Analysis of the role of histamine in inhibitory avoidance in goldfish.

1. Teleost fish have histaminergic cell bodies on the posterior part of the basal hypothalamus. It was suggested that they are homologous to the tuberomammillary E group in rats. However, unlike in rats, fish have fewer ascending fibers. The main projection runs through the ventral telencephalic area reaching the dorsal telencephalon. This projection is considered homologous to the prosencephalic forebrain bundle. 2. The aim of this study was to verify if the histaminergic system has an inhibitory action on learning and memory in goldfish, as suggested previously for higher vertebrates. 3. A two-compartment aquarium with a central sliding door was used. The animals were placed in one of them, the central door was opened after 30 sec and the time spend for crossing between compartments was recorded. After the fish dorsal fin crossed the line between the compartments a 45 g weight was dropped into the compartment the fish entered. 4. On the training day this procedure was done 3 times. Immediately after the 3rd trial the fish was injected i.p. with either vehicle (2 ml/kg), chlorpheniramine (CPA; 1.0, 4.0 and 8.0 mg/kg) or histidine (500 mg/kg). On the next day, fishes were placed in the start compartment and the latency to cross between compartments was again recorded. 5. The group treated with CPA at the dose of 8 mg/kg, presented a significant increase in the latency to leave the start compartment (Wilcoxon rank sum test, p<0.0232). On the other hand, the vehicle and 1-histidine (500 mg/kg) treated groups, presented a decrease in test latency. 6. Thus, we suggest that also in fish, the histaminergic system has an inhibitory role on learning and memory.