Identification of ?-aminobutyric acid-immunoreactive axon endings associated with mesencephalic periodontal afferent terminals and morphometry of the two types of terminals in the cat supratrigeminal nucleus

Abstract

A previous study has shown that mesencephalic periodontal afferent terminals receive contacts more frequently from axonal endings containing pleomorphic, synaptic vesicles (P-endings) in the supratrigeminal nucleus (Vsup) than in the trigeminal motor nucleus, suggesting that interneurons 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 cat Vsup through the use of intracellular staining of horseradish peroxidase combined with the postembedding immunogold methods. A morphometric analysis was carried out to compare the ultrastructural features of these two types of terminals. Serial sections of 31 labeled boutons and of their associated 38 P-endings were examined. They were processed for postembedding immunogold labeling with antibodies to the neurotransmitter gamma-aminobutyric acid (GABA). The 38 P-endings presynaptic to periodontal afferents showed GABA-like immunoreactivity, but the afferent 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 correlated in a positive, linear manner with bouton volume. These observations provide evidence that P-endings presynaptic to mesencephalic periodontal afferents contain the neurotransmitter GABA and that their axoaxonic 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.

Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

“…In terms of the chemical properties of presynaptic Ptype terminals, most P-type terminals presynaptic to the intracellularly identified and stained boutons of either group Ia or II muscle spindle afferents were GABAergic or glycinergic (Maxwell et al, 1990;Maxwell and Riddell, 1999) in the spinal cord. In the brainstem, a similar result was reported by Bae et al (1997) showing that P-type terminals presynaptic to the periodontal afferents of Vme neurons were GABA immunoreactive. Although we did not attempt to label P-type terminals using anti-GABA or anti-glycine immunohistochemistry in this study, the ultrastructure of presynaptic P-type terminals that contain pleomorphic and flattened vesicles imply their inhibitory properties.…”

Ultrastructure of jaw muscle spindle afferents within the rat trigeminal mesencephalic nucleus

“…In agreement with a comparable study [9], recurrent axon terminals synaptic to the Vmes somata were very small and contained a small number of vesicles. This was a general feature common to other synapses made on the somata of Vmes neurons but which contrasted with large terminals synapsing HRP-labeled dendrites in the Vmes presented here, and axon terminals on dendrites or the somata of masseteric motoneurons in the Vmo [13]. This suggests that a smaller amount of neurotransmitter is released from the recurring axon terminals on the somata of Vmes neurons than on dendrites of masseter motoneurons.…”

Synaptic relationships between hair follicle afferents and neurones expressing GABA and glycine‐like immunoreactivity in the spinal cord of the rat

“…GABA‐like immunoreactivity (albeit often rather weak) has frequently been reported in dendrites postsynaptic to the central terminals of glomeruli in laminae I and II (Carlton and Hayes, 1990; Todd and Lochhead, 1990; Carlton et al, 1996; Todd, 1996) and to intracellularly labelled C and Aδ fibres (Alvarez et al, 1992, 1993; Bernardi et al, 1995). Only rarely did we see this in dendrites postsynaptic to hair follicle afferents, and immunoreactivity for GABA has not been reported by other immunohistochemical studies of physiologically identified Aα or Aβ fibres such as hair follicle afferents (Maxwell and Noble, 1987), type Ia or type II muscle spindle afferents (Maxwell et al, 1990; Maxwell and Riddell, 1999; Watson and Bazzaz, 2001), slowly adapting periodontal afferents (Bae et al, 1997), or for terminals of large‐diameter myelinated afferents labelled by retrograde transport of Cholera toxin B (Bae et al, 2000). There may therefore be a fundamental difference in the circuitry underlying PAD involving unmyelinated and small myelinated afferents compared with that involving large‐diameter myelinated afferents.…”