Effects of morphine on the rat striatal dopamine metabolism

Substantia nigra opiate receptors on basal ganglia efferents

Many behavioral effects of opiate narcotics and peptides have been linked to effects on dopamine neurons originating in the substantia nigra pars compacta and ventral tegmental area. Selective brain lesions were combined with quantitative autoradiography to determine whether opiate receptors are on dopaminergic somata and/or processes in the substantia nigra pars compacta and ventral tegmental area. 6-Hydroxydopamine lesions that eliminated dopamine neurons produced little change in the pattern or density of [3H]-naloxone binding in the substantia nigra pars compacta or ventral tegmental area. Radiofrequency lesions of the internal capsule or globus pallidus and kainic acid lesions of the striatum markedly decreased [3H]-naloxone binding in the pars compacta and pars reticulata. These results are consistent with a dense distribution of opiate receptors on pallido-nigral and/or striato-nigral fibers and strengthen the likelihood that local effects of opiates on dopamine function in the nigrostriatal pathway are mediated indirectly by actions on nondopaminergic processes.

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.

In vivo electrochemical studies of rat striatal dopamine and serotonin release after morphine

The effect of the reference opiate, morphine (d-morphine-sulfate), on endogenously released striatal dopamine and serotonin was studied in male, adult, anesthetized Sprague-Dawley rats. The intraperitoneal administration of morphine produced a biphasic effect on striatal dopamine release. A significant increase in the dopamine signal was seen in the first hour after drug administration; a significant decrease in the dopamine signal was seen in the second and third hour after drug administration. On the other hand, the effect of morphine on striatal serotonin release was monophasic. Morphine significantly increased serotonin release from rat striatum. The effect lasted three hours after morphine administration, i.e., the effect persisted significantly throughout the study. These data show a simultaneous opiate-dopaminergic and opiate-serotonergic interaction in rat striatum. These data further extend studies which have suggested that the pharmacological mechanism of action of morphine may have its etiology in the concurrent modulation of more than one neurotransmitter.

Naloxone pretreatment alters the local cerebral metabolic effect of gamma-hydroxybutyrate in rats

The effect of naloxone on gamma-hydroxybutyrate (GHB)-induced cerebral metabolic depression was studied in rats with the 2-[14C]deoxyglucose method. Naloxone pretreatment statistically significantly antagonized the cerebral metabolic effect of GHB in 10 of 38 structures examined. These results are consistent with previous data showing reversal of the dopaminergic, electroencephalographic, and behavioral effects of GHB by naloxone and suggest that some of the neuropharmacological effects of GHB are mediated by the endogenous opiate system.

Interaction of opiates with dopamine receptors: receptor binding and behavioral assays

We examined the hypothesis that opiates act as dopamine (DA) receptor-blocking agents thereby inducing a compensatory increase in DA receptor density during chronic administration, and that increased receptor density could account for the behavioral hypersensitivity to DA agonists seen after treatment with opiates. Morphine and methadone did not inhibit the specific binding of 3H-spiroperidol to DA receptors in vitro, nor did they decrease affinity or apparent receptor density in the striatum when administered acutely in vivo in behaviorally effective doses. In contrast, neuroleptics had the expected inhibitory effect in both these experiments. Stereotypy and locomotion in response to apomorphine were measured before and after a 3-week treatment with saline or methadone. About half the methadone-treated rats showed significant increases over predrug baselines in stereotypy or locomotion, as did a few saline-treated animals. However, in those animals showing enhanced stereotypy or locomotion, DA receptor density was not elevated in striatum or mesolimbic areas respectively. These results indicate that opiates do not act as antagonists at DA receptor sites, and that changes in DA receptor density cannot account for opiate-induced behavioral hypersensitivity.

The effect of long-term concurrent administration of chlorpromazine and lithium on the striatal and frontal cortical dopamine metabolism in rats

The effects of lithium and chlorpromazine chronically administered alone and together on the dopamine metabolism in the rat striatum and frontal cortex were investigated by measurement of the levels of dopamine (DA) and its main metabolite, homovanillic acid (HVA). Long-term chlorpromazine administration caused a significant increase in the striatal DA level and a decrease in that of HVA and HVA/DA ratio without any changes in those of the frontal cortex. The prolonged administration of lithium elevated the striatal levels of both HVA and DA, but no change in the frontal cortex was observed. The concurrent administration of chlorpromazine and lithium caused a significant increase in the frontal cortical DA level and a decrease in that of HVA and HVA/DA ratio. The striatal DA and HVA levels increased under the effect of the combined treatment, while the HVA/DA ratio remained unchanged.

Effect of L-prolyl-L-leucyl-glycinamide (MIF-I) on some neutrotransmitter-receptor interactions

Some neurotransmitter-receptor interactions have been studied in an attempt to determine how L-prolyl-L-leucyl-glycinamide (MIF-I) exerts its antiparkinson effect. MIF-I affected neither the contractile responses of isolated mouse vas deferens and guinea pig ileum to noradrenaline, acetylcholine, substance P and histamine, nor the inhibitory effects of dopamine and GABA on the rat vas deferens and guinea pig ileum. MIF-I, as well as L-leucine and Pro-Leu, antagonized the contractile response of the ileum to 5-hydroxytryptamine (5-HT). Behavioural tests were used to examine the action of MIF-I on CNS transmitter-receptor interactions. MIF-I did not modify the circling produced by either dopamine agonists in nigro-striatal lesioned rats of 5-HT agonists in rats with a lesion of the medial raphe nucleus. MIF-I affected neither 5-hydroxytryptophan-induced head twitches in mice, which is a measure of 5-HT receptor stimulation, nor striatally-evoked head turning in the rat, which is a model for brain GABA function. It is concluded that MIF-I, at the doses used, does not directly modify the function of any of the CNS transmitter examined. Other possibilities to explain its antiparkinson action are discussed.