A Focal Inactivation and Computational Study of Ventrolateral Periaqueductal Gray and Deep Mesencephalic Reticular Nucleus Involvement in Sleep State Switching and Bistability

Abstract

Neurons of the ventrolateral periaqueductal gray (vlPAG) and adjacent deep mesencephalic reticular nucleus (DpMe) are implicated in the control of sleep-wake state and are hypothesized components of a flip-flop circuit that maintains sleep bistability by preventing the overexpression of NREM/REM sleep intermediary states (NRt). To determine the contribution of vlPAG/DpMe neurons in maintaining sleep bistability we combined computer simulations of flip-flop circuitry with focal inactivation of vlPAG/DpMe neurons by microdialysis delivery of the GABA_A receptor agonist muscimol in freely behaving male rats (n = 25) instrumented for electroencephalographic and electromyographic recording. REM sleep was enhanced by muscimol at the vlPAG/DpMe, consistent with previous studies; however, our analyses of NRt dynamics in vivo and those produced by flop-flop circuit simulations show that current thinking is too narrowly focused on the contribution of REM sleep-inactive populations towards vlPAG/DpMe involvement in REM sleep control. We found that much of the muscimol-mediated increase in REM sleep was more appropriately classified as NRt. This loss of sleep bistability was accompanied by fragmentation of REM sleep, as evidenced by an increased number of short REM sleep bouts. REM sleep fragmentation stemmed from an increased number and duration of NRt bouts originating in REM sleep. By contrast, NREM sleep bouts were not likewise fragmented by vlPAG/DpMe inactivation. In flip-flop circuit simulations, these changes could not be replicated through inhibition of the REM sleep-inactive population alone. Instead, combined suppression of REM sleep active and inactive vlPAG/DpMe subpopulations was required to replicate the changes in NRt dynamics.

Significance Statement Sleep separates into two distinct states: rapid eye movement (REM) and non-REM (NREM) sleep. The circuit mechanisms underlying this bistability have not been defined. Neurons in the ventrolateral periaqueductal gray and adjacent deep mesencephalic reticular nucleus (vlPAG/DpMe), namely those with REM sleep-inactive discharge profiles, are hypothesized to be negative-regulators of NREM-to-REM sleep transitions. We show that the main effect of vlPAG/DpMe inactivation is a loss of sleep bistability that originates mainly in REM sleep, which suggests the important added involvement of the vlPAG/DpMe REM sleep-active cell population. Simulations of flip-flop circuitry support the participation of REM sleep-active and inactive vlPAG/DpMe cell groups in a noisy, asymmetric flip-flop switch that contributes to the bistability and robustness of sleep switching.