A Genetically Defined Morphologically and Functionally Unique Subset of 5-HT Neurons in the Mouse Raphe Nuclei

Abstract

Heterogeneity of central serotonin (5-HT) raphe neurons is suggested by numerous lines of evidence, but its genetic basis remains elusive.The transcription factor Pet1 is required for the acquisition of serotonergic identity in a majority of neurons in the raphe nuclei. Nevertheless, a subset of 5-HT neurons differentiates in Pet1 knock-out mice. We show here that these residual 5-HT neurons outline a unique subpopulation of raphe neurons with highly selective anatomical targets and characteristic synaptic differentiations. In Pet1 knock-out mice, 5-HT innervation strikingly outlines the brain areas involved in stress responses with dense innervation to the basolateral amygdala, the paraventricular nucleus of the hypothalamus, and the intralaminar thalamic nuclei. In these regions, 5-HT terminals establish asymmetric synaptic junctions. This target selectivity could not be related to altered growth of the remaining 5-HT neurons, as indicated by axon tracing and cell culture analyses. The residual 5-HT axon terminals are functional with maintained release properties in vitro and in vivo. The functional consequence of this uneven distribution of 5-HT innervation on behavior was characterized. Pet1 knock-out mice showed decreased anxiety behavior in novelty exploration and increased fear responses to conditioned aversive cues. Overall, our findings lead us to propose the existence of Pet1-dependent and Pet1-resistant 5-HT neurons targeting different brain centers that might delineate the anatomical basis for a dual serotonergic control on stress responses.

Severe Serotonin Depletion after Conditional Deletion of the Vesicular Monoamine Transporter 2 Gene in Serotonin Neurons: Neural and Behavioral Consequences

“…All recent genetic hyposerotonergic models focused on reducing 5-HT production either by targeting the central 5-HT synthesis enzyme, TPH2 (Alenina et al, 2009;Beaulieu et al, 2008;Gutknecht et al, 2008;Savelieva et al, 2008) or by invalidating the transcription factors controlling differentiation of raphe neurons such as pet1 and lmx1B (Dai et al, 2008;Hendricks et al, 2003;Kiyasova et al, 2011;Zhao et al, 2006). The more severe 5-HT depletion observed here (À95%), compared with previous models (À70 to 90%), is likely due to the fact that increased 5-HT degradation, when VMAT2 is absent, affects all sources of 5-HT (eg, produced by TPH1/TPH2).…”

Transcriptional regulation of the 5-HT1Areceptor: implications for mental illness

“…Hence, changes in 5-HT 1A autoreceptor levels may be secondary to or enhanced by alterations in 5-HT levels. Furthermore, regional diversity of the raphe nuclei has been suggested, with a Pet-1-insensitive population of 5-HT neurons regulating anxiety behaviour [133]: differential regulation of 5-HT 1A receptors within these populations may predispose to anxiety versus depression. Thus, certain populations of 5-HT neurons may display similar levels of 5-HT 1A autoreceptors, while others may be affected by the rs6295 polymorphism, stress or other factors.…”

A Subpopulation of Serotonergic Neurons That Do Not Express the 5-HT1A Autoreceptor

“…Strong EGFP expression was detected across all subfield of the raphe nuclei, with an approximate 95% colocalization with 5-HT immunoreactivity. 20 Double transgenic mice (5-HT1A-iCre/R26R; C57B/L6 background) were obtained by crossing 5-HT1A-iCre transgenic mice 21 and R26R reporter mice harboring a LacZ transgene. 36 Brains obtained from 5-HT1A-iCre/R26R mice were examined for β-galactosidase expression, monitored by X-Gal staining, and colocalization with 5-HT.…”

“…20 In these mice, all the GFP-expressing neurons contained 5-HT. We used this model to evaluate the expression of 5-HT1A receptors in serotonergic cells.…”