Integrated neuromodulatory network in the dorsal striatum-amygdala connectional hub

The dorsal striatum (DS) integrates sensorimotor circuits via cortico- and thalamic inputs, and motivational inputs from the substantia nigra pars compacta (SNc), thereby regulating voluntary movements and goal-directed control of behavior. The DS is also connected to key emotional circuits such as the CeA, a brain region involved in the processing of both positive and negative valence stimuli. The connectional hub, formed by the sensory DS and the CeA is shaped by the temporal dynamics of the neuromodulator serotonin (5-HT) released by projections originating from the Dorsal Raphe Nucleus (DRN), one of the main neuromodulatory nucleus in the brain. In addition to serotonergic neurons (DRN5-HT), the DRN contains dopaminergic cells (DRNDA). DRNDA neurons are anatomically placed in the proximity to DRN5-HT cells, suggesting a modulatory functional crosstalk. Anatomical evidence also shows that the DRN receives projections from the noradrenergic locus coeruleus (LC). However, how the DRN processes different monoaminergic signals to shape striatal 5-HT release remains unexplored. In this study, we combined pharmacological and optogenetic approaches ex-vivo together with fiber photometry in-vivo to examine how DRNDA neurons and LC projections to the DRN (LC→DRN) modulate the activity of DRN5-HT cells, and thereby 5-HT release in the DS and CeA. We found that exogenous DA application in the DRN boosted the firing activity of DS- and CeA-projecting DRN5-HT neurons (DRN5-HT→DS and DRN5-HT→CeA). While this DA-mediated increase in firing activity was insensitive to dopamine D1 or D2 receptor antagonism, it was prevented by the application of the adrenaline α1 receptor (α1R) antagonist prazosin. Accordingly, in tyrosine hydroxylase (TH)-CRE mice infused into the DRN with an adeno-associated viral (AAV) vector encoding the opsin Chrimson, we demonstrated that light-activation of DRN TH+ neurons increased DRN5-HT→DS and DRN5-HT→CeA firing rate. Furthermore, in dopamine beta-hydroxylase (DBH)-CRE mice injected with a Chrimson-expressing virus into the LC, we observed that LC preferentially activate DRNDA neurons compared to DRN5-HT. These observations suggest that DRNDA neurons represent a potential source for local DA-mediated modulation of DRN microcircuit that can be shaped by afferent noradrenergic LC projections. By using genetically encoded sensors for monitoring the temporal dynamics of 5HT signal, we found that exogenous DA infusion in the DRN as well as the optogenetic activation of either DRNDA neurons or LC→DRN results in 5-HT release in the DS and CeA. In summary, our results show how local monoaminergic modulation of DRN microcircuit shapes 5-HT release in the DS and CeA, which may affect motor behavior in health and disease states.