Developmental study of N-methyl-D-aspartate-induced firing activity and whole-cell currents in nucleus tractus solitarii neurons.

Whole-cell recordings of rat nucleus tractus solitarii (NTS) neurons were performed on a slice preparation. We investigated possible postnatal changes in firing activities and currents induced by N-methyl-D-aspartate (NMDA) application. A total of 42 neurons were selected and fell into the following age groups: 0-5 days (n = 15), 10-15 days (n = 9) and 30-60 days (adult, n = 18). During this period, input resistance and spike duration decreased by approximately 40%. At all ages, bath application of NMDA elicited a bursting firing activity when the membrane potential was held between -60 and -75 mV. However, in the youngest cells the rhythmic bursting activity was irregular and was characterized by a progressive firing inactivation during a burst. In a tetrodotoxin-containing saline, NMDA-induced oscillations of membrane potential were retained in all age groups. The membrane current-voltage relationship of the NMDA-induced inward current (INMDA) was characterized by a region of negative slope conductance which was similar in all age groups. Thus the voltage-dependent block of INMDA is present in NTS neurons from birth, allowing NTS neurons to display membrane potential oscillations. However, postnatal maturation of repolarizing conductances, as suggested by changes in spike characteristics, could render the oscillatory activity more stable than at birth.