The time of appearance and distribution of neuropeptide Y (NPY) in the
developing and adult rat heart has been studied by immunohistochemica
l method using the Avidin biotin technique. At 18 days of gestation an
d postnatal day 1, no immunoreactivity was detected. Immunoreactivity
was first observed on postnatal day 5 around the large blood vessels,
developing coronary arteries and very sparsely along the blood vessels
in the musculature. At later stages up to adulthood the intensity of
immunostaining and density of fibres along the intramyocardial blood v
essels increased. After postnatal day 10 the NPY immunoreactive fibres
were also seen amongst the muscle fibres. Immunopositive NPY neurons
were seen in the ganglionic clusters close to the great vessels at pos
tnatal day 20 and were visualised in between the ventricular muscle bu
ndles near the atrioventricular region only in the adult heart. The ve
ntricular musculature was always more richly innervated than the atria
by NPY immunopositive nerve fibres. Thus the present study demonstrat
es that NPY-like immunoreactivity makes its appearance postnatally, fi
rst around the blood vessels and later on the muscle fibres too.