Acute and repeated administration of sulpiride alters Met-and Leu-enkephalin content of rat brain

Glutamate receptors participate in the nicotine-induced changes of met-enkephalin in striatum

A single dose of nicotine given to mice induces first a rapid decrease (presumed release/enhanced degradation) and then a rise (presumed synthesis/enhanced accumulation) of met-enkephalin (Met-Enk) in dorsal and ventral striatum observed at 30 and 60 min post-treatment, respectively. These studies investigated whether the nicotine effect on Met-Enk was mediated indirectly, in part, via other neurotransmitters known to be released by nicotine. Based on the ability of selective antagonists of dopamine (Sch 23390, D1; Sulpiride, D2), glutamate (CPP, competitive NMDA; dizocilpine, non-competitive NMDA; NBQX, AMPA) and GABA (bicuculline, GABA(A); Sch 50911, GABA(B)) receptors, to inhibit or enhance the response to nicotine, we conclude that nicotine alters striatal Met-Enk, in part, via glutamate NMDA and AMPA receptors. These findings further support the notion that glutamate might play a role in the pharmacology of nicotine.

Modulation of basal ganglia neurotransmission by the classical antipsychotic fluphenazine is due in part to the blockade of dopamine D1-receptors

Classical antipsychotics, such as fluphenazine, influence neurotransmission by blocking both dopamine D1- and D2-receptors which in turn results in widespread adaptive changes in the neurochemistry of the basal ganglia. The purpose of the present study was to determine the role of D1-receptors in mediating some of these neurochemical events, including changes in D1- and D2-receptor binding, and the expression of preproenkephalin and glutamic acid decarboxylase mRNAs. For these experiments, rats were given a depot injection of fluphenazine decanoate or injected twice daily for 21 days with the D1-receptor antagonist SCH-23390. An additional group received both fluphenazine and SCH-23390 and controls were given saline. Fluphenazine administration decreased D2-receptor binding throughout the basal ganglia while SCH-23390 was without effect. In contrast to the uniform reduction in D2-receptor binding, fluphenazine altered D1-receptor binding in a region-dependent manner. Region-dependent changes were also observed in animals given SCH-23390 which increased binding in the entopeduncular nucleus and posterior caudate-putamen without affecting other brain regions. Both fluphenazine and SCH-23390 significantly enhanced preproenkephalin and glutamic acid decarboxylase (GAD) mRNA expression in the anterior striatum. Fluphenazine also increased GAD mRNA levels in the entopeduncular nucleus. Together, these results indicate that the attenuation of D1-receptor-mediated neurotransmission modulates a number of clinically relevant neurochemical processes in the basal ganglia.

Effect of long-term haloperidol treatment on striatal neuropeptides: relation to stereotyped behavior

Behavioral and biochemical responses to D1 and D2 dopamine (DA) agonists were used to evaluate the participation of striatal peptidergic mechanisms in the motor function alterations that attend chronic neuroleptic treatment. Rats, given haloperidol (1 mg/kg, i.c.) for 21 consecutive days, were randomly allocated to one of the following treatments: the D1 agonist SKF 38393, the D2 agonist quinpirole, their combination or saline. Stereotyped behavior and neuropeptide levels were evaluated after 5 days treatment and 4 days washout. Haloperidol increased most oral behaviors including licking, chewing and biting as well as striatal enkephalin and somatostatin levels. Subsequent treatment with SKF 38393 diminished the haloperidol-induced increase in licking and chewing; quinpirole reduced chewing behavior. The administration of both agonists together decreased chewing and biting. Neither DA agonist alone, nor their combination, reduced the haloperidol-induced increase in enkephalin levels. Both SKF 38393 and quinpirole, when given alone, tended to decrease the haloperidol-induced increase in somatostatin levels; when both D1 and D2 agonists were administered together, somatostatin levels declined significantly. These results suggest that somatostatin- but not enkephalin-containing striatal neurons contribute to the expression of haloperidol-induced stereotypies.

Changes in methionine-enkephalin levels in specific rat brain regions following repeated treatment with selective dopaminergic agonists and antagonists

The present study was conducted to investigate the effects of repeated treatment with selective dopaminergic agents on the level of methionine-enkephalin (Met-Enk) in rat brain cortex (CTX), hypothalamus, (HYPO), hippocampus (HIPP) and midbrain (MID). Male Sprague-Dawley rats were kept under controlled conditions for at least one week. After adaptation period, rats were randomly assigned into nine groups (7-9 rats per group) for intraperitoneal treatment with dopaminergic agents. Group 1 served as control, while, groups 2, 3 and 4 were treated with either SKF-81297, SCH 23390 or their combination, respectively. Groups 5, 6 and 7 received either LY 171555, (-)-sulpiride or their combination, whereas groups 8 and 9 were treated with nomifensine or selegiline, respectively. One hour after the last injection, rats were sacrificed, brains were removed and dissected into different regions, then extracted and their Met-Enk levels determined by radioimmunoassay (RIA). Administration of SKF-81297 or SCH 23390 significantly elevated Met-Enk levels in all brain regions examined, while their combination elevated Met-Enk levels in HYPO and HIPP only. On the other hand, treatment with LY 171555 or (-)-sulpiride, but not their combination, markedly increased Met-ENK levels in all brain regions investigated, whereas, treatment with nomifensine increased Met-Enk levels in all brain regions investigated, whereas, treatment with selegiline significantly elevated Met-Enk in HYPO, HIPP and MID but not in CTX. These findings clearly indicate that dopaminergic agonists and antagonists alter Met-Enk levels in specific rat brain regions.

The effect of prolonged treatment with imipramine and electroconvulsive shock on the levels of endogenous enkephalins in the nucleus accumbens and the ventral tegmentum of the rat

The present study was designed to find out whether the prolonged administration of imipramine (IMI) or electroconvulsive shock (ECS) influences levels of endogenous enkephalins in the nucleus accumbens (NAS) and the ventral tegmentum (VTA) of the rat. Ressults indicate that treatment with IMI as well as with ECS has a profound effect on the levels of enkephalins in both structures. In the NAS both treatments lead to an increase in the levels of endogenous enkephalins and this effect is accompanied by an increase in mRNA coding for proenkephalin (measured by in situ hybridization) in this structure, indicating the enhancement of biosynthesis of endogenous enkephalinergic peptides following antidepressant treatment. The results are discussed in the light of the hypothesis concerning the influence of endogenous enkephalins on mesolimbic dopamine neurons, the activity of which plays a crucial role in the etiology of depression.

Effect of sulpiride on somatostatin receptors, somatostatin-like immunoreactivity and modulation of adenylyl cyclase activity in the rat brain

The present study investigates the effects of the administration of an intracerebroventricular (i.c.v.) dose of 500 micrograms/rat of the neuroleptic (-) sulpiride on somatostatin-like immunoreactivity (SSLI) levels, 125I-Tyr11-SS binding to its specific receptors, SS-modulated adenylyl cyclase (AC) activity and the pertussis toxin (PTX) substrates measured by toxin-catalysed ADP ribosylation of the alpha-subunits from G-proteins. (-) Sulpiride significantly decreased the SSLI levels in the frontoparietal cortex at 30 min but was without effect on the SSLI concentration in the striatum. This decrease had disappeared within 24 hr. The administration of (-) sulpiride produced a significant increase in the number of 125I-Tyr11-SS receptors and a significant reduction in their affinity at 30 min after injection in the striatum without affecting the frontoparietal cortex. The effects of the (-) sulpiride injection had disappeared after 24 hr. This change in SS binding was not due to a direct effect of (-) sulpiride on these receptors since no effect on binding was produced by high concentrations of (-) sulpiride (10(-5) M) when added in vitro. No significant differences were seen in either brain region for the basal or the forskolin (FK)-stimulated AC enzyme activities in the control and (-) sulpiride groups. In the (-) sulpiride group, the capacity of SS to inhibit FK-stimulated AC in the frontoparietal cortex was significantly higher than in the control group with no significant difference in the striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Gamma-hydroxybutyrate: an overview of the pros and cons for it being a neurotransmitter and/or a useful therapeutic agent

Gamma-hydroxybutyrate (GHB) is a catabolite in brain of gamma-aminobutyrate (GABA) and is also found in nonneuronal tissues. It is present in the brain at about one thousandth of the concentration of its parent compound. High affinity and specific uptake, and energy dependent transport systems for GHB have been described in brain in addition to a class of high affinity binding sites, functional at a rather unphysiologically low pH. Administration of large doses of GHB to animals and man leads to sedation, and at the highest doses, anaesthesia. These effects are prominent when GHB brain levels are over one hundred-fold the endogenous levels. In some animals, GHB administration also induces an electroencephalographic and behavioural changes resembling that of human petit mal epilepsy. GHB has been used in man as an anaesthetic adjuvant. GHB lowers cerebral energy requirements and may play a neuroprotective role. Administered GHB profoundly effects the cerebral dopaminergic system by a mechanism which remains to be unravelled. GHB has been tested with success on alcoholic patients where it attenuates the withdrawal syndrome. It is indicated here that in this situation, it may owe its effect by acting as a pro-drug of the neurotransmitter GABA into which it can be transformed. As administration of GHB, a GABAB receptor agonist and a natural opioid peptide all elicit similar abnormal EEG phenomena, it may be suggested that they are acting via a common pathway. The petit mal epileptic effects of GHB might be ascribed to its direct, or indirect agonist properties after transformation to a pool of GABA at the GABAB receptor or via interactions at its own binding sites linked to a similar series of biochemical events. Some anticonvulsant drugs, the opiate antagonist naloxone and a synthetic structural GHB analogue antagonise certain behavioural effects of GHB administration. It is postulated that GHB exerts some of its effects via transformation to GABA pools, and that substances which inhibit this process antagonise its effects by blocking GABA formation. GHB has been proposed as a neurotransmitter, although straightforward evidence for this role is lacking. Evidence for and against GHB, as a neurotransmitter, is reviewed here together with a discussion of its potential as a therapeutically useful drug.

Acute effects of D-1 and D-2 dopamine receptor agonist and antagonist drugs on basal ganglia [Met5]- and [Leu5]-enkephalin and neurotensin content in the rat

The effects of acute systemic injection of the D-1 agonist SKF 38393 (2.5-20 mg/kg) or the D-1 antagonist SCH 23390 (0.25-2.0 mg/kg), and of the D-2 agonist quinpirole (0.12-1.0 mg/kg) or the D-2 antagonist sulpiride (25-100 mg/kg) on the neuropeptide content of rat basal ganglia were investigated. In striatum, the [Met5]- and [Leu5]-enkephalin content was unaffected by administration of SKF 38393 or SCH 23390. Quinpirole had no effect on [Met5]- and [Leu5]-enkephalin levels but sulpiride produced an increase in both [Met5]- and [Leu5]-enkephalin content. In the nucleus accumbens, SKF 38393 decreased and SCH 23390 increased [Met5]- and [Leu5]enkephalin levels. Quinpirole decreased [Met5]- and [Leu5]-enkephalin levels, while sulpiride decreased [Leu5]-enkephalin levels alone. The content of [Leu5]- but not [Met5]-enkephalin levels in the substantia nigra was increased by administration of SKF 38393, and decreased by SCH 23390. Quinpirole and sulpiride were without effect on the [Met5]- or [Leu5]-enkephalin content of substantia nigra. Neurotensin levels in striatum were increased by administration of SKF 38393 and decreased by SCH 23390. Similarly, quinpirole decreased the neurotensin content while sulpiride caused an increase. In the nucleus accumbens, the neurotensin content was not affected by administration of SKF 38393 but increased by SCH 23390. Neither quinpirole nor sulpiride altered neurotensin levels in the nucleus accumbens. Neurotensin levels in substantia nigra were unaffected by the administration of SKF 38393 and SCH 23390, or by quinpirole and sulpiride. These results indicate that acute administration of D-1 and D-2 agonist and antagonist drugs can alter the levels of [Met5]- and [Leu5]-enkephalin and neurotensin in basal ganglia. However, there are marked differences between brain regions in the regulation of peptide levels by acute D-1 and D-2 receptor occupation.

Increases in rat striatal preproenkephalin mRNA levels following chronic treatment with the depot neuroleptic, haloperidol decanoate

Studies of the effects of chronic neuroleptic drug treatment have consistently demonstrated enhanced transcription and translation of the preproenkephalin gene in the rat striatum. However, all of these studies have used daily ip drug treatments and none have evaluated the effects of chronic depot neuroleptics. With these drug treatments, dopamine receptor blockade undergoes less variability as a result of sustained steady-state blood levels of the neuroleptic. Therefore, as a result of the increasing utilization of depot neuroleptics therapeutically, we examined the effects of haloperidol decanoate on striatal preproenkephalin mRNA levels. As with daily ip drug injections, the depot preparation was found to increase the levels of this mRNA to an apparent new steady-state level twice that of controls, by 3 days and sustaining this steady-state for the 14 day observation period. These data indicate that both continuous and fluctuating patterns of dopamine receptor blockade result in activation of the preproenkephalin gene.

Evaluation of selective actions of dopamine D-1 and D-2 receptor agonists and antagonists on opioid antinociception

The effect of the selective dopamine receptor agonists SKF 38393 (D-1) and quinpirole (D-2) on nociception was studied in the mouse tail immersion test. The D-1 receptor agonist induced mild hyperalgesia whereas the D-2 agonist produced antinociception. Pretreatment with either the selective D-1 receptor antagonist SCH 23390 or the D-2 receptor antagonist (-)-sulpiride converted the hyperalgesia produced by the D-1 agonist into an antinociceptive response whereas the effect of the D-2 receptor agonist was significantly antagonised. The antinociceptive response of selective opioid agonists was also studied in combination with selective dopamine receptor agonists and antagonists. Sufentanil (mu-opioid) antinociception was enhanced in animals pretreated with (-)-sulpiride but not SCH 23390. In animals co-administered sufentanil with SKF 38393 there was a reduced antinociceptive effect whilst quinpirole enhanced the action of sufentanil. Likewise, antinociception induced by the kappa-opioid agonist U50,488H was unaltered in animals pretreated with SCH 23390, increased by (-)-sulpiride, and reduced by SKF 38393. delta-Opioid antinociception induced by [D-Ala2,D-Leu5]enkephaline remained unmodified following pretreatment with either (-)-sulpiride or SCH 23390 but was potentiated in animals which received both the delta-agonist and the D-2 receptor agonist. It is concluded that D-2 receptor agonists not only have intrinsic antinociceptive activity, but can also potentiate opioid-induced antinociception. Similarly, dopamine D-2 receptor antagonists appear to potentiate opioid-induced antinociception in this nociceptive model.

Multiple factors influencing the in vitro release of [Met5]-enkephalin from rat hypothalamic slices

This study examined several in vivo and in vitro factors which influence the release of [Met5]-enkephalin (Met-ENK) from male rat hypothalamic slices superfused in vitro. Met-ENK release was significantly stimulated by corticotropin-releasing hormone (CRH; 10(-12)-10(-8) M), an effect which was abolished in the presence of the CRH-receptor antagonist, alpha-helical CRF9-41 (10(-6) M). The amount of Met-ENK release diminished with time in experiments in which the slices were continuously exposed to CRH. The opioid receptor antagonist naloxone (10(-6) M) stimulated Met-ENK release, even in the presence of the Na+ -channel blocker tetrodotoxin (10(-6) M), a result indicating presynaptic opioid feedback inhibition of Met-ENK release. The role of gonadal steroids in the control of Met-ENK release in vitro was also examined. It was found that the basal and CRH-induced release of Met-ENK was not changed 1 week after castration. However, a significant increase in the basal release of this peptide was observed 4 weeks after gonadectomy, and the Met-ENK-releasing efficacy of CRH was found to be reduced. The Met-ENK content of hypothalami from 1-week castrates was not significantly changed from control levels, but was significantly reduced in those from 4-week castrates. These long-term effects of castration could be overcome by the subcutaneous implantation of testosterone-containing capsules at the time of castration.