Electrophysiological and microiontophoretic studies with buspirone : influence on the firing rate of central monoaminergic neurons and their responsiveness to dopamine, clonidine or GABA

The anti-dyskinetic effect of dopamine receptor blockade is enhanced in parkinsonian rats following dopamine neuron transplantation

Graft-induced dyskinesia (GID) is a serious complication induced by dopamine (DA) cell transplantation in parkinsonian patients. We have recently shown that DA D2 receptor blockade produces striking blockade of dyskinesia induced by amphetamine in grafted 6-OHDA-lesioned rats, a model of GID. This study was designed to investigate whether blockade of DA D1 receptors could produce similar outcome, and to see whether the effect of these treatments in grafted rats was specific for dyskinesia induced by amphetamine, or could also influence L-DOPA-induced dyskinesia (LID). L-DOPA-primed rats received transplants of fetal DA neurons into the DA-denervated striatum. Beginning at 20weeks after transplantation rats were subjected to pharmacological treatments with either L-DOPA (6mg/kg) or amphetamine (1.5mg/kg) alone, or in combination with the D1 receptor antagonist SCH23390, the D2 receptor antagonist eticlopride, and the 5-HT1A agonist/D2 receptor antagonist buspirone. Grafted rats developed severe GID, while LID was reduced. Both eticlopride and SCH23390 produced near-complete suppression of GID already at very low doses (0.015 and 0.1mg/kg, respectively). Buspirone induced similar suppression at a dose as low as 0.3mg/kg, which is far lower than the dose known to affect LID in non-grafted dyskinetic rats. In agreement with our previous results, the effect of buspirone was independent from 5-HT1A receptor activation, as it was not counteracted by the selective 5-HT1A antagonist WAY100635, but likely due to D2 receptor blockade. Most interestingly, the same doses of eticlopride, SCH23390 and buspirone were found to suppress LID in grafted but not in control dyskinetic rats. Taken together, these data demonstrate that the DA cell grafts strikingly exacerbate the effect of DA D1 and D2 receptor blockade against both GID and LID, and suggest that the anti-GID effect of buspirone seen in patients may also be due to blockade of DA D2 receptors.

Serotonergic and dopaminergic mechanisms in graft-induced dyskinesia in a rat model of Parkinson's disease

Dyskinesia seen in the off-state, referred as graft-induced dyskinesia (GID), has emerged as a serious complication induced by dopamine (DA) cell transplantation in parkinsonian patients. Although the mechanism underlying the appearance of GID is unknown, in a recent clinical study the partial 5-HT(1A) agonist buspirone was found to markedly reduce GID in three grafted patients, who showed significant serotonin (5-HT) hyperinnervation in the grafted striatum in positron emission tomography scanning (Politis et al., 2010, 2011). Prompted by these findings, this study was performed to investigate the involvement of serotonin neurons in the appearance of GID in the rat 6-hydroxydopamine model. L-DOPA-primed rats received transplants of DA neurons only, DA plus 5-HT neurons or 5-HT neurons only into the lesioned striatum. In DA cell-grafted rats, with or without 5-HT neurons, but not in 5-HT grafts, GID was observed consistently after administration of amphetamine (1.5mg/kg, i.p.) indicating that grafted DA neurons are required to induce GID. Strikingly, a low dose of buspirone produced a complete suppression of GID. In addition, activation of 5-HT(1A) and 5-HT(1B) receptors by 8-OH-DPAT and CP 94253, known to inhibit the activity of 5-HT neurons, significantly reduced GID, whereas induction of neurotransmitter release by fenfluramine administration significantly increased GID, indicating an involvement of the 5-HT system in the modulation of GID. To investigate the involvement of the host 5-HT system in GID, the endogenous 5-HT terminals were removed by intracerebral injection of 5,7-dihydroxytryptamine, but this treatment did not affect GID expression. However, 5-HT terminal destruction suppressed the anti-GID effect of 5-HT(1A) and 5-HT(1B) agonists, demonstrating that the 5-HT(1) agonist combination exerted its anti-GID effect through the activation of pre-synaptic host-derived receptors. By contrast, removal of the host 5-HT innervation or pre-treatment with a 5-HT(1A) antagonist did not abolish the anti-GID effect of buspirone, showing that its effect is independent from activation of either pre- or post-synaptic 5-HT(1A) receptors. Since buspirone is known to also act as a DA D(2) receptor antagonist, the selective D(2) receptor antagonist eticlopride was administered to test whether blockade of D(2) receptors could account for the anti-dyskinetic effect of buspirone. In fact, eticlopride produced complete suppression of GID in grafted animals already at very low dose. Together, these results point to a critical role of both 5-HT(1) and D(2) receptors in the modulation of GID, and suggest that 5-HT neurons exert a modulatory role in the development of this side effect of neuronal transplantation.

Behavioral and biochemical effects of L-tryptophan and buspirone in a model of cerebellar atrophy

The Lurcher mutant mouse can be considered an adequate model of autosomal dominant spinocerebellar atrophy because of the severe degeneration of its cerebellar cortex and inferior olive. The purpose of this study was to determine whether the motor coordination deficits of Lurcher mutants could be improved after chronic administration of the serotonin (5-hydroxytryptamine; 5-HT) precursor, L-tryptophan, or of the 5-HT(1A) agonist, buspirone. During these treatments, the mice were submitted to behavioral evaluations using the coat hanger and the rotorod tests, as well as an inclined screen and a vertical grid test. At the end of treatments, 5-HT and 5-hydroxindole-3-acetic acid (5-HIAA) were measured in six brain regions. On the coat hanger test, administration of L-tryptophan accelerated movements along the horizontal bar by 44%, while buspirone increased the time spent on the apparatus by 11%. Neither drug had an effect on climbing ability or on the time spent on a rotating beam. Administration of L-tryptophan increased 5-HIAA levels in frontal cortex, neostriatum, thalamus, brainstem, cerebellum and spinal cord, but elevated 5-HT only in neostriatum, brainstem and cerebellum. In contrast, buspirone led to 5-HT increases in cerebellum and augmented 5-HIAA in the spinal cord. The modest test-specific improvements are consistent with some of the clinical data concerning 5-HT pharmacotherapy in patients suffering from cerebellar atrophy.

Effects of buspirone on brain indoleamines and catecholamines in wild-type mice and Lurcher mutants

The effects of a chronic serotoninergic stimulation on brain monoamine levels and metabolism were studied in wild-type (+/+) mice and Lurcher (Lc/+) mutants. Endogenous serotonin, dopamine, noradrenaline and some of their major metabolites were measured in the frontal cortex, neostriatum, thalamus, brainstem, cerebellum and spinal cord. In +/+ mice, buspirone (1 mg/kg; i.p.) treatment during 40 days increased indoleamines, albeit with moderate changes in the ratios between tissue serotonin metabolites and endogenous serotonin, augmented noradrenaline contents in the spinal cord, and caused elevations of dopamine metabolites in most regions. In Lc/+ mutants, the effects of buspirone were attenuated, but higher L-tryptophan and indoleamine levels, suggest a storage of serotonin in a non-releasable compartment. In the hypoplastic Lc/+ cerebellum, indoleamine content was accrued, but with a decreased [serotonin metabolites]/[serotonin] ratio, indicating that the reorganized nerve terminals in Lc/+ mutants although they can synthesize and accumulate serotonin, may not utilize it efficiently in synaptic transmission.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

The future of 5-HT1A receptor agonists. (Aryl-piperazine derivatives)

At present the dominant position among anti-anxiety medications has changed from meprobamate to the benzodiazepine derivatives. In order to avoid benzodiazepine's (BZ) undesirable side effects such as impairment of psycho-motor function, memory impairment, low dose dependence and withdrawal symptoms, a third generation anxiolytic agent, buspirone, the focus of the aryl-piperazine group of anti-anxiety agents, has been introduced recently. Aryl-piperazine derivatives work as 5-HT1A receptor partial agonists and are known as serotonin normalizers. Therefore, they are expected to have not only an anxiolytic function but also an anti-depressant effect as well. A characteristic of the aryl-piperazine derivatives is that they have no sedative and muscle relaxant effects, and they do not have BZ's undesirable side-effects, especially in regard to withdrawal symptoms. However they have a rather weak anxiolytic action and a slow onset of action. Aryl-piperazine derivatives will not take the place of BZ, but the use of BZ and buspirone as bridge medications, making the most of the strong points of both, can be proposed as a way to compensate for their respective clisadvantages.