SUBCHRONIC COLD STRESS REDUCES 5-HT1A RECEPTOR RESPONSIVENESS IN THE OLD BUT NOT IN THE YOUNG-RAT

The inhibitory effect of the prototypical 5-hydroxytryptamine (5-HT)(1 A) receptor agonist 8-hydroxy-2-(di-n-propyl amino)tetraline (8-OH-DPA T) on forskolin-stimulated adenylyl cyclase activity, has been examine d as an index of the functional activity of 5-HT1A receptors in the hi ppocampus of young (3 months) and old (18 months) rats exposed during 24 h or 5 days to cold. In both young and old rats exposed to cold str ess during 24 h, there was a reduction in the potency (EC(50)) and/or the maximal inhibitory effect (E(max)) of 8-OH-DPAT in reducing forsko lin-induced cAMP accumulation. The properties of the hippocampal 5-HT1 A sites labelled by [H-3]8-OH-DPAT were not affected by these stressfu l conditions. Moreover, while the sensitivity of 5-HT1A receptors to 8 -OH-DPAT in young rats returned to control values after 5 days of cold exposure, old rats still exhibited a significant desensitization of 5 -HT1A receptors as compared to naive animals. These results point out the capacity of young but not of old rats to adapt to the aversive eff ects of a subchronic stressor.