CHEMICALLY-DEFINED NEURON GROUPS AND THEIR SUBPOPULATIONS IN THE GLOMERULAR LAYER OF THE RAT MAIN OLFACTORY-BULB

Chemically-defined neuron groups and their subpopulations in the glome rular layer of the rat main olfactory bulb were revealed immunocytoche mically using antibodies against gamma-amino butyric acid (GABA), tyro sine hydroxylase (TH), methionin-enkephalin-Arg(6)-Gly(7)-Leu(8) (ENK) , calretinin (CR), calbindin-D28K (calbindin) and thyrotropin-releasin g hormone (TRH). GABA-like immunoreactive (GABA-LIR) neurons and CR im munoreactive (CR-IR) neurons were most numerous; they were about 1.5-3 times more numerous than calbindin immunoreactive (calbindin-IR), TH immunoreactive (TH-IR), ENK-like immunoreactive (ENK-LIR) and TRH-like immunoreactive (TRH-LIR) neurons. We identified at least three distin ct chemically-defined neuron groups, GABA-LIR neurons, CR containing n eurons and calbindin containing neurons; since these three neuron grou ps were almost separate from one another, On the other hand, TH-IR and ENK-LIR neurons were nearly included in and thus considered to be sub populations of GABA-LIR and CR-IR neurons, respectively, for about 80% of these two neuron groups contained GABA-L and CR immunoreactivities , respectively. TRH-LIR neurons appeared to be divided into two subpop ulations, one containing the GABA-L immunoreactivity and the other con taining the CR immunoreactivity. Thus in the glomerular layer of the r at olfactory bulb, GABA-LIR, CR-IR and calbindin-Ia cells could be con sidered to be three distinct chemically-defined neuron groups, whereas TH-IR, TRH-LIR and ENK-LIR neurons were regarded as their subpopulati ons. Furthermore, some neurons are supposed to contain three substance s (e.g. GABA + TH + TRH, GABA + TRH + ENK, CR + TRH + ENK, GABA + TRK + CR) or a few might even contain four substances (e.g. GABA + TRH + C R + ENK). Preliminary quantitative analysis using the optical disector method showed percentages of these three main neuron groups to total cells in the glomerular layer; that is, neuron groups containing GABA, CR and calbindin were about 20%, 20% and 10%, respectively,