β2-Adrenoreceptor immunoreactivity in cardiac ganglia of the guinea pig

Previous pharmacological studies in co-culture systems have indicated the presence of β-adrenoreceptors on intrinsic cardiac neurons of the guinea pig (Horackova et al., 1993) but radioligand binding studies on tissue sections failed to provide a definite answer as to the presence of such receptors on cardiac neurons in situ, due to the iodine-binding properties of cardiac nerve bundles and ganglia (Molenaar et al., 1992). We therefore addressed this question by immunohistochemistry, using antisera raised against synthetic peptides of the β2-adrenoreceptor. For comparison, cholinergic and catecholaminergic neurons were identified immunohistochemically by means of antibodies against the enzymes involved in the synthesis of acetylcholine (choline acetyltransferase), and of catecholamines (tyrosine hydroxylase). Virtually all intrinsic cardiac neurons contained both β2-adrenoreceptor- and choline acetyltransferase-immunoreactivities. In addition, some nerve fibre bundles exhibited β2-adrenoreceptor-immunoreactivity. Several ganglia were innervated by tyrosine hydroxylase-immunoreactive axons, but the majority of ganglia did not receive tyrosine hydroxylase-immunoreactive nerve terminals, and additional intraganglionic sources of catecholamine synthesis could not be identified. Thus, the results are in favour of β-adreneregic modulation of guinea pig cardiac ganglia by humorally and, partially, by locally released catecholamines.