IDENTIFICATION OF GAMMA-AMINOBUTYRIC ACID-IMMUNOREACTIVE AXON ENDINGSASSOCIATED WITH MESENCEPHALIC PERIODONTAL AFFERENT TERMINALS AND MORPHOMETRY OF THE 2 TYPES OF TERMINALS IN THE CAT SUPRATRIGEMINAL NUCLEUS

A previous study has shown that mesencephalic periodontal afferent ter minals receive contacts more frequently from axonal endings containing pleomorphic, synaptic vesicles (P-endings) in the supratrigeminal nuc leus (Vsup) than in the trigeminal motor nucleus, suggesting that inte rneurons in Vsup play an important role in modulating the jaw-closing reflex. The present study was attempted to identify neurotransmitters in P-endings associated with mesencephalic periodontal afferents in ca t Vsup through the use of intracellular staining of horseradish peroxi dase combined with the postembedding immunogold methods. A morphometri c analysis was carried out to compare the ultrastructural features of these two types of terminals. Serial sections of 31 labeled boutons an d of their associated 38 P-endings were examined. They were processed for postembedding immunogold labeling with antibodies to the neurotran smitter gamma-aminobutyric acid (GABA). The 38 P-endings presynaptic t o periodontal afferents showed GABA-like immunoreactivity, but the aff erent terminals were free from the labeling. The morphometric analysis indicated that bouton volume, apposed surface area, total active zone size, and mitochondrial volume were smaller in GABA-immunoreactive P- endings than in periodontal afferents, but the pooled data of the two types of terminals showed that each synaptic parameter was highly corr elated in a positive, linear manner with bouton volume. These observat ions provide evidence that P-endings presynaptic to mesencephalic peri odontal afferents contain the neurotransmitter GABA and that their axo axonic synapses are organized in accordance with the ultrastructural ' 'size principle'' proposed by Pierce and Mendell (Pierce and Mendell [ 1993] J. Neurosci. 13:4748-4763) on Ia-motoneuron synapses. (C) 1997 W iley-Liss, Inc.