Enhanced reactivity of 5-HT1A receptors in the rat dorsal periaqueductal gray matter after chronic treatment with fluoxetine and sertraline: evidence from the elevated T-maze.

Behavioral evidence indicates that sensitization of 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal gray (DPAG) may underlie the therapeutic effect of serotonin reuptake inhibitors (SRIs) in panic disorder. These results were obtained from studies using animal models that associate escape behavior with panic attacks, such as the elevated T-maze. In this test, chronic administration of the non-selective SRI imipramine enhances the inhibitory effect on escape caused by the intra-DPAG injection of the 5-HT1A receptor agonist 8-OH-DPAT. We here evaluated the generality of this finding by investigating the effect of chronic administration of two selective SRIs (SSRIs), fluoxetine and sertraline, on the reactivity of the rat DPAG 5-HT1A receptors. The results showed that both SSRIs inhibited escape behavior in the elevated T-maze, suggestive of a panicolytic-like effect. Whereas intra-DPAG injection of a low dose of 8-OH-DPAT (0.4nmol) had no effect on escape in control animals, it significantly enhanced the inhibitory effect caused by the SSRIs on this response. Microinjection of 8-OH-DPAT in SSRI-treated rats also inhibited the acquisition of inhibitory avoidance, another defensive response measured by the elevated T-maze. The results indicate that chronic administration of fluoxetine and sertraline sensitizes 5-HT1A receptors in the DPAG. Overall, they support the view that facilitation of 5-HT1A receptor-mediated neurotransmission in the DPAG is implicated in the therapeutic effect of SRIs on panic disorder.