Recent studies of autonomic ganglia have shown that specific combinati
ons of neuropeptides and other potential neurotransmitters distinguish
different functional types of neurons. In the present paper the patte
rns of coexistence of neurochemicals in guinea-pig cardiac ganglion ce
lls was examined, using multiple-labelling immunohistochemistry. Many
neurons were found to contain somatostatin immunoreactivity with vario
us combinations of immunoreactivity for dynorphin B, substance P, neur
opeptide Y and nitric oxide synthase. There were several small populat
ions of neurons without somatostatin immunoreactivity, which contained
combinations of immunoreactivity for vasoactive intestinal peptide, n
europeptide Y, dynorphin B, substance P and nitric oxide synthase. Pos
sible synaptic inputs to these populations of ganglion cells were iden
tified using multiple-labelling immunohistochemistry combined with lon
g-term organ culture. These experiments demonstrated that cardiac gang
lia contain prominent pericellular baskets of varicose nerve terminals
of sympathetic and sensory origin. In addition, populations of intrin
sic intraganglionic nerve terminals were identified which were immunor
eactive for vasoactive intestinal peptide, neuropeptide Y or both pept
ides. These terminals presumably originate from intrinsic neurons, wit
h the same combinations of neuropeptides, located in other cardiac gan
glia. These results have demonstrated that there are diverse populatio
ns of cardiac ganglion cells in the guinea-pig and that some of these
neurons may act as interneurons within the intrinsic cardiac plexuses.
Therefore it is highly likely that vagal transmission in the heart is
modified by sympathetic, sensory and intrinsic neurons and that cardi
ac ganglia are complex integrators of convergent neuronal activity rat
her than simple relays.