Regulation of glycine receptor binding in the mouse hypoglossal nucleus in response to axotomy

Recent studies have shown that muscarinic receptors in brain hypoglossal nuclei exhibit a loss of specific ligand binding in response to axotomy of the hypoglossal nerve. The mouse hypoglossal nucleus contains a high level of receptors for the inhibitory neurotransmitter, glycine; the ligand [3H]strychnine binds to the glycine receptor with high affinity. In the present study [3H]strychnine binding in mouse hypoglossal nuclei was examined at 1 to 150 days after unilateral lesions of the hypoglossal nerve. Brains were sectioned on a cryostat, thaw-mounted onto microscope slides, incubated with [3H]strychnine and processed for light microscopic autoradiography. Receptor density was assessed by counting silver grains in photomicrographs of operated and control nuclei. During the first 25 days after axotomy grain density fell to 50 percent of that of the control nucleus. After this time grain density slowly increased, returning to control levels by 150 days post lesion. These data indicate that glycine receptors on the axotomized cells of the hypoglossal nucleus are lost when connection with the target muscles of the tongue is interrupted, and that the receptors reappear when the hypoglossal nerve regenerates. It is suggested that excitatory and inhibitory neurotransmitter receptor systems may be regulated in a coordinated fashion by the functional state of the motoneuron.