Postnatal development of the rat intrinsic cardiac nervous system: a confocal laser scanning microscopy study in whole-mount atria

We used confocal laser scanning microscopy and fluorescent immunohistochemi stry to study the developmental pattern and distribution of specific neuron al phenotypes within the intrinsic cardiac nervous system in whole-mount at rial preparations from newborn to 5 week old rats. Individual ganglia and n euronal cell bodies were localized by means of two general neuronal markers : protein gene product 9.5 (PGP) and microtubule-associated protein two (MA P), In rats less than or equal to2 weeks old there were two main subpopulat ions of intrinsic neurons located in the intraatrial septum and around the origin of the superior vena cava. The more abundant was a population of str ongly tyrosine hydroxylase (TH) immunoreactive (IR) neurons (10-40 mum in d iameter) most of which were also PGP-IR. The second, less numerous (similar to 60-70% than the TH-IR group) type of neurons exhibited ChAT-IR which co localized with MAP-IR. Towards the end of the second postnatal week and dur ing the third, the ganglia containing these neurons became more numerous an d their localization also included tissues around the origins of the inferi or vena cava and the pulmonary veins, as well as both atrial walls close to the AV junction. During the second and third postnatal weeks, when the ext rinsic innervation of the adrenergic and cholinergic phenotypes largely inc reases, the intrinsic innervation also changed greatly, and around the 21st postnatal day it appeared to acquire mature characteristics. The TH-IR neu rons changed their characteristics and formed two types of ganglia. The lar ger ganglia containing large cells (20-40 mum in diameter) expressed TH-IR mostly close to their inner body surface (similar to 80-90% of identified n eurons). Most of these neurons also expressed neuropeptide Y (NPY)-IR, spec ifically around their nuclei. The second type of small strongly TH-IR neuro ns (similar to 10% of all identified neurons) were contained in smaller gro ups (20-50 cells) which were usually embedded into much larger ganglia (100 -400 cells), containing large (20-50 mum) neurons. Unlike all other intrins ic neurons, these small TH-IR cells did not exhibit any PGP-IR or MAP-IR. T he number of ChAT-IR neurons increased at this stage, reaching similar to 9 0% of the neurons identified by the general neuronal markers. These neurons were surrounded by a rich network of cholinergic varicose nerve fibers, so me of which were likely of an extrinsic origin. We have also identified rel atively small ganglia expressing immunoreactivity to vasoactive intestinal polypeptide (VIP), and to substance P (SP). The presented data indicate tha t the phenotypes of intrinsic neurons in the rat heart change greatly durin g the first month of postnatal development. This may be at least partially related to the development and maturation of functional extrinsic nervous c ontrol of the heart. (C) 2000 Harcourt Publishers Ltd.