Repeated histamine pretreatment decreased amnesia induced by post-training administration of the drug in a step-down inhibitory avoidance test in mice.

BACKGROUND: Repeated administration of certain drugs could result in an enhancement of the behavioral effects of those drugs. In the present study, the effect of repeated administration of histamine on amnesia induced by post-training administration of the drug was examined. METHODS: A single trial step-down inhibitory (passive) avoidance task was used for memory assessment in male NMRI mice. RESULTS: The results showed that post-training administration of different doses of histamine (5, 10, and 20 microg/mouse, i.c.v.) decreased the step-down latency on the test day. Repeated pretreatment of histamine (10 and 20 microg/mouse) for three days followed by five days of no drug treatment prevented amnesia due to post-training histamine (20 microg/mouse). In contrast, repeated administration of histamine H1 receptor antagonist, pyrilamine (5, 10, and 20 mg/kg) or histamine H2 receptor antagonist, ranitidine (12.5 and 25 mg/kg) 10 minutes prior to histamine injections, decreased the effect of repeated histamine administration. Moreover, a similar pattern was seen in animals which received dopamine D1 receptor antagonist, SCH 23390 (0.025, 0.5, and 1 mg/kg) or dopamine D2 receptor antagonist, sulpiride (0.2, 1, and 5 mg/kg) 10 minutes prior to histamine injections during the repeated pretreatment. CONCLUSION: The results indicated that both the histamine and dopamine receptor mechanisms may be involved in the effects of repeated pretreatment of histamine on drug induced amnesia.

Influence of morphine- or apomorphine-induced sensitization on histamine state-dependent learning in the step-down passive avoidance test.

Effects of morphine- or apomorphine-induced sensitization on histamine state-dependent memory of passive avoidance task were examined in mice. Pre-training intracerebroventricular (i.c.v.) administration of histamine (20 microg/mouse) decreased the learning of a one-trial passive avoidance task. Pre-test administration of histamine (10 and 20 microg/mouse) reversed amnesia induced by pre-training of histamine, with maximum response at 20 microg/mouse. Pre-training histamine-induced amnesia was also reversed in morphine- or apomorphine-sensitized mice that had previously received once daily injections of morphine (20 and 30 mg/kg) or apomorphine (0.5 and 1 mg/kg) for 3 days. The reversion of histamine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5 and 1 mg/kg), SCH 23390 (0.05 and 0.1 mg/kg) or sulpiride (25, 50 and 100 mg/kg) prior to injection of morphine (30 mg/kg/day, 3 days). Furthermore, once daily administration of sulpiride (50 and 100 mg/kg) but not SCH 23390 (0.01, 0.05 and 0.1 mg/kg) prior to apomorphine (1 mg/kg, for 3 days) decreased the reversion of pre-training histamine-induced amnesia by apomorphine. The results suggest that apomorphine or morphine sensitization affects the impairment of memory induced by histamine and thus it is postulated that opioid and dopamine receptors may play an important role in this effect.