GABA in Paraventricular Nucleus Regulates Adipose Afferent Reflex in Rats

Abstract

Background: Chemical stimulation of white adipose tissue (WAT) induces adipose afferent reflex (AAR), and thereby causes a general sympathetic activation. Paraventricular nucleus (PVN) is important in control of sympathetic outflow. This study was designed to investigate the role of γ-aminobutyric acid (GABA) in PVN in regulating the AAR.

Methodology/principal findings: Experiments were carried out in anesthetized rats. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were continuously recorded. AAR was evaluated by the RSNA and MAP responses to electrical stimulation of the right epididymal WAT (eWAT) afferent nerve. Electrical stimulation of eWAT afferent nerve increase RSNA. Bilateral microinjection of the GABAA receptor agonist isoguvacine or the GABAB receptor agonist baclofen attenuated the AAR. The effect of isoguvacine on the AAR was greater than that of baclofen. The GABAA receptor antagonist gabazine enhanced the AAR, while the GABAB receptor antagonist CGP-35348 had no significant effect on the AAR. Bilateral PVN microinjection of vigabatrin, a selective GABA-transaminase inhibitor, to increase endogenous GABA levels in the PVN abolished the AAR. The inhibitory effect of vigabatrin on the AAR was attenuated by the pretreatment with gabazine or CGP-35348. Pretreatment with combined gabazine and CGP-35348 abolished the effects of vigabatrin.

Conclusions: Activation of GABAA or GABAB receptors in the PVN inhibits the AAR. Blockade of GABAA receptors in the PVN enhances the AAR. Endogenous GABA in the PVN plays an important role in regulating the AAR.

Fig 1. AAR induced by electrical stimulation of right eWAT.

A, effects of different frequency of stimulation (0, 3, 10, 30 and 60 Hz, 5V) on the RSNA and MAP; B, effects of different voltage of stimulation (0, 1, 5 and 10 V, 30 Hz) on the RSNA and MAP; C, stimulation of the peripheral end of the right eWAT or the WAT artery had no significant effect on the RSNA and MAP. D, difference between the AAR induced by stimulation of central end of eWAT nerve and the AAR induced by infusion of capsaicin into the iWAT. Control means sham electrical stimulation. Values are mean±SE. * P<0.05 vs. Control; † P<0.05 vs. Stimulation of eWAT artery; ‡ P<0.05 vs. Stimulation of peripheral end of eWAT nerve. n = 6 for each group.

Fig 2. Representative traces showing the effects of pretreatment with the PVN microinjection of saline, isoguvacine (a GABA_A receptor agonist, 10 nmol) or baclofen (a GABA_B receptor agonist, 10 nmol) on the RSNA induced by electrical stimulation of right eWAT (30 Hz, 5 V, 1 min).

Fig 3. Effects of PVN microinjection of different doses of a GABA_A receptor agonist isoguvacine or a GABA_B receptor agonist baclofen (0.1, 1.0 or 10.0 nmol) on the basal RSNA and MAP, and the AAR induced by electrical stimulation of right eWAT.

A, baseline RSNA and MAP changes; B, changes in electrical stimulation-induced AAR. Values are mean±SE. *P<0.05 vs. Saline; †P<0.05 vs. isoguvacine. n = 6 for each group.

Fig 4. Effects of the PVN microinjection of gabazine (a GABA_A receptor antagonist, 0.1 nmol) or CGP35348 (a GABA_B receptor antagonist, 10 nmol) on the basal RSNA and MAP, and the AAR induced by electrical stimulation of right eWAT.

A, baseline RSNA and MAP changes; B, changes in electrical stimulation-induced AAR. Control rats received the PVN microinjection of saline and sham electrical stimulation. Gabazine or CGP35348 was administered 10 min before isoguvacine (a GABA_A receptor agonist, 10 nmol) or baclofen (a GABA_B receptor agonist, 10 nmol). Electrical stimulation of right eWAT was carried out 20 min after the PVN microinjection. Values are mean±SE. *P<0.05 vs. Control; †P<0.05 vs. Saline; ‡P<0.05 vs. isoguvacine; #P<0.05 vs. baclofen. n = 6 for each group.

Fig 5. Effects of the PVN microinjection of saline, vigabatrin (a GABA-transminase inhibitor, 10 nmol) on the basal RSNA and MAP, and the AAR induced by electrical stimulation of right eWAT.

A, baseline RSNA and MAP changes; B, changes in electrical stimulation-induced AAR. Control rats received the PVN microinjection of saline and sham electrical stimulation. Gabazine (a GABA_A receptor antagonist, 0.1 nmol), CGP35348 (a GABA_B receptor antagonist, 10 nmol) or combined gabazine and CGP35348 was administered 10 min before vigabatrin administration. Electrical stimulation of right eWAT was carried out 20 min after the PVN microinjection. Values are mean±SE. *P<0.05 vs. Control; †P<0.05 vs. Saline; ‡P<0.05 vs. Vigabatrin; #P<0.05 vs. Gabazine+Vigabatrin. n = 6 for each group.