Enhanced activation of RVLM-projecting PVN neurons in rats with chronic heart failure

Abstract

Previous studies have indicated that there is increased activation of the paraventricular nucleus (PVN) in rats with chronic heart failure (CHF); however, it is not clear if the preautonomic neurons within the PVN are specifically overactive. Also, it is not known if these neurons have altered responses to baroreceptor or osmotic challenges. Experiments were conducted in rats with CHF (6-8 wk after coronary artery ligation). Spontaneously active neurons were recorded in the PVN, of which 36% were antidromically activated from the rostral ventrolateral medulla (RVLM). The baseline discharge rate in RVLM-projecting PVN (PVN-RVLM) neurons from CHF rats was significantly greater than in sham-operated (sham) rats (6.0 ± 0.6 vs. 2.6 ± 0.3 spikes/s, P < 0.05). Picoinjection of the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid significantly decreased the basal discharge of PVN-RVLM neurons by 80% in CHF rats compared with 37% in sham rats. Fifty-two percent of spontaneously active PVN-RVLM neurons responded to changes in the mean arterial pressure (MAP). The changes in discharge rate in PVN-RVLM neurons after a reduction in MAP (+52 ± 7% vs. +184 ± 61%) or an increase in MAP (-42 ± 8% vs. -71 ± 6%) were significantly attenuated in rats with CHF compared with sham rats. Most PVN-RVLM neurons (63%), including all barosensitive PVN-RVLM neurons, were excited by an internal carotid artery injection of hypertonic NaCl (2.1 osmol/l), whereas a smaller number (7%) were inhibited. The increase in discharge rate in PVN-RVLM neurons to hypertonic stimulation was significantly enhanced in rats with CHF compared with sham rats (134 ± 15% vs. 92 ± 13%). Taken together, these data suggest that PVN-RVLM neurons are more active under basal conditions and this overactivation is mediated by an enhanced glutamatergic tone in rats with CHF. Furthermore, this enhanced activation of PVN-RVLM neurons may contribute to the altered responses to baroreceptor and osmotic challenges observed during CHF.

Fig. 1.

A–C: approximate locations of neurons that were antidromically activated from the rostral ventrolateral medulla (RVLM). The distance (in mm) posterior to the bregma is shown for each section. AH, anterior hypothalamic nucleus; f, fornix; 3V, third ventricle; OX, optic tract; SO, supraoptic nucleus. D: histological photo showing the recording site in the paraventricular nucleus (PVN) of one rat. Arrowhead shows a marked recording location.

Fig. 2.

Two neurons indentified as RVLM-projecting PVN (PVN-RVLM) neurons. One neuron was indentified as a PVN-RVLM neuron with consistent onset latency (B) and a collision test (C). A “latency jumping” test was performed in another neuron (D). A: five superimposed sweeps of neuron activity with constant latency (22 ms) antidromic spikes evoked by stimulation of the RVLM. B: peristimulus histogram of spike occurrence triggered by electric stimulation of the RVLM with 40 sweeps. C: antidromic stimulation-evoked action potential with constant latency (22 ms; a, b, and d) for the same neuron as in C. RVLM stimulation evoked an antidromic spike that was cancelled when the interval between the spontaneous action potential and stimulation was reduced to <21 ms and high frequency (333 Hz, 3 ms) after the test (e). All segments (a–e) represent two superimposed sweeps. D: latency jumping in the PVN, a gradual increase in RVLM stimulation from 700 to 800 μA, produced an abrupt decrease (≈4 ms) of the antidromic onset latency. This latency jump indicates that the action potential shifted to a nearby branch of the axon site on the axon. ↑, electric stimulation; ●, spontaneous action potentials; #, stimulus artifacts; ○, antidromic spikes.

Fig. 3.

A and B: segments of original recordings (left) and spike discriminator output demonstrating a single-unit discharge (right) from an individual sham-operated (sham) rat (A) and a rat with chronic heart failure (CHF; B). C: composite data of the baseline discharge rate of PVN-RVLM neurons in rats with CHF and in sham rats. Values are presented as means ± SE. *P < 0.05 vs. sham rats.

Fig. 4.

A and B: tracings of changes in the discharge of PVN-RVLM neurons after picoinjection of N-methyl-d-aspartate (NMDA; A) or the NMDA receptor antagonist d,l-2-amino-5-phosphonovaleric acid (d-AP5; B) in sham and CHF rats. C and D: mean data of changes in the discharge of PVN-RVLM neurons after picoinjection of NMDA (C) or d-AP5 (D) in sham and CHF rats. E: integral from 10 to 120 s of the difference between each NMDA treatment and the baseline discharge of PVN-RVLM neurons in sham or CHF rats separately [change in area under the curve (ΔAUC)]. Values are presented as means ± SE. †P < 0.05 after treatment in sham rats vs. the first 10 s before treatment in sham rats (n = 5); #P < 0.05 after treatment in CHF rats vs. the first 10 s before treatment in CHF rats (n = 5); *P < 0.05, CHF rats vs. sham rats (n = 5 rats/group).

Fig. 5.

A: time course of changes in the discharge of PVN-RVLM neurons after picoinjection of NMDA (10 pmol) only and NMDA (10 pmol) + N-monomethyl-l-arginine (l-NMMA; 10 pmol). n = 5 rats/group. B: integral from 10 to 120 s of difference between each NMDA treatment and the baseline discharge of PVN-RVLM neurons (ΔAUC). Values are presented as means ± SE. *P < 0.05 vs. NMDA-only treatment.

Fig. 6.

Segments of original recordings from an individual sham rat (A) and a rat with CHF (B), demonstrating changes in heart rate (HR; in beats/min), arterial pressure (AP; in mmHg), and PVN-RVLM neuronal activity and discharge rate (in spikes/s) to intravenous injections of sodium nitroprusside (SNP; left) or phenylephrine (PE; right).

Fig. 7.

A and B: percent changes in discharge after intravenous injections of SNP to lower mean AP (MAP; A; sham rats: n = 8 and CHF rats: n = 14) or PE to increase MAP (B; sham rats: n = 5 and CHF rats: n = 7) in PVN-RVLM neurons. C: baroreflex response sensitivity of PVN-RVLM neurons defined as the change in discharge for a given change in MAP in sham and CHF rats. D: baroreflex response sensitivity of HR defined as the change in HR for a given change in MAP in sham and HF rats. Values are presented as means ± SE. *P < 0.05 vs. sham rats.

Fig. 8.

A and B: segments of original recordings from an individual sham rat and a rat with CHF demonstrating HR (in beats/min), AP (in mmHg), PVN-RVLM neuronal activity, and nerve discharge (spikes/s) responses to an internal carotid artery (ICA) injection of 2.1 osmol/l NaCl. C: mean data of changes in discharge, MAP, and HR after an ICA injection of 2.1 osmol/l NaCl in sham and CHF rats. Values are presented as means ± SE. *P < 0.05 vs. sham rats.