Basal ganglia-diencephalon synpatic relations in the cat. II. Intracellular recordings from dorsal thalamic neurons during low frequency stimulation of the caudatothalamic projection systems and the nigrothalamic pathway

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.

Regulation of cerebello-cortical transmission in the rat ventromedial thalamic nucleus

On the basis of antidromic stimulation we have identified two distinct neuronal populations in the rat ventromedial thalamic nucleus. The largest population (96%) are thalamo-cortical relay cells which project via the internal capsule to the cerebral cortex. The smaller population of cells (4%) project caudally to the reticular formation and superior colliculus. These two cell types could be distinguished further on the basis of their patterns of spontaneous discharge. Relay cells fluctuate between two activity patterns (i) a rhythmic pattern characterized by periods of high-frequency bursting, and (ii) a more tonic discharge pattern of single spikes. The caudally projecting cells had a characteristic fast, regular type of spontaneous firing. Brachium conjunctivum stimulation evokes two distinct responses in thalamic relay cells. (i) a short-latency single spike, (ii) a longer latency, rhythmic response of 2-3 spikes. Both excitatory responses are followed by a period of cell quiescence. The type of response is dependent upon the cell's firing pattern. The short-latency response occurs during tonic, single-spike activity whilst the longer latency response occurs during high-frequency bursting activity. The short-latency response can be altered to the long latency response by increasing the level of anaesthesia or by applying a conditioning shock to known inhibitory pathways. Conversely the long latency response can be altered to the short-latency response by decreasing anaesthesia or by stimulation of the reticular formation. It is argued that both response types are evoked monosynaptically by activation of the same cerebello-thalamic fibres but that different ionic conductances which are active at different levels of membrane polarization are responsible for the two response patterns. Efficient time-locked cerebello-thalamo-cortical transmission occurs only during tonic single-spike activity, when cerebellar stimulation evokes a short-latency response. Such transmission is allowed or disallowed by the fine balance between converging excitatory and inhibitory afferents. In addition to a monosynaptic excitatory input from the cerebellar nuclei, relay cells received converging synaptic inputs from the substantia nigra, cerebral cortex, reticular formation and superior colliculus. Due to the anatomical arrangement in the rat it proved impossible to assess the role of the pallidum. The population of caudally projecting cells also received several converging synaptic inputs, but unlike those influencing relay cells, these inputs were all excitatory.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological and morphological studies on thalamic neurons receiving entopedunculo- and cerebello-thalamic projections in the cat

Electrophysiological studies on the entopedunculo- and cerebello-thalamic projections were performed by intracellular recordings in the thalamic VA, VL and VM nuclei of cats under sodium pentobarbital anesthesia. Identification of the thalamic neurons were performed electrophysiologically by antidromic activation on stimulation of the precruciate cortex (areas 4 and 6) and the caudate nucleus, and morphologically by intracellular staining with HRP through recording microelectrodes. One hundred and sixty-three neurons were collected in the VA, VL and VM nuclei. In 79 neurons penetrated in the medial and ventral parts of the VA and VL nuclei, stimulation of the entopeduncular nucleus induced monosynaptic IPSPs (latency of 1.1-3.5 ms, mean 2.07 ms). Sixteen neurons were identified as thalamo-cortical relay neurons and 3 were activated only orthodromically by precruciate stimulation. Seventy-eight neurons located dorsolaterally to the entopeduncular-influenced neurons received only cerebellar EPSPs. Only 6 neurons showed convergence of entopeduncular and cerebellar inputs. They were scattered around the border between the entopeduncular and cerebellar projection areas. Sixteen neurons could be stained intracellularly by HRP injection. From the pattern of dendritic arborization, two types of neurons can be distinguished: neurons whose dendrites spread radially in all directions and neurons whose dendrites extend mainly along the long axis of the soma for a long distance in the frontal plane, respectively. The former are relay cells to the cerebral cortex or the caudate nucleus (i.e. projection neurons) and the latter appear to be interneurons in the thalamus.

Effects of caudate nuclear or frontal cortical ablation in neonatal kittens or adult cats on the spontaneous firing of forebrain neurons

In this paper we have determined the long-lasting consequences of caudate and frontal cortical lesions on spontaneous neuronal firing. Lesions were made both in neonatal and adult cats. All recordings were made in adults. Qualitatively, the effects of the caudate ablations were similar whether they had been carried out in kittens or in adult cats. Caudate lesions produced long-lasting (greater than or equal to 1 year) decreases in the spontaneous firing of cortical neurons. These changes were more pronounced when made in neonates than in adults. The distributions of mean interspike intervals were also altered by these caudate lesions in the pallidum and in the ventral lateral nucleus of the thalamus. Again these effects were more marked if the animals were lesioned as neonates than as adults. Frontal cortical lesions inflicted upon adult cats produced more widespread changes in spontaneous firing rates than similar lesions made in neonates. In both groups frontal lesions slowed spontaneous firing and changed the distributions of mean interspike intervals of caudate neurons. These effects were long-lasting (greater than or equal to 1 year in neonatally-ablated animals). Cortical lesions made in adult cats markedly altered thalamic and pallidal spontaneous activity. Similar lesions made in neonates produced relatively small changes in thalamic and pallidal activity.

Unit responses of intralaminar thalamus to midbrain and medullary stimulation and effects of conditioning caudate and hippocampal stimuli

Single units responding to heterotopic somatic stimuli, on extracellular recording in thalamic intralaminar and neighbouring nuclei, also responding to stimulation of the midbrain tegmentum or the medullary magnocellular reticular formation. Consideration of response latencies suggested that some monosynaptic projections from both midbrain and medulla may be received in nuclei centralis lateralis, centrum medianum-parafascicularis complex, and medial ventralis lateralis. Responses to brainstem of nuclei medialis dorsalis, lateralis posterior were of considerably longer latency. There was no correlation between shortness of latency and following-rate of unit responses; the ability of intralaminar neurons to follow rapidly-repeated brainstem stimuli is inferred to be limited by inhibitory processes rather than by synaptic interruptions in the afferent pathway. Conditioning stimuli to caudate nucleus or hippocampus suppressed most intralaminar responses to midbrain stimuli, the shortest-latency responses included, suggesting that inhibitory effects could be exerted at the thalamic level, perhaps directly on the responsive neurone.

Behavioral alterations and loss of caudate modulation in the centrum medianum--parafascicular complex of the cat following electrolytic lesions of the substantia nigra

Extracellular activity in the centrum medianum--parafascicular complex was examined following electrolytic lesions of the substantia nigra in the cat. Three major changes were observed in the activity of these polysensory medial thalamic neurons: (1) normally quiescent in the intact animal, about 70% of the neurons now displayed spontaneous activity; (2) many of these spontaneously active neurons no longer responded to caudate stimulation and the responses to sensory stimulation were not changed; and (3) while caudate stimulation alone did inhibit some neurons, such stimulation in a conditioning test procedure often failed to inhibit the test response to limb stimulation which 'broke through' the inhibitory period. All of these changes were correlated with the extent of damage to the ipsilateral substantia nigra. Additionally, a noticeable change was observed in the behavior of the lesioned animals; intense circling, contralateral to the side of the lesion, persisted for several days following surgery. Other symptoms were arrest reactions, abnormal rhythmic pawing and sluggishness of movement. The results indicate that the substantia nigra is an important element in funneling caudatofugal activity to the centromedian--parafascicular complex where it interacts with, and modulates incoming sensory activity. The loss of this modulation and the removal of a tonic background inhibition, indicated by the increased proportion of spontaneously active neurons, may be important factors in producing the observed behavioral abnormalities.

The afferent projections to the centrum medianum of the cat as demonstrated by retrograde transport of horseradish peroxidase

The afferent projections of nucleus centrum medianum (CM) of the thalamus were studied, in the cat, by means of retrograde transport of electrophoretically ejected horseradish peroxidase. Several variations of method--survival time, fixatives, substrates, etc.--were tried to improve the amount of visible reaction product. Labeled neurons were localized primarily in two categories of nuclei in the brain. The first consisted of structures making up or closely related to the basal ganglia: the entopeduncular nucleus, the pars reticulata of the substantia nigra, and motor cortex. The second category was made up of nuclei closely related to postural and orienting functions: the deep layers of the superior colliculus ipsilaterally, and the medial and lateral vestibular nuclei bilaterally. Other nuclei containing retrogradely labeled neurons were the periaqueductal gray and locus coeruleus. Brain stem reticular projections were sparse and widely scattered. These results identify CM as an important element in the loop system linking medial thalamus and neostriatrum; the probable attention and orientation related functions of this system are discussed.

Antidromic identification of dopaminergic and other output neurons of the rat substantia nigra

In the present study dopamine (DA)-containing and other output neurons of the substantia nigra (SN) wer identified by antidromic stimulation from postulated target nuclei, the caudate-putamen, the thalamus, the cortex and the pontine reticular formation. To guide electrode placements, the topography of the nigrostriatal projection system was determined by retrograde tracing methods. Spontaneously active cells present in the SN were then classified in two groups according to the shape of their action potentials and their firing rate. Type I cells were located mainly in the pars compacta and could be antidromically-activated (AD-activated) from various locations along the course of the nigrostriatal pathway (caudate-putamen, globus pallidus, MFB) but not from other brain areas (ventromedial thalamus, motor cortex, pontine reticular formation). These neurons had a slow bursting pattern of firing, a very slow conduction velocity (0.58 m/sec), and a wide action potential. Their firing rate was dramatically reduced following the intravenous administration of apomorphine (ID 50: 9.3 microgram/kg), or the iontophoretic application of DA and GABA. Type II cells were located predominantly in the pars reticulata; most of them could be AD-activated from the ventromedial thalamus and the MFB but not from the motor cortex. A few of these cells could be AD-activated from the pontine reticular formation and the thalamus. A minority of type II cells, located in or near the pars compacta could be AD-activated from the caudate-putamen. In addition, their conduction velocuty was much higher (2.8 m/sec) and their firing rate far in excess of that exhibited by type I neurons. These neurons were inhibited by the iontophoretic application of GABA but not of DA. The microinjection of 6-hydroxydopamine (a neurotoxin relatively specific against catecholamine-containing neurons) in the vicinity of the MFB blocked selectively the propagation of antidromic spikes in type I but not type II cells. It is concluded that type I cells are the DA neurons of the nigrostriatal pathway. Type II cells are mainly oupput neurons that project to the ventromedial thalamus, the pons and the forebrain. This telencephalic projection most likely constitutes a second, non-DA, fast-conducting nigrostriatal pathway.

Effect of substantia nigra stimulation on identified neurons in the VL-VA thalamic complex: comparison between intact and chronically decorticated cats

The effects of substantia nigra (SN) stimulation on the activity of the nucleus ventralis lateralis of the thalamus (VL) have been studied. The VL cells were identified as relay or non-relay cells among the cerebellothalamocortical pathway on the basis of orthodromical activation from the cerebellum and antidromical activation from the cortex. Certain experiments were performed after wide pericruciate decortication in order to eliminate the response due to unavoidable activation of corticofugal fibers. These results show that nigrothalamic neurons exert an inhibitory effect on VL cells. This inhibition was observed on the relay as well as on non-relay cells and was strong enought to suppress the cerebellar monosynaptic excitatory input. Thus the SN can modulate the information running along the cerebellothalamocortical pathway. The topographic localization of inhibited cells suggests that the SN controls the activity of the thalamic neurons which interfere with axial and proximal musculature.

Effects of intrastriatal injections of atropine and methacholine on the apomorphine-induced gnawing in the rabbit

To find out the anatomical location of the target point of cholinergic-dopaminergic equilibrium, atropine (40 microng) or methacholine (10 microng) were injected through previously implanted cannulas into various places of caudate nucleus and putamen of the rabbit, and the effect of the injections on stereotype gnawing induced by subcutaneously or intravenously administered apomorphine (1--2 mg/kg) was assessed. The intensity of gnawing was measured using a special apparatus, counting each bite. Atropine inhibited the stereotype, while metacholine potentiated it. The effects were evident with the method used, but difficult to reveal with the classical method of assessing the intensity of stereotyped behavior, based on visual observation. The results suggest that the striatum is not a target point for the cholinergic component of the cholinergic-dopaminergic equilibrium in the central nervous system.

Modification of visual signals by nigral stimulation

Responses of the lateral geniculate neurons to light were modified by stimulation of the substantia nigra. Nigral stimulation often caused enhancement of firing in neurons responding primarily to flash, but it usually had the contrary effect on units inhibited by light.