorphin to be 6-10 times more potent than morphine
on a per mole basis. In addition, morphine tolerance did not reduce dynorphin-induced analgesia. Ren et al. demonstrated some of the complexities related to dynorphin induced analgesia. The authors found that combining subanalgesic levels of morphine and dynorphin A1-13, a version of dynorphin A containing only the first 13 amino acids of the peptide, in the rat spinal cord had additive effects. However, when dynorphin A1-13 was injected into the intracerebroventriulcar (ICV) region of the brain, it had an antagonist effect on morphine-induced analgesia.
A study by Lai et al. found that dynorphin might actually stimulate pain. The group found that dynorphin acts on the bradykinin receptor as well as KOR. The N-terminal tyrosine of dynorphin A is necessary to activate opioid receptors such as KOR, but is unnecessary in binding to bradykinin receptors. Lai et al. studied the effects of dynorphin A2-13 that did not contain the N-terminal tyrosine. Based on the results of dynorphin A2-13, the authors proposed a mechanism in which dynorphin A activates bradykinin receptors and thus stimulates pain response. According to this mechanism, dynorphin activates bradykinin receptors, which triggers the release of calcium ions into the cell through voltage-sensitive channels in the cell membrane. Blocking bradykinin receptors in the lumbar region of the spinal cord reversed persistent pain. A multiple pathway system might help explain the conflicting effects of dynorphin in the CNS.
Svensson et al. provided another possible mechanism by which dynorphin might cause pain in the spinal cord. The authors found that administration of truncated dynorphin A2-17, which does not bind to opioid receptors, causes an increase in phosphorylated p38 mitogen-activated protein kinase (MAPK) in microglia
in the dorsal horn of the spinal cord. Activated p38 has been previously linked to the NMDA-evoked prostaglandin
release, which causes pain. Thus, dynorphin could also induce pain in the spinal cord through a non-opioid p38 pathway.
Other studies have identified a role for dynorphin and kappa opioid receptor stimulation in neuropathic pain. This same group also showed that the dynorphin-KOR system mediates astrocyte
proliferation through the activation of p38 MAPK that was required for the effects of neuropathic pain on analgesic responses. Taken together, these reports suggest that dynorphin can elicit multiple effects on both Kappa opioid, and non-opioid pathways to modulate analgesic responses.
and substantia nigra
in rats. One proposed molecular mechanism for increased dynorphin levels involves transcriptional regulation by CREB
(3’, 5’-monophosphate response element binding protein). According to the model proposed by Carlezon et al., use of cocaine increases the expression of cAMP
and cAMP-dependent protein kinase (PKA). PKA leads to the activation of CREB, which increases the expression of dynorphin in the nucleus accumbens and dorsal striatum, brain areas important in addiction. Dynorphin decreases dopamine
release by binding to KORs on dopamine nerve terminals, which leads to drug tolerance and withdrawal symptoms (Berke and Hyman, 2000).
Carlezon et al. performed several experiments to validate this model. They found that, when mice were injected with cocaine, they preferred to be in the place where they were injected (showed stronger place preference) significantly more than control mice (injected with saline) did. However, in mice overexpressing CREB under a constitutive promoter, place aversion was observed. This indicates that increasing CREB reverses the positive effects of cocaine. Northern blot
analysis several days after CREB overexpression showed a marked increase in dynorphin mRNA in the nucleus accumbens. Blocking KORs with an antagonist (norBNI) blocked the aversive effects caused by CREB overexpression. Thus, cocaine use ultimately appears to lead to an increase in the transcription of prodynorphin mRNA. Dynorphin inhibits dopamine release, that would counter the pleasurable effects of cocaine caused by the release of dopamine. There is also evidence suggesting that increased amounts of dynorphin can protect humans from cocaine addiction. According to research at Rockefeller University, the gene for dynorphin is present in two versions: a “high output” and a “low output” functional variation. The high output functional variation of the gene contains polymorphisms in the promoter regions that are speculated to cause it to produce more copies of dynorphin mRNA, which would give people carrying this variation a “built-in defense system” against drug addiction.
Because of its role in mediating dysphoria, dynorphin has also been investigated in relation to depression
. Newton et al. studied the effects of CREB and dynorphin on learned helplessness
(an animal model for depression) in mice. Overexpression of dominant negative CREB (mCREB) in transgenic mice had an antidepressant effect (in terms of behavior), whereas overexpressing wild-type CREB caused an increase in depression-like symptoms. As described previously, CREB increases transcription of prodynorphin, which gives rise to different dynorphin subtypes. Newton et al. supported this mechanism, as the mCREB was colocalized with decreased expression of prodynorphin. Also, direct antagonism of dynorphin caused antidepressant-like effects similar to those seen with mCREB expression. Thus, the CREB-dynorphin pathway regulates mood as well as cocaine rewards.
Shirayama et al. used several animal depression models in rats to describe the effects of dynorphins A and B in depression. The authors found that learned helplessness increases the levels of dynorphins A and B in the hippocampus and nucleus accumbens and that injecting KOR antagonist norBNI induces recovery from learned helplessness. Immobilization stress causes increases levels of both dynorphins A and B in the hippocampus and nucleus accumbens. Forced swim stress increases the levels of dynorphin A in the hippocampus. Shirayama et al. concluded that both dynorphins A and B were important in stress response. The authors proposed several mechanisms to account for the effects of the KOR antagonist norBNI on learned helplessness. First, increased dynorphin levels block the release of glutamate, a neurotransmitter involved in plasticity
in the hippocampus, which would inhibit new learning. Blocking dynorphin effects would allow glutamate to be released and restore functional plasticity in the hippocampus, reversing the phenomenon of learned helplessness. In addition, blocking dynorphin would enhance dopamine signaling and thus reduce depressive symptoms associated with stress. The authors suggest that KOR antagonists might have potential in treating depression in humans.
through appetite control and circadian rhythms. Przewlocki et al. found that, during the day, dynorphins are naturally elevated in the neurointermediate lobe of the pituitary (NI pituitary) and depressed in the hypothalamus. This pattern is reversed at night. In addition, mice deprived of food and water, or of water alone, had increased levels of dynorphin in the hypothalamus during the day. Deprivation of water alone also decreased the dynorphin levels in the NI pituitary. These findings led Przewlocki et al. to conclude that dynorphins are essential in maintaining homeostasis.
Dynorphin has been implicated as an appetite stimulant. A number of studies in rats have shown that increasing the dynorphin levels stimulates eating. Opioid antagonists, such as naloxone
, can reverse the effects of elevated dynorphin. This inhibition is especially strong in obese animals or animals that have access to particularly appealing food. Inui et al. found that administering dynorphin to dogs increased both their food and water intake. Dynorphin plays a role in the eating behavior of hibernating animals. Nizeilski et al. examined dynorphin levels in the ground squirrel, which undergoes periods of excessive eating and periods of starvation before winter. They found that dynorphin levels increased during the starvation periods. Berman et al. studied the levels of dynorphin during periods of food restriction. The group found that while food did not alter the expression of dynorphin B, it increases dynorphin A levels in several rat brain regions (hypothalamus
, nucleus accumbens
, and bed nucleus of the stria terminalis).
Recent research on dynorphin knockout mice did not find differences between knockout and control animals in food intake, but found that fat storage was reduced in male knockout mice. Fatty acids were oxidized more quickly in knockout animals.
Studies have also shown that ingesting a high-fat diet increases the gene expression of dynorphin in the hypothalamus. Thus, dynorphin may cause overeating when a high-fat diet is available. were the first to describe the role of opioid peptides in stress-related eating. In their study, mice had their tails pinched (causes stress), which induced eating. Stress-related eating was reduced by injecting naloxone, an opioid peptide antagonist. Mandenoff et al. proposed that, although endogenous opioids are not necessary to maintain body weight and energy expenditure under predictable circumstances, they become activated under stressful conditions. They found that endogenous opioids, such as dynorphin, stimulate appetite and decrease energy expenditure. Taken together, the studies above suggest an important evolutionary mechanism in which more food is eaten, more nutrients are stored, and less energy is expended by an organism during times of stress.
, where it was found that MOR agonists stimulate this response when injected into the periaqueductal gray (PAG) region of the brain. Xin et al. showed that delivery of dynorphin A1-17 (a KOR agonist) through microdialysis
into the PAG region induced hypothermia in rats. The authors found that the severity of hypothermia
was proportional to the dose of dynorphin A1-17 administered. Hypothermia could be prevented by administering KOR antagonist norBNI to the rat. Xin et al. hypothesized that while MOR agonists mediate hyperthermia, KOR agonists, such as dynorphin, mediate hypothermia. Sharma and Alm found that subjecting rats to heat (38˚C) caused dynorphins to be upregulated in the cerebral cortex, hippocampus, cerebellum, and the brain stem. Further, authors found that administration of nitric oxide synthase (NOS) inhibitors reduced dynorphin A1-17 levels in the brain and attenuated symptoms related to heat stress. Sharma and Alm concluded that hyperthermia increases dynorphin levels, which may cause damage and promote heat stress reaction. They further hypothesized that nitric oxide
was part of this mechanism. Ansonoff et al. found that hypothermic effects are mediated through K1 (κ-opioid receptor 1), but not K2. The authors applied a KOR agonist to K1 knockout mice, which eliminated hypothermic response. Thus, K2 does not appear to have a role in the hypothermic mechanism
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
prodynorphin
Prodynorphin
Prodynorphin, also known as proenkephalin B, is an opioid polypeptide hormone involved with chemical signal transduction and cell communication. The gene for prodynorphin is expressed in the endometrium and the striatum, and its gene map locus is 20pter-p12...
. When prodynorphin is cleaved during processing by proprotein convertase 2
Proprotein convertase 2
Proprotein convertase 2 also known as prohormone convertase 2 or neuroendocrine convertase 2 is a serine protease and proprotein convertase PC2, like proprotein convertase 1 , is an enzyme responsible for the first step in the maturation of many neuroendocrine peptides from their precursors, such...
(PC2), multiple active peptides are released: dynorphin A
Dynorphin A
Dynorphin A is a form of dynorphin, it is an endogenous opioid peptide whose sequence is: Tyr - Gly - Gly - Phe - Leu - Arg - Arg - Ile - Arg - Pro - Lys - Leu - Lys....
, dynorphin B
Dynorphin B
Dynorphin B is a form of dynorphin, it is an endogenous opioid peptide whose sequence is: Tyr - Gly - Gly - Phe - Leu - Arg - Arg - Gln - Phe - Lys - Leu - Leu - Thr....
, and α
Α-neo-endorphin
α-Neoendorphin is an endogenous opioid peptide with a decapeptide structure and the amino acid sequence Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro-Lys....
/β-neo-endorphin
Β-neo-endorphin
β-Neoendorphin is an endogenous opioid peptide with a nonapeptide structure and the amino acid sequence Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro....
. Depolarization of a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
containing prodynorphin stimulates PC2 processing, which occurs within synaptic vesicles in the presynaptic terminal. Occasionally, prodynorphin is not fully processed, leading to the release of “big dynorphin.” This 32-amino acid molecule consists of both dynorphin A and dynorphin B.
Dynorphin A, dynorphin B, and big dynorphin
Big dynorphin
Big dynorphin is a form of dynorphin consisting of dynorphin A and dynorphin B.It binds to the kappa-opioid receptor....
all contain a high proportion of basic amino acid residues, in particular lysine and arginine (29.4%, 23.1%, and 31.2% basic residues, respectively), as well as many hydrophobic residues (41.2%, 30.8%, and 34.4% hydrophobic residues, respectively). Although dynorphins are found widely distributed in the CNS, they have the highest concentrations in the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
, pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
, midbrain, and spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
. Dynorphins are stored in large (80-120 nm diameter) dense-core vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
that are considerably larger than vesicles storing neurotransmitters. These large dense-core vesicles differ from small synaptic vesicles in that a more intense and prolonged stimulus is needed to cause the large vesicles to release their contents into the synaptic cleft. Dense-core vesicle storage is characteristic of opioid peptides storage.
The first clues to the functionality of dynorphins came from Goldstein et al. in their work with opioid peptides. The group discovered an endogenous opioid peptide in the porcine pituitary that proved difficult to isolate. By sequencing the first 13 amino acids of the peptide, they created a synthetic version of the peptide with a similar potency to the natural peptide. Goldstein et al. applied the synthetic peptide to the guinea ileum longitudinal muscle and found it to be an extraordinarily potent opioid peptide. The peptide was called dynorphin (from the Greek dynamis=power) to describe its potency.
Dynorphins exert their effects primarily through the κ-opioid receptor (KOR), a G-protein-coupled receptor. Two subtypes of KORs have been identified: K1 and K2. Although KOR is the primary receptor for all dynorphins, the peptides do have some affinity for the μ-opioid receptor (MOR), δ-opioid receptor (DOR), N-methyl-D-aspartic acid (NMDA
NMDA
N-Methyl-D-aspartic acid or N-Methyl-D-aspartate is an amino acid derivative which acts as a specific agonist at the NMDA receptor mimicking the action of glutamate, the neurotransmitter which normally acts at that receptor...
)-type glutamate receptor Different dynorphins show different receptor selectivities and potencies at receptors. Big dynorphin and dynorphin A have the same selectivity for human KOR, but dynorphin A is more selective for KOR over MOR and DOR than is big dynorphin. Big dynorphin is more potent at KORs than is dynorphin A. Both big dynorphin and dynorphin A are more potent and more selective than dynorphin B.
Production
Dynorphin is produced in many different parts of the brain, including the hypothalamusHypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
and the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, and has many different physiological actions, depending upon its site of production.
- For example, dynorphin that is made in magnocellular vasopressinVasopressinArginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
neurons of the supraoptic nucleusSupraoptic nucleusThe supraoptic nucleus is a nucleus of magnocellular neurosecretory cells in the hypothalamus of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm...
is important in the patterning of electrical activity. Dynorphin produced in magnocellular oxytocinOxytocinOxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...
neurons is a negative feedbackNegative feedbackNegative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
inhibitor of oxytocinOxytocinOxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...
secretion. - Dynorphin produced in the arcuate nucleusArcuate nucleusThe arcuate nucleus is an aggregation of neurons in the mediobasal hypothalamus, adjacent to the third ventricle and the median eminence...
and in orexinOrexinOrexins, also called hypocretins, are the common names given to a pair of excitatory neuropeptide hormones that were simultaneously discovered by two groups of researchers in rat brains....
neurons of the lateral hypothalamusLateral hypothalamusThe lateral hypothalamus or lateral hypothalamic area is a part of the hypothalamus.It is concerned with hunger. Damage to this area can cause reduced food intake...
affects the control of appetiteAppetiteThe appetite is the desire to eat food, felt as hunger. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Decreased desire to eat is...
.
Analgesia
Dynorphin has been shown to be a modulator of pain response. Han and Xie found that injecting dynorphin into the subarachnoid space of the rat spinal cord produced dose-dependent analgesia that was measured by tail-flick latency. Analgesia was partially eliminated by opioid antagonist naloxone.Han and Xie found dynorphin to be 6-10 times more potent than morphine
Morphine
Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...
on a per mole basis. In addition, morphine tolerance did not reduce dynorphin-induced analgesia. Ren et al. demonstrated some of the complexities related to dynorphin induced analgesia. The authors found that combining subanalgesic levels of morphine and dynorphin A1-13, a version of dynorphin A containing only the first 13 amino acids of the peptide, in the rat spinal cord had additive effects. However, when dynorphin A1-13 was injected into the intracerebroventriulcar (ICV) region of the brain, it had an antagonist effect on morphine-induced analgesia.
A study by Lai et al. found that dynorphin might actually stimulate pain. The group found that dynorphin acts on the bradykinin receptor as well as KOR. The N-terminal tyrosine of dynorphin A is necessary to activate opioid receptors such as KOR, but is unnecessary in binding to bradykinin receptors. Lai et al. studied the effects of dynorphin A2-13 that did not contain the N-terminal tyrosine. Based on the results of dynorphin A2-13, the authors proposed a mechanism in which dynorphin A activates bradykinin receptors and thus stimulates pain response. According to this mechanism, dynorphin activates bradykinin receptors, which triggers the release of calcium ions into the cell through voltage-sensitive channels in the cell membrane. Blocking bradykinin receptors in the lumbar region of the spinal cord reversed persistent pain. A multiple pathway system might help explain the conflicting effects of dynorphin in the CNS.
Svensson et al. provided another possible mechanism by which dynorphin might cause pain in the spinal cord. The authors found that administration of truncated dynorphin A2-17, which does not bind to opioid receptors, causes an increase in phosphorylated p38 mitogen-activated protein kinase (MAPK) in microglia
Microglia
Microglia are a type of glial cell that are the resident macrophages of the brain and spinal cord, and thus act as the first and main form of active immune defense in the central nervous system . Microglia constitute 20% of the total glial cell population within the brain...
in the dorsal horn of the spinal cord. Activated p38 has been previously linked to the NMDA-evoked prostaglandin
Prostaglandin
A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....
release, which causes pain. Thus, dynorphin could also induce pain in the spinal cord through a non-opioid p38 pathway.
Other studies have identified a role for dynorphin and kappa opioid receptor stimulation in neuropathic pain. This same group also showed that the dynorphin-KOR system mediates astrocyte
Astrocyte
Astrocytes , also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord...
proliferation through the activation of p38 MAPK that was required for the effects of neuropathic pain on analgesic responses. Taken together, these reports suggest that dynorphin can elicit multiple effects on both Kappa opioid, and non-opioid pathways to modulate analgesic responses.
Addiction
Cocaine addiction results from complex molecular changes in the brain following multiple exposures to cocaine. Dynorphins have been shown to be an important part of this process. Although a single exposure to cocaine does not affect brain dynorphin levels, repeated exposures to the drug increases dynorphin concentrations in the striatumStriatum
The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of the forebrain. It is the major input station of the basal ganglia system. The striatum, in turn, gets input from the cerebral cortex...
and substantia nigra
Substantia nigra
The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in...
in rats. One proposed molecular mechanism for increased dynorphin levels involves transcriptional regulation by CREB
CREB
CREB is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements , thereby increasing or decreasing the transcription of the downstream genes....
(3’, 5’-monophosphate response element binding protein). According to the model proposed by Carlezon et al., use of cocaine increases the expression of cAMP
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
and cAMP-dependent protein kinase (PKA). PKA leads to the activation of CREB, which increases the expression of dynorphin in the nucleus accumbens and dorsal striatum, brain areas important in addiction. Dynorphin decreases dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...
release by binding to KORs on dopamine nerve terminals, which leads to drug tolerance and withdrawal symptoms (Berke and Hyman, 2000).
Carlezon et al. performed several experiments to validate this model. They found that, when mice were injected with cocaine, they preferred to be in the place where they were injected (showed stronger place preference) significantly more than control mice (injected with saline) did. However, in mice overexpressing CREB under a constitutive promoter, place aversion was observed. This indicates that increasing CREB reverses the positive effects of cocaine. Northern blot
Northern blot
The northern blot is a technique used in molecular biology research to study gene expression by detection of RNA in a sample. With northern blotting it is possible to observe cellular control over structure and function by determining the particular gene expression levels during differentiation,...
analysis several days after CREB overexpression showed a marked increase in dynorphin mRNA in the nucleus accumbens. Blocking KORs with an antagonist (norBNI) blocked the aversive effects caused by CREB overexpression. Thus, cocaine use ultimately appears to lead to an increase in the transcription of prodynorphin mRNA. Dynorphin inhibits dopamine release, that would counter the pleasurable effects of cocaine caused by the release of dopamine. There is also evidence suggesting that increased amounts of dynorphin can protect humans from cocaine addiction. According to research at Rockefeller University, the gene for dynorphin is present in two versions: a “high output” and a “low output” functional variation. The high output functional variation of the gene contains polymorphisms in the promoter regions that are speculated to cause it to produce more copies of dynorphin mRNA, which would give people carrying this variation a “built-in defense system” against drug addiction.
Stress and depression
Land et al. first described a mechanism of dysphoria in which corticotropin-releasing factor (CRF) provokes dynorphin release. While control mice displayed aversive behaviors in response to forced swim tests and foot shocks, mice lacking dynorphin did not show any such signs of aversion. They noted that injecting CRF led to aversive behaviors in mice with functional genes for dynorphin even in the absence of stress, but not in those with dynorphin gene deletions. Place aversion was eliminated when the receptor CRF2 was blocked with an antagonist. Together these results led Land et al. to conclude that dysphoric elements of stress occur when CRF2 stimulates dynorphin release and activates KOR. The group further postulated that this pathway might be involved in drug seeking behavior. In support of this, it was shown previously that stress can reinstate cocaine-seeking behavior in mice through a CRF mechanism. Dynorphin has also been shown to influence drug seeking behavior and is required for stress-induced, but not prime-induced, reinstatement of cocaine seeking. A downstream element of this pathway was later identified by Bruchas et al. The authors found that KOR activates p38, a member of the mitogen-activated protein kinase (MAPK) family, through phosphorylation. Activation of p38 is necessary to produce KOR-dependent behaviors.Because of its role in mediating dysphoria, dynorphin has also been investigated in relation to depression
Depression (mood)
Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behaviour, feelings and physical well-being. Depressed people may feel sad, anxious, empty, hopeless, helpless, worthless, guilty, irritable, or restless...
. Newton et al. studied the effects of CREB and dynorphin on learned helplessness
Learned helplessness
Learned helplessness, as a technical term in animal psychology and related human psychology, means a condition of a human person or an animal in which it has learned to behave helplessly, even when the opportunity is restored for it to help itself by avoiding an unpleasant or harmful circumstance...
(an animal model for depression) in mice. Overexpression of dominant negative CREB (mCREB) in transgenic mice had an antidepressant effect (in terms of behavior), whereas overexpressing wild-type CREB caused an increase in depression-like symptoms. As described previously, CREB increases transcription of prodynorphin, which gives rise to different dynorphin subtypes. Newton et al. supported this mechanism, as the mCREB was colocalized with decreased expression of prodynorphin. Also, direct antagonism of dynorphin caused antidepressant-like effects similar to those seen with mCREB expression. Thus, the CREB-dynorphin pathway regulates mood as well as cocaine rewards.
Shirayama et al. used several animal depression models in rats to describe the effects of dynorphins A and B in depression. The authors found that learned helplessness increases the levels of dynorphins A and B in the hippocampus and nucleus accumbens and that injecting KOR antagonist norBNI induces recovery from learned helplessness. Immobilization stress causes increases levels of both dynorphins A and B in the hippocampus and nucleus accumbens. Forced swim stress increases the levels of dynorphin A in the hippocampus. Shirayama et al. concluded that both dynorphins A and B were important in stress response. The authors proposed several mechanisms to account for the effects of the KOR antagonist norBNI on learned helplessness. First, increased dynorphin levels block the release of glutamate, a neurotransmitter involved in plasticity
Neuroplasticity
Neuroplasticity is a non-specific neuroscience term referring to the ability of the brain and nervous system in all species to change structurally and functionally as a result of input from the environment. Plasticity occurs on a variety of levels, ranging from cellular changes involved in...
in the hippocampus, which would inhibit new learning. Blocking dynorphin effects would allow glutamate to be released and restore functional plasticity in the hippocampus, reversing the phenomenon of learned helplessness. In addition, blocking dynorphin would enhance dopamine signaling and thus reduce depressive symptoms associated with stress. The authors suggest that KOR antagonists might have potential in treating depression in humans.
Appetite and circadian rhythms
Dynorphins are important in maintaining homeostasisHomeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
through appetite control and circadian rhythms. Przewlocki et al. found that, during the day, dynorphins are naturally elevated in the neurointermediate lobe of the pituitary (NI pituitary) and depressed in the hypothalamus. This pattern is reversed at night. In addition, mice deprived of food and water, or of water alone, had increased levels of dynorphin in the hypothalamus during the day. Deprivation of water alone also decreased the dynorphin levels in the NI pituitary. These findings led Przewlocki et al. to conclude that dynorphins are essential in maintaining homeostasis.
Dynorphin has been implicated as an appetite stimulant. A number of studies in rats have shown that increasing the dynorphin levels stimulates eating. Opioid antagonists, such as naloxone
Naloxone
Naloxone is an opioid antagonist drug developed by Sankyo in the 1960s. Naloxone is a drug used to counter the effects of opiate overdose, for example heroin or morphine overdose. Naloxone is specifically used to counteract life-threatening depression of the central nervous system and respiratory...
, can reverse the effects of elevated dynorphin. This inhibition is especially strong in obese animals or animals that have access to particularly appealing food. Inui et al. found that administering dynorphin to dogs increased both their food and water intake. Dynorphin plays a role in the eating behavior of hibernating animals. Nizeilski et al. examined dynorphin levels in the ground squirrel, which undergoes periods of excessive eating and periods of starvation before winter. They found that dynorphin levels increased during the starvation periods. Berman et al. studied the levels of dynorphin during periods of food restriction. The group found that while food did not alter the expression of dynorphin B, it increases dynorphin A levels in several rat brain regions (hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, nucleus accumbens
Nucleus accumbens
The nucleus accumbens , also known as the accumbens nucleus or as the nucleus accumbens septi , is a collection of neurons and forms the main part of the ventral striatum...
, and bed nucleus of the stria terminalis).
Recent research on dynorphin knockout mice did not find differences between knockout and control animals in food intake, but found that fat storage was reduced in male knockout mice. Fatty acids were oxidized more quickly in knockout animals.
Studies have also shown that ingesting a high-fat diet increases the gene expression of dynorphin in the hypothalamus. Thus, dynorphin may cause overeating when a high-fat diet is available. were the first to describe the role of opioid peptides in stress-related eating. In their study, mice had their tails pinched (causes stress), which induced eating. Stress-related eating was reduced by injecting naloxone, an opioid peptide antagonist. Mandenoff et al. proposed that, although endogenous opioids are not necessary to maintain body weight and energy expenditure under predictable circumstances, they become activated under stressful conditions. They found that endogenous opioids, such as dynorphin, stimulate appetite and decrease energy expenditure. Taken together, the studies above suggest an important evolutionary mechanism in which more food is eaten, more nutrients are stored, and less energy is expended by an organism during times of stress.
Temperature regulation
In addition to their role in weight control, dynorphins have been found to regulate body temperature. Opioid peptides were first investigated in hyperthermiaHyperthermia
Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...
, where it was found that MOR agonists stimulate this response when injected into the periaqueductal gray (PAG) region of the brain. Xin et al. showed that delivery of dynorphin A1-17 (a KOR agonist) through microdialysis
Microdialysis
- The microdialysis technique:Microdialysis is a semi-invasive sampling technique that is used for continuous measurement of free, unbound analyte concentrations in the extracellular fluid of virtually any tissue. Analytes may include endogenous molecules to assess their biochemical functions in...
into the PAG region induced hypothermia in rats. The authors found that the severity of hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
was proportional to the dose of dynorphin A1-17 administered. Hypothermia could be prevented by administering KOR antagonist norBNI to the rat. Xin et al. hypothesized that while MOR agonists mediate hyperthermia, KOR agonists, such as dynorphin, mediate hypothermia. Sharma and Alm found that subjecting rats to heat (38˚C) caused dynorphins to be upregulated in the cerebral cortex, hippocampus, cerebellum, and the brain stem. Further, authors found that administration of nitric oxide synthase (NOS) inhibitors reduced dynorphin A1-17 levels in the brain and attenuated symptoms related to heat stress. Sharma and Alm concluded that hyperthermia increases dynorphin levels, which may cause damage and promote heat stress reaction. They further hypothesized that nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
was part of this mechanism. Ansonoff et al. found that hypothermic effects are mediated through K1 (κ-opioid receptor 1), but not K2. The authors applied a KOR agonist to K1 knockout mice, which eliminated hypothermic response. Thus, K2 does not appear to have a role in the hypothermic mechanism