Role of striatal dopamine D2 receptors in the paw test, an animal model for the therapeutic efficacy and extrapyramidal side effects of neuroleptic drugs

Spiraling dopaminergic circuitry from the ventral striatum to dorsal striatum is an effective feed-forward loop

Central dopamine systems are key players in the cerebral organization of behavior and in various neurological and psychiatric diseases. We demonstrate the presence of a neurochemical feed-forward loop characterized by region-specific changes in dopamine efflux in serially connected striatal regions, providing evidence in favor of the existence of so-called spiraling striato-nigro-striatal connections. Using in vivo microdialysis of rats, we show that simultaneous stimulation of dopamine D1 and D2 receptors in the accumbal shell decreased dorsal striatal dopamine efflux via a direct or indirect feed-forward loop involving shell, core, ventrolateral and dorsal part of the striatum: simultaneous stimulation of dopamine D1 and D2 receptors in the shell decreased dopamine efflux in the core; flupenthixol-induced inhibition of dopamine D1 and D2 receptors in the core increased dopamine efflux in the ventrolateral part of the striatum, and simultaneous stimulation of dopamine D1 and D2 receptors in the ventrolateral part of the striatum decreased dopamine efflux in the dorsal part of the striatum. Finally, simultaneous stimulation of dopamine D1 and D2 receptors in the shell decreased dopamine efflux in the dorsal part of the striatum. Thus, distinct striatal regions act also in series, providing a better understanding of the neural mechanisms underlying dopamine-dependent behaviors and the progression of dopamine-dependent disorders such as depression, schizophrenia, attention deficit hyperactivity disorder (ADHD), and addiction.

JL 13, an atypical antipsychotic: a preclinical review

The extensive pharmacological evaluation of JL 13 as an atypical antipsychotic drug has revealed a close similarity to clozapine, however with some major advantages. JL 13 was characterized as a weak D(2) antagonist, both in vitro and in vivo, with a strong affinity for the D(4) and the 5-HT(2A) receptors. It has no affinity for the 5-HT(2C) receptor. In vivo microdialysis experiments in rat showed that JL 13, like clozapine, preferentially increased extracellular dopamine concentrations in the prefrontal cortex compared to nucleus accumbens or striatum. Behavioral studies showed that JL 13, like clozapine, has the profile of an atypical antipsychotic. Thus, JL 13 did not antagonize apomorphine-induced stereotypy nor did it produce catalepsy, but it antagonized apomorphine-induced climbing in rodents. It was inactive against d-amphetamine-induced stereotypy but antagonized d-amphetamine-induced hyperactivity in the mouse. Likewise, in the paw test, it was more effective in prolonging hindlimb retraction time than prolonging forelimb retraction time. Like other antipsychotic drugs, JL 13 reversed the apomorphine- and amphetamine-induced disruption of prepulse inhibition. In a complex temporal regulation schedule in the dog, JL 13 showed a high resemblance with clozapine without inducing sialorrhea, palpebral ptosis or any significant motor side effects. In rats and squirrel monkeys JL 13 induced a high degree of generalization (70%) to clozapine. Regarding behavioral toxicology, JL 13 did not produce dystonia or Parkinsonian symptoms in haloperidol-sensitized monkeys. After acute administration, again like clozapine, JL 13 induced only a transient increase in circulating prolactin. Last but not the least, regarding a possible hematological toxicity, unlike clozapine, JL 13 did not present sensitivity to peroxidase-induced oxidation. Moreover, its electrooxidation potential was close to that of loxapine and far from that of clozapine. Taking all these preclinical data into account, it appears that JL 13 is a promising atypical antipsychotic drug.

Effects of risperidone and haloperidol on tachykinin and opioid precursor peptide mRNA levels in the caudate-putamen and nucleus accumbens of the rat

We investigated whether the two output pathways of the striatum are differently affected by the novel atypical drug risperidone and the conventional typical antipsychotic drug haloperidol. To this end, changes in mRNA levels of preproenkephalin-A, preproenkephalin-B, and preprotachykinin were determined in the rat striatum following chronic drug treatment for 14 days, using quantitative in situ hybridization. Furthermore, we studied the contribution of the dopamine D2 and serotonin 5-HT2A antagonist components of risperidone in establishing its effects on neuropeptide mRNA levels in the striatum. The results showed that both risperidone and haloperidol had major effects on the preproenkephalin-A mRNA and thus on the indirect striatal output route, whereas they had minor effects on preproenkephalin-B and preprotachykinin mRNA, contained by the direct output route. When both drugs were administered in the same dose, preproenkephalin-A mRNA was much more elevated by haloperidol than by risperidone. However, when doses of risperidone and haloperidol were modified to attain comparable dopamine D2 receptor occupancy, the drugs had comparable effects on preproenkephalin-A mRNA levels. It was further found that 5-HT2A/C receptor blockade with ritanserin had only modest effects on preproenkephalin-B and preprotachykinin mRNA levels and did not affect preproenkephalin-A mRNA levels. We conclude that risperidone and haloperidol, administered in the same dose, differently affect the striatal output routes. Furthermore, the results suggest that the effects of risperidone on neuropeptide mRNA levels are fully accounted for by its D2 antagonism and that no indication exists for a role of 5-HT2A receptor blockade in this action.