Influence of non-L-type calcium channel antagonists on phencyclidine-induced effects in rats

T-type calcium channel Cav3.2 deficient mice show elevated anxiety, impaired memory and reduced sensitivity to psychostimulants

The fine-tuning of neuronal excitability relies on a tight control of Ca²⁺ homeostasis. The low voltage-activated (LVA) T-type calcium channels (Ca_v3.1, Ca_v3.2 and Ca_v3.3 isoforms) play a critical role in regulating these processes. Despite their wide expression throughout the central nervous system, the implication of T-type Ca_v3.2 isoform in brain functions is still poorly characterized. Here, we investigate the effect of genetic ablation of this isoform in affective disorders, including anxiety, cognitive functions as well as sensitivity to drugs of abuse. Using a wide range of behavioral assays we show that genetic ablation of the cacna1h gene results in an anxiety-like phenotype, whereas novelty-induced locomotor activity is unaffected. Deletion of the T-type channel Ca_v3.2 also triggers impairment of hippocampus-dependent recognition memories. Acute and sensitized hyperlocomotion induced by d-amphetamine and cocaine are dramatically reduced in T-type Ca_v3.2 deficient mice. In addition, the administration of the T-type blocker TTA-A2 prevented the expression of locomotor sensitization observed in wildtype mice. In conclusion, our data reveal that physiological activity of this specific Ca²⁺ channel is required for affective and cognitive behaviors. Moreover, our work highlights the interest of T-type channel blockers as therapeutic strategies to reverse drug-associated alterations.

T-type calcium channel antagonism produces antipsychotic-like effects and reduces stimulant-induced glutamate release in the nucleus accumbens of rats

T-type calcium channels are important in burst firing and expressed in brain regions implicated in schizophrenia. Therefore, we examined the effects of novel selective T-type calcium channel antagonists in preclinical assays predictive of antipsychotic-like activity. TTA-A2 blocked the psychostimulant effects of amphetamine and MK-801 and decreased conditioned avoidance responding. These effects appeared mechanism based, rather than compound specific, as two structurally dissimilar T-type antagonists also reduced amphetamine-induced psychomotor activity. Importantly, the ability to reduce amphetamine's effects was maintained following 20 days pre-treatment with TTA-A2. To explore the neural substrates mediating the observed behavioral effects, we examined the influence of TTA-A2 on amphetamine-induced c-fos expression as well as basal and stimulant-evoked dopamine and glutamate release in the nucleus accumbens. TTA-A2 decreased amphetamine-induced c-fos expression as well as MK-801-induced, but not basal, glutamate levels in the nucleus accumbens. Basal, amphetamine- and MK-801-induced dopamine efflux was altered. These findings suggest that T-type calcium channel antagonism could represent a novel mechanism for treating schizophrenia.

Effects of sigma ligands on the ability of rimcazole to inhibit PCP hsp70 induction

Phencyclidine (PCP) can result in schizophrenia-like behavior. It binds at the PCP site on the NMDA-receptor calcium channel and at the sigma receptor. PCP also induces the heat shock gene hsp7O in retrosplenial cortex neurons. An antipsychotic drug, rimcazole, inhibits PCP hsp7O induction. Rimcazole binds predominantly to sigma-2 sites. It is hypothesized that sigma ligands without antipsychotic properties and with some sigma-2 affinity should partially reverse the effects of rimcazole. (+)-3-PPP, (+)-cyclazocine, and (+)-pentazocine bind predominantly to sigma-I sites. (+)-3-PPP is also a modest sigma-2 ligand. Female Sprague-Dawley rats (200-260 g) were injected intraperitoneally (IP) with (+)-3-PPP (50 mg/kg), rimcazole (60 mg/kg) and, after 5 min, with PCP (40 mg/kg). Brains were sectioned (100 mu m) and presence of the hsp7O gene protein product, HSP7O, was determined immunocytochemically. (+)-3-PPP significantly (0 <0.05) diminished the ability of rimcazole to inhibit PCP hsp7O induction in the retrosplenial cortex. (+)-Cyclazocine (15mg/kg, IP) and (+)-pentazocine (8Omg/kg, IP) given in an analogous manner did not diminish the ability of rimcazole to inhibit PCP hsp7O induction.

DNQX inhibits phencyclidine (PCP) and ketamine induction of the hsp70 heat shock gene in the rat cingulate and retrosplenial cortex

Phencyclidine (PCP) and ketamine are known to block NMDA receptor mediated excitotoxicity by non-competitively blocking the NMDA receptor calcium channel. PCP and ketamine have the paradoxical effect of also inducing the heat shock gene, hsp70, in the cingulate and retrosplenial cortex of the rat. The present study shows that DNQX, a specific AMPA receptor antagonist, given as either a 5 mg/kg or 10 mg/kg intraperitoneal dose or into the lateral cerebral ventricle (5 microliters of 0.5 mg/ml) significantly diminished PCP (40 mg/kg) and ketamine (80, 100, 120 mg/kg) hsp70 induction in the posterior cingulate and retrosplenial cortex. The most dramatic decrease of hsp70 induction was seen with the intraventricular dose of DNQX. Present findings show that the AMPA receptor has a role in PCP/ketamine induction of hsp70 in the cortex. DNQX inhibition of PCP/ketamine hsp70 induction was likely related to AMPA receptor antagonism which prevented excess calcium influx via voltage-gated calcium channels.