GABA and glycine-like immunoreactivity at axoaxonic synapses on 1a muscle afferent terminals in the spinal cord of the rat

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.