Diazepam potentiates the effect of neuroleptics on behavioural activity as well as dopamine and norepinephrine turnover: Do benzodiazepines have antipsychotic potency?

A single injection of diazepam (10 mg/kg, s.c.), haloperidol (2 mg/kg, i.p.) or chlorpromazine (10 mg/kg, i.p.) decreased the ambulatory as well as sniffing behaviour of rats. These behavioural responses were further decreased when diazepam was administered concurrently with the neuroleptic. Acute haloperidol or chlorpromazine treatment increased striatal dopamine as well as cerebro-cortical norepinephrine turnover. In contrast, diazepam diminished the release of both of these catecholamines. When diazepam was administered together with haloperidol or chlorpromazine, a further decrease particularly in dopamine release was seen in striatum. This effect of diazepam on norepinephrine and dopamine turnover persisted even after 21 days of daily treatment, Similarly, the sedative effect of diazepam elicited in the form of depressed locomotor activity was also apparent after long-term administration of this benzodiazepine. However, chronic administration of neuroleptics enhanced the spontaneous locomotor activity and sniffing behaviour by about 25%. Furthermore, repeated neuroleptic treatment decreased the synthesis and turnover of dopamine and norepinephrine. This was reflected in decreased tyrosine hydroxylase and homovanillic acid level in striatum as well as by low concentration of 3-methoxy-4-hydroxyphenylethylene glycol in the cerebral cortex. When diazepam was administered together with haloperidol or chlorpromazine for 21 days, behavioural activity remained elevated and was comparable to groups of rats receiving neuroleptics alone. The cocomitant injection of diazepam and neuroleptics for 21 days elicited a synergistic effect on decreased synthesis and release of dopamine as well as norepinephrine. These data provide neurochemical evidence for potentiation of the neuroleptic effects by a benzodiazepine.