Distribution pattern of inhibitory and excitatory synapses in the dendritic tree of single masseter ?-motoneurons in the cat

Abstract

Little is known about the differences in the distributions of inhibitory and excitatory synapses in the dendritic tree of single motoneurons in the brainstem and spinal cord. In this study, the distribution of γ‐aminobutyric acid (GABA)‐, glycine‐, and glutamate‐like immunoreactivity in axon terminals on dendrites of cat masseter α‐motoneurons, stained intracellularly with horseradish peroxidase, was examined by using postembedding immunogold histochemistry in serial ultrathin sections. The dendritic tree was divided into three segments: primary (Pd) and distal (Dd) dendrites and intermediate (Id) dendrites between the two segments. Quantitative analysis of 175, 279, and 105 boutons synapsing on 13 Pd, 54 Id, and 81 Dd, respectively, was performed. Fifty percent of the total number of studied boutons were immunopositive for GABA and/or glycine and 48% for glutamate. Among the former, 27% showed glycine immunoreactivity only and 14% were immunoreactive to both glycine and GABA. The remainder (9%) showed immunoreactivity for GABA only. As few as 3% of the boutons were immunonegative for the three amino acids. Most boutons immunoreactive to inhibitory amino acid(s) contained a mixture of spherical, oval, and flattened synaptic vesicles. Most boutons immunoreactive to excitatory amino acid contained clear, spherical, synaptic vesicles with a few dense‐cored vesicles. When comparisons of the inhibitory and excitatory boutons were made between the three dendritic segments, the proportion of the inhibitory to the excitatory boutons was high in the Pd (60% vs. 37%) but somewhat low in the Id (46% vs. 52%) and Dd (44% vs. 53%). The percentage of synaptic covering and packing density of the inhibitory synaptic boutons decreased in the order Pd, Id, and Dd, but this trend was not applicable to the excitatory boutons. The present study provides possible evidence that the spatial distribution patterns of inhibitory and excitatory synapses are different in the dendritic tree of jaw‐closing α‐motoneurons. J. Comp. Neurol. 414:454–468, 1999. © 1999 Wiley‐Liss, Inc.

Synaptic Transmission From the Supratrigeminal Region to Jaw-Closing and Jaw-Opening Motoneurons in Developing Rats

“…Apparently, MMNs and DMNs receive GABAergic inputs that are weaker than glycinergic inputs from the SupV. The number of glycine-immunoreactive boutons synapsing on MMNs and DMNs in cats is close to twice the number of GABA-immunoreactive boutons (Bae et al 1999;Shigenaga et al 2005). The SupV may be the origin of glycinergic and GABAergic axon terminals at MMNs and DMNs.…”

Excitatory Effects of Hypocretin-1 (Orexin-A) in the Trigeminal Motor Nucleus Are Reversed by NMDA Antagonism

“…Two lines of evidence support this contention. First, both Hcrt and glutamatecontaining presynaptic terminals are found within the V motor nucleus (Bae et al 1999;Fung et al 2001). Second, Hcrt has been shown to modulate amino acids release.…”

A method for the three-dimensional reconstruction of Neurobiotin™-filled neurons and the location of their synaptic inputs

“…To date, the location and distribution of synapses on neurons that have been morphologically analyzed have been determined by processes such as electron microscopy (EM; Bae et al, 1999; Megias et al, 2001; Shigenaga et al, 2005; Arthur et al, 2007; Chen et al, 2008b), or by light microscopy of cultured neurons. However, these techniques are limited by the lack of three-dimensional morphology of the entire post-synaptic neuron, replete with all of the surrounding cellular inputs in its physiological setting [e.g., cultured neurons; (Cullen et al, 2010; Ivenshitz and Segal, 2010; Schatzle et al, 2012)], as well as the limited and labor-intensive nature of being able to identify the synaptic type, including the molecular make up of its postsynaptic specialization [i.e., neurotransmitter type or post-synaptic adaptor proteins, as in EM analyses; (Chen et al, 2008b; Dani et al, 2010)].…”