Peripheral projections of the chick primary sensory neurons expressing gamma-aminobutyric acid immunoreactivity

The expression of gamma-aminobutyric acid was studied in sensory neurons and peripheral target tissues of the chick dorsal root ganglia by combining immunocytochemistry and electron microscopy. In the chick embryos, the first immunoreaction was observed at embryonic day 12 in 1.4% of ganglion cell bodies. The intensity of immunostaining gradually increased during development and the percentage of immunostained neurons reached an average of 7.3% after hatching. These immunostained cell bodies could be identified as sensory neurons belonging either to some large neurons of the A1 subclass or to a few small neurons of the B1 subclass. The other neuronal cell bodies, corresponding to the A2 and B2 subclasses, as well as the satellite and glial cells were apparently devoid of any gamma-aminobutyric acid immunostaining. Among the peripheral tissues innervated by the primary sensory neurons, the nerve endings of Achilles' tendon and the paravertebral autonomic ganglia appeared devoid of immunoreactivity. In contrast, immunoreactivity was found within nerve endings located in some neuromuscular spindles of the skeletal muscles and within some Herbst's corpuscles in the subcutaneous tissue of the skin. Thus, the present results provide evidence that gamma-aminobutyric acid may be expressed by neuronal cell bodies belonging to two subclasses of primary sensory neurons and could be a putative neurotransmitter involved in the peripheral sensory innervation of, at least in part, skin and skeletal muscles.