Local inhibitory control of Median Raphe output
Median Raphe (MR) exerts an important neuromodulatory control over various behavioral and emotional processes. While modulation by MR projection neurons was extensively studied, the local circuit mechanisms regulating their activity are largely unknown. Inhibition, as suggested by preceding studies, may be pivotal for shaping the output of the MR. Therefore we aimed to reveal the role of the local inhibitory circuit controlling state- and behavior-dependent activity of MR projection neurons. We performed simultaneous whole cell or multichannel recordings from MR and unit recordings from the CA1 region of the dorsal hippocampus combined with optogenetics in awake, head-restrained mice. We found brain state dependent and cell type specific but heterogenous firing pattern (frequency ranged from 0.5 Hz to 25 Hz) in MR cells. Surprisingly, there was no difference in the average resting membrane potential (Vm) and each cell type showed large subthreshold activity but GABAergic cells displayed greater Vm variability than excitatory neurons. Next, we revealed ripple-correlated firing rate changes in half of MR cells: about 10% was facilitated whereas about 40% of neurons decreased activity around ripple events. Remarkably, a subset of neurons displayed sharp increase or drop in activity accompanied by marked depolarization or hyperpolarization in Vm within 50 ms of ripple peaks. GABAergic cells with fast transient activation during ripples preferred to fire around pyramidal layer theta peaks while cells with intermittently suppressed activity preferred theta trough, thereby revealing an unexpected correlation between ripple-coupled activity and theta phase preference. Optogenetic silencing of GABAergic cells uncovered the presence of a robust tonic inhibition both on glutamatergic and serotonergic cells while transient activation of inhibitory cells caused marked hyperpolarization. The response of excitatory neurons to sensory stimuli and rewards also has significantly changed upon optogenetic inhibition of the local GABAergic circuit, underlining the role of local inhibition in the regulation of the salient event-triggered response of MR neurons. Our experiments are the first to characterize the in vivo membrane potential dynamics of MR cells in behaving animals. We demonstrated that the local GABAergic network efficiently controls the output of the MR on multiple timescales coupled to different operational states of forebrain circuits. Moreover, our recordings revealed a rapidly changing pattern of activity both among and within cell types implying highly complex and dynamic, targeted neuromodulation instead of a global modulatory signal.