Ontogeny of phenylethanolamine N‐methyltransferase‐ and tyrosine hydroxylase‐like immunoreactivity in presumptive adrenaline neurones of the foetal rat central nervous system

Abstract

The appearance of phenylethanolamine N‐methyltransferase (PNMT)‐and tyrosine hydroxylase (TH)‐like immunoreactivity (LI) in the foetal rat central nervous system has been investigated. Antibodies raised against PNMT and TH were used in an indirect immunofluorescence method. Attention was focussed on areas containing putative adrenaline‐containing nerve cell bodies or fibres and, using an elution‐restaining technique, it was possible to analyze whether neurones contained PNMT‐LI, TH‐LI, or both. PNMT‐immunoreactive neurones could first be visualized on day 13 of gestation, in the ventrolateral and dorsal medulla oblongata, and probably corresponding to those of the C1 and C2 groups. The number of positive cell bodies and the intensity of their fluorescent staining in these areas were not dissimilar at this stage to the number and intensity observed at 1 day postnatal, the final age studied. At day 16, PNMT‐positive cells were observed for the first time in midline areas of the rostral dorsal medulla oblongata‐caudal pons, associated with the medial longitudinal fasciculus. These cells probably composed the C3 cell group. Many PNMT‐immunoreactive fibres could be seen at day 13 of gestation, in the medulla, and coursing both in an ascending bundle around the mesencephalic flexure and in a descending bundle toward the spinal cord. The extent of the bundles increased with gestational age, such that dense meshworks of PNMT‐immunoreactive varicose fibres were visible ventral to the aqueductus Sylvii, and the periventricular and lateral regions of the hypothalamus by days 18 to 19, and in the paraventricular nucleus, the septum, and the thoracic spinal cord by day 1 after birth. A sparser fibre plexus was observed in the amygdala at day 1 postnatal. In contrast to the explosive appearance of PNMT‐immunoreactive cells at day 13 of gestation, the development of TH‐LI within these same neurones was much more protracted. Only rarely were TH‐LI awl PNMT‐LI colocalized at day 13, and even at birth TH‐LI could not be visualized in 5% of PNMT‐immunoreactive cells in the ventrolateral medulla oblongata, and in 50% of those in the dorsal vagal complex. A similar tardy appearance of TH‐LI in PNMT‐immunoreactive fibres was observed also. It should be emphasized that strongly TH‐immunoreactive neurones were found already at gestational day 10.5 in the medulla oblongata, caudal to the PNMT‐immunoreactive cells. It is concluded that the expression of enzymes involved in the synthesis of adrenaline is not unitarily controlled, and may be partly dependent on other than epigenetic factors. Furthermore, ascription of the term adrenergic to a class of neurones appears to require the identification of more than a single synthetic enzyme.