Genotypic differences in mesolimbic enkephalin gene expression in DBA/2J and C57BL/6J inbred mice

Alcohol Withdrawal and the Associated Mood Disorders-A Review

Recreational use of alcohol is a social norm in many communities worldwide. Alcohol use in moderation brings pleasure and may protect the cardiovascular system. However, excessive alcohol consumption or alcohol abuse are detrimental to one's health. Three million deaths due to excessive alcohol consumption were reported by the World Health Organization. Emerging evidence also revealed the danger of moderate consumption, which includes the increased risk to cancer. Alcohol abuse and periods of withdrawal have been linked to depression and anxiety. Here, we present the effects of alcohol consumption (acute and chronic) on important brain structures-the frontal lobe, the temporal lobe, the limbic system, and the cerebellum. Apart from this, we also present the link between alcohol abuse and withdrawal and mood disorders in this review, thus drawing a link to oxidative stress. In addition, we also discuss the positive impacts of some pharmacotherapies used. Due to the ever-rising demands of life, the cycle between alcohol abuse, withdrawal, and mood disorders may be a never-ending cycle of destruction. Hence, through this review, we hope that we can emphasise the importance and urgency of managing this issue with the appropriate approaches.

Hypothalamic neuropeptide signaling in alcohol addiction

The hypothalamus is now known to regulate alcohol intake in addition to its established role in food intake, in part through neuromodulatory neurochemicals termed neuropeptides. Certain orexigenic neuropeptides act in the hypothalamus to promote alcohol drinking, although they affect different aspects of the drinking response. These neuropeptides, which include galanin, the endogenous opioid enkephalin, and orexin/hypocretin, appear to stimulate alcohol intake not only through mechanisms that promote food intake but also by enhancing reward and reinforcement from alcohol. Moreover, these neuropeptides participate in a positive feedback relationship with alcohol, whereby they are upregulated by alcohol intake to promote even further consumption. They contrast with other orexigenic neuropeptides, such as melanin-concentrating hormone and neuropeptide Y, which promote alcohol intake under limited circumstances, are not consistently stimulated by alcohol, and do not enhance reward. They also contrast with neuropeptides that can be anorexigenic, including the endogenous opioid dynorphin, corticotropin-releasing factor, and melanocortins, which act in the hypothalamus to inhibit alcohol drinking as well as reward and therefore counter the ingestive drive promoted by orexigenic neuropeptides. Thus, while multiple hypothalamic neuropeptides may work together to regulate different aspects of the alcohol drinking response, excessive signaling from orexigenic neuropeptides or inadequate signaling from anorexigenic neuropeptides can therefore allow alcohol drinking to become dysregulated.

Endogenous opioids as substrates for ethanol intake in the neonatal rat: The impact of prenatal ethanol exposure on the opioid family in the early postnatal period

BACKGROUND

Despite considerable knowledge that prenatal ethanol exposure can lead to devastating effects on the developing fetus, alcohol consumption by pregnant women remains strikingly prevalent. Both clinical and basic research has suggested that, in addition to possible physical, behavioral, and cognitive deficits, gestational exposure to alcohol may lead to an increased risk for the development of later alcohol-related use and abuse disorders. The current work sought to characterize alterations in endogenous opioid signaling peptides and gene expression produced by ethanol exposure during the last days of gestation.

METHODS

Experimental subjects were 4-, 8-, and 12-day old infant rats obtained from pregnant females that were given daily intubations of 0, 1, or 2g/kg ethanol during the last few days of gestation (GDs 17-20). Using real-time RT-PCR, western blotting analysis, and enzyme immunoassays, we examined mRNA and protein for three opioid receptors and ligands in the nucleus accumbens, ventral tegmental area, and hypothalamus.

RESULTS

Three main trends emerged - (1) mRNA for the majority of factors was found to upregulate across each of the three postnatal ages assessed, indicative of escalating ontogenetic expression of opioid-related genes; (2) prenatal ethanol significantly reduced many opioid peptides, suggesting a possible mechanism by which prenatal exposure can affect future responsiveness towards ethanol; and (3) the nucleus accumbens emerged as a key site for ethanol-dependent effects, suggesting a potential target for additional assessment and intervention towards understanding the ethanol's ability to program the developing brain.

CONCLUSION

We provide a global assessment of relatively long-term changes in both opioid gene expression and protein following exposure to only moderate amounts of ethanol during a relatively short window in the prenatal period. These results suggest that, while continuing to undergo ontogenetic changes, the infant brain is sensitive to prenatal ethanol exposure and that such exposure may lead to relatively long-lasting changes in the endogenous opioid system within the reward circuitry. These data indicate a potential mechanism and target for additional assessments of ethanol's ability to program the brain, affecting later responsiveness towards the drug.

Adenosine signaling in striatal circuits and alcohol use disorders

Adenosine signaling has been implicated in the pathophysiology of alcohol use disorders and other psychiatric disorders such as anxiety and depression. Numerous studies have indicated a role for A1 receptors (A1R) in acute ethanol-induced motor incoordination, while A2A receptors (A2AR) mainly regulate the rewarding effect of ethanol in mice. Recent findings have demonstrated that dampened A2AR-mediated signaling in the dorsomedial striatum (DMS) promotes ethanol-seeking behaviors. Moreover, decreased A2AR function is associated with decreased CREB activity in the DMS, which enhances goal-oriented behaviors and contributes to excessive ethanol drinking in mice. Interestingly, caffeine, the most commonly used psychoactive substance, is known to inhibit both the A1R and A2AR. This dampened adenosine receptor function may mask some of the acute intoxicating effects of ethanol. Furthermore, based on the fact that A2AR activity plays a role in goal-directed behavior, caffeine may also promote ethanol-seeking behavior. The A2AR is enriched in the striatum and exclusively expressed in striatopallidal neurons, which may be responsible for the regulation of inhibitory behavioral control over drug rewarding processes through the indirect pathway of the basal ganglia circuit. Furthermore, the antagonistic interactions between adenosine and dopamine receptors in the striatum also play an integral role in alcoholism and addiction-related disorders. This review focuses on regulation of adenosine signaling in striatal circuits and the possible implication of caffeine in goal-directed behaviors and addiction.

Acute ethanol administration differentially alters enkephalinase and aminopeptidase N activity and mRNA levels in regions of the nigrostriatal pathway

Opioid peptides play a key role in ethanol reinforcement and may also represent important determinants in brain sensitivity to ethanol through modulation of nigrostriatal dopaminergic activity. Regulation of opioid levels by peptidase-degrading enzymes could be relevant in ethanol's actions. The aim of this work was to study the acute ethanol (2.5 g/kg) effects on the activity and mRNA expression of enkephalinase (NEP) and aminopeptidase N (APN) in the rat substantia nigra (SN) and the anterior-medial (amCP) and medial-posterior (mpCP) regions of the caudate-putamen (CP). Enzymatic activities were measured by fluorometric assays and mRNA expression by reverse transcriptase polymerase chain reaction. Acute ethanol administration differentially altered peptidase activities and mRNA expression with different kinetics. Ethanol increased and decreased NEP mRNA levels in the SN and amCP, respectively, but produced biphasic effects in the mpCP. APN mRNA levels were increased by ethanol in all brain regions. Ethanol induced a transient and long-lasting increase in NEP (mpCP) and APN (amCP) activities, respectively. Peptidase activities were not changed by ethanol in the SN. Our results indicate that striatal NEP and APN are important ethanol targets. Ethanol-induced changes in these neuropeptidases in the CP could contribute to the mechanisms involved in brain sensitivity to ethanol.

Activity and expression of enkephalinase and aminopeptidase N in regions of the mesocorticolimbic system are selectively modified by acute ethanol administration

Opioid peptides play a key role in ethanol reinforcement and alcohol drinking behavior. However, regulation of opioid levels by peptidase-degrading activities in ethanol's actions in brain is still unclear. The aim of this work was to study the acute effects of ethanol (2.5 g/kg) on enkephalinase (NEP) and aminopeptidase N (APN) activities and expression in regions of the mesocorticolimbic system, as well as on corticosterone levels in serum for up to 24 h after administration. Enzymatic activities were measured by fluorometric assays, mRNA's expression by reverse transcriptase polymerase chain reaction (RT-PCR) and corticosterone levels by radioimmunoassay. Acute ethanol administration modified peptidase activity and expression with different kinetics. Ethanol induced a transitory increase and decrease in NEP and APN activities in the frontal cortex (FC) and ventral tegmental area (VTA), whereas only increases in these activities were observed in the nucleus accumbens (NAcc). Ethanol induced an increase in NEP mRNA in the FC and decreases in APN mRNA in the FC and NAcc. In contrast, ethanol produced biphasic effects on both enzymes expression in the VTA. Corticosterone levels were not changed by ethanol. Our results suggest that NEP and APN could play a main role in ethanol reinforcement through regulation of opioid levels in mesolimbic areas.

Ethanol exposure selectively alters beta-endorphin content but not [3H]-DAMGO binding in discrete regions of the rat brain

The dopaminergic mesocorticolimbic system plays an important role in the reinforcing effects of ethanol. Opioid peptides modulate the activity of this system and have been suggested to mediate, at least in part, the reinforcing properties of ethanol. Thus, beta-endorphin (beta-END) could participate in the development of ethanol reinforcement and addiction. The aim of this work was to investigate the acute and chronic ethanol effects on beta-END content in regions of the mesolimbic system and to examine if chronic ethanol treatment alters ligand binding to mu opioid receptor (muOR). Male Wistar rats received a single acute ethanol dose of 2.5 g/kg or water by intra-gastric administration. For chronic ethanol treatment experiments, one group of rats was given ethanol (10% v/v solution) to drink, two groups were given equivalent volumes of sucrose (14.14% isocaloric solution) or water, respectively, and a fourth group had ad libitum access to food and water. Treatment was followed for 4 weeks. Beta-endorphin content in brain regions was quantified by radioimmunoassay and ligand binding studies to muOR were performed by quantitative autoradiography using 8 nM [(3)H]-DAMGO as radioligand. Acute ethanol decreased beta-END content in the hypothalamus (26%) 1h after administration. No ethanol effects were observed in the midbrain, ventral tegmental area, substantia nigra, nucleus accumbens, nucleus accumbens-septum and prefrontal cortex. Chronic ethanol treatment neither changed beta-END levels nor [(3)H]-DAMGO binding to mu opioid receptors in any of the regions studied. However, beta-END levels in the sucrose group were significantly increased in the nucleus accumbens and substantia nigra, in comparison to all other groups. These findings suggest that different neural mechanisms and specific brain regions may be involved in the reinforcing effects of ethanol and sucrose.

Motivational effects of opiates in conditioned place preference and aversion paradigm--a study in three inbred strains of mice

RATIONALE

Interstrain differences in the motivational properties of morphine and heroin have been previously reported in mice, suggesting the involvement of a genotype-dependent modulation of the rewarding effects of opiates. Yet, interstrain differences in the motivational effects of naloxone have not been described.

OBJECTIVES

The aim of our study was to examine genotype modulation of the motivational effects of opiates in inbred stains of mice with known, distinct, opiate-induced phenotypes, and morphine-induced striatal transcriptional responses.

METHODS

We studied the rewarding properties of morphine (5, 10, and 20 mg/kg i.p.) and heroin (1, 5, and 10 mg/kg i.p.) in conditioned place preference (CPP) as well as the aversive properties of naloxone (1, 10, and 20 mg/kg i.p.) in the conditioned place aversion (CPA) paradigm in C57Bl/6J (C57), DBA/2J (DBA), and SWR/J (SWR) inbred strains of mice.

RESULTS

Our results show that morphine and heroin as well as naloxone induce CPP and CPA, respectively, in a genotype- and dose-dependent manner in these studied inbred strains of mice. Interestingly, C57 mice are the most sensitive in the case of the rewarding properties of morphine and heroin but are the least sensitive to the aversive effects of naloxone, whereas the DBA strain exhibit the opposite behavioral effects.

CONCLUSIONS

We suggest that motivational homeostasis can be modulated by mu opioid receptors in mice, with the C57 mice representing a genotype that is more sensitive to processes related to rewards, whereas the genotype of DBA is more sensitive to aversion.

Morphological correlates of emotional and cognitive behaviour: insights from studies on inbred and outbred rodent strains and their crosses

Every study in rodents is also a behavioural genetic study even if only a single strain is used. Outbred strains are genetically heterogeneous populations with a high intrastrain variation, whereas inbred strains are based on the multiplication of a unique individual. The aim of the present review is to summarize findings on brain regions involved in three major components of rodent behaviour, locomotion, anxiety-related behaviour and cognition, by paying particular attention to the genetic context, genetic models used and interstrain comparisons. Recent trends correlating gene expression in inbred strains with behavioural data in databases, morpho-behavioural-haplotype analyses and problems arising from large-scale multivariate analyses are discussed. Morpho-behavioural correlations in multiple strains are presented, including correlations with projection neurons, interneurons and fibre systems in the striatum, midbrain, amygdala, medial septum and hippocampus, by relating them to relevant transmitter systems. In addition, brain areas differentially activated in different strains are described (hippocampus, prefrontal cortex, nucleus accumbens, locus ceruleus). Direct interstrain comparisons indicate that strain differences in behavioural variables and neuronal markers are much more common than usually thought. The choice of the appropriate genetic model can therefore contribute to an interpretation of positive results in a wider context, and help to avoid misleading interpretations of negative results.

Genetic liability increases propensity to prime-induced reinstatement of conditioned place preference in mice exposed to low cocaine

RATIONALE

Relapse to drug use after periods of forced or self-imposed abstinence is a central problem in the treatment of addiction; therefore, identification of factors modulating the risk to relapse is a relevant goal of preclinical research.

OBJECTIVES

These experiments evaluated the influence of the amount of drug experienced, the duration of drug withdrawal, and individual liability on the propensity to cocaine-induced reinstatement of conditioned place preference (CPP).

MATERIALS AND METHODS

Mice from the inbred strains C57BL/6J and DBA/2J were trained for CPP with a high (20 mg/kg) or low (5 mg/kg) effective dose of cocaine. After CPP testing, all groups underwent extinction. Twenty-four hours after the extinction test, mice were challenged with saline, a cocaine dose unable to induce CPP (2.5 mg/kg) or an intermediate effective dose (10 mg/kg), and tested for CPP reinstatement. Additional groups of mice trained with the low cocaine dose were left undisturbed for 8 days after extinction test (long withdrawal), retested for extinction, and evaluated for prime-induced reinstatement (0, 2.5, 10 mg/kg of cocaine).

RESULTS

Mice trained with the high cocaine dose, but not with the low one, showed prime-induced reinstatement 24 h after the extinction test; DBA/2J mice trained with the low dose showed reinstatement after long withdrawal.

CONCLUSIONS

These results indicate that reinstatement of CPP by cocaine prime depends on the amount of drug experienced and on an interaction between individual liability and duration of drug abstinence and suggest that the risk to relapse into drug seeking is not prevented by moderated drug consumption.

Ethanol-induced changes in proenkephalin mRNA expression in the rat nigrostriatal pathway

Endogenous opioid systems have been suggested to play a key role in ethanol reinforcement mechanisms and alcohol-drinking behavior. Ethanol induces differential alterations in opioid peptide expression in brain areas of the reward circuits, which may be linked to the reinforcing effects of ethanol. In addition, ethanol-induced alterations in opioidergic nigrostriatal transmission could be involved in brain sensitivity to ethanol and play a role in addictive processes. The aim of this work was to study the effects of acute ethanol administration on proenkephalin (proenk) mRNA expression in the rat substantia nigra and caudate-putamen (CP) for up to 24 h post treatment. Male Wistar rats received ethanol (2.5 g/kg) or distilled water by intragastric administration, and proenk mRNA expression was studied by in situ hybridization and densitometry. Ethanol transiently increased proenk mRNA expression in the CP 1 h after drug administration. Proenk mRNA levels remained elevated 2 h post treatment in the anterior-medial and medial-posterior regions of the CP. In contrast, ethanol decreased proenk mRNA expression in the substantia nigra pars compacta and pars reticulata 2 h after drug exposure. Alterations in enkephalin expression in the substantia nigra and CP in response to ethanol exposure could be involved in the mechanisms underlying brain sensitivity to the drug.

Analysis of behavior using genetical genomics in mice as a model: from alcohol preferences to gene expression differences

Most familial behavioral phenotypes result from the complex interaction of multiple genes. Studies of such phenotypes involving human subjects are often inconclusive owing to complexity of causation and experimental limitations. Studies of animal models argue for the use of established genetic strains as a powerful tool for genetic dissection of behavioral disorders and have led to the identification of rare genes and genetic mechanisms implicated in such phenotypes. We have used microarrays to study global gene expression in adult brains of four genetic strains of mice (C57BL/6J, DBA/2J, A/J, and BALB/c). Our results demonstrate that different strains show expression differences for a number of genes in the brain, and that closely related strains have similar patterns of gene expression as compared with distantly related strains. In addition, among the 24 000 genes and ESTs on the microarray, 77 showed at least a 1.5-fold increase in the brains of C57BL/6J mice as compared with those of DBA/2J mice. These genes fall into such functional categories as gene regulation, metabolism, cell signaling, neurotransmitter transport, and DNA/RNA binding. The importance of these findings as a novel genetic resource and their use and application in the genetic analysis of complex behavioral phenotypes, susceptibilities, and responses to drugs and chemicals are discussed.

The role of neuropeptide FF (NPFF) in the expression of sensitization to hyperlocomotor effect of morphine and ethanol

Neuropeptide FF (NPFF) has been characterized as an endogenous anti-opioid peptide because its intraventricular injection (icv) reversed morphine- and stress-induced analgesia, and precipitates withdrawal syndrome in morphine-dependent rats. The role of NPFF in other aspects of drug dependence is unknown. Therefore, the aim of this study was to determine NPFF influence on the expression of sensitization to the morphine-induced hyperlocomotion. As the opioid system plays a role in ethanol effects, the influence of NPFF on the expression of sensitization to hyperlocomotor effect of ethanol was also investigated. Our study indicated that acute administration of NPFF (5, 10, 20nmol, icv) inhibited the expression of morphine-induced sensitization at doses of 10 (P<0.05) and 20nmol (P<0.01), and also inhibited ethanol-induced sensitization at a dose of 20nmol (P<0.01). Furthermore, NPFF inhibited the acute locomotor effect of morphine (10 and 20nmol) but not that of ethanol. NPFF, given alone, did not change the locomotor activity of mice and did not disturb motor coordination of animals in the rotarod test. In conclusion, our experiments indicated that NPFF attenuated the acute morphine locomotion and the expression of sensitization to locomotion. We anticipate that NPFF may be involved in both of these effects.

Cannabinoid-serotonin interactions in alcohol-preferring vs. alcohol-avoiding mice

Because cannabinoid and serotonin (5-HT) systems have been proposed to play an important role in drug craving, we investigated whether cannabinoid 1 (CB1) and 5-HT(1A) receptor ligands could affect voluntary alcohol intake in two mouse strains, C57BL/6 J and DBA/2 J, with marked differences in native alcohol preference. When offered progressively (3-10% ethanol) in drinking water, in a free-choice procedure, alcohol intake was markedly lower (approximately 70%) in DBA/2 J than in C57BL/6 J mice. In DBA/2 J mice, chronic treatment with the cannabinoid receptor agonist WIN 55,212-2 increased alcohol intake. WIN 55,212-2 effect was prevented by concomitant, chronic CB1 receptor blockade by rimonabant or chronic 5-HT(1A) receptor stimulation by 8-hydroxy-2-(di-n-propylamino)-tetralin, which, on their own, did not affect alcohol intake. In C57BL/6 J mice, chronic treatment with WIN 55,212-2 had no effect but chronic CB1 receptor blockade or chronic 5-HT(1A) receptor stimulation significantly decreased alcohol intake. Parallel autoradiographic investigations showed that chronic treatment with WIN 55,212-2 significantly decreased 5-HT(1A)-mediated [35S]guanosine triphosphate-gamma-S binding in the hippocampus of both mouse strains. Conversely, chronic rimonabant increased this binding in C57BL/6 J mice. These results show that cannabinoid neurotransmission can exert a permissive control on alcohol intake, possibly through CB1-5-HT(1A) interactions. However, the differences between C57BL/6 J and DBA/2 J mice indicate that such modulations of alcohol intake are under genetic control.

Ethanol exposure differentially alters pro-enkephalin mRNA expression in regions of the mesocorticolimbic system

RATIONALE

Opioid peptides have been suggested to play a major role in ethanol reinforcement mechanisms and alcohol drinking behaviour. However, in non-selected strains of rodents, it is not known whether opioid biosynthesis is a critical event in these processes.

OBJECTIVE

The aim of this work was to study the effects of a high dose of ethanol (2.5 g/kg body weight) on pro-enkephalin (pro-enk) mRNA expression in brain regions of the mesocorticolimbic system for up to 24 h after drug administration.

MATERIALS AND METHODS

Male Wistar rats were administered with ethanol (2.5 g/kg body weight) or distilled water and were killed 30 min, 1, 2, 4, 8 or 24 h after treatment. Coronal brain sections (20 mu) were obtained and pro-enk mRNA expression was studied by in situ hybridization and densitometry.

RESULTS

Acute ethanol administration induced a transient decrease and increase in pro-enk mRNA expression in the ventral tegmental area (33.2%) and prefrontal cortex (26.5%) 2 and 4 h after treatment, respectively. In contrast, ethanol induced prolonged increases in pro-enk mRNA expression in the core and shell regions of the nucleus accumbens, with different kinetics. Maximal effects were observed 2 h after ethanol exposure (core, 70.0%; shell, 60.0%).

CONCLUSIONS

Our results indicate that enkephalin expression in regions of the rat mesocorticolimbic system is differentially altered by acute ethanol treatment and suggest that enkephalins may play a key role in ethanol reinforcement mechanisms.

Effects of chronic ethanol administration on brain interstitial fluid levels of Methionine-enkephalin as measured by microdialysis in vivo

The level of Met-enkephalin in the brain is inversely correlated with ethanol consumption and is controlled partially through efflux activity of peptide transport system-1 (PTS-1) at the blood-brain barrier (BBB). Prolonged alcohol drinking can perturb aspects of this system, including a loss of control of Met-enkephalin levels at the transcriptional and translational levels, and impaired release of Met-enkephalin from tissue sources. Met-enkephalin levels in whole brain homogenates often first paradoxically increase after a few days of ethanol drinking and then decrease with the development of physical dependence. Which of those various changes drives the others is unclear. To clarify these interactions, we here determined the levels of Met-enkephalin in striatal interstitial fluid (ISF) by microdialysis, striatal tissue homogenates, and serum after chronic ethanol treatment and alcohol withdrawal. Mice received ethanol (5%) in liquid diet for 7 days (ethanol-treated) and others withdrawn for a day following 7-day treatment (withdrawal). There was a significant (P<0.05) difference in the levels of Met-enkephalin in striatal microdialysate between the control (79.1+/-5.9 pg/ml) and ethanol-treated group (94.9+/-4.3 pg/ml), which was lost by withdrawing ethanol (83.9+/-3.8 pg/ml). In contrast, ethanol treatment did not affect Met-enkephalin levels in the striatal tissue. In the ethanol-treated group, there was a significant (P<0.05) reduction of the levels of Met-enkephalin in serum to 70.5% of control levels. This decrease was restored to the level of control by withdrawing ethanol. These reversible changes in ISF and serum are readily explained by the known changes in the efflux activity of PTS-1 at the BBB.

Comparison of ethanol preference and neurochemical measures of mesolimbic dopamine and adenosine systems across different strains of mice

BACKGROUND

To extend the known phenotype of strains commonly used in the development of mutant mice, ethanol, saccharin, and caffeine preferences were examined in C57Bl/6J, CD-1, and hybrid C57Bl/6J x CD-1 mice. As dopaminergic mechanisms are inherently involved in the neuronal processing of many drugs of abuse (including ethanol), and an important role for adenosine-dopamine interactions has also been reported, the dopaminergic and purinergic neurochemical profiles of mice were compared against the consummatory phenotype observed.

METHODS

Ethanol (5% v/v), saccharin (0.1% w/v), and caffeine (0.1% w/v) consumption and preference were examined using a 2-bottle free-choice paradigm. Dopamine and adenosine receptor and transporter mRNA and protein density were quantified using in situ hybridization histochemistry and in vitro autoradiography, respectively.

RESULTS

C57Bl/6J and hybrid C57Bl/6J x CD-1 mice demonstrated a clear ethanol preference, voluntarily consuming large quantities of ethanol when given the choice between drinking vessels containing either ethanol or water. Conversely, CD-1 mice were characterized as ethanol-avoiding under the present paradigm. Differences in D(1) receptor mRNA between the strains were consistent with the observed behavioral differences in ethanol preference. The high ethanol-preferring phenotype of C57Bl/6J mice could not be directly linked to alterations in dopamine transporter neurochemistry and/or enkephalin levels as proposed by earlier researchers. Ethanol-seeking behavior appeared to correlate with D2 receptor expression, however, with evidence that ethanol-preferring mice also exhibit an increased density of D2 receptors within limbic dopaminergic projection nuclei. Interestingly, strain differences in the expression of the ethanol-sensitive nucleoside transporter paralleled differences in ethanol consumption, a novel finding consonant with purinergic involvement in dopamine-related behaviors.

CONCLUSIONS

This study has highlighted the relevance of alterations in dopamine receptor expression and purinergic modulation within the mesolimbic pathway and predisposition toward the development of ethanol-seeking behavior.

A neuropeptide FF agonist blocks the acquisition of conditioned place preference to morphine in C57Bl/6J mice

Neuropeptide FF behaves as an opioid-modulating peptide that seems to be involved in morphine tolerance and physical dependence. Nevertheless, the effects of neuropeptide FF agonists on the rewarding properties of morphine remain unknown. C57BL6 mice were conditioned in an unbiased balanced paradigm of conditioned place preference to study the effect of i.c.v. injections of 1DMe (D-Tyr1(NMe)Phe3]NPFF), a stable agonist of the neuropeptide FF system, on the acquisition of place conditioning by morphine or alcohol (ethanol). Morphine (10 mg/kg, i.p.) or ethanol (2 g/kg, i.p.) induced a significant place preference. Injection of 1DMe (1-20 nmol), given 10 min before the i.p. injection of the reinforcing drug during conditioning, inhibited the rewarding effect of morphine but had no effect on the rewarding effect of ethanol. However, a single injection of 1DMe given just before place preference testing was unable to inhibit the rewarding effects of morphine. By itself, 1DMe was inactive but an aversive effect of this agonist could be evidenced if the experimental procedure was biased. These results suggest that neuropeptide FF, injected during conditioning, should influence the development of rewarding effects of morphine and reinforce the hypothesis of strong inhibitory interactions between neuropeptide FF and opioids.

Influence of Ethanol Dependence and Methionine Enkephalin Antisense on Serum Endomorphin-1 and Methionine Enkephalin Levels

BACKGROUND

Opiate peptides are involved in the physical dependence on ethanol. Levels of methionine enkephalin (MEnk), for example, are affected by ethanol. No study on the effect of ethanol on endomorphin, the endogenous ligand for the mu-opiate receptor, has yet been conducted.

METHODS

We examined the effect of ethanol ingestion on serum endomorphin (EM)-1 and MEnk levels. We also determined the effect of antisense directed at MEnk on serum levels of EM-1 and MEnk.

RESULTS

Serum EM-1 levels steadily decreased about 20% during 56 days of ethanol ingestion in liquid feed, whereas a similar decrease in serum MEnk levels was not statistically significant. Serum MEnk levels decreased about 20% by 48 hr after antisense injection and then returned to baseline, whereas serum EM-1 levels increased by about 80% and remained elevated for about 2 weeks. In mice not treated with antisense or alcohol, there was no correlation between the serum levels of EM-1 and MEnk.

CONCLUSIONS

These results show that serum levels of EM-1 are decreased by physical dependence on ethanol and that this effect is not directly mediated through MEnk.

A Microdialysis Profile of Met-Enkephalin Release in the Rat Nucleus Accumbens Following Alcohol Administration

BACKGROUND

Pharmacological studies have implicated the endogenous opioid system in mediating alcohol intake. Other evidence has shown that alcohol administration can influence opioid activity. In this regard, the majority of studies have concentrated on endorphinergic systems, whereas other opioid systems have been granted comparably less attention. This is the case despite some compelling evidence that has implicated enkephalinergic peptide systems, particularly Met-enkephalin, in mediating alcohol preference. The aim of the present study was to investigate the effect of alcohol administration on extracellular levels of Met-enkephalin in the rat nucleus accumbens--a brain region that plays a significant role in the processes underlying reinforcement and stress.

METHODS

Male Sprague-Dawley rats were implanted with a microdialysis probe aimed at the shell region of the nucleus accumbens. Artificial cerebrospinal fluid was pumped at a rate of 1.75 mul/min in awake and freely moving rats and dialysates were collected at 30-minute intervals. After several baseline collections, rats were injected intraperitoneally with either physiological saline or one of four doses of alcohol: 0.8, 1.6, 2.4, or 3.2 g/kg ethanol body weight. The levels of Met-enkephalin in the dialysates were analyzed with solid-phase radioimmunoassay.

RESULTS

Within the first 30 minutes of administration, an alcohol dose of 1.6 g/kg caused a significant and prolonged elevation in the extracellular levels of Met-enkephalin. Alcohol did not have a major effect on the release of Met-enkephalin at any other dose.

CONCLUSIONS

In this experiment, only a moderate dose of alcohol was capable of stimulating Met-enkephalin release in the nucleus accumbens. Enkephalins may modulate local neurotransmitter release by binding to presynaptic Delta-opioid receptors, or, they may inhibit effector cells by binding to postsynaptic Delta- or mu-opioid receptors. This may be one of multiple neurological mechanisms that modulate alcohol-drinking behavior.

The opioidergic-alcohol link : implications for treatment

Preclinical and clinical data implicate the endogenous opioid system in alcohol dependence. In vitro studies show that rodent pituitary and hypothalamic tissue responds to acute exposure to alcohol by releasing beta-endorphins. In vivo studies suggest differential activity of endogenous opioid receptors in rodents with high and low alcohol preference. Similarly, humans with a family history of alcohol dependence also show a heightened endorphin response to an acute challenge of alcohol compared with those with no family history of alcohol dependence.The effects of opioid agonists and antagonists on rodent and human alcohol consumption further support the opioid-alcohol link. In rodents and humans, small doses of opioid agonists increase alcohol consumption, while pretreatment with large doses decreases consumption. The opioid antagonist naltrexone decreases rodent alcohol consumption, particularly in low doses under acute and intermittent schedules. Most clinical trials in patients with alcohol dependence support modest therapeutic effects of naltrexone in decreasing alcohol consumption. Efforts to identify subgroups of alcohol-dependent patients responsive to naltrexone, as well as psychosocial and pharmacological augmentation strategies, may further improve the clinical usefulness of the drug.

Behavioral phenotype of pre-proenkephalin-deficient mice on diverse congenic backgrounds

RATIONALE

The phenotype of genetically modified animals is thought to result from an interaction of gene manipulation with the genetic background and environmental factors.

OBJECTIVES

To test the behavioral and drug responses of Penk1(-/-) mice on different genetic backgrounds.

METHODS

Congenic C57BL/6J and DBA/2J mouse strains with a targeted deletion of the Penk1 gene were generated. Behavior and drug effects were tested in models of pain and anxiety.

RESULTS

Penk1(-/-) mice showed exaggerated responses to painful or threatening environmental stimuli, but the expressivity of the mutant phenotype was strongly dependent on the behavioral paradigm and on the genetic background. For example, elevated levels of anxiety were readily detectable in C57BL/6J-Penk1(-/-) mice in the light-dark and startle response tests, but not in the social interaction test. In contrast, we found elevated levels of anxiety in DBA/2J-Penk1(-/-) mice only in the zero-maze and social interaction tests. In some cases, the idiosyncratic behavior masked the appearance of the knockout gene effect. The activity of the anxiogenic drug, m-chlorophenylpiperazine, but not the anxiolytic drug diazepam, was strain and genotype dependent. Mice with the Penk1 mutation on the DBA/2J, but not on other genetic backgrounds, showed an increased opioid-dependent stress-induced analgesia.

CONCLUSIONS

(1) The behavioral effects of the Penk1 gene deletion persists on different genetic backgrounds, but its detection sometimes requires the use of different behavioral paradigms. (2) The behavior of the background strain should be considered in the analysis of knockout mice to avoid floor and ceiling effects, which may mask the phenotype.

Effects of abused inhalants and GABA-positive modulators in dizocilpine discriminating inbred mice

There is in vitro evidence that some of the effects of abused volatile solvents may be produced by actions at the NMDA receptor. In addition, some solvents produce phencyclidine-like discriminative stimulus effects. The major goal of the present study was to further compare abused solvents to NMDA antagonists by testing them in two strains of mice trained to discriminate 0.17 mg/kg of the very selective uncompetitive NMDA antagonist, dizocilpine, from saline and contrast those results with several GABA(A)-positive modulators, PCP and ethanol. The results indicated that the discriminative stimulus produced by 0.17 mg/kg dizocilpine was highly specific in both mouse strains. PCP produced 91% dizocilpine-lever responding in C57BL/6J mice, but only 56% dizocilpine-lever responding in DBA/2J mice. Pentobarbital, midazolam and ethanol produced at least some overlap in discriminative stimulus effects with dizocilpine in one or both mouse strains. In contrast, toluene, 1,1,1-trichloroethane (TCE), xylene and methoxyflurane produced saline-appropriate responding almost exclusively. These data indicate that, at least under the specific conditions tested, abused volatile solvents do not have substantial dizocilpine-like discriminative stimulus effects in either C57BL/6J or DBA/2J mice, providing little support that NMDA antagonism plays a central role in the production of this abuse-related effect.

Opioids and alcoholism

Although far from conclusive, evidence implicating the endogenous opioid system in the development and maintenance of alcoholism is growing. Currently available data suggest that ethanol increases opioid neurotransmission and that this activation is part of the mechanism responsible for its reinforcing effects. Findings from preclinical research indicate that ethanol consumption and ethanol-induced dopamine (DA) release are both reduced by opioid antagonists. Individual differences in endogenous opioid activity have been linked to inherited risks for alcoholism in studies comparing ethanol-preferring and nonpreferring rats, as well as in studies using targeted gene mutation (knockout) strategies. To a large extent, findings from human studies have paralleled those from the preclinical work. Persons who differ in family history of alcoholism have been shown to also differ in basal beta-endorphin activity, beta-endorphin response to alcohol, and subjective and HPA axis hormonal response to opioid antagonists. Findings from clinical trials indicate that opioid antagonists may reduce ethanol consumption in alcoholics, particularly in persons who have resumed drinking. Nevertheless, many questions remain unanswered about the use of opioid antagonists in alcoholism treatment and about the exact role of the opioid system in ethanol preference and reward. The progression of knowledge in this field suggests that many of these questions are imminently answerable, as our ability to characterize relationships between opioid activity and human behavior continues to develop. This paper summarizes both the progress that has been made and the gaps that remain in our understanding of the interactions between the endogenous opioid system and risk for alcoholism.

Differences in basal levels of CREB and NPY in nucleus accumbens regions between C57BL/6 and DBA/2 mice differing in inborn alcohol drinking behavior

An increasing body of evidence suggests that genetic factors play a role in alcohol drinking behaviors. C57BL/6J (C57) mice innately consume larger amounts of alcohol compared to that consumed by DBA/2J (DBA) mice. Furthermore, alterations in cAMP-responsive element binding (CREB) protein function in the brain have been implicated in alcohol drinking behaviors. The present investigation examined innate expression and phosphorylation of CREB in various brain structures of C57 and DBA mice. It was found that CREB expression and phosphorylation was lower, specifically in the shell structure of the nucleus accumbens, in C57 mice compared to that in DBA mice. CREB expression and phosphorylation were similar in other brain regions such as the nucleus accumbens core and the cortical, amygdaloid, hippocampal, and striatal structures of C57 and DBA mice. The expression of a cAMP-inducible gene, neuropeptide Y (NPY), was also investigated in the nucleus accumbens region of C57 and DBA mice. It was found that in C57 mice, NPY protein levels were lower in the shell but not in the core structure of the nucleus accumbens compared to that in DBA mice. It was also found that C57 mice are not innately anxious, but they consume larger amounts of alcohol than do DBA mice. Because the shell structure of the nucleus accumbens has been implicated in reward mechanisms of alcohol, it is possible that lower CREB function in this brain structure may be in part associated with the excessive alcohol drinking behavior of C57 mice.

Ultrastructural localization of Leu5-enkephalin immunoreactivity in mesocortical neurons and their input terminals in rat ventral tegmental area

Enkephalin (ENK) immunoreactivity is widely distributed in the ventral tegmental area (VTA), where endogenous ENK and dynorphin opioid peptides are known to have opposing actions in reward, stress, cognition, and fear-related behaviors. Many neurons in the VTA give rise to mesocortical projections terminating in the medial prefrontal cortex (mPFC), and these projections have been implicated to varying extents in all these functions. To determine whether there is a synaptic basis for ENK and/or dynorphin modulation of cortically projecting neurons within the VTA, we combined retrograde tract-tracing from the mPFC with dual immunocytochemical-labeling electron microscopy in the rat VTA. The retrograde tracer Fluorogold (FG) was microinjected into mPFC. At optimal survival periods, sections through the VTA were processed for immunolabeling of anti-FG and a Leu(5)-ENK antibody recognizing both ENK and dynorphin peptides. Over 26% of the retrogradely labeled neuronal somatodendritic profiles (n = 177) were contacted by ENK-immunoreactive axonal profiles including small axons and axon terminals. The axon terminals varied in their subcellular distribution of ENK immunoreactivity and also differed in forming either inhibitory-type (symmetric) or excitatory-type (asymmetric) synapses. Many of the axonal profiles also were apposed to FG-labeled somata or dendrites without forming recognizable synapses. Approximately one-third of the mesocortical neuronal perikarya also showed sparsely distributed somatodendritic ENK-immunoreactivity. Our results provide ultrastructural evidence that ENK and possibly dynorphin in the rat VTA have distributions consistent with involvement in diverse physiological actions affecting the output of mesocortical neurons, some of which also contain one or both peptides.

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.

Effects of chronic ethanol on brain and serum level of methionine enkephalin

Most evidence agrees that levels of methionine enkephalin (Met-Enk) in brain are inversely correlated with ethanol drinking and withdrawal seizures. One area of discrepancy is the effect of chronic ethanol administration on the level of immunoactive Met-Enk in brain, with some authors reporting increased and others reporting decreased levels. These reports differed greatly in terms of method of ethanol administration, species used, length of time ethanol was administered, and the region of brain examined. We found that all studies could be resolved by considering only length of time ethanol was administered, with Met-Enk levels first increasing and then decreasing. We tested this finding by determining the effect of 4-56 days of ethanol delivered in liquid feed on levels of brain Met-Enk. We found that brain levels of Met-Enk peaked after 7 days of ethanol ingestion and declined to levels lower than control by 28 days. Exposure to ethanol abolished a correlation between brain and serum levels of Met-Enk which occurred in controls. HPLC showed that whereas 100% of immunoactivity eluted in the position of Met-Enk in controls, only about 50% eluted as Met-Enk in mice exposed to ethanol. These results support the hypothesis that exposure to ethanol alters brain Met-Enk in a way consistent with the reinforcement of physical dependence.

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.

Ethanol consumption patterns and conditioned place preference in mice lacking preproenkephalin

There is a great deal of evidence suggesting that endogenous opioid systems are involved in the control of ethanol-seeking behavior and reward. To ascertain the role of the enkephalinergic opioid peptide system in these processes, we examined voluntary ethanol consumption patterns in mice lacking the preproenkephalin (Penk) gene using a two-bottle choice paradigm with free access to water and increasing concentrations of ethanol (2, 4, 8, and 10% v/v). We also examined the ability of ethanol (2 g/kg i.p.) to establish a conditioned place preference in these mice. No differences in ethanol consumption or preference were observed between wildtypes and Penk null mutant mice. In addition, both genotypes displayed a similar conditioned place preference to ethanol. These data suggest that the preproenkephalin system is not involved in voluntary ethanol consumption patterns or ethanol reward.

Novel approaches to targeting neuropeptide systems

Generation and/or interruption of cell signalling by neuropeptides has been shown to be essentially, although not exclusively, mediated by one or several membrane-bound enzymes, giving rise to the concept of selective versus dual enzyme inhibitors. Because most of these enzymes are zinc metallopeptidases, novel inhibitors are now being designed based on the structure of these proteins. The physiological role of neuropeptides and their relationships with other peptide systems can be investigated by comparing results obtained using peptidase inhibitors and selective receptor antagonists with those obtained using mice in which genes encoding the various components of a peptide system have been deleted. The potential use of peptidase inhibitors, compared with exogenous agonists, as therapeutic agents (particularly as analgesics or antidepressants) and their use in the investigation of the neurobiology of drug abuse will be discussed with particular focus on enkephalins and cholecystokinin 8 (CCK-8).

Acute ethanol administration induces changes in TRH and proenkephalin expression in hypothalamic and limbic regions of rat brain

Thyrotropin releasing hormone (TRH) present in several brain areas has been proposed as a neuromodulator. Its administration produces opposite effects to those observed with acute ethanol consumption. Opioid peptides, in contrast, have been proposed to mediate some of the effects of alcohol intoxication. We measured TRH content and the levels of its mRNA in hypothalamic and limbic zones 1-24 h after acute ethanol injection. We report here fast and transient changes in the content of TRH and its mRNA in these areas. The levels of proenkephalin mRNA varied differently from those of proTRH mRNA, depending on the time and region studied. Wistar rats were administered one dose of ethanol (intraperitoneal, 3 g/kg body weight) and brains dissected in hypothalamus, hippocampus, amygdala, n. accumbens and frontal cortex, for TRH quantification by radioimmunoassay or for proTRH mRNA measurement by RT-PCR. After 1 h injection, TRH levels were increased in hippocampus and decreased in n. accumbens; after 4 h, it decreased in the hypothalamus, frontal cortex and amygdala, recovering to control values in all regions at 24 h. ProTRH mRNA levels increased at 1 h post-injection in total hypothalamus and hippocampus, while they decreased in the frontal cortex. The effect of ethanol was also studied in primary culture of hypothalamic cells; a fast and transient increase in proTRH mRNA was observed at 1 h of incubation (0.001% final ethanol concentration). Changes in the mRNA levels of proTRH and proenkephalin were quantified by in situ hybridization in rats administered ethanol intragastrically (2.5 g/kg). Opposite alterations were observed for these two mRNAs in hippocampus and frontal cortex, while in n. accumbens and the paraventricular nucleus of the hypothalamus, both mRNA levels were increased but with different kinetics. These results give support for TRH and enkephalin neurons as targets of ethanol and, as possible mediators of some of its observed behavioral effects.

Neutral endopeptidase and alcohol consumption, experiments in neutral endopeptidase-deficient mice

Alcohol consumption was investigated in mice which were rendered deficient in the peptide-degrading enzyme neutral endopeptidase (EC 3.4.24.11) (NEP-/-) by gene targeting and compared to alcohol consumption in corresponding wild type mice (NEP+/+). Mice were offered a free choice to drink tap water or 10% alcohol. The NEP-/- mice consumed significantly more alcohol ( approximately 42%) than the NEP+/+ mice, whereas no significant differences were observed in the total fluid consumption. The daily food consumption of alcohol naive NEP-/- animals was elevated ( approximately 29%). Furthermore, the activities of peptidases closely related to neutral endopeptidase were analysed ex vivo in several brain regions from NEP-/- and NEP+/+ mice not treated with alcohol. There was no obvious compensation for the total loss of neutral endopeptidase by the functionally related peptidases angiotensin-converting enzyme and aminopeptidase N. In vitro, the degradation of exogenously applied [Leu(5)]enkephalin was not reduced in membrane preparations of those brain regions assayed in NEP-/- mice. A small reduction in [Leu(5)]enkephalin degradation was detected in striatal membrane preparations of NEP-/- mice, if aminopeptidase N was additionally blocked by bestatin or amastatin.

Opioid propeptide mRNA content and receptor density in the brains of AA and ANA rats

Recent evidence has indicated an association between the rewarding effects of ethanol intake and endogenous opioid activity. The present studies examine the presence of differences in opioid peptide mRNA content and mu and kappa opioid receptor densities, between ethanol naive AA and ANA rats bred selectively for their high and low alcohol consumption, respectively. In situ hybridization was used to compare the content of proopiomelanocortin, proenkephalin and prodynorphin mRNA in distinct brain regions known to be involved in the reinforcing properties of addictive drugs, between rats from each line. Results indicated that AA rats had a significantly greater content of proopiomelanocortin mRNA in the arcuate nucleus of the hypothalamus, of proenkephalin mRNA in the prefrontal cortex and of prodynorphin mRNA in the mediodorsal nucleus of the thalamus (p < or = .05). Receptor autoradiography was performed using 3H-labeled ligands specific for mu and kappa opioid receptors. AA rats were found to have a greater density of mu opioid receptors in the shell region of the nucleus accumbens and prefrontal cortex, but a lower density of kappa opioid receptors in the ventromedial hypothalamus, compared to ANA rats. The present data demonstrate the presence of inherited differences in the activity of distinct components of the endogenous opioid system in some brain regions associated with the processes of reward and reinforcement; and as such, may play a role in determining differences in ethanol drinking between AA and ANA rats.

The preoptic area/anterior hypothalamus of different strains of mice: sex differences and development

While sex differences in neural morphology in the preoptic area/anterior hypothalamus (POA/AH) have been demonstrated in many species, their existence in mice have been controversial. Given the increased use of transgenic and gene-disrupted mice, we characterized sex differences using Nissl stains, and the immunocytochemical location of estrogen receptor-alpha (ER-alpha) and galanin in the POA/AH of two widely used strains, C57BL/6 and 129SvEv, and a mixed strain (C57BL/6x129Sv); the wild-type littermates of steroidogenic factor-1 (SF-1) gene-disrupted mice. Cell grouping was not a reliable marker of sex. In adults, cells located beneath the anterior commissure (AC) were reliably larger in females than males in 129SvEv, but not in the other strains. Caudally, cells in a group medial to the medial extension of the bed nucleus of the stria terminalis (BST) were significantly larger in males than females in C57BL/6J and SF-1 gene-disrupted wild-types. Cell groups discernible by embryonic day (E) 18 were not sexually dimorphic for cell size in C57BL/6J mice at E18 or postnatal day (P) 4. The pattern of distribution of cells containing ER-alpha was similar among the strains, reduced in the group medial to the BST; a pattern established by P0. Galanin-containing cells and fibers were seen from E15 to adulthood ventral to the AC. Caudally, a smaller group ventromedial to the BST was found only in 129SvEv adults. Sex differences in neural morphology which develop within the POA/AH depend upon multiple factors, particularly including genetic background.

Comparison of the proopiomelanocortin and proenkephalin opioid peptide systems in brain regions of the alcohol-preferring C57BL/6 and alcohol-avoiding DBA/2 mice

Differences in the activity of distinct components of the endogenous opioid system between ethanol-preferring and ethanol-avoiding animals may be important in controlling their voluntary alcohol consumption. The objective of the present studies was to compare the activity of two opioid peptide systems (enkephalin and beta-endorphin) in distinct regions of the brain, between the C57BL/6 and DBA/2 mice, using sensitive radioimmunoassays, in situ hybridization, and immunohistochemical techniques. The immunohistochemical studies indicated that there was no significant difference in the number of either beta-endorphin or enkephalin immunopositive cells between the C57BL/6 and DBA/2 mice. The in situ hybridization studies demonstrated a 27% higher content of proopiomelanocortin mRNA in the arcuate nucleus of the C57BL/6 than DBA/2 mice, p < 0.02. The content of proenkephalin mRNA was 25% higher in the nucleus accumbens, p < 0.005 and 23% higher in the caudate putamen, p < 0.01, of the C57BL/6 than DBA/2 mice. There was no significant difference in the content of beta-endorphin peptides in the distinct brain regions investigated. The content of met-enkephalin-arg6-phe7 in the nucleus accumbens and caudate was similar between the two strains of mice, while it was significantly lower in the amygdala, hippocampus, ventral tegmental area, and periaqueductal grey of the C57BL/6 than DBA/2 mice. Thus, there are significant differences in the activity of these two endogenous opioid peptide systems in distinct regions of the brain, between ethanol naive C57BL/6 and DBA/2 mice, which may play a role in controlling their alcohol consumption.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Repeated administration of cocaine, nicotine and ethanol: effects on preprodynorphin, preprotachykinin A and preproenkephalin mRNA expression in the dorsal and the ventral striatum of the rat

It is established that dopamine (DA) controls the expression of preprodynorphin (PPDYN), preprotachykinin A (PPT-A) and preproenkephalin (PPE) mRNAs in striatal structures. Since cocaine, nicotine and ethanol enhance extracellular DA concentration, we have examined whether their repeated administration produced common changes in the expression of these mRNAs. Quantitative in situ hybridization histochemistry was performed in rats 2 h after a final challenge subsequent to repeated subcutaneous injections (3 X a day) of cocaine (12.5 mg/kg), nicotine (0.4 mg/kg) for 14 days and ethanol (160 mg/kg) for 7 days. In the dorsal striatum, cocaine produced simultaneous PPDYN and PPT-A mRNA increases without PPE mRNA change whereas nicotine and ethanol produced no modification. After cocaine, PPDYN mRNA was preferentially increased in striatal patch compartment. In the nucleus accumbens, the effects were more complex. In cocaine-treated rats, we measured concomitant increases of PPDYN and PPE mRNA in the rostral pole, an isolated induction of PPT-A mRNA signals in the core without any change in the two shell subregions: the cone and the ventral shell. In contrast, after nicotine and ethanol, the ventral shell was the only accumbal subregion which showed a neuropeptide mRNA alteration, nicotine leading to decreased PPDYN mRNA and ethanol to increased PPT-A mRNA contents. The neuropeptide regulation after chronic treatment with these psychostimulant drugs does not strictly conform to a general DA control scheme in the dorsal and the ventral striatum. The cocaine effects can be clearly distinguished from those of nicotine and ethanol in terms of neuropeptide regulation and striatal subregions affected.