Blunted sympathoinhibitory responses in obesity-related hypertension are due to aberrant central but not peripheral signalling mechanisms

Abstract

The gut hormone cholecystokinin (CCK) acts at subdiaphragmatic vagal afferents to induce renal and splanchnic sympathoinhibition and vasodilatation, via reflex inhibition of a subclass of cardiovascular-controlling neurons in the rostroventrolateral medulla (RVLM). These sympathoinhibitory and vasodilator responses are blunted in obese, hypertensive rats and our aim in the present study was to determine whether this is attributable to (i) altered sensitivity of presympathetic vasomotor RVLM neurons, and (ii) aberrant peripheral or central signalling mechanisms. Using a diet-induced obesity model, male Sprague-Dawley rats exhibited either an obesity-prone (OP) or obesity-resistant (OR) phenotype when placed on a medium high fat diet for 13-15 weeks; control animals were placed on a low fat diet. OP animals had elevated resting arterial pressure compared to OR/control animals (P < 0.05). Barosensitivity of RVLM neurons was significantly attenuated in OP animals (P < 0.05), suggesting altered baroreflex gain. CCK induced inhibitory responses in RVLM neurons of OR/control animals but not OP animals. Subdiaphragmatic vagal nerve responsiveness to CCK and CCK1 receptor mRNA expression in nodose ganglia did not differ between the groups, but CCK induced significantly less Fos-like immunoreactivity in both the nucleus of the solitary tract and the caudal ventrolateral medulla of OP animals compared to controls (P < 0.05). These results suggest that blunted sympathoinhibitory and vasodilator responses in obesity-related hypertension are due to alterations in RVLM neuronal responses, resulting from aberrant central but not peripheral signalling mechanisms. In obesity, blunted sympathoinhibitory mechanisms may lead to increased regional vascular resistance and contribute to the development of hypertension.

Figure 1

A, CCK administration (0.05–4 μg kg⁻¹ close arterially) induced dose-dependent inhibitory responses in RVLM neurons of control/OR animals that were significantly reduced/abolished in OP animals (OP n = 8; OR n = 3; control n = 6). OP vs. control: *P < 0.05, **P < 0.01, ***P < 0.001; OP vs. OR: †P < 0.05. B, representative traces of RVLM neuronal responses in a control (upper traces; latency: 6 ms, conduction velocity, 1.9 m s^–1; firing rate, 7 spikes s^–1) and an OP (lower traces; latency, 3.5 ms, conduction velocity, 8.8 m s^–1; firing rate, 18 spikes s^–1) animal. Pulse triggered histograms (to the right of the traces) demonstrate poor pulse-synchronicity in the OP animal compared to the control animal. Proof that neurons are bulbospinal is determined by the positive collision tests (to the far right-hand side of the traces). While both neurons are barosensitive and inhibited by increases in arterial pressure induced by aortic occlusion (AOc), the OP animal is less sensitive to these changes. The neuron from the control animal is clearly inhibited by CCK administration (1 μg kg⁻¹) whereas in the OP animal, CCK induces a modest excitatory response.

Figure 2

Animals with higher AP (predominantly OP animals) had reduced inhibitory, or excitatory, responses to CCK (at 0.1 μg kg⁻¹), whereas those with lower APs had clear inhibitory responses to CCK.

Figure 3

A, arterial pressure (AP) required for 20% inhibition in neuronal firing rate of RVLM neurons in control, OP and OR animals. Horizontal line represents mean resting AP for animals from Study 1 (OP n = 8; OR n = 5; control n = 6). Both the absolute AP (†P < 0.05, ††P < 0.01) and the change in AP from baseline (*P < 0.05) for 20% inhibition in neuronal firing rate were significantly higher in OP animals compared to OR or controls. B, dot plot representing individual basal firing rates (spikes s^–1) of neurons characterised in A for OP, OR or control animals. Horizontal lines represent mean values.

Figure 4

A, CCK administered close to the coeliac artery induced a dose-dependent increase in SVND that was not significantly different between OP (n = 7), OR (n = 7) or control (n = 8) animals. B, SVND responses to varying doses of CCK in a control animal, demonstrating the dose-dependent effects.

Figure 5

Values represented in graphs are means ± SEM of Fos immunoreactive (Fos-IR) neurons per section in the nucleus of the solitary tract (NTS; A) and caudal ventrolateral medulla (CVLM; B) between Bregma −14.6 to −13.68 according to the Rat Brain Atlas (Paxinos et al. 1999). In the NTS, there were significantly fewer Fos-IR neurons in OP compared to control animals. In the CVLM, there were significantly fewer Fos-IR neurons in OP compared to either control or OR animals. Values are means ± SEM; n = 4 per group; *P < 0.05, **P < 0.01.

Figure 6

CCK administered at 8 μg kg⁻¹ in control (left-hand sections), OP (middle sections) and OR animals (right-hand sections). A–C, Fos-IR in the nucleus of the solitary tract (NTS); D–F, Fos-IR in the caudal ventrolateral medulla (CVLM). In both the NTS and the CVLM there were fewer Fos-IR neurons in OP compared to control and/or OR animals. Scale bar = 100 μm; magnification ×10. Schematic representation to the far right indicates typical areas from which Fos-IR was quantified (roughly within shaded areas) according to the Rat Brain Atlas (Paxinos et al. 1999).