Exercise training normalizes an increased neuronal excitability of NTS-projecting neurons of the hypothalamic paraventricular nucleus in hypertensive rats

Abstract

Elevated sympathetic outflow and altered autonomic reflexes, including impaired baroreflex function, are common findings observed in hypertensive disorders. Although a growing body of evidence supports a contribution of preautonomic neurons in the hypothalamic paraventricular nucleus (PVN) to altered autonomic control during hypertension, the precise underlying mechanisms remain unknown. Here, we aimed to determine whether the intrinsic excitability and repetitive firing properties of preautonomic PVN neurons that innervate the nucleus tractus solitarii (PVN-NTS neurons) were altered in spontaneously hypertensive rats (SHR). Moreover, given that exercise training is known to improve and/or correct autonomic deficits in hypertensive conditions, we evaluated whether exercise is an efficient behavioral approach to correct altered neuronal excitability in hypertensive rats. Patch-clamp recordings were obtained from retrogradely labeled PVN-NTS neurons in hypothalamic slices obtained from sedentary (S) and trained (T) Wistar-Kyoto (WKY) and SHR rats. Our results indicate an increased excitability of PVN-NTS neurons in SHR-S rats, reflected by an enhanced input-output function in response to depolarizing stimuli, a hyperpolarizing shift in Na(+) spike threshold, and smaller hyperpolarizing afterpotentials. Importantly, we found exercise training in SHR rats to restore all these parameters back to those levels observed in WKY-S rats. In several cases, exercise evoked opposing effects in WKY-S rats compared with SHR-S rats, suggesting that exercise effects on PVN-NTS neurons are state dependent. Taken together, our results suggest that elevated preautonomic PVN-NTS neuronal excitability may contribute to altered autonomic control in SHR rats and that exercise training efficiently corrects these abnormalities.

Fig. 1.

Efficacy of the exercise training protocol in Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. A: time course changes on treadmill performance in WKY and SHR groups submitted to sedentary (S) and training (T) protocols. B–D: comparison of performance gain (B), body weight gain (C), and tail pressure (D) in the 4 groups at the end of S and T protocols. Values are means ± SE; n = 8 for each group. *P < 0.05 vs. WKY, †P < 0.05 vs. S.

Fig. 2.

Exercise training differentially affects the input-output (I/O) function of paraventricular neurons that innervate the nucleus tractus solitarii (PVN-NTS) in WKY and SHR rats. A1: plot of the mean number of spikes (including action potential and “spikelets”) as a function of current injection obtained in WKY-S and WKY-T rats. Neurons in WKY-T rats responded to current stimulation by generating a higher number of action potentials than in WKY-S rats. A2: representative traces of repetitive firing evoked in PVN-NTS neurons from WKY-S and WKY-T rats in response to increasing depolarizing steps. B1: plot of the mean number of spikes as a function of current injection obtained in SHR-S and SHR-T rats. Neurons in SHR-T rats responded to current stimulation by generating a lower number of action potentials than in SHR-S rats. Note that repetitive firing in SHR-S neurons was basally as high as that in WKY-T and that exercise training normalized the I/O function back to control levels. B2: representative traces of repetitive firing evoked in PVN-NTS neurons from SHR-S and SHR-T rats in response to increasing depolarizing steps. Values are means ± SE. *P < 0.05; #P < 0.01 vs. respective current step in the S group. Filled arrow in A2 points to failed action potentials, whereas open arrows in B2 point to representative spikelets.

Fig. 3.

Changes in the number and proportion of spikelets in WKY and SHR rats from S and T groups. A–C: the mean total number of action potentials (A), mean number of spikelets (B), and mean proportion of spikelets (C) following a depolarizing pulse of 150 pA for WKY-S, WKY-T, SHR-S, and SHR-T rats. Note that the data shown in A correspond to the same points displayed in Fig. 2, A1 and B1, for 150 pA. Values are means ± SE. *P < 0.05; **P < 0.01 vs. respective S control.

Fig. 4.

Exercise training diminishes the magnitude of the afterhyperpolarizing potential (AHP) in both WKY and SHR rats. A: representative traces showing the effects of exercise training on evoked AHPs (arrows) in WKY (top) and SHR rats (bottom). Each trace represents an average of 8 sweeps. Action potentials were cropped. B: on average, the AHP magnitude in PVN-NTS neurons from both WKY and SHR rats was significantly diminished by training compared with sedentary rats [2-way ANOVA results: blood pressure F = 0.04; exercise F = 13.5 (P < 0.0005), interaction F = 0.1]. Values are means ± SE. *P < 0.05.

Fig. 5.

Exercise training affected the Na⁺ action potential waveform of PVN-NTS neurons in WKY and SHR rats. A: representative traces showing the effects of exercise training on Na⁺ action potentials evoked in PVN-NTS neurons from WKY (left) and SHR rats (right). The thin and thick traces correspond to rats from S and T protocols, respectively. Action potentials were aligned to better compare their waveform. Arrows point to hyperpolarizing afterpotentials (HAP) that followed each evoked action potential; arrowheads point to spike threshold. Traces represent an average of 15 sweeps. B: bar graphs depicting mean values for action potential amplitude (top left), width (top right), threshold (bottom left), and HAP amplitude (bottom right) in PVN-NTS neurons from WKY and SHR rats. Two-way ANOVA results are as follows: amplitude: blood pressure F = 0.01, exercise F = 5.9 (P < 0.02), interaction F = 1.7; width: blood pressure F = 3.5, exercise F = 0.7, interaction F = 2.6; threshold: blood pressure F = 0.3, exercise F = 0.3, interaction F = 13.3 (P < 0.001); and HAP amplitude: blood pressure F = 0.5, exercise F = 0.4, interaction F = 6.5 (P < 0.01).*P < 0.05; #P < 0.05.