Regulation of conditioned and unconditioned fear in rats by 5-HT 1A receptors in the dorsal periaqueductal gray

Abstract

Studies on the involvement of 5-HT 1-mediated mechanisms in the dorsal periaqueductal gray (dPAG) of animals with past stressful experiences have not been conducted so far. We investigated the role of 5-HT 1 receptors in the dPAG of rats previously submitted to contextual fear conditioning. Defensive behaviors induced by activation of the dPAG were assessed by measuring the lowest electric current applied to this structure (threshold) able to produce freezing and escape responses during testing sessions of contextual fear conditioning, in which animals were placed in a context previously paired to footshocks. The 5-HT 1A function of the dPAG was evaluated by local injections of 8-OH-DPAT (4 and 8 nmol/0.2 µL) and WAY-100635 (10 nmol/0.2 µL), selective agonist and antagonist of 5-HT 1A receptors, respectively. In accordance with previous studies, 8-OH-DPAT increased aversive thresholds (antiaversive effects) but injections of WAY 100635 into the dPAG did not produce significant effects on the aversive thresholds in naive rats. However, the aversive thresholds of animals exhibiting contextual fear remained unchanged with both treatments. Moreover, 8-OH-DPAT and WAY 100635 did not change the dPAG post-stimulation freezing. The present results suggest that the stressful experience of being fear conditioned has an effect on the role of the 5-HT 1A receptors in mediating unconditioned fear. Also, the reduction in the regulation of the defensive behaviors by 5-HT 1A-mediated mechanisms in the dPAG of these animals may underlie the stress precipitated psychopathology associated with the neural substrates of aversion of the dPAG.