[Membrane properties and cholecystokinin sensitivity of neurons in the gastrointestinal medial region of the nucleus tractus solitarius].

The medial region of the nucleus tractus solitarius (mNTS) has been shown to receive inputs from gastrointestinal afferents and to possess type A receptors of cholecystokinin (CCK) which mediate a satiety signal. In the present study, we examined the electrophysiological properties of neurons of the rat mNTS and their CCK responsiveness by means of in vitro intracellular recording. The mNTS neurons (n = 51) were classified into three groups, based on their different firing response patterns to injection of depolarizing current, showing repetitive firing ("repetitive" type, n = 21), delayed excitation ("delayed" type, n = 12) or a single action potential ("single" type, n = 18). Current-voltage (I-V) relations were different among the three types. "Repetitive" type neurons had anomalous rectification, while "delayed" type neurons showed delayed rectification. "Single" type neurons had linear I-V relations. Although the levels of the resting membrane potential, the sizes of the action potentials and the afterhyperpolarizations did not differ among the groups, input resistances of the "single" type neurons (103 +/- 12 M omega) were significantly smaller than those of "repetitive" (161 +/- 11 M omega) and "delayed" types (143 +/- 17 M omega) of neurons (p < 0.05). Bath application of CCK-8S (100nM) induced depolarization in 15 of 51 mNTS neurons. Fifty-two percent of "repetitive" type neurons, but only 25% of "delayed" type and 6% of "single" type neurons responded to CCK. Although "delayed" and "single" types of neurons were widely distributed in the mNTS, "repetitive" type neurons were localized mainly in the subnucleus centralis of the mNTS. These results suggest that the "repetitive" type neurons in the mNTS may play a predominant role in CCK-mediated functions.