Nitrergic and peptidergic innervation in the developing rat heart

The phenotypic expression and anatomic distribution of nitrergic and peptid ergic innervation in the developing rat heart was localized by reduced nico tinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and immunohistochemistry using antibodies against neuronal isoform of nitr ic oxide synthase (nNOS), neuropeptide Y (NPY) and calcitonin-gene-related peptide (CGRP). NPY-immunoreactive nerve fibers showed the earliest express ion by 16 days of gestation, with preferential innervation of the nodal and perinodal areas, followed by the innervation of the valves and ventricles by postnatal day 7. NPY immunoreactivity was also localized to a large prop ortion of the intrinsic cardiac ganglia from 16 days of gestation onwards w ith a progressive increase in the number of neuronal cell bodies per gangli a with age. CGRP-positive nerve fibers appeared by 19 days of gestation and were less dense during the gestational and early postnatal periods, and sh owed a quantitative increase in density by 7 days, followed by a decrease b y 3 weeks postnatal. None of the intrinsic ganglia were stained positive fo r CGRP, indicating the extrinsic sensory origin of these stained fibers. Ni trergic innervation paralleled the sensory innervation, with the cardiac ga nglia and nerve fibers showing a positive labeling from 19 days of gestatio n onwards. NADPH-d and nNOS were partially co-localized. Double-label immun ohistochemistry showed that a considerable proportion of sensory CGRP-immun opositive fibers were also immunoreactive for NOS. The results of the prese nt study show that neuropeptides and nitric oxide are expressed by the late gestational period and that autonomic efferent innervation precedes sensor y and nitrergic innervation in the developing heart.