Neurochemical differences between target-specific populations of rat dorsal raphe projection neurons

Abstract

Serotonin (5-HT)-containing neurons in the dorsal raphe (DR) nucleus project throughout the forebrain and are implicated in many physiological processes and neuropsychiatric disorders. Diversity among these neurons has been characterized in terms of their neurochemistry and anatomical organization, but a clear sense of whether these attributes align with specific brain functions or terminal fields is lacking. DR 5-HT neurons can co-express additional neuroactive substances, increasing the potential for individualized regulation of target circuits. The goal of this study was to link DR neurons to a specific functional role by characterizing cells according to both their neurotransmitter expression and efferent connectivity; specifically, cells projecting to the medial prefrontal cortex (mPFC), a region implicated in cognition, emotion, and responses to stress. Following retrograde tracer injection, brainstem sections from Sprague-Dawley rats were immunohistochemically stained for markers of serotonin, glutamate, GABA, and nitric oxide (NO). 98% of the mPFC-projecting serotonergic neurons co-expressed the marker for glutamate, while the markers for NO and GABA were observed in 60% and less than 1% of those neurons, respectively. To identify potential target-specific differences in co-transmitter expression, we also characterized DR neurons projecting to a visual sensory structure, the lateral geniculate nucleus (LGN). The proportion of serotonergic neurons co-expressing NO was greater amongst cells targeting the mPFC vs LGN (60% vs 22%). The established role of 5-HT in affective disorders and the emerging role of NO in stress signaling suggest that the impact of 5-HT/NO co-localization in DR neurons that regulate mPFC circuit function may be clinically relevant.

Anatomical, Physiological, and Functional Heterogeneity of the Dorsal Raphe Serotonin System

“…Our retrograde tracing from eight diverse target sites, combined with image registration to compare the data in the same reference brain, provides a more comprehensive view of the spatial organization of DR serotonergic projections in the mouse. We did not find a prominent anterior-posterior topographic organization except that entorhinal cortex-projecting serotonin neurons tended to be localized to the posterior DR. We did observe a prominent division along the dorsal-ventral axis in the anterior DR, where ventral and dorsal serotonin neurons preferentially innervate cortical and subcortical targets, respectively, confirming and extending previous reports (Lowry et al, 2005;Muzerelle et al, 2016;Prouty et al, 2017). We further uncovered a strong correlation between cortical projections and Vglut3 co-expression in DR serotonin neurons.…”

Selective vulnerability of dorsal raphe‐medial prefrontal cortex projection neurons to corticosterone‐induced hypofunction

“…The electrophysiological properties of the FG‐labeled neurons closely aligned with established properties characteristic of DR serotonergic neurons, including long AP duration and large amplitude AHP with slow recovery time (Calizo et al., ; Tuckwell, ; Vandermaelen & Aghajanian, ). Those signature electrophysiological features in conjunction with our previous neurochemical study showing that the vast majority of DR neurons projecting to mPFC or LGN were tryptophan hydroxylase‐positive (Prouty et al., ), provide strong support for the putative serotonergic identity of the recorded neurons.…”

“…Using the same rat strain, tracer, and stereotaxic procedures, we have previously shown that 90% of 2238 labeled DR‐mPFC neurons and 95% of 2117 DR‐LGN neurons (total counts from six animals per group) were positive for tryptophan hydroxylase, indicating that both structures receive strong, primarily serotonergic projections from the DR (Prouty et al., ). The distribution of labeled cells in the current experiments matched closely with our previous report.…”

“…As described in (Prouty, Chandler, & Waterhouse, ). Young adult (P56, ~250 g) rats received stereotaxic injections of Fluoro‐Gold (FG, Fluorochrome Inc., Denver, CO) into the mPFC or LGN.…”

Serotonin improves behavioral contrast sensitivity of freely moving rats

“…Serotonin is secreted from afferent terminals originating from neurons in the dorsal and median raphe nuclei, which act on subcortical and cortical areas [3,4] to regulate various brain functions [5][6][7]. For example, serotonergic neurons regularly fire at low frequencies (0.2-4 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111…”