The inhibitory synaptic inputs to the superior salivatory nucleus

Objectives: The superior salivatory (SS) nucleus includes a group of parasympathetic preganglionic neurons innervating the submandibular and sublingual salivary glands and tongue. The activities of the SS neurons are controlled by the higher and lower centers. Recently, an electrophysiological study shows that the SS neurons are received not only the excitatory synaptic inputs, but also the inhibitory synaptic inputs to the SS neurons. However, the origins of inhibitory inputs, or relationships of inhibitory inputs between the higher and lower centers have not yet been examined. In the present study, the inhibitory synaptic inputs to the SS neurons were investigated electrophysiologically using the normal and decerebrate rats. Methods: We used neonatal Wistar rats (9-14days old). The SS neurons were labeled by retrograde axonal transport of a fluorescent dye, Texas Red. Some rats were decerebrated to shut off the inputs from the higher centers. Using a whole-cell patch-clamp technique, the inhibitory postsynaptic currents (IPSCs) were recorded at 0 mV in brainstem slices. IPSCs were evoked by application of the agonists (GABA, Glycine) into the bath in the presence of tetrodotoxin, or by electrical stimulation of tissue surrounding the recording cell. Results: After decebration, the amplitude of IPSCs evoked by application of GABA and glycine into the bath were increased. This increase resulted from the up-regulation of the receptors at the postsynaptic membrane. In 83% (n=34/41) neurons electrically stimulated, the peak amplitude of IPSCs were increased, suggesting that neurotransmitters released from the lower centers bound the postsynaptic receptors. In 17% neurons (n=7/41), IPSCs were disappeared, suggesting that the neurons were received only the descending inputs from the higher centers. Conclusion: It is suggested that most neurons were received the inhibitory synaptic inputs from the higher and lower centers, and that some neurons were received from only the higher centers.