Peptide-dopamine interactions in the central nervous system: implications for neuropsychiatric disorders

The potential cardiotoxicity of antipsychotic drugs as assessed by heart rate variability

Most antipsychotic drugs have cardiac effects as a consequence of their pharmacological actions. Recently, thioridazine has been subjected to a restricted indications notice and sertindole had its license withdrawn because of concerns about their potential cardiotoxicity. In the development of new atypical agents, heart-rate corrected QT effects are evaluated but it is unclear how predictive these are of clinically significant cardiotoxicity or sudden death. Heart rate variability (HRV) is a potential index of cardiotoxicity which has been found to be decreased following antidepressants and clozapine. We studied acute HRV changes following antipsychotic agents. Sixteen healthy male volunteers received risperidone (4 mg), olanzapine (10 mg), thioridazine (50 mg) or placebo in a randomized cross-over design. Subjective effects and psychomotor function were assayed at 2 h and both linear (summary statistics) and non-linear (scatterplot) measures of HRV were evaluated by continuous electrocardiogram recording over 10 h. Differential effects of single doses of the three antipsychotic drugs on HRV were found, and these were independent of their sedative effects. Olanzapine increased, and thioridazine decreased HRV, while risperidone had no effect. HRV is sensitive to the acute effects of antipsychotics. It may prove to be a reliable index of their potential for cardiotoxicity. Further studies in both healthy volunteers and patients on antipsychotic medication will be valuable.

In vivo voltammetry and concomitant electrophysiology at a single micro-biosensor to analyse ischaemia, depression and drug dependence

Electrochemical methods such as voltammetry can be used to understand patho-physiological mechanisms of action and, therefore, develop therapeutic approaches. In particular, voltammetry with treated micro-biosensors (carbon fibre micro-electrodes, mCFE) has been used to study models of (1) ischaemia; (2) drug dependence, and in particular craving; (3) depression. In addition, in studies (1) and (3) concomitant in vivo voltammetric and electrophysiological analysis has been performed by means of the same mCFE. Original data concerning ascorbate release in ischaemia, peptidergic activity during craving for drugs of abuse and concomitant voltammetric and electrophysiological changes of the serotonergic system in rats submitted to forced swimming test or to pharmacological treatment with the selective serotonin reuptake inhibitor fluoxetine are shown and discussed.

Somatostatin modulates the behavioral effects of dopamine receptor activation in parkinsonian rats

Somatostatin may play a role in several neuropsychiatric disorders, including Parkinson's disease. Although functional interactions between somatostatinergic and dopaminergic transmitter systems have been well documented, no study has been conducted in animals with experimental Parkinsonism to explore the effects of somatostatin on dopamine receptor-mediated behavior. In the present study, rats with unilateral 6-hydroxydopamine-induced destruction of the medial forebrain bundle were assessed following administration of the dopamine(1/2) receptor agonist apomorphine. Ipsilateral intrastriatal infusion of somatostatin produced a dose-related inhibition of apomorphine-induced rotations with maximal effect at a dose of 7.5 microg in 2 microl. This inhibitory effect of somatostatin was antagonized by the somatostatin antagonist cyclo-somatostatin (0.1 microg in 2 microl, intrastriatally). Neither somatostatin (up to 15 microg in 2 microl) nor cyclo-somatostatin on its own induced rotations; similarly, this dose of cyclo-somatostatin did not affect apomorphine-induced rotations. From these results we suggest that exogenous somatostatin, by directly acting on its specific receptors in the striatum, inhibits the effects of dopamine receptor activation in parkinsonian rats. We conclude that therapies based on modulation of somatostatin may be worth exploring in the management of Parkinson's disease and other disorders of the basal ganglia.

Involvement of cholecystokinin within craving for cocaine: role of cholecystokinin receptor ligands

In the brain, cholecystokinin (CCK) has been described to act as a central neurotransmitter or neuromodulator involved in functions such as food consumption, stress and anxiety. Recently, the CCK system has been involved in drug dependence phenomena and proposed to be correlated to a putative state of 'drug preferring' phenotype within free choice tests. CCK exerts its action in the CNS through at least two different G-protein coupled high affinity receptors, CCK1 and CCK2. Various selective CCK receptor agonists and antagonists have been synthesised. In particular, L-364,718 has been demonstrated to be a potent and selective CCK1 receptor antagonist, whereas L-365,260 is a potent and selective CCK2 receptor antagonist. More recently, GV150013 has been reported to be a highly selective CCK2 receptor antagonist. This paper reviews the putative role of the CCK system within drug dependence phenomena. In particular, it analyses the relationship between central CCK activity and the exhibition of spontaneous preference for drugs of abuse, such as cocaine or alcohol. The potential therapeutic role for CCK receptor antagonists is also discussed.

Substance P decreases extracellular concentrations of acetylcholine in neostriatum and nucleus accumbens in vivo: possible relevance for the central processing of reward and aversion

It has been shown that peripherally administered substance P has reinforcing effects and can promote functional recovery after unilateral partial lesion of the nigrostriatal system. Furthermore, peripheral injection of substance P induces an increase in extracellular striatal dopamine. To obtain further information about the central mechanisms of these properties we used the in vivo microdialysis technique to investigate changes in the extracellular concentrations of acetylcholine in neostriatum and nucleus accumbens after intraperitoneal (i.p.) administration of substance P or vehicle in freely moving rats. The i.p. administration of 50 micrograms/kg substance P induced a steady, long-lasting decrease in the extracellular concentrations of acetylcholine in neostriatum, while no changes were observed in the nucleus accumbens. In comparison, substance P in a dose of 250 micrograms/kg i.p. acutely decreased the extracellular levels of acetylcholine in both nuclei. Interestingly, after the administration of vehicle, an acute increase in acetylcholine levels was observed in the nucleus accumbens, but not in the neostriatum. This effect did not occur after the injection of substance P indicating that the neurokinin blocked the increase in acetylcholine levels induced by the vehicle injection. These effects of substance P on striatal acetylcholine are discussed with respect to their relationship with dopamine and endogenous opiates, and with respect to the functional role of substance P, such as in reward, aversion, motor activity, and functional recovery.

CCK antagonists: pharmacology and therapeutic interest

CCK was first identified and characterized in the digestive tract where it is known to be a factor involved in the control of gut motility. Later, CCK and CCK receptors were identified in regions of the central nervous system that are associated with the control of emotion, motivation and sensory processing. The recent discovery and development of CCK-receptor antagonists having selective affinity for either CCKA or CCKB receptors has led to a better understanding of the functional role of CCK and its binding sites in the brain and periphery. Some of these compounds are being examined in man for their therapeutic usefulness in mental as well as in digestive disorders. This review will highlight the results from both basic and clinical investigations that have examined the effects of selective CCK receptor ligands. The focus will be on the central nervous system pharmacology of CCK antagonists and the involvement of CCK in gastrointestinal and colonic motility.

The opiate antagonist naloxone suppresses a rodent model of tardive dyskinesia

The effects of both opiate agonists and the opiate antagonist naloxone were examined in a rodent model of tardive dyskinesia (TD). Chronic (approximately 20 weeks) administration of fluphenazine resulted in the emergence of vacuous chewing mouth movements (VCMs), a response which may be a useful model for this disorder. Fluphenazine-induced VCMs were not affected by a variety of selective opiate agonists administered intracerebroventricularly, but were potently suppressed by subcutaneous administration of the opiate antagonist naloxone. These findings suggest that increased opiate transmission may contribute to the pathogenesis of TD. Further investigation of the role of opiate antagonists in treating this disorder are warranted.

Effects of acute and chronic desipramine treatment on somatostatin receptors in brain

The effects of acute (5 mg/kg, IP twice daily for 2 days) and chronic (5 mg/kg IP twice daily for 21 days) administration of desipramine (DMI) on [125I]-Tyr11-somatostatin binding sites in brain were examined. There was no change in [125I]Tyr11-somatostatin binding in membranes prepared from the frontal cortex, striatum, and hippocampus of rats acutely or chronically treated with DMI as compared to non treated animals. [125I]Tyr11-somatostatin binding was increased in membranes prepared from the rat nucleus accumbens only after chronic DMI administration. Scatchard analysis of the binding data from the nucleus accumbens showed that [125I]Tyr11-somatostatin labels a single population of somatostatin binding sites with an affinity constant, Kd, of 1.8 +/- 0.60 nM and a Bmax of 330 +/- 90 fmol/mg protein. Chronic treatment with DMI increased the Bmax (500 +/- 140 fmol/mg protein) but had no effect on the Kd. This finding shows a regional effect of DMI on [125I]Tyr11-somatostatin binding sites in rat brain and suggests that somatostatin may play a role in the pathophysiology of depression.

Neurotensin and neurotensin analogues modify the effects of chronic neuroleptic administration in the rat

The effects of intracerebroventricular neurotensin and the neurotensin analogues neuromedin N and [D-Trp 11]neurotensin on the behavioural responses to chronic neuroleptic administration were investigated in the rat. Chronic (18 weeks) administration of a low dose (12.5 mg/kg, i.m., every 3 weeks) of fluphenazine decanoate alone failed to elicit the vacuous chewing mouth movements (VCMs) which have previously been reported following higher doses of this drug, but VCMs were seen in neuroleptic-treated animals following the additional administration of neurotensin. A higher dose of fluphenazine (25 mg/kg, i.m., every 3 weeks) greatly increased the VCM response, and this potentiation was suppressed to control levels by [D-Trp11]neurotensin, but unaffected by neuromedin N. These findings suggest that alterations in neurotensin may contribute to the deleterious extrapyramidal effects of long-term neuroleptic administration, and that [D-Trp11]neurotensin may attenuate these effects by blockade of neurotensin receptors within the central nervous system.

Cholecystokinin-dopamine interactions

Cholecystokinin (CCK) coexists with dopamine in a large proportion of the ventral tegmental and substantia nigra neurons in rodents and primates. In this review Jacki Crawley integrates the neurophysiological, behavioral, and release studies which demonstrate both excitatory effects of CCK, and facilitatory modulating effects of CCK on the inhibitory actions of dopamine, in the mesolimbic pathway. Nonpeptide antagonists selective for the CCKA and CCKB receptors have recently been developed, and provide long-awaited tools to test hypotheses about the role of endogenous CCK as a modulator of dopaminergic function, and the potential of CCK-based drugs as treatments for neuropsychiatric disorders.

Effect of gamma-butyrolactone on the retrograde axonal transport of neurotensin in dopaminergic neurones

gamma-Butyrolactone is a drug known to inhibit the flow of electrical impulses in dopaminergic neurones and to prevent dopamine release in the striatum. In this study, we investigated the effects of this compound on the retrograde transport of neurotensin in the nigrostriatal dopaminergic pathway. The amount of radioactivity measured in the substantia nigra after injection of iodinated neurotensin into the striatum and the time course of its accumulation were not found to be modified in gamma-butyrolactone-treated rats. We therefore conclude that the retrograde axonal transport of neurotensin in the nigrostriatal pathway is not affected by reduced dopaminergic firing or by inhibition of the striatal release of dopamine.