THE FETAL EXPRESSION OF PROENKEPHALIN MESSENGER-RNAS AND MET-ENKEPHALIN IMMUNOREACTIVITY IN THE HYPOTHALAMOSEPTAL TRACT AND ADJACENT HYPOTHALAMIC AREAS OF THE GUINEA-PIG BRAIN

The development of the enkephalinergic hypothalamoseptal tract in the guinea pig brain was studied from embryonic day 30 until birth. Proenk ephalin (PE) mRNAs were detected in the hypothalamic magnocellular dor sal nucleus (MDN) by in situ hybridization with a synthetic S-35-label ed oligonucleotide. The Met-enkephalin-like immunoreactivity (Met-enk- LI) in the MDN and the lateral septum (LS) was detected with antibodie s against Met-enkephalin, on adjacent cryostat sections. At the same t ime, an immunohistochemical study of the arrangement of enkephalinergi c axon terminals in the LS at birth was performed at the electron micr oscopic level. PE mRNAs were first found to be expressed in the MDN at embryonic day 32 (E32) and increased to reach a maximal level at E48. Met-enk-LI was consistently detectable from E38 in numerous perikarya of the MDN as well as in nerve terminals of the LS. The number of Met -enk-LI cells of the MDN decreased after this stage until birth, where as positive nerve endings in the LS increased. At the electron microsc opic level, numerous cell bodies of the LS at birth were consistently surrounded by Met-enk immunoreactive nerve terminals. Cells expressing the PE gene and Met-enk-LI were also observed from E38 to E44 in the periventricular area. Some of these cells were found double-labeled wi th Met-enkephalin and Somatostatin antisera. The enkephalinergic syste m of the hypothalamoseptal tract appears at early embryonic stages and may be essential in regulating septal neuronal functions early in ges tation. Differing ontogenic onsets of the enkephalinergic hypothalamos eptal and periventricular-median eminence tracts suggest possible deve lopmental and functional differences.