Catecholaminergic neurons in the rat intrinsic cardiac nervous system

Immunoreactivities (IR) for catecholamine-synthesizing enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DbetaH), phenylethanolamine N-methyl transferase (PNMT), serotonin-synthesizing enzyme tryptophan hydroxylase, and neuropeptide Y were investigated in the intrinsic cardiac nervous system of 27-40-day-old rats using fluorescent immunohistochemistry. Individual neurons were identified by the general neuronal marker protein gene product 9.5. The presence of DbetaH and PNMT in the atrial specimens was verified using reverse transcriptase-polymerase chain reaction. Two types of catecholamine-handling intrinsic ganglion neurons were observed: small intensely fluorescent (SIF) cells and large-diameter neurons. SIF cells exhibited TH- and tryptophan hydroxylase-IR, but they were not positive for DbetaH. In contrast, large-diameter intrinsic TH-positive neurons, showing in majority also NPY-IR, displayed also DbetaH- and PNMT-IR, thus indicating the capacity for the synthesis of norepinephrine and epinephrine, respectively. In conclusion, the SIF cells are most probably dopaminergic and serotonergic neurons, whereas large-diameter intrinsic cells seem to represent a subpopulation of norepinephrine- and/or epinephrine-secreting neurons.