Stimulation of 5-HT(1A) receptors in the dorsal raphe ameliorates the impairment of spatial learning caused by intrahippocampal 7-chloro-kynurenic acid in naive and pretrained rats.

Abstract

The present study investigated the effect of stimulating 5-HT(1A) receptors in the dorsal raphe on the impairment of spatial learning caused by intrahippocampal 7-chloro-kynurenic acid (7-Cl-Kyn) in naive rats and in rats familiar with the general requirements of the task. A week after implantation of cannulae to give access to the dorsal raphe (DR) and the CA1 region of the dorsal hippocampus, rats started their 5 days acquisition training on a two-platform spatial discrimination task in a water maze. On each acquisition day, WAY 100635 and 8-OH-DPAT alone or in combination were injected into the dorsal raphe (DR) 5 min before intrahippocampal injections of 7-Cl-Kyn which was given 10 min before the training session. Similar experiments were conducted in rats that had been familiarized with the general requirements of the task by pretraining them in the water maze in the absence of distal cues. 7-Cl-Kyn (3 microg/microl), injected bilaterally in the CA1 region of the dorsal hippocampus, impaired choice accuracy with no significant effect on choice latency. Rats treated with 7-Cl-Kyn tended to spend more time swimming close to the pool walls and made more errors of omission than controls in the first two sessions. Administered into the DR, the 5-HT1A receptor agonist 8-OH-DPAT (1 microg/0.5 microl) had no effect on any parameter of rats' performance but antagonized the impairment of choice accuracy caused by intrahippocampal 7-Cl-Kyn. Injected into the DR, 1 microg/0.5 microl WAY 100635, a 5-HT(1A) receptor antagonist, had no effect on rats' performance or on the impairment caused by intrahippocampal 7-Cl-Kyn, but antagonized the effect of 8-OH-DPAT on the 7-Cl-Kyn-induced deficit. The non-mnemonic behavioral disturbances shown by naive rats treated with 7-Cl-Kyn were greatly reduced in pretrained rats which, nevertheless, showed a marked impairment of choice accuracy similar to that of naive rats. As in previous experiments, administration of 1 microg/0.5 microl 8-OH-DPAT in the dorsal raphe antagonized the impairment of choice accuracy caused by intrahippocampal 7-Cl-Kyn without any effect on other parameters of rats' performance. The results show that stimulation of presynaptic 5-HT(1A) receptors in the dorsal raphe counteracts the deficit in spatial learning caused by a reduced NMDA-mediated excitatory input on pyramidal cells in the hippocampus. The possible mechanisms and the importance of these findings for the symptomatic treatment of memory disorders in man are discussed.