Ahd. Watson et Aa. Bazzaz, GABA and glycine-like immunoreactivity at axoaxonic synapses on 1a muscle afferent terminals in the spinal cord of the rat, J COMP NEUR, 433(3), 2001, pp. 335-348
The object of this study was to analyze the synaptic interactions of identi
fied muscle spindle afferent axon terminals in the spinal cord of the rat.
Group la muscle afferents supplying the gastrocnemius muscle were impaled w
ith microelectrodes in the dorsal white matter of the spinal cord and stain
ed by intracellular injection with Neurobiotin. Postembedding immunogold te
chniques were used to reveal GABA- and glycine-like immunoreactivity in bou
tons presynaptic to afferent terminals in the ventral horn and the deep lay
ers of the dorsal horn. Serial-section reconstruction was used to reveal th
e distribution of synaptic contacts of different types on the afferent term
inals. The majority of afferent boutons received axoaxonic and made axodend
ritic or axosomatic synaptic contacts. In the ventral horn, 91% of boutons
presynaptic to the afferent terminals were immunoreactive for GABA alone an
d 9% were immunoreactive for both GABA and glycine. The mean number of axo-
axonic contacts received per terminal was 2.7, and the mean number of synap
tic contacts at which the terminal was the presynaptic element was 1.4. In
the deep layers of the dorsal horn, 58% of boutons presynaptic to afferent
terminals were immunoreactive for GABA alone, 31% were immunoreactive for G
ABA and glycine, and 11% for glycine alone. The mean number of axoaxonic co
ntacts received per afferent terminal in this region was 1.6 and the mean n
umber of synaptic contacts at which the terminal was the presynaptic elemen
t was 0.86. This clearly establishes the principle that activity in la affe
rents is modulated by several neurochemically distinct populations of presy
naptic neuron. (C) 2001 Wiley-Liss, Inc.