EFFECTS OF AGING ON THE INTRINSIC MEMBRANE-PROPERTIES OF MEDIAL NTS NEURONS OF FISCHER-344 RATS

1. Recent studies have demonstrated that the arterial baroreflex is impaired with aging and have implicated central components of the baroreflex arc in this autonomic dysfunction. Neurons in the medial portion of the nucleus tractus solitarius (mNTS) receive a major input from the arterial baroreceptors. The present study was undertaken to characterize the intrinsic membrane properties of mNTS neurons in young rats and to test the hypothesis that these properties are altered with aging. An in vitro brain stem slice preparation was used to record intracellularly from mNTS neurons; passive membrane properties, action potential characteristics, and repetitive firing properties were examined and compared. 2. Neurons in the mNTS of young (3-5 mo old) Fischer-344 rats (F-344; n = 35) had a resting membrane potential of -57 +/- 6.9 mV (mean +/- SD), a membrane time constant of 18 +/- 9.0 ms, and an input resistance of 110 +/- 60 MOMEGA. Action potential amplitude was 81 +/- 7.5 mV with a duration at half-height of 0.83 +/- 0.15 ms. The spontaneous firing rate in 24 cells was 4.3 +/- 2.9 Hz. The amplitude and duration of the action potential afterhyperpolarization (AHP) were 6.6 +/- 3.0 mV and 64 +/- 34 ms, respectively. All neurons expressed spike frequency adaptation, action potential AHP, and posttetanic hyperpolarization. Delayed excitation and postinhibitory rebound were present in 34 and 14% of neurons tested, respectively. Neurons from adult (10-12 mo old) F-344 rats (n = 34) were similar to the young F-344 rats with respect to all of these variables. 3. Neurons from aged (21-24 mo old) F-344 rats (n = 32) were similar to those from young and adult rats, but there were two potentially important differences: the mean input resistance of the aged neurons was high, er (170 +/- 150 MOMEGA), with a larger proportion (46% of aged neurons vs. 20% of young neurons and 21% of adult neurons) having input resistances > 150 MOMEGA; and there was a tendency for a smaller percentage of aged neurons (16% of aged neurons vs. 34% of young neurons and 29% of adult neurons) to express delayed excitation. 4. The potential significance of a high input resistance was tested by comparing the steady-state current-voltage (I-V) relationships and the frequency-current (f-1) relationships among low-resistance (1-100 MOMEGA), medium-resistance (101-200 MOMEGA), and high-resistance (>201 MOMEGA) cells. High-resistance neurons tended to have relatively flat I-V relationships and steeper f-I relationships than low-resistance neurons. 5. We conclude that: 1) neurons in mNTS of young F-344 rats display intrinsic cellular properties similar to those previously described in other regions of NTS, although a clear separation of neurons into specific categories on the basis of these cellular properties not as apparent in mNTS as elsewhere (e.g., ventrolateral NTS); 2) mNTS neurons in the F-344 rat do not undergo the distinct changes in electrophysiological properties that have been described for aged neurons in higher centers (e.g., prolonged AHP in aged hippocampal neurons); 3) in aged animals, the presence of more high-resistance mNTS neurons, with fewer mNTS neurons having delayed excitation, might lead to an altered state of neuronal responsiveness and thereby contribute to the cardiovascular autonomic dysfunction associated with aging.