Bidirectional Communication between the Pontine Nucleus Incertus and the Medial Septum Is Carried Out by Electrophysiologically-Distinct Neuronal Populations

Theta oscillations are key brain rhythm involved in memory formation, sensorimotor integration, and control of locomotion and behavioral states. Generation and spatiotemporal synchronization of theta oscillations rely on interactions between brain nuclei forming a large neural network, which includes pontine nucleus incertus (NI). Here we identified distinct populations of NI neurons, based on the relationship of their firing to hippocampal waves, with a special focus on theta oscillations, and the direction and type of interaction with the medial septum (MS) in male, urethane-anesthetized rats. By recording NI neuronal firing and hippocampal LFP, we described NI neurons that fire action potentials in a theta phase-independent or theta phase-locked and delta wave-independent or delta wave-locked manner. Among hippocampal activity-independent NI neurons, irregular, slow-firing, and regular, fast-firing cells were observed, while hippocampal oscillation-/wave-locked NI neurons were of a bursting or nonbursting type. By projection-specific optotagging, we revealed that only fast-firing theta phase-independent NI neurons innervate the MS, rarely receiving feedback information. In contrast, the majority of theta-bursting NI neurons were inhibited by MS stimulation, and this effect was mediated by direct GABAergic input. Described NI neuronal populations differ in reciprocal connections with the septohippocampal system, plausibly forming separate neuronal loops. Our results suggest that theta phase-independent NI neurons participate in theta rhythm generation through direct innervation of the MS, while theta-bursting NI neurons further transmit the rhythmic signal received from the MS to stabilize and/or strengthen rhythmic activity in other structures. SIGNIFICANCE STATEMENT The generation and spatiotemporal synchronization of theta oscillations rely on interactions between nuclei forming a large neural network, part of which is the pontine nucleus incertus (NI). Here we describe that within NI there are populations of neurons that can be distinguished based on the relationship of their firing to hippocampal theta oscillations and delta waves. We show that these neuronal populations largely do not have reciprocal connections with the septohippocampal system, but form separate neuronal loops. Our results suggest that medial septum (MS)-projecting, fast-firing, theta phase-independent NI neurons may participate in theta rhythm generation through direct innervation of the MS, while theta-bursting NI neurons may further transmit the rhythmic signal received from the MS to other structures.