Evaluation of the effects of α2 adrenoceptor antagonism with the D2 receptor antagonist raclopride on conditioned avoidance responding in rats

Clozapine reconstructed: Haloperidol's ability to reduce alcohol intake in the Syrian golden hamster can be enhanced through noradrenergic modulation by desipramine and idazoxan

BACKGROUND

Alcohol use disorder commonly occurs in patients with schizophrenia. Most antipsychotic drugs do not lessen alcohol use; although the atypical antipsychotic clozapine has been shown to reduce alcohol use in patients with schizophrenia, its toxicity severely limits its use in patients. With an eye toward creation of a safer clozapine-like drug, we have investigated the pharmacological basis of the clozapine's effects on alcohol drinking in the Syrian golden hamster. In this animal, as in patients with schizophrenia, clozapine reduces alcohol drinking while the typical antipsychotic haloperidol does not. We have suggested that clozapine decreases alcohol drinking due to its weak dopamine D2 receptor blockade, its potent norepinephrine α-2 receptor antagonism, as well as its ability to elevate plasma norepinephrine.

METHODS

We recreated a clozapine-like drug to reduce alcohol drinking in the Syrian golden hamster by combining low dose haloperidol with a norepinephrine α-2 receptor antagonist, idazoxan, and a norepinephrine reuptake inhibitor, desipramine. Hamsters were given free access to water and alcohol (15% v/v) and were treated daily with each drug or with the three-drug combination for 23 days.

RESULTS

The drug combination reduced alcohol drinking and preference significantly as compared to vehicle or to haloperidol, idazoxan or desipramine, while not altering food-intake or body-weight.

CONCLUSION

These findings suggest that that haloperidol, which does not reduce alcohol drinking in patients with schizophrenia or the hamster, if combined with idazoxan and desipramine (producing a drug combination that mimics aspects of clozapine's pharmacology) is able to reduce alcohol drinking in the hamster.

α2-Adrenoceptors are targets for antipsychotic drugs

RATIONALE

Almost all antipsychotic drugs (APDs), irrespective of whether they belong to the first-generation (e.g. haloperidol) or second-generation (e.g. clozapine), are dopamine D2 receptor antagonists. Second-generation APDs, which differ from first-generation APDs in possessing a lower propensity to induce extrapyramidal side effects, target a variety of monoamine receptors such as serotonin (5-hydroxytryptamine) receptors (e.g. 5-HT1A, 5-HT2A, 5-HT2C, 5-HT6, 5-HT7) and α1- and α2-adrenoceptors in addition to their antagonist effects at D2 receptors.

OBJECTIVE

This short review is focussed on the potential role of α2-adrenoceptors in the antipsychotic therapy.

RESULTS

Schizophrenia is characterised by three categories of symptoms: positive symptoms, negative symptoms and cognitive deficits. α2-Adrenoceptors are classified into three distinct subtypes in mammals, α2A, α2B and α2C. Whereas the α2B-adrenoceptor seems to play only a minor role in the brain, activation of postsynaptic α2A-adrenoceptors in the prefrontal cortex improves cognitive functions. Preclinical models such as D-amphetamine-induced locomotion, the conditioned avoidance response and the pharmacological N-methyl-D-aspartate receptor hypofunction model have shown that α2C-adrenoceptor blockade or the combination of D2 receptor antagonists with idazoxan (α2A/2C-adrenoceptor antagonist) could be useful in schizophrenia. A potential benefit of a treatment combination of first-generation APDs with the α2A/2C-adrenoceptor antagonists idazoxan or mirtazapine was also demonstrated in patients with schizophrenia.

CONCLUSIONS

It is concluded that α2-adrenoceptors may be promising targets in the antipsychotic therapy.

The neurotensin-1 receptor agonist PD149163 inhibits conditioned avoidance responding without producing catalepsy in rats

Agonists for neurotensin (NT)-1 receptors have produced antipsychotic-like effects in many animals, including reversal of prepulse inhibition deficits and psychostimulant-induced increases in spontaneous activity. The present study sought to provide a basic assessment of the putative antipsychotic effects of PD149163 in rats using a two way conditioned avoidance response task, which is highly validated for screening antipsychotic drugs, and an inclined grid assessment, which is used to assess extrapyramidal side effect liability. PD149163 (0.0625-8.0 mg/kg) significantly suppressed conditioned avoidance responding (CAR) following administration of a 1.0 or 8.0 mg/kg dose. PD149163 failed to significantly increase catalepsy scores. The typical antipsychotic drug haloperidol (0.01-1.0 mg/kg) significantly suppressed CAR at a 0.1, 0.3, and 1.0 mg/kg dose, and a significant increase in catalepsy scores was found at the 1.0 mg/kg dose. The atypical antipsychotic drug clozapine (2.5-10.0 mg/kg) also produced a significant inhibition of CAR, which occurred following administration of a 10.0 mg/kg dose. Clozapine failed to significantly increase catalepsy scores. Finally, D-amphetamine (1.0 mg/kg), serving as a negative control, failed to suppress CAR or increase catalepsy scores. These data further suggest that PD149163 may have atypical antipsychotic-like properties.