TYROSINE HYDROXYLASE-CONTAINING NEURONS IN THE SPINAL-CORD OF THE CHICKEN .1. DEVELOPMENT AND ANALYSIS OF CATECHOLAMINE SYNTHESIS CAPABILITIES

1. The development of tyrosine hydroxylase-immunoreactive (TH-IR) neur ons was examined; in the spinal cord of the chick embryo and hatchling . 2. Two groups of TH-IR cells are described, both of which appear to reach their full complement in number relatively late in embryonic dev elopment. One group is comprised of numerous cells located ventral to the central canal which make direct contact with the lumen of the cana l. The other group consists of large multipolar neurons that reside in the dorsal horn, more commonly along the outer margin of the gray mat ter within lamina I and II, and less frequently deeper in the dorsal h orn within medial portions of laminae V, VI or VII. 3. TH-IR cells ven tral to the central canal in the chick are comparable in location to d opamine (DA)-containing spinal cord cells in lower vertebrate species. In contrast, the dorsally-suited TH-IR cells in the chick are known o nly to occur in similar positions in higher vertebrates. Therefore, th e chick is novel in that the presence of both groups of TH-IR cells ap pearing together in significant numbers within the spinal cord has not been shown in any other species studied to date. 4. The TH-containing cells in the chick cord do not appear to contain the catecholamine bi osynthesis enzymes, DBH or PNMT. Moreover, using anti-DA immunocytoche mistry, neither group of TH-IR cells demonstrated detectable levels of DA in control animals nor in animals pretreated with inhibitors of MA O (MAO-I). 5. However, a difference was noted though between the two T H-IR cell groups in terms of their responses to exogenously supplied L -DOPA, the immediate precursor to DA. With the administration of L-DOP A and a MAO-I to chick hatchlings, cells in the region ventral to the central canal stained intensely for DA. In contrast, the same treatmen t failed to produce DA-immunoreactive cells in the dorsal horn. 6. One reasonable hypothesis for these results is that the TH-IR cells ventr al to the central canal contain an active form of AADC, the enzyme tha t converts L-DOPA to DA. With this interpretation, if these cells can produce DA from L-DOPA, yet do not appear to synthesize DA endogenousl y, it would appear that the TH enzyme contained in these cells occurs in an inactive form, Whether the TH enzyme in the dorsally located imm unoreactive cells is also inactive is uncertain since it remains uncle ar whether they contain AADC.