Inhibition of noradrenaline-dependent synaptic transmission in the dorsal raphe nucleus by alpha2-adrenergic receptors

Abstract

In the central nervous system, noradrenaline transmission controls the degree to which we are awake, alert, and attentive. Aberrant noradrenaline transmission is associated with pathological forms of hyper- and hypo-arousal that present in numerous neuropsychiatric disorders often associated with dysfunction in serotonin transmission. In vivo, noradrenaline regulates the release of serotonin because noradrenergic input drives the serotonin neurons to fire action potentials via activation of excitatory 1-adrenergic receptors (1-AR). Despite the critical influence of noradrenaline on the activity of dorsal raphe serotonin neurons, the source of noradrenergic afferents has not been resolved and the presynaptic mechanisms that regulate noradrenaline-dependent synaptic transmission have not been described. Using an acute brain slice preparation from male and female mice and electrophysiological recordings from dorsal raphe serotonin neurons, we found that selective optogenetic activation of locus coeruleus terminals in the dorsal raphe was sufficient to produce an 1-AR-mediated excitatory postsynaptic current (1-AR-EPSC). Activation of inhibitory 2-adrenergic receptors (2-AR) with UK-14,304 eliminated the 1-AR-EPSC via presynaptic inhibition of noradrenaline release, likely via inhibition of voltage-gated calcium channels. In a subset of serotonin neurons, activation of postsynaptic 2-AR produced an outward current through activation of potassium conductance. Further, in vivo activation of 2-AR by systemic administration of clonidine reduced the expression of c-fos in the dorsal raphe serotonin neurons, indicating reduced neural activity. Thus, 2-AR are critical regulators of serotonin neuron excitability.