Immunocytochemical studies using antibodies raised against a number of probable synaptic transmitters of the mesodiencephalic area, and fibre-tracing studies using wheat germ agglutinin-horseradish peroxidase (WGA-HRP), have been performed in adult cats. Glutamate and aspartate immunoreactivity produced a strong labelling of many cell bodies and terminals in the nucleus of Darkschewitsch (ND). gamma-Aminobutyrate (GABA) immunoreactivity in the ND appeared as a moderate label in some small neurones, and as a strong label in a few glial-like cells, in addition to being present in high levels to produce strong labelling in many GABA-immunopositive terminals that possessed pleomorphic vesicles. Some choline acetyltransferase-positive terminals and dendrites and a few substance P-positive fine fibres possessing varicosities also were observed in the ND. Following WGA-HRP injection in the ND, dense terminal labelling was seen ipsilaterally in the rostral half of the medial accessory olive, suggesting that there may be a certain degree of mediolateral and dorsoventral topographic correspondance within the ND-olive projection. In the same cases, many cell bodies containing HRP reaction product also were found 1) ipsilaterally in the motor cortex, anterior pretectal nucleus, and a restricted area of the caudal part of the substantia nigra pars reticulata; 2) contralaterally in the anterior and posterior interposed cerebellar nuclei as well as in a portion of the lateral cerebellar nucleus; and 3) bilaterally in the zona incerta, the posterior pretectal nucleus, the pedunculopontine tegmental nuclei, the spinal trigeminal nucleus, the dorsal column nuclei, and the spinal cord. Details of the interrelationships and functional considerations amongst the ND, adjacent nuclei, and longitudinal zones of the cerebellum are discussed.

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The contribution of NMDA and non-NMDA receptors to excitatory subthreshold receptive fields was examined in callosal efferent neurons (CC neurons) in primary somatosensory cortex of the fully awake rabbit. Only neurons showing no traditional (suprathreshold) receptive fields were examined. Subthreshold responses were examined by monitoring the thresholds of efferent neurons to juxtasomal current pulses (JSCPs) delivered through the recording microelectrode. Changes in threshold following a peripheral conditioning stimulus signify a subthreshold response. Using this method, excitatory postsynaptic potentials and inhibitory postsynaptic potentials are manifested as decreases and increases in JSCP threshold, respectively. NMDA and non-NMDA agonists and antagonists were administered iontophoretically via a multibarrel micropipette assembly attached to the recording/stimulating microelectrode. Receptor-selective doses of both AMPA/kainate and NMDA antagonists decreased the excitability of CC neurons in the absence of any peripheral stimulation. Threshold to JSCPs rose by a mean of 20% for both classes of antagonist. Despite the similar effects of NMDA and non-NMDA antagonists on baseline excitability, these antagonists had dramatically different effects on the subthreshold excitatory response to activation of the receptive field. Whereas receptor-selective doses of AMPA/kainate antagonists either eliminated or severely attenuated the subthreshold excitatory responses to peripheral stimulation, NMDA antagonists had little or no effect on the subthreshold evoked response.