Loss of adult 5-HT1A autoreceptors results in a paradoxical anxiogenic response to antidepressant treatment.

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are first-line antidepressants but require several weeks to elicit their actions. Chronic SSR1 treatment induces desensitization of 5-HT 1A autoreceptors to enhance 5-HT neurotransmission. Mice (both sexes) with gene deletion of 5-HT1A autoreceptors in adult 5-HT neurons (lAcKO) were tested for response to SSRIs. Tamoxifen-induced recombination in adult lAcKO mice specifically reduced 5-HT1A autoreceptor levels. The lAcKO mice showed a loss of 5-HT1A autoreceptormediated hypothermia and electrophysiological responses, but no changes in anxiety- or depression-like behavior. Subchronic fluoxetine (FLX) treatment induced an unexpected anxiogenic effect in lAcKO mice in the novelty suppressed feeding and elevated plus maze tests, as did escitalopram in the novelty suppressed feeding test. No effect was seen in wild-type (WT) mice. Subchronic FLX increased 5-HT metabolism in prefrontal cortex, hippocampus, and raphe of lAcKO but not WT mice, suggesting hyperactivation of 5-HT release. To detect chronic cellular activation, FosB+ cells were quantified. FosB+ cells were reduced in entorhinal cortex and hippocampus (CA2/3) and increased in dorsal raphe 5-HT cells of lAcKO mice, suggesting increased raphe activation. In WTbut not lAcKO mice, FLX reduced FosB+ cells in the median raphe, hippocampus, entorhinal cortex, and median septum, which receive rich 5-HT projections. Thus, in the absence of 5-HT 1A autoreceptors, SSRIs induce a paradoxical anxiogenic response. This may involve imbalance in activation of dorsal and median raphe to regulate septohippocampal or fimbria-fornix pathways. These results suggest that markedly reduced 5-HT1A autoreceptors may provide a marker for aberrant response to SSRI treatment. [ABSTRACT FROM AUTHOR]