Cholinergic nigrotectal projections in the rat

Topographic and functional neuroanatomical study of GABAergic disinhibitory striatum–nigral inputs and inhibitory nigrocollicular pathways: Neural hodology recruiting the substantia nigra, pars reticulata, for the modulation of the neural activity in the inferior colliculus involved with panic-like emotions

Considering the influence of the substantia nigra on mesencephalic neurons involved with fear-induced reactions organized in rostral aspects of the dorsal midbrain, the present work investigated the topographical and functional neuroanatomy of similar influence on caudal division of the corpora quadrigemina, addressing: (a) the neural hodology connecting the neostriatum, the substantia nigra, periaqueductal gray matter and inferior colliculus (IC) neural networks; (b) the influence of the inhibitory neostriatonigral-nigrocollicular GABAergic links on the control of the defensive behavior organized in the IC. The effects of the increase or decrease of activity of nigrocollicular inputs on defensive responses elicited by either electrical or chemical stimulation of the IC were also determined. Electrolytic or chemical lesions of the substantia nigra, pars reticulata (SNpr), decreased the freezing and escape behaviors thresholds elicited by electrical stimulation of the IC, and increased the behavioral responses evoked by the GABAA blockade in the same sites of the mesencephalic tectum (MT) electrically stimulated. These findings were corroborated by similar effects caused by microinjections of the GABAA-receptor agonist muscimol in the SNpr, followed by electrical and chemical stimulations of the IC. The GABAA blockade in the SNpr caused a significant increase in the defensive behavior thresholds elicited by electrical stimulation of the IC and a decrease in the mean incidence of panic-like responses induced by microinjections of bicuculline in the mesencephalic tectum (inferior colliculus). These findings suggest that the substantia nigra receives GABAergic inputs that modulate local and also inhibitory GABAergic outputs toward the IC. In fact, neurotracing experiments with fast blue and iontophoretic microinjections of biotinylated dextran amine either into the inferior colliculus or in the reticular division of the substantia nigra demonstrated a neural link between these structures, as well as between the neostriatum and SNpr.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Projections from the substantia nigra pars reticulata to the motor thalamus of the rat: single axon reconstructions and immunohistochemical study

This is a study in the rat of the distribution of specific neurotransmitters in neurones projecting from the substantia nigra reticulata (SNR) to the ventrolateral (VL) and ventromedial (VM) thalamic nuclei. Individual axons projecting from the SNR to these thalamic nuclei have also been reconstructed following small injection of the anterograde tracer dextran biotin into the the SNR. Analysis of reconstructions revealed two populations of SNR neurones projecting onto the VL and VM thalamic nuclei. One group projects directly onto the VM and VL, and the other projects to the VM/VL and to the parafascicular nucleus. In another set of experiments Fluoro-Gold was injected into the VL/VM to label SNR projection neurones retrogradely, and immunohistochemistry was performed to determine the distribution of choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT), gamma-aminobutyric acid (GABA), and glutamate in Fluoro-Gold-labelled SNR projection neurones. Most SNR-VL/VM thalamic projection neurones were immunoreactive to acetylcholine or glutamate, whereas only 25% of the projection neurones were found to be immunoreactive to GABA.

Hippocampal theta-related cellular activity in the superior colliculus of the urethane-anesthetized rat

In the present study, 93 cells in the superior colliculus (SC) were recorded extracellularly during the simultaneous occurrence of spontaneous theta field activity, sensory-induced (tail pinch) theta field activity, and large amplitude irregular (LIA) field activity, recorded from an electrode located in the stratum moleculare of the hippocampal formation (HPC). The effect of the intravenous administration of atropine sulfate (ATSO4) was also tested on SC cellular activity. The field activities of theta and LIA were recorded from all layers of the SC and were found to be temporally coherent with the same activities recorded simultaneously from the HPC, during all conditions tested. By using the criteria of Colom and Bland (1987) for the classification of theta-related cells, 75 of 93 cells (81%) were found to be related to the generation of theta field activity in the HPC and 18 of 93 (19%) were nonrelated. All cells recorded discharged in a tonic, nonrhythmic pattern during the theta HPC field states. Of the 75 theta-related cells, 61 (81%) were classified as tonic theta-ON cells and 14 (19%) as tonic theta-OFF cells. Although these cell types were found in all three layers of the SC, the majority of tonic theta-ON cells were recorded in the intermediate layer, and the tonic theta-OFF cells were dispersed evenly between the intermediate layer and the deep layer of the SC. The intravenous administration of ATSO4 abolished theta field activity in the HPC and SC, and the theta-related increase in the discharge rate of all tonic theta-ON cells tested. However, the same treatment did not have any effect on the discharge properties of tonic theta-OFF cells. The same stimuli that resulted in the inhibition of the discharge rates of these cells (tail pinch and electrical stimulation of the PH) in the predrug condition did so after the administration of ATSO4.

Cytoarchitecture of the substantia nigra pars lateralis in the opossum (Didelphis virginiana): a correlated light and electron microscopic study

BACKGROUND

The substantia nigra has been divided into three subdivisions. However, the cytoarchitecture of one of these subdivisions, the pars lateralis (SNl), has not been previously examined in detail at the light and electron microscopic levels in any species. In the adult opossum, the three nigral subdivisions can be easily distinguished as distinct, rostrocaudally oriented cell groups separated by neuron-free zones. Thus it was possible to determine the boundaries of the SNl unambiguously. This report covers the results of an examination of the morphology and organization of the SNl in the opossum.

METHODS

Material from 13 opossums was used for this study. Eight of the animals had been previously stained for Nissl substance (n = 4) or impregnated by the Golgi technique (n = 4). The remaining five animals were prepared for electron microscopic studies using standard procedures.

RESULTS

Two cell types were identified on the basis of morphological differences, small and medium-large neurons. Small neurons (10-18 microns long axis) have large nuclei with moderate amounts of heterochromatin and a thin rim of cytoplasm. They have long (up to 500 microns), spine-free dendrites. Medium-large neurons (18-54 microns long axis) have rounded nuclei with electron-lucent nucleoplasm. Few indentations of the nuclear envelope were observed. The surrounding cytoplasm has dense arrays of organelles. Nissl bodies are particularly prominent in the form of pyramids with their bases at juxtanuclear positions and their apices directed toward emerging dendrites. Dendrites of medium-large neurons are long (some > 1 mm in length), are primarily oriented in the frontal plane, and extend along the dorsal surface of or into the cerebral peduncle. Some cells have dendrites that are moderately spinous, whereas other neurons possess sparsely spinous dendrites. Relatively few synaptic profiles are observed to contact somata and proximal dendrites.

CONCLUSION

This report provides added morphological support for the idea that the SNl is a distinct subdivision of the substantia nigra, a distinction previously made on the basis of the physiologically characterized relationship between the lateral substantia nigra and orienting behaviors and seizure-related function.