Induction of fos-like immunoreactivity by opioids in guinea-pig brain

In the present study the effects of intracerebroventricular (i.c.v.) administration of 100 nmol of morphine, the selective μ-receptor agonist DAMGO, the δ-receptor agonist DPDPE and the κ-receptor agonist U50,488H, on the induction of Fos-like immunoreactivity (Fos-LI) in the guinea-pig brain were investigated using immunohistochemical techniques. Guinea-pigs given i.c.v. injection of opioids showed marked increases in the number of Fos-LI nuclei within a large number of brain regions, several of which, including hypothalamic nuclei, paraventricular thalamic nucleus, the amygdala, periaqueductal gray, superior and inferior colliculi, the piriform and entorhinal cortices, have been shown to be activated under stressful or aversive conditions. Pretreatment with the opioid antagonist, naltrexone, before administration of morphine or U50,488H, inhibited Fos-LI induction indicating that the effects of the opioids were mediated by opioid receptors. U50,488H administration resulted in higher numbers of Fos-LI stained neurons compared to morphine in most regions other than the nucleus accumbens and interpeduncular nucleus. Morphine and DAMGO produced significantly higher numbers of Fos-LI neurons in the nucleus accumbens shell region than U50,488H, which may reflect the more powerful reinforcing/rewarding effects of μ-receptor agonists. Thus the present study supports a critical role for the nucleus accumbens shell region in the reinforcing/rewarding effects of opioids.     

Patterns of glucose use after bicuculline-induced convulsions in relationship to γ-aminobutyric acid and μ-opioid receptors in the ventral pallidum — functional markers for the ventral pallidum

Bicuculline-induced convulsions increased glucose use throughout the brain and sharply demarcated the ventral pallidum and globus pallidus. Glucose use in the nucleus accumbens also increased after bicuculline-induced convulsions, except for a circumscribed region in the dorsomedial shell. Since the projection from the nucleus accumbens to the ventral pallidum contains γ-aminobutyric acid (GABA) and the opioid peptide, enkephalin, the pattern of increased glucose use in the ventral pallidum and nucleus accumbens after bicuculline-induced convulsions was compared to the topography of GABA_A and μ-opioid receptors. The pattern of glucose use in the nucleus accumbens and ventral pallidum resembled the topography of GABA_A, but differed from that of μ-opioid receptors. Bicuculline may disinhibit GABAergic efferents to the ventral pallidum resulting in a dramatic increase in glucose use within striatopallidal synaptic terminals as well as in local terminals of the pallidal projection neurons.     

GABAA receptor subunits in the human amygdala and hippocampus: Immunohistochemical distribution of 7 subunits

GABAergic neurotransmission in the amygdala contributes to the regulation of emotional processes in anxiety, stress, reward, mnestic functions, addiction, and epilepsy. Species‐specific differences in the distribution and composition of GABA_A receptors may account for distinct effects and side‐effects of GABAergic agents. However, data on the distribution and composition of GABA_A receptors in the human amygdala are lacking. Here, the expression of GABA_A receptor subunits α1, α2, α3, α5, β2, β2/3, and γ2 was studied in the human amygdala using immunohistochemistry. Hippocampi were evaluated as a reference structure. Neuronal counts and field fraction analyses were performed, and subcellular expression of GABA_A receptor subunits was analyzed semiquantitatively. In the amygdala, field fraction analyses showed the highest α1 expression in the lateral nucleus (La), whereas α3 was prominent in intercalated nuclei (IC), and α5 and γ2 in the cortical nuclei, and amygdalo‐hippocampal/parahippocampal‐amygdala transition areas. In the hippocampus, α1 and α3 were accentuated in the dentate gyrus, CA1 region, and subiculum, whereas α5 expression was rather uniform. In both regions, α2 was homogenously distributed, and the two β subunits and γ2 showed faint immunostaining. The intensity of subunit expression also varied in the neuropil, neuronal somata, and/or cellular processes in the subregions. GABA_A receptors containing subunit α1, showing the strongest expression in the La, and α3, with the strongest expression in the IC and subiculum, could be targets for treating amygdala‐related disorders. Differences in GABA_A receptor subunit expression between the human and rodent amygdala should be taken into consideration when developing subunit‐selective 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.     

Dopamine and opioids inhibit synaptic outputs of the main island of the intercalated neurons of the amygdala

Neural circuits in the amygdala are important for associating the positive experience of drug taking with the coincident environmental cues. During abstinence, cue re‐exposure activates the amygdala, increases dopamine release in the amygdala and stimulates relapse to drug use in an opioid dependent manner. Neural circuits in the amygdala and the learning that underlies these behaviours are inhibited by GABAergic synaptic inhibition. A specialised subtype of GABAergic neurons in the amygdala are the clusters of intercalated cells. We focussed on the main‐island of intercalated cells because these neurons, located ventromedial to the basolateral amygdala, express very high levels of dopamine D1‐receptor and μ‐opioid receptor, release enkephalin and are densely innervated by the ventral tegmental area. However, where these neurons project to was not fully described and their regulation by opioids and dopamine was incomplete. To address this issue we electrically stimulated in the main‐island of the intercalated cells in rat brain slices and made patch‐clamp recordings of GABAergic synaptics from amygdala neurons. We found that main‐island neurons had a strong GABAergic inhibitory output to pyramidal neurons of the basolateral nucleus and the medial central nucleus, the major output zones of the amygdala. Opioids inhibited both these synaptic outputs of the intercalated neurons and thus would disinhibit these target zones. Additionally, dopamine acting at D1‐receptors inhibited main‐island neuron synapses onto other main‐island neurons. This data indicates that the inhibitory projections from the main‐island neurons could influence multiple aspects of addiction and emotional processing in an opioid and dopamine dependent manner.     

NR2B-containing NMDA receptor is required for morphine-but not stress-induced reinstatement

Glutamate receptors are known to be densely distributed in the forebrain rewarding circuits, and glutamatergic transmission is actively involved in the regulation of rewarding and reinstating effects of drugs of abuse. Here we investigated the possible involvement of the N-methyl-D-aspartate (NMDA) receptors in the reinstatement of extinguished morphine conditioned place preference (CPP) in rats. We found that previously extinguished morphine (3 mg/kg, i.p.) CPP was markedly reinstated by a priming injection of morphine (2 mg/kg, i.p.) or an acute environmental stressor (forced swim for 10 min), but not by the stress induced by a 24-h food deprivation. Parallel with this, protein levels of the NMDA receptor 2B subunit (NR2B) were elevated in the nucleus accumbens (NAc) and the hippocampus, but not the prefrontal cortex, of reinstated rats. Systemic administration of an NR2B selective antagonist ifenprodil (1, 3, 10 mg/kg, i.p.) attenuated the reinstatement induced by a priming morphine injection, although not by the forced swim. Ifenprodil (2.0 microg/rat) directly injected into the NAc shell or the CA1 region of the dorsal hippocampus produced a similar effect. These results indicate that the NR2B-containing NMDA receptors in the NAc and the dorsal hippocampus play a significant role in mediating the reinstatement of rewarding responses to morphine.     

The role of glucocorticoids in the uncontrollable stress-induced potentiation of nucleus accumbens shell dopamine and conditioned place preference responses to morphine

Exposure to stressors can impact on the responsiveness to drugs of abuse, and glucocorticoid hormones (CORT) may interact with dopamine (DA) within the nucleus accumbens shell (NAcs) to mediate these responses. We have previously shown that the CORT response to morphine, but not to a previous uncontrollable stressor, is necessary for the stress-induced potentiation of morphine's rewarding effects. Here, we test (1) the necessity of CORT during inescapable stress (IS) and/or morphine for IS potentiation of morphine-induced NAcs DA and (2) the sufficiency of enhanced CORT, in the absence of prior IS, to potentiate morphine-induced NAcs DA as well as morphine conditioned place preference (CPP) in male Sprague-Dawley rats. In the first experiment, we administered the CORT synthesis inhibitors metyrapone and aminoglutethimide (100mg/kg each, sc) to suppress the CORT response to either IS (100 1 mA tailshocks) or subsequent morphine (3 mg/kg, sc) treatment. Twenty-four hour after IS, microdialysis was performed and morphine was administered. In the next experiments, CORT (1 mg/kg, sc) was injected 20 or 30 min before morphine during either microdialysis or CPP testing, respectively, in non-stressed rats. We found that IS potentiated subsequent morphine-induced NAcs DA and this was completely blocked by CORT suppression before morphine, but not before IS. However, elevated levels of CORT concurrent with morphine, but in the absence of a stressor, failed to potentiate NAcs DA or CPP. These results suggest that the CORT response to morphine is necessary, but not sufficient in the absence of prior IS, for sensitized NAcs DA and CPP responding to morphine, and provide further evidence that CORT is involved in the expression, but not the induction, of this sensitization.     

Distribution of GABAA and GABAB binding sites in the cochlear nucleus of the guinea pig

We compared the distribution of GABA_A and GABA_B binding sites in the cochlear nucleus using quantitative receptor autoradiography with [³H]GABA. To visualize GABA_A binding sites, GABA_B binding sites were blocked with±baclofen. To visualize GABA_B binding sites, isoguvacine was used to block GABA_A binding sites. GABA_A binding sites predominated over GABA_B, although there were marked regional differences in the distribution of binding. In the ventral cochlear nucleus, GABA_A and GABA_B binding sites were concentrated in the peripheral granule cell cap, with low binding levels in the central region. In the dorsal cochlear nucleus, binding was concentrated in the superficial (fusiform and molecular) layers, with a distinct laminar pattern. GABA_A binding sites predominated in the fusiform cell layer. The molecular layer contained the highest level of GABA_B binding sites in the entire cochlear nucleus. These results suggest that GABAergic inhibition in the cochlear nucleus is mediated both by GABA_A and GABA_B receptors, particularly in the dorsal cochlear nucleus. However, low levels of binding in areas such as the magnocellular regions of the ventral cochlear nucleus, known to contain abundant GABAergic synapses, suggest heterogeneity of GABA receptors in this auditory nucleus.     

Lower GABAA receptor binding in the amygdala and hypothalamus of spontaneously hypertensive rats

The central GABAergic system is associated with normal blood pressure regulation, but the role of GAGA receptors in genetic hypertension remains unclear. This study was conducted to investigate GABA_A receptor binding in several brain regions of spontaneously hypertensive (SHR) rats during development of hypertension. GAGA_A receptor binding was labeled with [³⁵S]TBPS and was assessed by quantitative autoradiography with the aid of a computer-assisted image analysis system. Densities of GABA_A receptor binding sites were significantly lower in all hypothalamic and amygdaloid nuclei evaluated in 4-week-old SHR rats, when compared with their age-matched normotensive Wistar-Kyoto rats. At 12 weeks of age, GABA_A receptor binding remained significantly lower in the central amygdaloid nucleus and paraventricular hypothalamic nucleus of SHR rats. Collectively, the results suggest that GABA_A receptors in these nuclei are likely to be involved in the initiation and maintenance of hypertension. In conclusion, this study supports a notion that downregulation of GABA_A receptor binding occurs in the hypothalamus and amygdala of SHR rats and may play a role in genetic hypertension.     

Brain dynorphin and enkephalin systems in Fischer and Lewis rats: effects of morphine tolerance and withdrawal

Lewis rats are more likely to self-administer various drugs of abuse than Fischer rats. Here these two strains of rats were compared with regard to basal brain opioid peptide levels and the response to chronic morphine treatment and to naloxone-precipitated withdrawal. Lewis rats had lower basal dynorphin peptides in the substantia nogra, striatum (not Leu-enkephalinArg⁶) and VTA (not dynorphin B) and the pituitary gland. Leu-enkephalinArg⁶ levels were also lower in these structures (with the exception of striatum which had higher levels) and in the nucleus accumbens. There were also strain differences in the response to chronic morphine treatment; in the nucleus accumbens, morphine treatment increased dynorphin A levels in Fischer rats only, in the ventral tegmental area effects were opposite with increased dynorphin levels in Fischer and decreased levels in Lewis rats, in the hippocampus dynorphin levels were markedly reduced in Lewis rats only. In Fischer rats, chronic morphine strongly affected peptide levels in the substantia nigra and striatum, whereas Lewis rats responded less in these areas. Leu-enkephalin, which derives from both prodynorphin and proenkephalin, and Met-enkephalin, which derives from proenkephalin, were effected by chronic morphine mainly in Fischer rats, increasing levels in most of the brain areas examined. The results in this study show (1) strain differences in basal levels of prodynorphin-derived opioid peptides, (2) the prodynorphin system to be differently influenced by morphine in Lewis rats than in Fischer rats and 3) the proenkephalin system to be influenced by chronic morphine in brain areas related to reward processes only in Fischer rats.     

Redistribution and increased specificity of GABAB receptors during development of the rostral nucleus of the solitary tract

Recent results show that there is an abundance of γ-aminobutyric acid (GABA) before GABAergic synapses have formed in the gustatory zone of the nucleus of the solitary tract. These results suggest that a non-synaptic, developmental function may exist for GABA prior to synaptogenesis. However, GABA exerts its physiological effect via its receptors, the development of which is a largely unknown process. The developmental expression of one of the GABA receptors in the young nucleus of the solitary tract is the focus of this study. The development of GABA_B receptors was investigated by light and electron microscopy. The results suggest that before the development of GABAergic synapses, GABA_B receptors are diffusely distributed. When GABAergic synapses form, the receptors become clustered. Quantitative postembedding immunohistochemical studies at the electron microscopic level show that extrasynaptic labeling for GABA_B receptors decreases during development, but synaptic labeling increases. Increased specificity of neurotransmitter receptors at synapses has been shown in other systems during development, including other central nervous system structures, but this may be the first demonstration of the phenomenon using quantitative electron microscopy.     

Food deprivation facilitates reinstatement of morphine‐induced conditioned place preference: Role of intra‐accumbal dopamine D2‐like receptors in associating reinstatement of morphine CPP with stress

The high rate of relapse to drug use is one of the main problems in the treatment of addiction. Stress plays the essential role in drug abuse and relapse; nevertheless, little is known about the mechanisms underlying stress and relapse. Accordingly, the effects of intra‐accumbal administration of Sulpiride, as a dopamine D2‐like receptor antagonist, on an ineffective morphine dose + food deprivation(FD)‐ and morphine priming‐induced reinstatement of conditioned place preference (CPP). About 104 adult male albino Wistar rats weighing 200–280 g were bilaterally implanted by cannula into the nucleus accumbens (NAc). Subcutaneous (sc) injection of morphine (5 mg kg^?1) was used daily during a 3‐day conditioning phase. After a 24‐hr “off” period following achievement of extinction criterion, rats were tested for FD‐ and priming‐induced reinstatement of morphine CPP by an ineffective (0.5 mg kg^?1, sc) and priming (1 mg kg^?1, sc) dose of morphine, respectively. In the next experiments, animals received different doses of intra‐accumbal Sulpiride (0.25, 1, and 4 µg/0.5 µL saline) bilaterally and were subsequently tested for morphine reinstatement. Our findings indicated that the 24‐hr FD facilitated reinstatement of morphine CPP. Furthermore, the D2‐like receptor antagonist attenuated the ineffective morphine dose+ FD‐ and priming‐induced reinstatement of morphine CPP dose‐dependently. Also, contribution of D2‐like receptors in mediation of the ineffective morphine dose+ FD‐induced reinstatement of CPP was greater than morphine priming‐induced reinstatement of CPP. The role of dopaminergic system in morphine reinstatement through a neural pathway in the NAc provides the evidence that D2‐like receptor antagonist can be useful therapeutic targets for reinstatement of morphine CPP.     

Acupuncture suppresses morphine self-administration through the GABA receptors

The neurobiological substrate for morphine self-administration in animals is believed to involve the dopamine system of the nucleus accumbens. Our previous study has shown that acupuncture at the acupoint Shenmen (HT7) reduced dopamine release in the nucleus accumbens and behavioral hyperactivity induced by systemic administration of morphine. Here we investigated the effect of acupuncture on morphine self-administration and potential roles of GABA receptors in the mechanisms behind acupuncture. Male Sprague-Dawley rats were trained to self-administer morphine (0.1 mg/kg per infusion) during daily 1-h session under fixed-ratio 1 schedule. Following the stable responding on morphine self-administration, acupuncture was applied to HT7 points bilaterally (1 min) prior to the testing session. Another groups of rats were given the GABA_B receptor antagonist SCH 50911 (3.0 mg/kg, i.p.), the GABA_A receptor antagonist bicuculline (1.0 mg/kg, i.p.) or saline 30 min prior to the acupuncture treatment. We have found that acupuncture at the acupoint HT7, but not at the control point Yangxi (LI5), significantly decreased morphine self-administration. Moreover, either SCH 50911 or bicuculline blocked the inhibitory effects of acupuncture on morphine self-administration. Taken together, the current results suggest that acupuncture at specific HT7 points regulates the reinforcing effects of morphine via regulation of GABA receptors.     

Alterations of GABAA receptor subunit mRNA levels associated with increases in punished responding induced by acute alprazolam administration: an in situ hybridization study

Changes in the mRNA encoding α1, α2, β2 and γ2 subunits of the GABA_A receptor associated with the anxiolytic effects of alprazolam were measured in 20 brain regions using in situ hybridization techniques. Compared to non-punished controls, punishment decreased α1 mRNA levels in two nuclei of the amygdala, the cerebral cortex, and the mediodorsal thalamic nucleus and decreased α2 mRNA levels in the hippocampus. Punishment increased β2 mRNA levels in ventroposterior thalamic nucleus and γ2 mRNA levels in the CA2 area of the hippocampus. All of these effects were reversed when alprazolam increased punished responding, while alprazolam alone had no effect on either non-punished responding or GABA_A receptor subunit regulation in these brain regions. Some brain regions that were unaffected by punishment were altered by alprazolam plus punishment. These results demonstrate that punishment and alprazolam can produce reciprocal changes in the mRNA levels for some subunits of the GABA_A receptor. These changes may alter GABAergic synaptic inhibition by altering the density of GABA_A receptors or their efficacy to bind drugs. They suggest that the underlying mechanisms by which drugs affect behavior can depend upon the conditions under which behavior is assessed.     

Synergistic interactions of endogenous opioids and cannabinoid systems

Cannabinoids and opioids are distinct drug classes historically used in combination to treat pain. Δ⁹-THC, an active constituent in marijuana, releases endogenous dynorphin A and leucine enkephalin in the production of analgesia. The endocannabinoid, anandamide (AEA), fails to release dynorphin A. The synthetic cannabinoid, CP55,940, releases dynorphin B. Neither AEA nor CP55,940 enhances morphine analgesia. The CB1 antagonist, SR141716A, differentially blocks Δ⁹-THC versus AEA. Tolerance to Δ⁹-THC, but not AEA, involves a decrease in the release of dynorphin A. Our preclinical studies indicate that Δ⁹-THC and morphine can be useful in low dose combination as an analgesic. Such is not observed with AEA or CP55,940. We hypothesize the existence of a new CB receptor differentially linked to endogenous opioid systems based upon data showing the stereoselectivity of endogenous opioid release. Such a receptor, due to the release of endogenous opioids, may have significant impact upon the clinical development of cannabinoid/opioid combinations for the treatment of a variety of types of pain in humans.     

Conditioned place preference associated with level of palmitoylation of PSD-95 in rat hippocampus and nucleus accumbens

Psychostimulant-mediated synaptic plasticity in the hippocampus and nucleus accumbens is one of the pathological features of addiction, a disease of learning and memory. Dynamic palmitoylation of PSD-95 modulates synaptic plasticity, but its role in addiction is not fully understood. Using a morphine-conditioned place preference (CPP) rat model and Acyl-biotin exchange (ABE) labeling we found a correlation between CPP and levels of palmitoylated PSD-95 in the hippocampus and nucleus accumbens. Rats that developed significant CPP had higher levels of palmitoylation of PSD-95 in the hippocampus and nucleus accumbens. Furthermore, palmitoylation of PSD-95 was significantly decreased in the hippocampus but increased in the nucleus accumbens during the beginning of withdrawal. With long-term withdrawal, palmitoylated PSD-95 in these regions recovered, while CPP waned and physical signs gradually disappeared. However, morphine reinjection restored strong CPP without producing any significant changes in palmitoylation of PSD-95. Our findings suggest that CPP is correlated with the dynamics of PSD-95 palmitoylation in rat hippocampus and nucleus accumbens, and could be one of the mechanisms for morphine-dependent synaptic plasticity.     

Repeated peripheral electrical stimulations suppress both morphine-induced CPP and reinstatement of extinguished CPP in rats: accelerated expression of PPE and PPD mRNA in NAc implicated

Previous studies have shown that peripheral electrical stimulation (PES) can suppress morphine-induced conditioned place preference (CPP) and the reinstatement of extinguished CPP in the rat. The present study was performed to elucidate if preproenkephalin (PPE) and preprodynorphin (PPD) mRNAs in the nucleus accumbens (NAc) play a role in this event. Rats were trained with morphine for 4 days to establish CPP paradigm. They were then given 15-min test once a day for eight consecutive days for extinction trial. Twenty-four hours after the 8th session of extinction trials, rats were given peripheral electrical stimulation (PES) at 2 or 100 Hz once a day for 3 days, then a morphine-priming injection at a dose of 1, 2, or 4 mg/kg to reinstate the extinguished CPP. At the end of the experiment, PPE and PPD mRNA levels in the nucleus acccumbens (NAc) were determined by the semiquantitative RT-PCR technique. The results showed that PES at 2- and 100-Hz administered 30 min a day for 3 days suppressed both the expression of morphine-induced CPP and the reinstatement of extinguished CPP. PES at 2 Hz increased preproenkephalin (PPE) mRNA levels, whereas PES of 100 Hz that of preprodynorphin (PPD) mRNA levels in the NAc. These findings suggest that enkephalin and dynorphin in NAc may play important roles in the mechanisms underlying the inhibitory effect of PES on the expression and reinstatement of morphine-induced CPP in rats.     

Nucleus accumbens dopamine release modulation by mesolimbic GABAA receptors—an in vivo electrochemical study

The role of GABA receptors in regulating the mesolimbic dopamine (DA) system and drug reinforced behaviors has not been well characterized. Using fast-cyclic voltammetry, the effects of specific GABA receptor modulation on DA release in the nucleus accumbens (NAcc) and heroin self-administration (SA) behavior was investigated. The GABA_A agonist muscimol, administered either intravenously or directly into the ventral tegmental area (VTA), significantly increased DA release in the NAcc in 7 of the 10 rats tested. DA release decreased in the remaining three rats; both effects were blocked by pretreatment with the GABA_A receptor antagonist bicuculline. In contrast, the GABA_B agonist baclofen decreased, while 2-OH-saclofen (a GABA_B antagonist) increased DA release in the NAcc. However, when VTA GABA_B receptors were previously activated or inactivated by microinjections of baclofen or 2-OH-saclofen, systemic injections of muscimol caused an inhibition of NAcc DA release. These results suggest that GABA_A receptors may be co-localized on both DA neurons and non-DA (GABAergic) interneurons in the VTA, with the effects of GABA_A determined by the net effect of both direct inhibition and indirect disinhibition of DA neurons. Finally, although a DA releaser, muscimol was neither self-administered in drug naive rats, nor did it substitute for heroin in rats previously trained to self-administer heroin, suggesting that GABA_A receptors appear to play a complex role in mediating drug reinforcement, depending upon the dynamic functional state of GABA_A receptors on both tegmental DA and non-DA neurons.     

Prenatal experience and postnatal stress modulate the adult neurosteroid and catecholaminergic stress responses

Allopregnanolone (3α-hydroxy-5α-regnan-20-one) is a neuroactive steroid recently shown to be involved in the neurochemical stress response via its positive modulation of the GABA_A receptor complex. This experiment investigated the effects of postnatal stress (daily maternal separation during the first week of life) on the subsequent adult response to a stressor (10 min forced swim) in Long–Evans rats from one of three prenatal treatment groups (alcohol, pair-fed and control). Indices of stress response were allopregnanolone concentrations in plasma, cortex and hippocampus, and dopamine and norepinephrine concentrations in prefrontal cortex, nucleus accumbens and striatum. Females had higher levels of allopregnanolone than males in both plasma and brain. Prenatal alcohol exposure combined with early maternal separation stress resulted in an increase in the endogenous levels of allopregnanolone in the prefrontal cortex and hippocampus of adult offspring in response to a stressor compared to subjects without a prior history of postnatal stress ; this effect was greater in females. This increased allopregnanolone was also associated with decreased dopamine and norepinephrine levels in the prefrontal cortex. In the prenatal alcohol-exposed offspring, postnatal maternal separation blunted the increase in dopamine levels in the striatum seen in both control groups. Postnatal maternal separation increased norepinephrine levels in the nucleus accumbens regardless of prenatal experience, while in the prefrontal cortex only prenatal diet condition (pair-feeding and alcohol) resulted in lower norepinephrine levels. The results of this experiment suggest that experience, both pre- and postnatal, can have long-term consequences for the developing neurochemical responses to stressors.