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Journal of Neuroscience, Vol 9, 2718-2736, Copyright © 1989 by Society for Neuroscience
Evidence for glycine as an inhibitory neurotransmitter of vestibular, reticular, and prepositus hypoglossi neurons that project to the cat abducens nucleus
RF Spencer, RJ Wenthold and R Baker
Department of Anatomy, Medical College of Virginia, Richmond 23298.
The localization and distribution of brain-stem afferent neurons to the cat
abducens nucleus has been examined by high-affinity uptake and retrograde
transport of 3H-glycine. Injections of 3H-glycine selectively labeled (by
autoradiography) only neurons located predominantly in the ipsilateral
medial vestibular and contralateral prepositus hypoglossi nuclei, and in
the contralateral dorsomedial reticular formation, the latter corresponding
to the location of inhibitory burst neurons. The specificity of uptake and
retrograde transport of 3H-glycine was indicated by the absence of labeling
of the dorsomedial medullary reticular neurons ipsilateral and in close
proximity to the injection site, where local uptake by diffusion could have
occurred. The selectivity of uptake and transport was demonstrated by the
absence of retrograde labeling following injections of 3H-GABA or
3H-leucine into the abducens nucleus. The immunohistochemical localization
of glycine and GABA revealed a differential distribution of the 2
inhibitory neurotransmitter candidates in the extraocular motor nuclei.
Glycine-immunoreactive staining of synaptic endings in the abducens nucleus
was dense with a widespread soma-dendritic distribution but was sparse in
the trochlear and oculomotor nuclei. By contrast, GABA-immunoreactive
staining within the oculomotor and trochlear nuclei was associated with
synaptic endings that were particularly prominent on the somata of
motoneurons. GABA- immunoreactive staining in the abducens nucleus,
however, was sparse. These differences between glycine- and
GABA-immunoreactive staining in the extraocular motor nuclei were
correlated with differences in the immunoreactivity of axons in the
descending (glycine) and ascending (GABA) limbs of the medial longitudinal
fasciculus. Glycine- immunoreactive neurons, furthermore, were observed in
the same locations as neurons that were labeled autoradiographically by
retrograde transport of 3H-glycine from the abducens nucleus.
Electrophysiological recordings from abducens motoneurons and internuclear
neurons revealed a marked reduction in the slow positivity of the
orthodromic extracellular potential elicited by ipsilateral vestibular
nerve stimulation following systemic administration of strychnine, an
antagonist of glycine. Intracellular recordings demonstrated that the
vestibular-evoked disynaptic inhibitory postsynaptic potentials in abducens
neurons were effectively blocked by strychnine but were unaffected by
picrotoxin, an antagonist of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)
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