Overlap of nigrothalamic terminals and thalamostriatal neurons in the feline lateralis medialis-suprageniculate nucleus.

The lateralis medialis-suprageniculate nucleus (LM-Sg) of the feline posterior thalamus is a relay nucleus with a clear visuomotor function. In this study, we examined the distribution of axon terminals of the nigral afferent to the LM-Sg following injection of an anterograde tracer, biocytin, into the substantia nigra pars reticulata, and the distribution of the thalamostriatal projection neurons in the LM-Sg following the injection of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) as a retrograde tracer into the caudate nucleus. The biocytin-labeled terminal-like puncta were located in the ventromedial portion of this nucleus in such a way that most of the labeled elements took the form of swellings having boutons in places, while a minority appeared in clusters of 3-5 large terminal-like puncta. The retrograde WGA-HRP-labeled neurons were also found in the ventromedial part of the LM-Sg, and the distributions of labeled nigrothalamic axon terminals and labeled thalamostriatal projection neurons therefore overlapped in this region. The present results indicate that the nigral afferent may make synaptic contacts directly with the thalamostriatal projection neurons within the LM-Sg.

Visual receptive field properties of excitatory neurons in the substantia nigra.

The substantia nigra has been widely regarded as a structure involved in visuomotor co-ordination, but little is known about the sensory background of its function. Here we give a detailed description of the visual receptive field properties of excitatory substantia nigra neurons. The visual responses of 59 excitatory neurons were recorded in both the substantia nigra pars reticularis and the pars compacta of halothane-anesthetized, immobilized, artificially respirated cats. The substantia nigra neurons were not responsive or exhibited very low sensitivity to stationary visual stimulation. The units responded optimally to small stimuli moving at intermediate or high velocities in their extremely large receptive field. We observed no signs of retinotopical organization within the substantia nigra. A majority of the units exhibited narrow direction tuning and high direction selectivity, while a smaller proportion of them were broadly tuned and not direction-sensitive. Our results suggest that the visual properties of the excitatory substantia nigra units are quite similar to those of the superior colliculus and other extrastriatal structures that receive tectal afferents. This supports the notion that the substantia nigra processes dynamic visual information and that its excitatory visual neurons are modulated by the extrageniculate tectal visual system of the mammalian brain.