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 in vitro. 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(1B) autoreceptors in DRN neurons may be an important mediator of pathological responses to stressful events.

In major depression in humans and in animal models of depression, there is a defect in serotonergic neurotransmission that can be relieved by chronic antidepressant treatment. One possibility is that this pathologic state is caused by excessive presynaptic autoreceptor activity in serotonergic neurons, and that antidepressants down-regulate the number of these inhibitory receptors, allowing more normal serotonin release to occur. To evaluate this hypothesis, we measured the effects of the antidepressant fluoxetine on neuronal levels of 5-HT1B receptor mRNA, the putative serotonin terminal autoreceptor in rat brain, and on serotonin transporter mRNA, the direct site of fluoxetine binding. Fluoxetine reduced serotonin transporter mRNA briefly, but this was not sustained after 21 days of treatment. However, fluoxetine reduced dorsal raphe 5-HT1B mRNA levels in a time-dependent and washout-reversible manner. This reduction in 5-HT1B mRNA was specific to dorsal raphe nucleus and was not found in several postsynaptic (nonserotonergic) regions. These results suggest that chronic fluoxetine may increase serotonin release from axonal terminals by down-regulating the messenger RNA coding for presynaptic 5-HT1B autoreceptors while causing only transient effects on serotonin transporter mRNA.