Pa. Steele et al., PROJECTIONS OF INTRINSIC CARDIAC NEURONS TO DIFFERENT TARGETS IN THE GUINEA-PIG HEART, Journal of the autonomic nervous system, 56(3), 1996, pp. 191-200
We set out to determine the projections of the major immunohistochemic
ally-defined populations of intrinsic cardiac neurons to different tar
get tissues within the guinea-pig heart. Ultrastructural studies, and
immunoreactivity to the neuronal marker, neuron-specific enolase, sugg
ested that the number of axons of intrinsic neurons in most regions of
the heart was low when compared with the populations of axons project
ing from extrinsic sensory and sympathetic ganglia. Multiple-labelling
immunofluorescence was used to demonstrate the terminals of the major
populations of peptide-containing intrinsic neurons. The intrinsic na
ture of peptide-containing axons was confirmed by long-term organotypi
c culture of cardiac tissue, which resulted in degeneration of axons o
f extrinsic neurons. The relative density and peptide content of intri
nsic axons throughout the heart was not consistent with the relative p
roportions of peptide-containing intracardiac nerve cell bodies observ
ed previously. The most commonly-encountered axons contained immunorea
ctivity (IR) to vasoactive intestinal peptide (VIP) alone, although ne
rve cell bodies with VIP constituted less than 5% of the total populat
ion of intrinsic neurons. Populations of axons containing IR to somato
statin alone, somatostatin and substance P, neuropeptide Y (NPY) alone
, somatostatin and NPY, or VIP and NPY, also were observed. Intrinsic
axons containing substance P-IR were very rare, much more so than woul
d be predicted from the peptide content of intrinsic nerve cell bodies
. The regions of the heart with the most dense innervation by axons of
intrinsic neurons were the cardiac valves, the atrio-ventricular node
and the sino-atrial node, Each of these targets was innervated by sev
eral populations of peptide-containing axons. Thus, each population of
peptide-containing intrinsic neurons projected to a variety of target
tissues within the heart. One possible interpretation of these result
s is that immunohistochemically-distinct populations of intrinsic neur
ons belong to different functional classes of neurons (sensory neurons
, interneurons, final motor neurons), each of which innervates many re
gions of the heart.