Nucleus of solitary tract mediates cardiac sympathetic afferent reflex in rats.

This study is to determine whether the nucleus of solitary tract (NTS) is an important component of the central neurocircuitry of the cardiac sympathetic afferent reflex (CSAR) and whether the gamma-aminobutyric acid (GABA) in the NTS modulates the CSAR. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded in sinoaortic-denervated and cervical-vagotomized anesthetized rats. The CSAR was evaluated by the RSNA response to epicardial application of capsaicin. The NTS microinjection of lidocaine or NTS electrolytic lesion inhibited the CSAR without significant effect on the RSNA and MAP. Selective lesion of the neuronal perikarya in NTS with kainic acid inhibited the CSAR and induced rapid decreases in RSNA and MAP followed by a slight increase in MAP. The NTS microinjection of GABA(A) receptor agonist isoguvacine or GABA(B) receptor agonist baclofen enhanced the CSAR, and increased the RSNA and MAP. GABA(A) receptor antagonist gabazine or GABA(B) receptor antagonist CGP-35348 attenuated the CSAR, and decreased the RSNA and MAP. The effects of isoguvacine and baclofen were abolished by the pretreatment with gabazine and CGP-35348, respectively. Nine days after iontophoretic injection of biotin-dextran (a retrograde tracer) into the dorsal horn of upper thoracic spinal cord, biotin-dextran-labeled terminations were found in the vicinity of glutamic acid decarboxylase (a marker for GABA neurons) staining neurons. The results indicate that the NTS is an important component of the central neurocircuitry of the CSAR. Both GABA(A) and GABA(B) receptors in NTS modulate the CSAR and these receptors play a tonic role in enhancing the CSAR.