Expression of sensory nerves in perivascular adipose tissue

Introduction: Perivascular adipose tissue (PVAT) is known to be innervated by sympathetic nerves. Very little is known about the expression of sensory nerves in PVAT.

Objective: The present study was conducted to investigate the existence of sensory nerves within PVAT

Methodology: Male Wistar rats (220-250g, n≥4) were killed by stunning and exsanguination. Mesenteric arterial beds (with and without PVAT, removed by careful dissection) were perfused at a constant flow rate of 5 ml min-1 with Krebs’ solution in the absence/presence of agonists/antagonist. Electrical field stimulation (EFS; 0.5-12Hz, 0.1ms, 60V, 30s) in the presence of guanethidine (5µM) and methoxamine (1-10µM) (to block sympathetic neurotransmission and pre-constrict the preparations respectively) was applied to study neurogenic vasodilator responses. Myography (isometric tension) experiments were employed on superior mesenteric arteries (SMA) and second order mesenteric artery (2OMA) segments and the responses to EFS were investigated. Immunofluorescence staining was employed to investigate the localisation of sensory nerves and enzyme immunoassay (EIA) was conducted to quantify capsaicin-evoked CGRP release of the dissected PVAT from 2OMA and SMA. Vasodilator responses of the preparations were measured as decreases in perfusion pressure (mmHg). Data were analysed by two-way ANOVA with Bonferroni post test and Student’s t-test. P<0.05 was considered significant.

Results: In the presence of PVAT (PVAT+), mesenteric arterial beds responded to EFS with frequency-dependent relaxations. In the absence of PVAT (PVAT-), neurogenic relaxant responses were attenuated (P<0.0001). In contrast, dose-dependent vasodilator responses to exogenous CGRP (1.5x10-13-5x10-11 moles) were not reduced in PVAT- preparations. Myography experiments revealed that EFS-evoked vasodilator responses were greater in PVAT+ preparations than in PVAT- preparations in both MA and 2OMA segments. Immunofluorescence showed the presence of CGRPergic nerves on the surface of clean vessels and CGRP-immunoreactivity within the PVAT region. EIA indicated that CGRP release was greater in dissected PVAT with capsaicin (10 µM) compared to dissected PVAT without capsaicin in 2OMA preparations.

Conclusions: The present study has shown that neurogenic vasodilator responses to EFS were greater in the presence of PVAT, the immunofluorescence staining visualised CGRP-containing nerves within PVAT, and EIA demonstrated capsaicin-evoked CGRP release from dissected PVAT in 2OMA. Collectively, these data indicate that sensory nerves are present within PVAT and can modulate in vitro vascular tone of mesenteric arterial beds.