Forebrain GABAergic projections from the dorsal raphe nucleus identified by using GAD67–GFP knock‐in mice

Abstract

The dorsal raphe nucleus (DR) contains serotonergic (5-HT) neurons that project widely throughout the fore-brain. These forebrain regions also receive innervation from non–5-HT neurons in the DR. One of the main groups of non–5-HT neurons in the DR is γ-aminobutyric acid (GABA)ergic, but their projections are poorly understood due to the difficulty of labeling these neurons immunohistochemically. To identify GABAergic projection neurons within the DR in the current study, we used a knock-in mouse line in which expression of green fluorescent protein (GFP) is controlled by the glutamic acid decarboxylase (GAD)67 promotor. Projections of GAD67–GFP neurons to the prefrontal cortex (PFC), nucleus accumbens (NAC), and lateral hypothalamus (LH) were evaluated by using retrograde tract tracing. The location of GAD67–GFP neurons projecting to each of these areas was mapped by rostrocaudal and dorsoventral location within the DR. Overall, 16% of DR neurons projecting to either the PFC or NAC were identified as GAD67–GFP neurons. GAD67–GFP neurons projecting to the PFC were most commonly found ventrally, in the rostral two-thirds of the DR. NAC-projecting GAD67–GFP neurons had an overlapping distribution that extended dorsally. GAD67–GFP neurons made a larger contribution to the projection of the DR to the LH, accounting for 36% of retrogradely labeled neurons, and were widespread throughout the DR. The current data indicate that DR GABAergic neurons not only may have the capacity to influence local network activity, but also make a notable contribution to DR output to multiple forebrain targets.

Presynaptic Partners of Dorsal Raphe Serotonergic and GABAergic Neurons

“…To quantify the spatial distribution of inputs, we subdivided the DR and nearby regions into three structures based on the pattern of Gad1/2 ISH: dorsal wings (DWs), ventral wings (VWs), and the midbrain reticular nucleus (Figures 7C 1 –7C 3 ) and grouped these subregions into four bins across the anterior-posterior axis (similar to Bang and Commons, 2012). We then quantified the number of local inputs to serotonin neurons that were coming from each of these subregions.…”

“…Second, some GABA neurons may inhibit other GABA neurons or other local neurons such as glutamate and dopamine neurons. Third, many DR GABA neurons are known to send long-range projections (Bang and Commons, 2012). Given the abundant long-range GABAergic projections from the DR, it is intriguing to consider the DR as two parallel but interacting subsystems that integrate similar inputs and send either serotonergic or GABAergic outputs.…”

Serotonergic versus Nonserotonergic Dorsal Raphe Projection Neurons: Differential Participation in Reward Circuitry

“…This suggests that the rewarding effects of DRN stimulation are driven by GABAergic and/or glutamatergic neurons, the two remaining major DRN cell types. The DRN contains GABAergic projection neurons (Bang and Commons, 2012), which could promote reward-related behavior by disinhibiting mesolimbic circuitry. To test the ability of DRN GABA neurons to reinforce behavior, we injected cre-ON vectors expressing ChR2-eYFP or eYFP alone into the DRN of Vgat iCre mice (Figure 4E).…”

Localisation and stress-induced plasticity of GABAA receptor subunits within the cellular networks of the mouse dorsal raphe nucleus

“…Scale bars 10 lm serotonin-expressing neurons (Challis et al 2013), or to other brain regions such as the prefrontal cortex, nucleus accumbens, and lateral hypothalamus (Bang and Commons 2012). Local circuit GABAergic neurons mono-synaptically contact DRN serotonergic neurons and inhibit their activity, impacting on DRN-associated behaviours (Challis et al 2013;Soiza-Reilly et al 2013).…”