Overexpression of 5-HT1B Receptor in Dorsal Raphe Nucleus Using Herpes Simplex Virus Gene Transfer Increases Anxiety Behavior after Inescapable Stress

Abstract

5-HT_1B autoreceptors have been implicated in animal models of stress and are regulated selectively by serotonin-selective reuptake inhibitors such as fluoxetine. These terminal autoreceptors regulate serotonin release from dorsal raphe nucleus (DRN) projections throughout rat forebrain. However, it has not been previously possible to manipulate 5-HT_1B autoreceptor activity selectively without also changing 5-HT_1B activity in other neurons mediating different behavioral responses. Therefore, we have developed a viral-mediated gene transfer strategy to express hemagglutinin-tagged 5-HT_1B and manipulate these autoreceptors in DRN. Green fluorescent protein (GFP) was coexpressed from a separate transcriptional unit on the same amplicon to assist in monitoring infection and expression. We confirmed the expression and biological activity of both transgenic proteins <i>in vitro</i>. When injected directly into DRN using stereotaxic procedure, HA-5-HT_1B receptors were expressed in serotonergic neurons and translocated to the forebrain. The effect of DRN expression of HA-5-HT_1B on stress-induced behaviors was compared with control rats that received GFP-only amplicons. There was no change in immobility in the forced swim test. However, HA-5-HT_1B expression significantly reduced entrances into the central region of an open-field arena after water-restraint stress without altering overall locomotor activity, but not in the absence of stress exposure. HA-5-HT_1B expression also reduced entries into the open arms of the elevated plus maze after water restraint. Because these tests are sensitive to increases in anxiety-like behavior, our results suggest that overactivity of 5-HT_1Bautoreceptors in DRN neurons may be an important mediator of pathological responses to stressful events.