INNERVATION OF THE PACEMAKER IN GUINEA-PIG SINOATRIAL NODE

Heart rate is regulated by the autonomic nervous system but little is known about the pattern of innervation of the pacemaker in the sinoatr ial node, or the subpopulations of nerves involved. Therefore in this study the pacemaker was located using electrophysiological methods and the pattern of innervation established by cholinesterase staining. In subsequent experiments, subpopulations of sympathetic, sensory and pa rasympathetic nerves were identified. Sympathetic nerves were labelled by glyoxylic acid-induced catecholamine fluorescence or an antiserum raised against tyrosine hydroxylase (TH). These experiments showed tha t the entire sinoatrial node was densely innervated by sympathetic axo ns, the majority of which were immunoreactive for neuropeptide Y (NPY) . There were a few axons which were only immunoreactive for TH. Sensor y nerves which were immunoreactive for both substance P (SP) and calci tonin gene-related peptide (CGRP) were also found throughout the sinoa trial node. In the absence of a selective marker for parasympathetic n eurons, hearts were extrinsically denervated by placing them in organo typic culture to allow degeneration of extrinsic axons. In this way in trinsic parasympathetic neurons could be characterised. These experime nts revealed several distinct populations of parasympathetic nerves wh ich innervated only a small, discrete part of the sinoatrial node. The se populations were immunoreactive for NPY, somatostatin (SOM) or vaso active intestinal peptide (VIP) alone, or SOM combined with NPY, SOM w ith dynorphin B, and SOM with SP. These results highlight a remarkable difference in the pattern of innervation of the sinoatrial node by th e sympathetic and parasympathetic nervous systems. Furthermore the pre sence of several distinct populations of autonomic cardiac neurons ind icates a further complexity in neuronal regulation of heart rate.