Pa. Steele et Jk. Choate, INNERVATION OF THE PACEMAKER IN GUINEA-PIG SINOATRIAL NODE, Journal of the autonomic nervous system, 47(3), 1994, pp. 177-187
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.