Down-regulation of proopiomelanocortin synthesis and beta-endorphin utilization in hypothalamus of morphine-tolerant rats

Effect of steady-state methadone on high fructose corn syrup consumption in rats

Patients undergoing methadone maintenance treatment self-report enhanced preferences for, and excessive consumption of, foods rich in sugar. However, it is unclear whether these are direct pharmacological effects of methadone or the consequences of metabolic dysfunctions induced by addiction to illicit opiates. Hence, the current study in drug-naïve male Sprague-Dawley rats explored the effects of steady-state methadone delivered by osmotic mini-pumps (13 days; 0, 10, 30 mg/kg/day) on consumption of rat chow and a palatable, sweet, liquid high fructose corn syrup solution. Six days after the removal of the pumps, mRNA expression of genes involved in responses to stress and rewards were quantified: pro-opiomelanocortin in the hypothalamus, mu-opioid receptor in the nucleus accumbens, and dopamine D2 receptor in the dorsal striatum. Taste reactivity and locomotion tests were also performed throughout the study. It was found that methadone increased caloric intake from high fructose corn syrup and reduced caloric intake from chow, effects that could not be directly ascribed to changes in high fructose corn syrup taste reactivity or motor functions. However, the changes in caloric intake displayed significant tolerance, and mRNA expression analysis suggested that methadone attenuated the effect of high fructose corn syrup on pro-opiomelanocortin mRNA, and possibly on dopamine D2 receptor mRNA. These findings in rats suggest that the pharmacological effect of methadone, administered to achieve steady-state maintenance, may not be the primary cause of dietary alterations reported by patients maintained on methadone.

Suppression of hypothalamic-pituitary-adrenal axis by acute heroin challenge in rats during acute and chronic withdrawal from chronic heroin administration

It is known that heroin dependence and withdrawal are associated with changes in the hypothalamic-pituitary-adrenal (HPA) axis. The objective of these studies in rats was to systematically investigate the level of HPA activity and response to a heroin challenge at two time points during heroin withdrawal, and to characterize the expression of associated stress-related genes 30 min after each heroin challenge. Rats received chronic (10-day) intermittent escalating-dose heroin administration (3 × 2.5 mg/kg/day on day 1; 3 × 20 mg/kg/day by day 10). Hormonal and neurochemical assessments were performed in acute (12 h after last heroin injection) and chronic (10 days after the last injection) withdrawal. Both plasma ACTH and corticosterone levels were elevated during acute withdrawal, and heroin challenge at 20 mg/kg (the last dose of chronic escalation) at this time point attenuated this HPA hyperactivity. During chronic withdrawal, HPA hormonal levels returned to baseline, but heroin challenge at 5 mg/kg decreased ACTH levels. In contrast, this dose of heroin challenge stimulated the HPA axis in heroin naïve rats. In the anterior pituitary, pro-opiomelanocortin (POMC) mRNA levels were increased during acute withdrawal and retuned to control levels after chronic withdrawal. In the medial hypothalamus, however, the POMC mRNA levels were decreased during acute withdrawal, and increased after chronic withdrawal. Our results suggest a long-lasting change in HPA abnormal responsivity during chronic heroin withdrawal.

Proopiomelanocortin (POMC) expression and conditioned place aversion during protracted withdrawal from chronic intermittent escalating-dose heroin in POMC-EGFP promoter transgenic mice

In heroin-dependent individuals, the drive to avoid or ameliorate the negative affective/emotional state associated with the discontinuation of heroin contributes to the chronic relapsing nature of the disease. Here, we investigate changes in proopiomelanocortin (POMC) expression at three time points across an extended period of heroin withdrawal in a clinically relevant rodent model of addiction using conditioned place aversion (CPA) in POMC-EGFP (POMC-enhanced green fluorescent protein) bacterial artificial chromosome (BAC) transgenic mice. Neurons expressing POMC-EGFP were found in the medial nucleus of the amygdala (MeA), basomedial amygdala (BMA) and dentate gyrus of hippocampus (DG), as well as the arcuate nucleus of hypothalamus (ARC). Heroin-treated mice displayed robust CPA after acute spontaneous withdrawal (12h), which persisted across the extended (14days) withdrawal period. After 12-h withdrawal, heroin-treated mice showed lower signal intensity of POMC-EGFP-positive cells in the ARC, higher levels of POMC mRNA in the amygdala but lower levels in the hippocampus than saline controls. After 7-d withdrawal, heroin-treated mice showed fewer POMC-EGFP-positive cells in the MeA and lower POMC mRNA in the amygdala than saline controls. After extended (14days) withdrawal, heroin-treated mice showed more POMC-EGFP-positive cells in BMA and DG, increased intensity of POMC-EGFP signal in DG, and higher POMC mRNA levels in the hippocampus compared to controls. Our results show dynamic changes in POMC in hypothalamic and extra-hypothalamic regions that may contribute to the negative affective/emotional state of heroin withdrawal shown by CPA from acute to extended periods of heroin withdrawal.

Pharmacological regulation of gene expression

Pharmacological regulation of gene expression was one of the top professional interests of Dr. Costa. He promoted the idea that drugs can improve the endogenous mechanisms of synaptic plasticity by modulating gene expression. In this article I reflect upon Dr. Costa's leadership in projects undertaken at FGIN that were aimed at elucidating how neurotransmitter receptor activation could affect brain function by modulating genes and their products. I will be presenting examples of how pharmacological tools can change gene expression. These include the ability of drugs of abuse to alter the synthesis of opioid peptides or an endogenous ligand for GABAA receptor. I will conclude with a brief summary of intriguing discoveries about the regulation of nerve growth factor (NGF) and its receptors by beta-receptor agonists, adrenal steroids and cytokines.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Drug-induced and genetic alterations in stress-responsive systems: Implications for specific addictive diseases

From the earliest work in our laboratory, we hypothesized, and with studies conducted in both clinical research and animal models, we have shown that drugs of abuse, administered or self-administered, on a chronic basis, profoundly alter stress-responsive systems. Alterations of expression of specific genes involved in stress responsivity, with increases or decreases in mRNA levels, receptor, and neuropeptide levels, and resultant changes in hormone levels, have been documented to occur after chronic intermittent exposure to heroin, morphine, other opiates, cocaine, other stimulants, and alcohol in animal models and in human molecular genetics. The best studied of the stress-responsive systems in humans and mammalian species in general is undoubtedly the HPA axis. In addition, there are stress-responsive systems in other parts in the brain itself, and some of these include components of the HPA axis, such as CRF and CRF receptors, along with POMC gene and gene products. Several other stress-responsive systems are known to influence the HPA axis, such as the vasopressin-vasopressin receptor system. Orexin-hypocretin, acting at its receptors, may effect changes which suggest that it should be properly categorized as a stress-responsive system. However, less is known about the interactions and connectivity of some of these different neuropeptide and receptor systems, and in particular, about the possible connectivity of fast-acting (e.g., glutamate and GABA) and slow-acting (including dopamine, serotonin, and norepinephrine) neurotransmitters with each of these stress-responsive components and the resultant impact, especially in the setting of chronic exposure to drugs of abuse. Several of these stress-responsive systems and components, primarily based on our laboratory-based and human molecular genetics research of addictive diseases, will be briefly discussed in this review.

Effects of prolonged treatment with the opiate tramadol on prodynorphin gene expression in rat CNS

A low abuse liability is reported for tramadol, an analgesic drug centrally acting through either opioid or nonopioid mechanisms. In this paper, we evaluated the effects of the repeated administration (7 d) of different doses of tramadol (10, 20, and 80 mg/kg, intraperitoneally) on the opioid precursor prodynorphin biosynthesis, in comparison with morphine (10 mg/kg, intraperitoneally), in the rat central nervous system (CNS). Northern analysis showed that morphine and tramadol produced different effects. While morphine caused a downregulation of prodynorphin mRNA levels in all investigated areas (hypothalamus, hippocampus, and striatum), tramadol did not cause any significant change in the striatum, and did not decrease prodynorphin biosynthesis in the hypothalamus and in the hippocampus, at nontoxic doses (10 and 20 mg/kg). The highest dose of tramadol (80 mg/kg) decreased prodynorphin mRNA levels in the hypothalamus and the hippocampus but not in the striatum. These data give some information on tramadol effects at molecular level in the CNS. They indicate that the alterations of prodynorphin gene expression caused by tramadol and morphine show a different pattern that may be related to the different abuse potential of the two analgesic drugs.

Alterations in prodynorphin gene expression and dynorphin levels in different brain regions after chronic administration of 14-methoxymetopon and oxycodone-6-oxime

Previous studies showed that opioid drugs-oxycodone-6-oxime and 14-methoxy-5-methyl-dihydromorphinone (14-methoxymetopon)-produced less respiratory depressive effect and slower rate of tolerance and dependence, respectively. It was also reported that morphine decreased the prodynorphin gene expression in the rat hippocampus, striatum and hypothalamus. In this study, we determined the prodynorphin gene expression and dynorphin levels in selected brain regions of opioid tolerant rats. We found that in the striatum morphine decreased, while oxycodone-6-oxime increased and 14-methoxymetopon did not alter the prodynorphin gene expression. In the nucleus accumbens, morphine and oxycodone-6-oxime did not change, while 14-methoxymetopon increased the prodynorphin gene expression. In the hippocampus both oxycodone-6-oxime and 14-methoxymetopon enhanced, whereas morphine did not alter the prodynorphin gene expression. In the rat striatum only oxycodone-6-oxime increased dynorphin levels significantly in accordance with the prodynorphin mRNA changes. In the hippocampus both opioid agonists increased the dynorphin levels significantly similarly to the augmented prodynorphin gene expression. In ventral tegmental area only 14-methoxymetopon increased dynorphin levels significantly. In nucleus accumbens and the temporal-parietal cortex the changes in the prodynorphin gene expression and the dynorphin levels did not correlate. Since the endogenous prodynorphin system may play a modulatory role in the development of opioid tolerance, the elevated supraspinal dynorphin levels appear to be partly responsible for the reduced degree of tolerance induced by the investigated opioids.

Opioid abuse and brain gene expression

Opiate addiction is a central nervous system disorder of unknown mechanism. Neuronal basis of positive reinforcement, which is essential to the action of opioids, relies on activation of dopaminergic neurons resulting in an increased dopamine release in the mesolimbic brain structures. Certain aspects of opioid dependence and withdrawal syndrome are also related to the activity of noradrenergic and serotonergic systems, as well as to both excitatory and inhibitory amino acid and peptidergic systems. The latter pathways have been recently proven to be involved both in the development of dependence and in counteracting the states related to relapse. An important role in neurochemical mechanisms of opioid reward, dependence and vulnerability to addiction has been ascribed to endogenous opioid peptides, particularly those acting via the mu- and kappa-opioid receptors. Opiate abuse leads to adaptive reactions in the nervous system which occur at the cellular and molecular levels. Recent research indicates that intracellular mechanisms of signal transmission-from the receptor, through G proteins, cyclic AMP, MAP kinases to transcription factors--also play an important role in opioid tolerance and dependence. The latter link in this chain of reactions may modify synthesis of target genes and in this manner, it may be responsible for opiate-induced long-lasting neural plasticity.

Time-dependent alterations in mRNA expression of brain neuropeptides regulating energy balance and hypothalamo-pituitary-adrenal activity after withdrawal from intermittent morphine treatment

Chronic stressors alter brain function and may leave traces after their relief. We used intermittent morphine treatment to examine the relationships between stress-induced changes in energy balance and hypothalamo-pituitary-adrenal (HPA) activity and the recovery thereafter. We studied the effects of morphine injections on energy balance, hormones and fat stores, brain neuropeptide expression, and the ACTH and corticosterone responses to restraint 12 hr after the final injection and 8 d later during recovery. Weight gain, food intake, and caloric efficiency decreased at morphine onset, and these were maintained throughout the morphine injections. At 12 hr, fat stores, leptin, insulin, and testosterone concentrations were reduced. Subsequently, body weight gain and food intake increased and caloric efficiency was above control during the final days. By the eighth recovery day, fat stores and peripheral hormones were no longer depressed. At 12 hr, an over-response of CRF mRNA to restraint occurred in the hypothalamus, similar to the facilitated ACTH and corticosterone responses. On day 8, the hypothalamic CRF mRNA response to restraint was still facilitated, opposite to inhibited ACTH responses. Hypothalamic CRF mRNA correlated highly with mesenteric fat weight in morphine-treated rats. We conclude that there is a prolonged recovery from chronic stressors involving interrelated changes in energy balance and HPA activity. Nonetheless, 8 d after withdrawal from morphine, rats still display facilitated central stress responses, similar to the HPA symptoms described in posttraumatic stress disorder patients. Repeated partial withdrawal associated with intermittent morphine treatment, compounded by complete withdrawal associated with termination of the treatment, is likely required for these metabolic and HPA derangements.

Expression of proopiomelanocortin and proenkephalin mRNA in sexually dimorphic brain regions are altered in adult male and female rats treated prenatally with morphine

The present study demonstrates that prenatal morphine exposure on gestation days 11-18 differentially alters proopiomelanocortin (POMC) and proenkephalin (pENK) mRNA in the hypothalamus and limbic system of adult male and female rats. In adult, prenatally morphine-exposed male rats POMC mRNA levels are decreased in the arcuate nucleus of the hypothalamus (ARC), while the pENK mRNA levels are increased in the paraventricular nucleus of the hypothalamus (PVN) and in the ventrolateral subdivision of the ventromedial nucleus of the hypothalamus (VMH), specifically in the ventrolateral subdivision of the VMH. In adult, prenatally morphine-exposed female rats, POMC mRNA levels in the ARC are increased in ovariectomized (OVX) but not in OVX, estradiol benzoate- (EB) or EB- and progesterone- (P) treated females. In contrast, pENK mRNA levels are decreased in the VMH of morphine-exposed, OVX females and increased in EB-treated females. Further, prenatal morphine exposure decreases pENK mRNA in the ARC and increases it in the medial pre-optic area independently of female gonadal hormones. Finally, POMC mRNA levels are increased in the ARC of saline-exposed, EB- or EB- and P-treated females but not in OVX females. Thus, the present study suggests that prenatal morphine exposure sex and brain region specifically alters the level of POMC and pENK mRNA.

Aminopeptidase activity in the postmortem brain of human heroin addicts

Several studies have reported that the chronic administration of opioids induces changes in the biosynthesis of endogenous opioid peptides or their precursors in specific brain regions of the adult central nervous system. However, little is known about the catabolic regulation of opioid peptides and its contribution to neuroadaptative changes underlying drug addiction. In the present study, we have analyzed the activity of two enkephalin-degrading enzymes (puromycin-sensitive aminopeptidase or PSA and aminopeptidase N or APN) and two functionally different, soluble aminopeptidases (aminopeptidase B and aspartyl-aminopeptidase) in postmortem samples of prefrontal cortex and caudate nucleus of eight human heroin addict brains and eight matched-controls. Enzyme activities were fluorimetrically measured using beta-naphthylamide derivatives. An increase in the activity of soluble PSA in the prefrontal cortex of heroin abusers was observed (heroin addict group: 51,452+/-3892 UAP/mg protein versus control group: 42,003+/-2597 UAP/mg protein; P<0.05), while the activity of the other peptidases in both brain regions remained unaltered. This result agrees with previous findings in morphine-tolerant rats, and indicates that soluble PSA may be involved in neurobiological processes which underlie heroin addiction.

Tolerance develops in spinal cord, but not in brain with chronic [Dmt1]DALDA treatment

Previously, we reported that H-2',6'-dimethyltyrosine [Dmt(1)]-d-Arg-Phe-Lys-NH(2) (DALDA), an analogue of the naturally occurring opioid peptide dermorphin, is a highly potent and selective mu receptor agonist with low cross-tolerance to morphine. In the present study, we investigated the effect of treating mice chronically with [Dmt(1)]DALDA. The AD(50) of [Dmt(1)]DALDA (s.c.) increased eight-fold in animals given this drug chronically; in contrast, the AD(50) increased two-fold in mice chronically treated with morphine. The AD(50) of morphine (s.c.) in these [Dmt(1)]DALDA-treated animals was increased more than 120 times, while that of the more selective mu agonist [d-Ala(2)-MePhe(4)-Gly-ol(5)]enkephalin (DAMGO) given intrathecally was increased more than 240 times. However, the AD(50) of DAMGO given intracerebroventricularly was essentially the same in animals treated chronically with [Dmt(1)]DALDA as in naive animals. The dose of naloxone required to precipitate withdrawal in [Dmt(1)]DALDA-treated animals was 20 times lower than that in morphine-tolerant animals. Using real-time quantitative PCR, we found that expression of the mu opioid receptor, delta opioid receptor, preproenkephalin and preprodynorphin genes was upregulated in the brain by [Dmt(1)]DALDA treatment. No significant changes in expression of opioid receptor or opioid peptide genes were detected in the spinal cord of [Dmt(1)]DALDA-treated mice, nor in the brain or spinal cord of morphine-treated mice. We conclude that a high degree of tolerance to [Dmt(1)]DALDA develops in the spinal cord but not brain, and cannot be accounted for by changes in expression of opioid receptors or opioid peptides in these tissues.

Alcoholism and obesity: overlapping neuropeptide pathways?

Ethanol is a caloric compound, and ethanol drinking and food intake are both appetitive and consummatory behaviors. Furthermore, both ethanol and food have rewarding properties. It is therefore possible that overlapping central pathways are involved with uncontrolled eating and excessive ethanol consumption. A growing list of peptides has been shown to regulate food intake and/or energy homeostasis. Peptides such as the melanocortins, corticotropin releasing factor, and cholecystokinin promote reductions of food intake while others such as galanin and neuropeptide Y stimulate feeding. The present review highlights research aimed at determining if ingestive peptides also regulate voluntary ethanol intake, with an emphasis on the melanocortins and neuropeptide Y. It is suggested that research directed at ingestive peptides may expand our understanding of the neurobiological mechanisms that drive ethanol self-administration, and may reveal new therapeutic candidates for treating alcohol abuse and alcoholism.

Modulation of proorphaninFQ/N gene expression by morphine in the rat mesocorticolimbic system

We studied the effects of acute and chronic morphine treatment on proorphaninFQ/N (proOFQ/N) gene expression in the mesocorticolimbic system, known to be a reward-relevant area, of the rat CNS. Northern blot analysis revealed that a single injection of morphine 10 mg/kg i.p. increased proOFQ/N mRNA levels in nucleus accumbens, temporo-parietal cortex and in striatum. The chronic administration of the opiate caused a significant increase of proOFQ/N mRNA levels in the ventral tegmental area and a decrease in the striatum and in the nucleus accumbens. No changes were observed in the prefrontal cortex. These data indicate for the first time that morphine alters proOFQ/N gene expression in mesocorticolimbic areas, supporting the direct interaction between the opioid and OFQ/N systems and the OFQ/N involvement in morphine-rewarding mechanisms.

Enkephalinergic neurons in the periaqueductal gray and morphine withdrawal

The effects of opioid (e.g., morphine) withdrawal on levels of endogenous opioid peptides and their mRNA in the various brain regions have been studied. However, the role of this opioidergic mechanism in the mediation of opioid withdrawal is not fully understood. Preproenkephalin (PPE) mRNA in the caudal periaqueductal gray (cPAG), an important brain region in opioid withdrawal, is increased by both opioid antagonist (naloxone)-precipitated and spontaneous morphine withdrawal, but not by various other stresses in rats, indicating a role of endogenous enkephalins in the cPAG in morphine withdrawal. In addition, PPE mRNA levels in the cPAG increase in the course of the dissipation of morphine withdrawal, and they are returned to the control levels after disappearance of morphine withdrawal signs. Local administration of an enkephalin analog or peptidase inhibitors into the cPAG suppresses morphine withdrawal signs. These facts suggest that enkephalinergic neurons in the PAG may have a critical role in the recovery phase of morphine withdrawal. Recently, an involvement of transcription factors in morphine withdrawal has been suggested. Thus, the possible role of transcription factors in the regulation of PPE gene expression in the cPAG during morphine withdrawal is also discussed.

Mu and delta opioid receptor regulation of pro-opiomelanocortin peptide secretion from the rat neurointermediate pituitary in vitro

We investigated the ability of selective opioid agonists and antagonists to influence pro-opiomelanocortin peptide secretion from the rat neurointermediate lobe in vitro. The mu-opioid agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]enkephalin) significantly stimulated beta-endorphin and alpha-melanocyte-stimulating hormone release relative to controls early (30 min) in the incubation period. Similar effects on beta-endorphin secretion were observed with the selective mu-opioid agonist dermorphin. The delta-opioid receptor agonist DPDPE ([D-Pen(2,5)]enkephalin) weakly inhibited beta-endorphin secretion relative to controls while the kappa-opioid receptor agonist U50488 had no effect. The mu-opioid selective antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)) inhibited basal beta-endorphin secretion while kappa- and delta-opioid receptor antagonists had no effect. Our data support a role for local mu-opioid receptor control of intermediate lobe pro-opiomelanocortin peptide secretion. Peptide secretion from melanotropes appears to be tonically stimulated by activation of mu-opioid receptors in the absence of intact neuronal innervation to the intermediate lobe.

The effect of repeated administration of morphine, cocaine and ethanol on mu and delta opioid receptor density in the nucleus accumbens and striatum of the rat

The present study was carried out to evaluate the effect of morphine, cocaine and ethanol on the density of opioid receptors in the nucleus accumbens and striatum of rat brain. The animals were injected i.p. with morphine in a single dose 20 mg/kg, or twice daily for 10 days in increasing doses of 20-100 mg/kg. Cocaine was administered in a dose of 60 mg/kg/day following the "binge" paradigm, every hour for 3 h, one day (single treatment) or five days (chronic treatment). Ethanol was administered in drinking water at increasing concentrations of 1-6% v/v, for one month. As shown by receptor autoradiography, single morphine and cocaine administration did not influence the binding density of the selective ligand of delta2 receptors [3H]Ile5,6deltorphin b, but single administration of cocaine decreased binding density of a highly selective antagonist of delta receptors, [3H]H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH. Repeated morphine administration decreased the receptor density after both ligands of the delta receptor in the nucleus accumbens after 3, 24 and 48 h, and in the striatum after 24 and 48 h. The density of [3H]Ile5,6deltorphin b binding remained unchanged in both structures following repeated cocaine administration. After repeated cocaine administration either no changes (3 h) or a decrease in the binding of [3H]H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH in the nucleus accumbens and striatum were observed after 24 and 48 h. Ethanol did not influence the binding density of [3H]H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH and [3H]Ile5,6deltorphin b in the nucleus accumbens and striatum at any time-point studied. In the nucleus accumbens and striatum, no changes were found in the binding density of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol following single or repeated morphine administration. At 3 h after single or repeated "binge" cocaine administration, the binding of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol was not changed in either structure, but after 24 h the density of mu opioid receptors was decreased in both structures. Ethanol given to rats in drinking water decreased the binding of [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol at the time of exposure to ethanol, yet in the nucleus accumbens only. Ethanol withdrawal decreased the density of the mu receptor in both structures after 24, 48 and 96 h. The above data indicate that repeated administration of morphine evokes a long-lasting down-regulation of the density of delta1 and delta2 opioid receptors, whereas cocaine affects in a similar way only the delta1 subtype in the nucleus accumbens, and to a lesser extent in the striatum. A long-term intake of ethanol solution down-regulates mu opioid receptors in both structures, but has no effect on any type of delta receptors. Thus changes in the particular opioid receptor depend on the type of drug used. Furthermore, the most profound changes are observed after late withdrawal, which may play some role in maintaining the state of dependence.

Methamphetamine alters prodynorphin gene expression and dynorphin A levels in rat hypothalamus

Chronic administration of morphine or cocaine affects opioid gene expression. To better understand the possible existence of common neuronal pathways shared by different classes of drugs of abuse, we studied the effects of methamphetamine on the gene expression of the opioid precursor prodynorphin and on the levels of peptide dynorphin A in the rat brain. Acute (6 mg/kg, intraperitoneally, i.p.) and chronic (6 mg/kg, i.p. for 15 days) methamphetamine markedly raised prodynorphin mRNA levels in the hypothalamus, whereas no effect was observed in the hippocampus. Dynorphin A levels increased after chronic treatment in the hypothalamus and in the striatum, whereas no significant changes were detected after acute treatment. These results indicate that methamphetamine affects prodynorphin gene expression in the hypothalamus, which may be an important site (also for its relevant neuroendocrine correlates) for opioidergic mechanisms activated by addictive drugs.

Proopiomelanocortin (POMC) mRNA expression: distribution and region-specific down-regulation by chronic morphine in female guinea pig hypothalamus

There is compelling evidence that endogenous opioid peptides are regulated by exogenous opiates. Our previous studies have shown that the mu-opioid receptor protein and mRNA are down-regulated in the mediobasal hypothalamus of the female guinea pig following chronic morphine treatment. In addition, electrophysiological studies have shown that hypothalamic beta-endorphin (beta-EP) neurons express mu-opioid receptors that are uncoupled and down-regulated following chronic morphine treatment. Currently, we tested the hypothesis that chronic morphine, which produces down-regulation of mu-opioid receptors, causes a down-regulation of pro-opiomelanocortin (POMC, the precursor of beta-EP) mRNA expression in female guinea pig hypothalamus. Female guinea pigs were ovariectomized and implanted subcutaneously (s.c.) with 4 x 75 mg pellets for 2 days plus six more pellets of either morphine (n = 6) or placebo (n = 6) for another 5 days. Animals were sacrificed between 1000 and 1100 h on day 7. The expression of POMC mRNA were investigated using in situ hybridization histochemistry with a guinea pig specific 35S-labeled cRNA probe in hypothalamic tissue sections. POMC mRNA was localized to the arcuate nucleus (Arc) and median eminence (ME) of the medial basal hypothalamus. The distribution pattern was the same in both morphine and placebo control animals. However, the density of silver grains was less in morphine treated animals versus placebo control animals. Overall, the level of POMC mRNA was decreased by 22% in the Arc of morphine-treated guinea pigs as compared with the placebo controls (p < 0.05). This decrease in POMC mRNA expression was even greater in the caudal Arc (28%, p < 0.01) in morphine-treated animals. These results suggested that the biosynthetic activity of POMC neurons is down-regulated with chronic exposure to morphine.

Effects of single and repeated morphine administration on the prodynorphin, proenkephalin and dopamine D2 receptor gene expression in the mouse brain

Effects of single (20 mg/kg i.p.) and repeated morphine administration (increasing doses: from 10 to 50 mg/kg i.p. twice daily for 7 days) on the proenkephalin (PENK), prodynorphin (PDYN) and dopamine D2 receptor (D2) mRNA levels in the nucleus accumbens and striatum of the mouse were investigated. As shown by an in situ hybridization, a single dose of morphine had no significant effect on the PDYN, PENK and D2 mRNA levels in the nucleus accumbens and striatum. Repeated treatment with morphine increased the PDYN mRNA level in both those structures after 2 and 72 h. In contrast to PDYN, the PENK mRNA level was reduced in the nucleus accumbens and remained unchanged in the striatum following repeated morphine administration. Repeated morphine had no effect on the D2 mRNA level in the nucleus accumbens and striatum after 2 h, and decreased it in the nucleus accumbens after 72 h only. The above results indicate that repeated morphine leads to long-lasting upregulation of the PDYN gene expression in the mouse nucleus accumbens and striatum; on the other hand, the PENK and D2 mRNA gene expressions are either inhibited or remain unchanged, significant changes being observed in the nucleus accumbens only.

Melanocortins and opiate addiction

Adrenocorticotropic hormone (ACTH) and alpha-melanocyte stimulating hormone (alpha-MSH) are centrally acting melanocortin peptides with numerous reported functions, including induction of excessive grooming and antipyresis, among others. Also reported is a role for melanocortins in aspects of opiate action. Although early work examined the effects of ACTH and MSH on opiate-induced behaviors, further progress has been limited. Recently, however, advances in the identification and characterization of melanocortin receptor (MC-R) subtypes have provided novel tools with which to study interactions between melanocortins and addiction. The present review discusses the effects of ACTH and MSH on opiate-induced behaviors and relates these findings to more recent reports on the regulation of melanocortin systems by exogenous opiates. Emerging from these data is the possibility that melanocortin receptor activation, specifically at the MC4-R subtype, may act to antagonize certain properties of exogenous opiates, including perhaps addiction.

Effect of morphine on proenkephalin gene expression in the rat brain

Previous studies indicate that an acute injection of morphine does not effect the level of opioid peptides and their mRNA in the brain. However, due to the presence of a large pool of mRNA and possible opposing changes in turnover rate it is often difficult to visualize the transitory and relatively small alterations in gene transcription by examining mRNA level. Therefore, in situ hybridization with probes directed against intronic sequences to measure the primary transcript of proenkephalin (PPE) mRNA (heteronucleic RNA, hnRNA) in the rat brain following morphine administration was used in this study. The distribution of the hybridization signal of probes against both the A and B intron of the PPE gene were identical and coincide with the distribution PPE mRNA. Thus, to increase the sensitivity of this assay both probes were concurrently hybridized. Female and male Sprague-Dawley rats were gonadectomized and injected with morphine (10 mg/kg, SC). We detected no changes in PPE mRNA levels in the striatum, olfactory tubercle (OT) and n. accumbens core (NAC) at any time following morphine administration. However, from 0.5 h until 24 h following morphine injection, the levels of PPE hnRNA in NAC and OT but not in the dorsal striatum were significantly decreased. The level of c-fos mRNA was increased only the dorsal striatum following morphine injections. These data show that morphine administration can acutely change opioid peptide gene transcription. The observed decrease of PPE hnRNA levels for 24 h following a single morphine injection may indicate its importance for the development of acute and chronic dependence. However, the significance of these alterations in PPE gene transcription in term of the acute effect of morphine is not clear, because the steady-state level of mRNA was not changed.

Increase of preproenkephalin mRNA in the caudal part of periaqueductal gray by morphine withdrawal in rats: a quantitative in situ hybridization study

Effects of morphine withdrawal on the levels of preproenkephalin (PPE) mRNA in the area from lateral to ventrolateral periaqueductal gray (PAG) were studied in rats by quantitative in situ hybridization. PPE mRNA in the rostral PAG was decreased by naloxone-precipitated morphine withdrawal but not affected by spontaneous morphine withdrawal. PPE mRNA in the caudal PAG was increased by both spontaneous and naloxone-precipitated morphine withdrawal.

Effect of morphine on proopiomelanocortin gene expression and peptide levels in the hypothalamus

Opiates have been reported to suppress POMC in the medial basal hypothalamus (MBH) but studies have been complicated by the fact that acutely, in the rat, opiates stimulate corticosterone and inhibit gonadal steroid release, which could both affect POMC in brain. We have therefore examined POMC gene expression and peptide levels in the MBH of castrated rats after 10 days of treatment with subcutaneous morphine or placebo pellets and after pellet removal. POMC mRNA was measured by solution hybridization assay and beta-endorphin (beta-EP) and alpha-MSH were measured by RIA. In castrated male rats, the mean POMC mRNA concentration in the MBH was 1.67 +/- 0.11 pg/microgram RNA in the control animals and decreased to 1.17 +/- 0.11 pg/microgram RNA in the morphine-treated animals (P < 0.01). Similarly in castrated, estradiol replaced female rats, the mean POMC mRNA level in the MBH was 1.36 +/- 0.19 pg/microgram RNA and decreased to 0.82 +/- 0.08 pg/microgram RNA after morphine treatment (P < 0.05). beta-EP levels were not significantly different in either study. When castrated male rats were similarly morphine pelleted and killed either on day 10 or 2 days later after pellet removal, the mean POMC mRNA level again fell from 1.83 +/- 0.21 in the controls to 1.28 +/- 0.20 pg/microgram RNA after 10 days of morphine; 2 days after pellet removal levels remained suppressed at 0.80 +/- 0.08 pg/microgram RNA (P < 0.01). In this study the concentrations of beta-EP and alpha-MSH were both noted to decline in the MBH after morphine treatment (P < 0.05). When the forms of beta-EP in the MBH were characterized by HPLC, a decrease in the concentration of beta-EP was again seen after morphine but no significant differences in the pattern of beta-EP processing or in the relative amounts of beta-EP1-31 compared to beta-EP1-27 and beta-EP1-26 were noted in morphine-treated animals. There was also no significant effect of 10(-6)-10(-4) M morphine on basal or KCl-stimulated release of beta-EP or gamma 3-MSH release from the perifused rat hypothalamus in vitro. We conclude that morphine suppresses POMC gene expression in the hypothalamus of chronically treated male and female rats. Persistent changes were also noted during morphine withdrawal. In some cases this was accompanied by a fall in beta-EP peptide content. These effects were seen in castrated animals with and without sex steroid replacement and are thus independent of the effects of morphine on the pituitary-gonadal axis. These results show that opiate drugs modify endogenous opioid systems in the brain and provide further support for the hypothesis that such changes may contribute to mechanisms of opiate dependence and withdrawal.

Chronic intracerebroventricular cocaine differentially affects prodynorphin gene expression in rat hypothalamus and caudate-putamen

We investigated the effects of sustained administration of cocaine on the regulation of prodynorphin gene expression in rat brain. Intracerebroventricular (i.c.v.) infusion of cocaine hydrochloride (30 micrograms/day) for 7 days, by means of osmotic minipumps, elicited a significant 35% decrease of prodynorphin mRNA levels in rat hypothalamus and increase (22%) in caudate-putamen. At the same time and in the same animals, no significant changes were detected in the hippocampus or in the nucleus accumbens. These results indicate that continuously infused cocaine is able to modulate expression of the prodynorphin gene in opposite directions or has no effect on prodynorphin expression, depending on the brain region analysed. Cocaine, as well as opiates, might activate specific neuronal pathways, shared by different classes of drugs of abuse, involving, at least in part, the endogenous opioid system.

The effect of single and repeated morphine administration on the prodynorphin system activity in the nucleus accumbens and striatum of the rat

Pharmacological data indicate that prodynorphin peptides and exogenous kappa agonists affect opioid tolerance and dependence. In order to elucidate the activity of the endogenous prodynorphin system during opiate tolerance and dependence, we investigated the effect of single and repeated morphine administration on the alpha-neoendorphin tissue level, its in vitro release, and the prodynorphin messenger RNA level in the nucleus accumbens and striatum of the rat. Acute and repeated morphine administration (14 days, increasing doses, 20-100 mg/kg, i.p.) increased the level of alpha-neoendorphin in the nucleus accumbens after 3 h; a similar effect was observed at 24 and 48 h after the last chronic morphine injection. On the other hand, the basal and stimulated (K+, 57 mM) release of alpha-neoendorphin from nucleus accumbens slices were significantly elevated only at 24 h after the last morphine injection. The prodynorphin messenger RNA hybridization signal in the nucleus accumbens was enhanced at 3 h after acute morphine injection, whereas repeated morphine administration decreased the messenger RNA level at that time point. Upon late chronic morphine withdrawal (at 24 and 48 h), the prodynorphin messenger RNA level in that tissue was significantly elevated. In the striatum, single morphine administration had no effect on the alpha-neoendorphin tissue level, release of the peptide, and prodynorphin messenger RNA level. On the other hand, chronic injection of morphine elevated all those parameters. The tissue level of alpha-neoendorphin was elevated at 3 h, and was back to normal at 24 and 48 h after the last drug injection. Both the basal and stimulated alpha-neoendorphin release from striatal slices was significantly increased at all the time points studied. Repeated morphine administration elevated the striatal prodynorphin messenger RNA level at 24 and 48 h after the drug withdrawal. Addition of morphine to the incubation medium reduced the basal release of alpha-neoendorphin in both the nucleus accumbens and striatal slices in naive animals, whereas the stimulated release was attenuated in the latter tissue only. The present study indicates that withdrawal of chronic morphine leads to enhancement of the prodynorphin neurons activity in the nucleus accumbens and striatum of the rat. It is suggested that these effects may participate in the mechanism of aversive reactions during withdrawal.

Effects of chronic opiate and opioid antagonist treatment on striatal opioid peptides

It has long been speculated that feedback inhibition of endogenous opioid neurons may have a role in opiate tolerance and dependence. However, in studies in which opiates or opioid antagonists have been administered to animals, mixed results have been obtained on the ability of these drugs to regulate endogenous opioids. The present studies were undertaken to determine the effects of chronic administration of opiate drugs on opioid peptides. These studies focused on the regulation of prodynorphin (Prodyn) and proenkephalin (Proenk) peptides in striatal tissue. Morphine, whether administered by chronic infusion or repeated injection, was found to increase the concentration of Prodyn peptides in striatum. Increases were statistically significant in the sensorimotor dorsal striatum (caudate-putamen) but not in the limbic-motor ventral striatum (nucleus accumbens-olfactory tubercle). No changes in Prodyn peptides were found following chronic administration of the opioid antagonist naltrexone. No changes in the Proenk peptide MERGL were found following chronic treatment with morphine or naltrexone. These studies are consistent with the suggestion that Prodyn neurons may have a role in the consequences of long-term opiate administration.

Long-term exposure to opioid antagonists up-regulates prodynorphin gene expression in rat brain

We investigated the effect of long-term administration of opioid antagonists on the regulation of prodynorphin gene expression in rat brain. Intracerebroventricular (i.c.v.) injections for seven days of nor-binaltorphimine (nor-BNI), the highly selective kappa opioid antagonist, naloxone and its longer acting analog naltrexone, both relatively selective antagonists for the mu opioid receptor, markedly raised prodynorphin mRNA levels in rat hypothalamus, hippocampus and striatum. Peptides, namely immunoreactive-dynorphin A (ir-dyn A), were unaffected after chronic treatment with all antagonists, in the same tissues. These results, taken together with our previous observations, suggest that chronic opioid antagonists, acting on kappa and mu opioid receptors, clearly up-regulate prodynorphin gene expression in discrete rat brain regions, activating its biosynthesis. Moreover, our data support the hypothesis that the endogenous opioid system plays a role in the mechanisms underlying the development of opiate tolerance.

Effect of opioid antagonism on beta-endorphin processing and proopiomelanocortin-peptide release in the hypothalamus

Previous studies have shown that chronic opioid receptor blockade has significant effects on POMC gene expression and peptide levels in the hypothalamus. We have now examined the effects of the opioid antagonist naltrexone on beta-EP processing in the hypothalamus and on the release of 2 POMC-derived peptides, beta-EP and gamma 3-MSH, from the perifused hypothalamus in vitro. The beta-EP immunoactivity in the medial basal hypothalamus (MBH) of 7 rats infused for 1 week with naltrexone by osmotic minipump, was individually analyzed by HPLC and compared to 7 control rats. The mean ratio of beta-EP1-31 compared to beta-EP1-27 plus beta-EP1-26 was 2.34 +/- 0.41 in the naltrexone treated rats, significantly higher than the ratio of 1.26 +/- 0.09 in the control rats (P < 0.02). Thus in the setting of chronic opioid antagonism although beta-EP content decreases, there is relatively more beta-EP1-31, the biologically active opioid form of the peptide, compared to the C-terminally cleaved forms of beta-EP which have reduced biological activity. To study the effects of naltrexone on beta-EP and gamma 3-MSH release, hypothalami were perifused in vitro with 10(-6) M naltrexone. Basal release of gamma 3-MSH was significantly higher from the naltrexone treated brains compared to the controls (221 +/- 20 pg/60 min vs. 161 +/- 6.7 pg/60 min) (P < 0.01); KCl stimulated gamma 3-MSH was also significantly higher in the naltrexone group (951 +/- 94 vs. 543 +/- 85 pg/60 min) (P < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of morphine withdrawal syndrome on endo-oligopeptidase (EC 3.4.22.19) activity

Endo-oligopeptidase (EC 3.4.22.19), an enzyme capable of generating enkephalin by single cleavage from enkephalin-containing peptides, was examined in several areas of the central nervous system (CNS) as well as in the immune and endocrine tissues of rats chronically treated with morphine and submitted to naloxone-induced withdrawal. A specific fluorogenic substrate was used to determine the endopeptidase 22.19 activity. A non-uniform increase in endopeptidase 22.19 activity was detected in the CNS. The highest increase in endopeptidase 22.19 specific activity was found in the dorsal hippocampus (about 3.5-fold higher than control), followed by occipital and frontal cortex, substantia nigra, thalamus and hypothalamus. In peripheral tissues, a significant decrease of endopeptidase 22.19 was observed in the pineal gland, whereas the morphine withdrawal syndrome caused a slight but significant increase in lymphoid tissues such as lymph nodes and thymus. These findings are indicative of a possible participation of endopeptidase 22.19 in naloxone-induced withdrawal.

Effects of morphine on prolactin receptors in the rat brain

The effect of chronically given morphine on the binding of ovine prolactin (oPRL) to specific areas in the male rat brain was studied. The drug was delivered through subcutaneously implanted miniosmotic pumps. The results indicated that the density of prolactin binding sites in the hypothalamus and the choroid plexus was significantly decreased in the acute phase of morphine administration but restored to control levels when tolerance to morphine was developed. The decrease in prolactin binding was contrasted by elevated plasma levels of the hormone. A negative correlation was found between the hormone concentration in plasma and the density of its binding sites in the hypothalamus and choroid plexus. The hormone-binding sites in these two regions were further characterized with regard to binding constants and molecular sizes. The relevance of the present results with respect to the hypothalamic control of prolactin secretion is discussed.

Opioid regulation of proopiomelanocortin (POMC) gene expression in the rat brain as studied by in situ hybridization

Proopiomelanocortin (POMC) is the precursor of the potent opioid peptide beta-endorphin as well as a number of other active peptides. On the basis of neuroanatomical data indicating the presence of contacts between POMC neurons in the rat arcuate nucleus, it has been proposed that POMC neurons could be autoregulated. In order to investigate the role of opiates in the regulation of POMC gene expression in the rat arcuate nucleus, we studied the effects of chronic administration of the opioid drug morphine and an opiate receptor antagonist naloxone on POMC mRNA levels as measured by in situ hybridization, 4-day treatment with naloxone (4 mg/kg/day) produced a 60% increase in the number of silver grains overlying POMC neurons. Conversely, morphine (40 mg/kg/day) also administered during 4 days decreased the hybridization signal by 30%. The concomitant administration of morphine and naloxone completely prevented the effect of morphine on POMC gene expression indicating that the inhibitory influence of morphine is likely to be mediated by opioid receptors. The data obtained clearly indicate that activation of opioid receptors decreased the biosynthetic activity of POMC neurons and that conversely opiate receptor blockade caused an increase in the activity of these neurons. They are consistent with the hypothesis of an autoregulation of the POMC neuronal system by endogenous opiate peptide(s).

Effects of chronic morphine treatment on beta-endorphin-related peptides in the caudal medulla and spinal cord

The effects of chronic morphine treatment on beta-endorphin (beta E)-immunoreactive (beta E-ir) peptide levels were determined in the rat caudal medulla and different areas of the spinal cord. Seven days of morphine pelleting had no effect on total beta E-ir peptides in the caudal medulla. In contrast, it significantly increased beta E-ir peptide concentrations in the cervical and thoracic regions of the spinal cord compared with placebo-pelleted controls, whereas in the lumbosacral region this trend did not reach statistical significance. Injections of the opiate receptor antagonist naloxone 1 h before the rats were killed had no effect on the morphine-induced increases in the cord. Chromatographic analyses revealed that enzymatic processing of beta E-related peptides in the spinal cord seemed unaffected by the morphine and/or naloxone treatments. In light of previous data showing that morphine down-regulates beta E biosynthesis in the hypothalamus, the present results suggest that the regulation of beta E-ir peptides in the spinal cord is distinct from that found in other CNS areas. These data provide support for previous results suggesting that beta E-expressing neurons may be intrinsic to the spinal cord.

Pre- and posttranslational regulation of beta-endorphin biosynthesis in the CNS: effects of chronic naltrexone treatment

There appear to be two anatomically distinct beta-endorphin (beta E) pathways in the brain, the major one originating in the arcuate nucleus of the hypothalamus and a smaller one in the area of the nucleus tractus solitarius (NTS) of the caudal medulla. Previous studies have shown that these two proopiomelanocortin (POMC) systems may be differentially regulated by chronic morphine treatment, with arcuate cells down-regulated and NTS cells unaffected. In the present experiments, we examined the effects of chronic opiate antagonist treatment on beta E biosynthesis across different CNS regions to assess whether the arcuate POMC system would be regulated in the opposite direction to that seen after opiate agonist treatment and to determine whether different beta E-containing areas might be differentially regulated. Male adult rats were administered naltrexone (NTX) by various routes for 8 days (subcutaneous pellets, osmotic minipumps, or repeated intraperitoneal injections). Brain and spinal cord regions were assayed for total beta E-ir, different molecular weight immunoreactive beta-endorphin (beta E-ir) peptides, and POMC mRNA. Chronic NTX treatment, regardless of the route of administration, reduced total beta E-ir concentrations by 30-40% in diencephalic areas (the arcuate nucleus, the remaining hypothalamus, and the thalamus) and the midbrain, but had no effect on beta E-ir in the NTS or any region of the spinal cord. At the same time, NTX pelleting increased POMC mRNA levels in the arcuate to approximately 140% of control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic opiate agonists down-regulate prodynorphin gene expression in rat brain

The effects of long-term administration of opioid agonists on the regulation of prodynorphin gene expression in rat brain were investigated. Chronic intracerebroventricular treatment with the synthetic opioid agonist acting on the kappa receptor, U-50,488H, and the classic mu agonist morphine markedly decreased prodynorphin mRNA levels in hypothalamus, hippocampus and striatum of tolerant rats. Levels of ir-Dynorphin A remained unchanged except in two cases. Chronic exposure to opiates thus appears to induce modifications of the endogenous opioid system, as regards gene expression regulation.

Effects of morphine treatment on pro-opiomelanocortin systems in rat brain

In previous studies to determine whether chronic opiate administration might negatively feedback upon endogenous opioid systems in the CNS, investigators found no changes in steady-state concentrations of opioid peptides following morphine pelleting. However, since only steady-state levels were measured, it was still not clear whether morphine treatment altered the release and/or biosynthesis of opioid-containing neurons. The goal of the present study was to assess the effects of chronic morphine pelleting on the dynamics of beta-endorphin (beta E) biosynthesis in rats. Hence, at several times during a 7-day morphine treatment, concentrations of total beta E-immunoreactivity (-ir), as well as chromatographically sieved forms of beta E, were determined by RIA, and mRNA levels of pro-opiomelanocortin (POMC) were measured by a solution phase protection assay using a mouse or rat POMC 32P-labelled riboprobe. Concentrations of total beta E-ir or different forms of beta E-ir peptides (i.e. beta-lipotropin, beta E1-31, or beta E1-27/beta E1-26) in the hypothalamus or midbrain following either 1 or 7 days of treatment were similar in morphine- and placebo-pelleted animals. However, a significant increase in total hypothalamic beta E-ir was observed following 3 days of morphine pelleting; chromatographic analyses indicated that this was primarily due to a selective increase in the opiate inactive forms of beta E, i.e. beta E1-27/beta E1-26. After 7 days of pelleting, morphine-treated animals tended to have lower POMC mRNA levels than those of placebo controls (20 to 50% decrease in different studies). The accumulation of hypothalamic beta E-ir at 3 days, and the apparent decline in POMC mRNA levels at 7 days, lend support to the hypothesis that morphine negatively feeds back upon POMC neurons in the brain by inhibiting beta E release and biosynthesis.

Pharmacology of neuronal gene expression

Pharmacological treatments were used to estimate trans-synaptic regulation of opioid peptide gene expression occuring at specific neurotransmitter receptors. In vitro and in vivo studies have shown that different signal-transduction mechanisms regulate the transcription of proenkephalin, proopiomelanocortin and nerve growth factor mRNA. The activation of receptors coupled to adenylate cyclase elicited the increase of proenkephalin and nerve growth factor gene expression. Therefore, a cAMP-dependent mechanism was suggested to be involved in such regulation. However, the temporal delay between the elevation of the intracellular cAMP content and the increase in nerve growth factor and proenkephalin mRNAs prompted us to investigate whether additional mechanisms associated with the second messenger were operative in the regulation of the expression of these two genes. We report evidence that a protein(s), probably functioning as a trans-acting factor, might be involved in the regulation of nerve growth factor gene transcription. The characterization and isolation of these DNA regulatory proteins will provide the pharmacologist with valuable information for the development of new compounds in the therapy of mental disorders.