RESUMO
Prolactin (PRL) exerts protective effects on the hippocampus against chronic stress exposure, or kainic acid insults. Further, PRL null mice were recently shown to exhibit learning and memory deficits. These findings suggest the hippocampus as a PRL target. To test this hypothesis, we analyzed the hippocampal formation function in dependence of serum PRL concentrations. Male Sprague-Dawley rats were either transplanted with 2 pituitary homografts (HPRL) or had a sham operation (SHAM). Other groups were given vehicle (VEH) or daily injections of bromocriptine (BRC, 4mg/kg) to reduce PRL levels. The animals were sequentially subjected to the Morris water maze (MWM), the object recognition test and the elevated plus maze. PRL concentrations were measured with ELISAs. Learning curves from the MWM showed a progressive reduction in distance over time but no significant differences were found between SHAM and HPRL, or between BRC and VEH groups. Also, memory retrieval was not affected in the probe trial between each pair of groups. However, in the object recognition test the HPRL group failed to discriminate between novel and familiar objects, and the discrimination index was significantly reduced from that of the VEH group. BRC rats exhibited a reduced exploration of both objects compared to VEH, but had a discrimination index similar to VEH. No significant differences were observed in the EPM. In conclusion, high PRL levels impair object recognition but do not alter spatial learning. These results show for the first time that PRL impairs object recognition, and thus modulates non-spatial cognitive tasks.
Assuntos
Hiperprolactinemia/complicações , Transtornos da Memória/etiologia , Reconhecimento Psicológico/fisiologia , Percepção Espacial/fisiologia , Análise de Variância , Animais , Bromocriptina/farmacologia , Discriminação Psicológica/efeitos dos fármacos , Modelos Animais de Doenças , Agonistas de Dopamina/farmacologia , Ensaio de Imunoadsorção Enzimática , Masculino , Aprendizagem em Labirinto/efeitos dos fármacos , Aprendizagem em Labirinto/fisiologia , Prolactina/metabolismo , Ratos , Ratos Sprague-DawleyRESUMO
The change in the percentage of rat hippocampal high-frequency theta activity from being immobile and awake to swimming behaviour was calculated for three groups of rats, trained in either place learning, cue learning or egocentric learning in the Morris water maze. The place-learning-trained rats showed an increase in the percentage of theta activity, along with a significant reduction in escape latency over the last 3 days of training. No changes were observed in the other two groups. Because the motor activity displayed by the three groups of rats was similar, we suggest that the increase in the percentage of theta activity concomitant with place-learning training could be related to the processing of information by the hippocampus, rather than to the displayed motor activity.