Activity of dorsal raphe neurons is controlled by noradrenaline afferents. In this brain region, noradrenaline activates Ga qcoupled a1-adrenergic receptors (a1-A R ), causing action potential (AP) firing and serotonin release. In vitro, electrical stimulation elicits vesicular noradrenaline release and subsequent activation of a1-A R to produce an EPSC (a1-A R -EPSC). The duration of the a1-A R -EPSC (;27 s) is much longer than that of most other synaptic currents, but the factors that govern the spatiotemporal dynamics of a1-A R are poorly understood. Using an acute brain slice preparation from adult male and female mice and electrophysiological recordings from dorsal raphe neurons, we found that the time course of the a1-A R -EPSC was slow, but highly consistent within individual serotonin neurons. The amount of noradrenaline released influenced the amplitude of the a1-A R -EPSC without altering the time constant of decay suggesting that once released, extracellular noradrenaline was cleared efficiently. Reuptake of noradrenaline via noradrenaline transporters was a primary means of terminating the a1-A R -EPSC, with little evidence for extrasynaptic diffusion of noradrenaline unless transporter-dependent reuptake was impaired. Taken together, the results demonstrate that despite slow intrinsic signaling kinetics, noradrenalinedependent synaptic transmission in the dorsal raphe is temporally and spatially controlled and noradrenaline transporters are critical regulators of serotonin neuron excitability. Given the functionally distinct types of neurons intermingled in the dorsal raphe nucleus and the unique roles of these neural circuits in physiological responses, transporters may preserve independence of each synapse to encode a long-lasting but discrete signal.

Previously, using electrophysiological recordings from adult male and female mouse brain slices containing the dorsal raphe nucleus, we showed that GluD1R channels carry ionic current and are modulated via activation of Gαq-coupled a1-adrenergic receptors (a1-AR) in a GTP-dependent manner (Gantz et al., 2020). GluD1R channels also carry a tonic cation current, generally ~-20 pA at subthreshold membrane potentials (Gantz et al., 2020). The origin of tonic GluD1R channel current is unknown. Here, using the same preparation, we show there is no role of on-going G protein-coupled receptor activity in generating or sustaining tonic GluD1R channel current. Neither augmentation nor disruption of G protein activity had an effect on tonic GluD1R current. These results reveal that tonic GluD1R current arises from a mechanism separate from on-going activity of G protein-coupled receptors. Under current clamp, block of GluD1R channels hyperpolarized the membrane by ~10 mV at subthreshold potentials leading to reduced excitability. Thus, GluD1R channels carry a G protein-independent tonic current that contributes to subthreshold drive of action potential firing in the dorsal raphe nucleus.

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