Chemically defined neuron groups and their subpopulations in the glomerular layer of the rat main olfactory bulb - IV. Intraglomerular synapses of tyrosine hydroxylase-immunoreactive neurons

Synapses of intraglomemlar processes of tyrosine hydroxylase-immunoreactive neurons in the rat main olfactory bulb were examined by electron microscop ic immunocytochemistry. Prominent characteristics of intraglomerular synaps es of tyrosine hydroxylase-immunoreactive elements were that the vast major ity (about 80%) of their synaptic inputs were asymmetrical synapses from ol factory nerve terminals and, though far smaller in proportion, one half of the remaining were asymmetrical synapses from mitral/tufted cell dendrites and the other half were symmetrical synapses from gamma-aminobutyric acid-l ike immunoreactive elements. So far, we have observed no typical reciprocal synapses between tyrosine hydroxylase-immunoreactive processes and mitral/ tufted dendrites: however, we have often identified serial synapses; that i s, asymmetrical synapsis from olfactory nerve terminals or mitral/tufted ce ll dendrites to tyrosine hydroxylase-immunoreactive processes, and then sym metrical synapses from the latter to different mitral/tufted cell dendrites . These synaptic connections of tyrosine hydroxylase-immunoreactive neurons were very different from those of Calbindin-D-28k-immunoreactive neurons, which received no synaptic contact directly from olfactory nerve terminals but formed reciprocal synapses with mitral/tufted cells as we analysed prev iously. Thus, our present and previous electron microscopic studies combined with c onfocal laser scanning light microscopy clearly indicated for the first tim e the heterogeneity of periglomerular neurons, not only in their chemical a nd morphological features, but also in their synaptic organization in the o lfactory glomerulus. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.