DEVELOPMENT OF CHOLINERGIC AND GABAERGIC NEURONS IN THE RAT MEDIAL SEPTUM - DIFFERENT ONSET OF CHOLINE-ACETYLTRANSFERASE AND GLUTAMATE-DECARBOXYLASE MESSENGER-RNA EXPRESSION

In the present study, we have investigated the developmental. expressi on of the transmitter-synthesizing enzymes choline acetyltransferase ( ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons b y using in situ hybridization histochemistry. In addition, we have emp loyed immunostaining for ChAT and the calcium-binding protein parvalbu min, known to be contained in septohippocampal GABAergic neurons. A la rge number of GAD67 mRNA-expressing neurons were already observed in t he septal complex on embryonic day (E) 17, the earliest time point stu died. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region, The adult pattern of ChAT mRNA-expr essing neurons was observed around postnatal day (P) 16. By using a mo noclonal ChAT antibody, the first immunoreactive cells were not seen b efore P 8. Similarly, the first weakly parvalbumin-immunoreactive neur ons were seen in the septal complex by the end of the Ist postnatal we ek. These results indicate that in situ hybridization histochemistry m ay be an adequate method to monitor the different development of trans mitter biosynthesis in cholinergic and GABAergic septal neurons. Moreo ver, the late onset of ChAT mRNA expression would be compatible with a rot of target-derived factors for the differentiation of the choliner gic phenotype. (C) 1996 Wiley-Liss, Inc.