Pontomedullary raphe neurons: monosynaptic excitation from midbrain sites that suppress the jaw opening reflex.

The intracellular responses of pontomedullary raphe neurons to midbrain stimulation were studied in chloralose anesthetized cats. Electrical stimulation of the periaqueductal gray region at sites that suppressed the nociceptive jaw opening reflex evoked monosynaptic excitatory postsynaptic potentials in the majority of raphe neurons recorded. Intracellular labeling of studied cells demonstrated that recordings were obtained primarily from large and medium sized neurons in nucleus raphe magnus and the rostral nucleus raphe obscurus. These results provide further evidence that analgesia induced by periaqueductal gray stimulation may result in part from the direct activation of caudal raphe neurons.