Electrophysiological study on development of inhibitory synaptic transmission in central neurons involved in oral function.

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We showed the excitatory (glutamate) and inhibitory (GABA and glycine) synaptic inputs to superior salivatory (SS) neurons in neonatal rat brainstem slices. There is a possibility that the synaptic transmission to SS neurons develop postnatally with the development of oral function. In the present study, we studied the development of the inhibitory synaptic transmission to SS neurons electrophysiologically. We used Wistar rats at postnatal day 2 (P2) to P14. The SS neurons innervating the submandibular salivary glands and tongue were labeled by retrograde axonal transport of a fluorescent dye. Gramicidin perforated patch-clamp recordings from the labeled cell were performed. To examine the reversal potential (V_<rev>), the inhibitory postsynaptic currents (IPSCs) were evoked by electrical stimulation near the recording cell at various potentials. Additionally, to examine the effect of GABA_A receptor activation on the excitability of the cells, GABA (1 mM, 100 ms) was applied via pressure ejection near the recording cell at its resting potential. The V_<rev> was shifted with age, and that at P2-P4, P5-P7 and P8-P14 were-52.6 ± 5.1 mV (n = 9),-62.3 ± 1.6 mV (n = 12) and-71.4 ± 1.5 mV (n = 12), respectively. Exogenous GABA application produced depolarization in 40 % cells at P2-P4, and produced hyperpolarization after P8. Such functional change of synaptic transmission might be involved in the transition from sucking to mastication.