Ventromedial Thalamus Research Articles

Gas chromatographic—mass spectrometric determination of glutamic acid decarboxylase activity in subregions of rat brain

A quantitative gas chromatographic—mass spectrometric method has been developed for the determination of glutamic acid decarboxylase (GAD) activity in subregions of rat brain. The five subregions analyzed, weighing approximately 2.51 mg each, were globus pallidus, entopeduncular nucleus, ventromedial thalamus, and substantia nigra medial and lateral. The activity of the GAD enzyme has been determined indirectly by measurement of γ-aminobutyric acid (GABA) using γ-[2,2- 2H 2]aminobutyric acid as the internal standard. Both compounds were quantitatively converted to trimethylsilyl-GABA and trimethylsilyl-[ 2H 2]GABA in 90 min with hexamethylchlorosilane, trimethylchlorosilane, pyridine and N,O-bis(trimethylsilyl)trifluoroacetamide silylating agents. Using selective ion monitoring and electron impact ionization at 70 eV, the limit of detection was 15 ng GABA per mg tissue. This method is compared with a fluorimetric procedure.

Bilateral asymmetry in brain GABA function?

The GABA system was studied in different regions of the rat's brain following inhibition of brain GABA catabolism with intracisternal ethanolamine-O-sulphate or intraperitoneal aminooxyacetic acid. Both treatments lowered GABA aminotransferase and glutamate decarboxylase activities to similar extents in equivalent nuclei on either side of the brain. However, GABA contents were elevated to a consistently higher level in the right-hand substantia nigra, superior colliculus and nucleus accumbens, and in the left-hand ventral tegmentum, ventromedial thalamus and caudate nucleus, with no bilateral asymmetry evident in globus pallidus. These findings are discussed with reference to possible inherent inequalities in the functional states of similar GABA systems on opposite sides of the brain.

Antinociceptive effect of centrally administered caerulein and cholecystokinin octapeptide (CCK-8)

When injected into the brain (periaqueductal gray (PAG), caudate nucleus, ventromedial thalamus) or subarachnoidal space of the lumbar spinal cord of rats, caerulein and CCK-8 produced a long-lasting (2 h) inhibition of the tail-flick response to thermal stimulation. The effective dose range for caerulin was from 1 to 5 ng per rat, for CCK-8 from 10 to 40 ng, and for morphine from 1 to 20 μg. On a molar basis and dependent upon the site of administration, caerulein was 4000–7000 times and CCK-8400-700 times more potent than morphine. Unsulphated caerulein (injected into the PAG) was less active than caerulin by a factor of 9. Sedation was seen after intracerebral, but not after spinal administration of the peptides; it was not observed after morphine or unsulphated caerulin. Naloxone (0.5 mg/kg i.p.) abolished the antinociceptive but not the sedative effect. It is concluded that caerulin and CCK-8 are very potent in reducing nociception by acting at both spinal and supraspinal levels.

Habenular projections in the monitor lizard (Varanus benegalensis).

The efferent connections of the medial (MHb) and the lateral (LHb) habenular nuclei in the monitor lizard were studied using experimental degeneration techniques. The MHb was found to project to the interpeduncular nucleus and the parvocellular nucleus of the superior raphe via the core portion of the habenulo-peduncular tract (HPT). The LHb fibers form the mantle portion of the HPT and curve laterally to collect again in the ventral tegmentum. From here, they follow either (1) the medial forebrain bundle to terminate in hypothalamus, ventromedial thalamus, preoptic area, and septum, or (2) they continue caudally to terminate in the superior raphe and paramedian reticular formation or (3) they decussate and follow in smaller numbers the ascending and descending pathways on the other side. Some fibers enter the midline and reach the periventricular zone of the midbrain. Short range projections exist to the dorsomedial thalamic nucleus and the paramedian central gray and pretectum. The habenular projections are bilateral, however, much smaller on the contralateral side. Although distinct terminal fields were not found in the substantia nigra and the central gray of the isthmic region, the overall pattern of habenular pathways is strikingly similar to those found in mammals which confirms a long presumed phylogenetic stability of habenular connections.U

Evidence for involvement of the frontal cortex in pain-related cerebral events in cats: increase in local cerebral blood flow by noxious stimuli

Noxious stimuli were shown to induce a remarkable increase in local cerebral blood flow restricted to the forepart of the cerebral hemispheres bilaterally anterior to the posterior sigmoid gyrus in cats. This increase in local cerebral blood flow was averted by lesions in the bilateral ventromedial thalamus and attenuated by pretreatment with an intraventricular injection of 6-hydroxydopamine.

Distribution of γ-aminobutyrate in the rat thalamus: Specific decreases in thalamic γ-aminobutyrate following lesion or electrical stimulation of the substantia nigra

Various subnuclei of the rat's thalamus were punched out or finely microdissected from frozen coronal sections of brain and assayed for γ-aminobutyrate by microdansylation. Highest levels were recorded in the ventromedial and parafascicular regions, corresponding to the sites of termination of nigrothalamic neurones. One week after placing a unilateral electrolytic lesion in the zona reticulata of the substantia nigra there were significant decreases in γ-aminobutyrate levels in the ipsilateral ventromedial (−14%) and parafascicular (−11%) nuclei, but not in other nuclei tested. Falls in the level of γ-aminobutyrate of 19–26% were found to be confined to the ventromedial region in microdissections of brain slices containing this nucleus, two weeks after injecting 1 μg kainic acid into the substantia nigra. The concentrations of glutamate, aspartate, glutamine, glycine and alanine were not altered in the deafferented ventromedial thalamus. Pathological effects of kainate were registered as a complete loss of neuronal perikarya at the injection site coupled with intense gliosis. Mild gliosis, but no postsynaptic neuronal damage, was visible in the corresponding ventromedial and parafascicular nuclei in the thalamus, although degenerating electron-dense terminal boutons were occasionally seen in electron micrographs of this tissue. In rats in which thalamic glutamate decar☐ylase activity was inhibited by systemic injection of 3-mercaptopropionic acid, trains of biphasic electrical pulses delivered to the nigra (100 Hz, 0.1 mA, width 0.6 ms for 15 min) evoked a stimulus-dependent decrease in ventromedial (−17%) and parafascicular (−10%) γ-aminobutyrate content, but not in that of other thalamic nuclei. Similar post-stimulus falls in the level of γ-aminobutyrate in the ventromedial nucleus were noted after direct stereotaxic injection of this nucleus with the enzyme inhibitor. These findings are consistent with the existence of γ-aminobutyrate-containing nigrothalamic fibres whose cell bodies lie in the medial part of the substantia nigra zona reticulata and which terminate in the ventromedial and parafascicular nuclear regions of the thalamus.

Brain mechanisms concerned with left-right differentiation in the white rat

Rats previously trained on a successive brightness discrimination—go left in the presence of two black cards and go right in the presence of two white cards—sustained bilateral lesions to one of nine brain regions (parietal cortex, anterior cingulate cortex, posterior cingulate cortex, cerebellum, anterior thalamus, ventromedial thalamus, ventrolateral thalamus, lateral thalamus, and ventrobasal thalamus) thought to be concerned with the maintenance of habits having a proprioceptive-kinesthetic basis. The retention test disclosed that each of the nine brain-damaged groups showed significant deficits in the performance of the successive problem. Damage to the motor cortex, hippocampus, septofornix, or medial longitudinal fasciculus also led to significant retention deficits, but mammillary body or mediodorsal thalamic lesions had no effect. These findings suggest that left-right differentiation has a proprioceptive-kinesthetic basis in addition to a possible vestibular basis.

Dopaminergic activation of reticulata neurones in the substantia nigra.

Dendritic release of dopamine (DA) in substantia nigra (SN) is well established in various experimental situations. Morphological substrates for DA storage exist in dendrites, as do dendro-dendritic and dendro-axonic contacts. DA receptors in SN are located on both cells and striato-nigral terminals. DA is thought to regulate the activity of neighbouring dopaminergic neurones through its dendritic release by a local feedback mechanism. However, dendrites of DA neurones also ramify close to the neuropil of non-dopaminergic reticulata neurones in SN. The question has arisen whether dendritically release DA might also influence these neurones which, to a large extent, project to ventromedial thalamus (VM) and superior colliculus. A necessary condition would be that they are sensitive to DA. In the experiments reported here this was found to be the case--a considerable proportion of nigrothalamic neurones were activated by iontophoretically applied DA. This contrasts with its known depressant effect on pars compacta DA neurones which we confirmed.

Odor discrimination performance as affected by cortical, thalamic and cerebellar lesions in rats

Blinded rats previously trained on an odor (tap water vs acetone) discrimination sustained bilateral parietal, cingulate, cerebellar, thalamic or olfactory bulb lesions and were subsequently tested for retention. Only the olfactory bulbectomized group showed significant deficits in retention. It is concluded that the parietal cortex, cingulate cortex, cerebellum, anterior thalamus, ventrolateral thalamus and ventromedial thalamus, rather than being included within a neural system concerned with the guidance of learned responses in nonvisual space, are components of the somatosensory system concerned with the performance of habits having a kinesthetic basis. Of some importance was the additional finding that the learning process underlying the tap water acetone problem may not be fundamentally different from that underlying a brightness discrimination problem.

Axonal branching patterns and location of nigrothalamic and nigrocollicular neurons in the cat.

1. Antidromic activation from the thalamus and the superior colliculus was used to identify the axonal projections of neurons in the substantia nigra. 2. Up to 43% of the nigral neurons studied in individual animals had branching axons that could be activated antidromically from both the ipsilateral ventromedial thalamus and the ipsilateral superior colliculus. 3. For several neurons, the conduction time spent in the unbranched axonal segment could be calculated using measurable collision and antidromic latency times. Time in the unbranched segment was less than 20% of the antidromic latency for most neurons studied. 4. Latencies for antidromic activation from the thalamus were measured and would be appropriate if these axons were to mediate the monosynaptic nigrothalamic inhibition reported earlier. 5. Stimulation of the caudate nucleus produced inhibition of spontaneous activity of both collicular- and thalamic-projecting neurons.

A horseradish peroxidase study of afferent connections of the globus pallidus in Macaca mulatta.

The high tonic discharge rates of globus pallidus neurons in awake monkeys suggest that these neurons may receive some potent excitatory input. Because most current electrophysiological evidence suggests that the major described pallidal afferent systems from the neostriatum are primarily inhibitory, we used retrograde transport of horseradish peroxidase (HRP) to identify possible additional sources of pallidal afferent fibers. The appropriate location was determined before HRP injection by mapping the characteristic high frequency discharge of single pallidal units in awake animals. In animals with injections confined to the internal pallidal segment, retrograde label was seen in neurons of the pedunculopontine nucleus, dorsal raphe nucleus, substantia nigra, caudate, putamen, subthalamic nucleus, parafascicular nucleus, zona incerta, medial and lateral subthalamic tegmentum, parabrachial nuclei, and locus coeruleus. An injection involving the external pallidal segment and the putamen as well resulted in additional labeling of cells in centromedian nucleus, pulvinar, and the ventromedial thalamus.

Fluorimetric Assay for the Determination of Glutamic Acid Decarboxylase Activity in Subregions of Rat Brain Tissue

Abstract A modification of the spectrofluorimetric method of Lowe et al 1 for the measurement of glutamic acid decarboxylase (GAD) activity in rat brain is described. This method involves the indirect determination of GAD activity by measurement of the formation of gamma-amino-butyric acid (GABA). The minimum activity of GAD observable was 0.45 μg GABA/hour/mg protein and the limit of detection was 0.20 × 10−6 M GABA. This method was developed for measurement of GAD activity of gabaergic terminals in globus pallidus, endopeduncular nucleus, substantia nigra, and ventromedial thalamus in rat brain.

Evidence for a gabaergic projection from the substantia nigra to the ventromedial thalamus and to the superior colliculus of the rat

Unilateral intranigral infusion of kainic acid (1.5 μg) produced neuronal loss in the lateral two-thirds of the nigra while sparing axon en passage. Fink-Heimer silver impregnation revealed dense terminal degeneration in the nigra itself (both in the compacta and in the reticulata) and in areas of non-dopaminergic nigral projection such as the ventromedial (VM) nucleus of the thalamus, the superior colliculus and the reticular formation; only sparse terminal degeneration was found in areas of dopaminergic projection such as the caudate and septum. In order to clarify the nature of the transmitter of the nigrothalamic and nigrocollicular neurons, the activity of glutamic decarboxylase (GAD), the marker of GABAergic neurons, and of choline acetyltransferer (CAT), the marker of cholinergic neurons, was measured in the VM and ventrobosal (VB) thalamus and in the nigra of each side, 7 days after unilateral intranigral injection of kainic acid. GAD activity was reduced significantly in the VM-thalamus (−33%), in the superior colliculus (−40%) and in the substantia nigra (−18%) but not in the VB-thalamus of the lesioned side. CAT remained unchanged in these areas. Similar results were obtained in the thalamus and in the superior colliculus after electrocoagulative lesions of the nigra. The results indicate the existence of a nigrothalamic and of a nigrocollicular GABAergic pathway. This projection might play an important role in motor coordination and gaze control.

Localization of a “passive avoidance memory system”in the white rat

Adult rats, previously trained to refrain from entering a small dark compartment from a large illuminated compartment, sustained lesions to 1 of 53 different brain sites and were subsequently tested for retention. Retention losses were associated with lesions to the neocortex, most elements of the limbic forebrain and basal ganglia, medial portions of the thalamus and hypothalamus, colliculi, median raphe, and cerebellum. Most of these structures were also found to be critical for retention of an active avoidance habit. It was hypothesized that avoidance habits depend upon the activities of four functional blocks of the brain: the first block (basal ganglia) is concerned with sensorimotor integration, the second block (dorsal midbrain, posterior thalamus, amygdala, nucleus accumbens, and frontal cortex) plays a role in fear-initiated responses, the third block (cerebellum, anterior and ventromedial thalamus, and parietal cortex) has somatosensory functions, and the fourth block (occipital cortex, hippocampal complex, and mamillary bodies) is involved with cognitive mapping. These functional blocks require the engagement of an inhibitory mechanism (medial hypothalamus and median raphe) or an excitatory mechanism (brainstem reticular formation and anterior cingulate cortex) in order to support passive and active avoidance habits, respectively.

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) were 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 (0.58m m/sec), and a wide action potential. Their firing rate was dramatically reduced following the intravenous administration of apomorphine ( ID50:9.3 μg/kg), or the iontophoretic application of DA and GABA. Type II cells were located predominantly in the pars reticula; 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, the their conduction velocity 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 output 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.

Properties of different functional types of neurones in the cat's rostral trigeminal nuclei responding to sinus hair stimulation.

1. Properties of neurones in the trigeminal nuclei principalis and oralis responding to movements of facial sinus hairs were studied in cats anaesthetized by I. V. infusion of pentobarbitone.2. Using electrophysiological methods trigeminal neurones were classified into primary afferent fibres, trigeminothalamic relay neurones, interneurones and other unspecified higher order neurones.3. When receptive fields of synaptically activated neurones were compared with those of primary afferent fibres, an often extensive convergence from first order on to higher order neurones was established. Out of 119 relay neurones six received input from one sinus hair only. Spontaneous activity was encountered about twice as often in synaptically activated neurones than in primary afferent fibres.4. The responsiveness of single neurones was unstable over time in about one fifth of the population and then the total number of impulses discharged in successive responses could vary by as much as 500%. Unstable responsiveness occurred sometimes alone but was often accompanied by marked changes in the size or the configuration of the receptive field. Such instabilities were observed in all kinds of synaptically activated neurones but not in primary afferent fibres.5. Afferent inhibition in relay neurones could be elicited from within the excitatory receptive field and appeared to be related to the activation of distinct receptor populations responding to specific stimulus parameters. Inhibition was also seen in interneurones following both mechanical stimulation of the skin and electrical stimulation of lemniscal fibre terminals in the contralateral ventromedial thalamus.6. The results are discussed and compared with previous findings about sinus hair representation in the trigeminal nucleus and the ascending lemniscal projection. The findings indicate that the concept of the ;static properties' of relay neurones is not adequate for all trigeminothalamic relay neurones and may require a critical reconsideration.7. It is suggested that the afferent input from sinus hairs is effectively controlled at the level of the rostral trigeminal nuclei. This control may affect the spatial input to relay neurones, the temporal components of their responses and the intensity dimension of their transmission capacity. It is postulated that by these mechanisms tactile information from the sinus hair system is modulated according to the instantaneous sensory requirements of the behaving cat.

Card displacement response as affected by brainstem lesions in the rat

Rats previously trained to push aside a stimulus card in order to gain access to an area of safety showed a marked impairment of the habit following lesions to either the corpus striatum, anterior thalamus, ventromedial thalamus, lateral hypothalamus, subthalamus, prerubral area, substantia nigra, ventral tegmental area, cerebral peduncle, or pontine reticular formation. Lesions to other brainstem areas had no deleterious effects on the card displacement habit.

Effect of Electrolysis at Different Levels of the Reticular Formation on the Synchronized Electrical Cortical Activity in Thoracic Spinal Cats

Shortly after spinal cord transection at T8 in cats immobilized with Flaxedil the electrocorticogram continuously showed synchronized activity intermingled with runs of desynchronized activity. Similar changes were seen in the mesencephalic reticular formation, thalamic reticular formation and in the nucleus ventromedialis thalami. No significant changes were seen in the nucleus reticularis pontis oralis. Electrolysis at different levels of the reticular formation did not alter the synchronized electrical cortical activity induced by the spinal cord section at T8.

Evoked photic responses from the human thalamus and midbrain.

Visual evoked potentials were recorded from a variety of sites in the mesencephalon and diencephalon of man with the exception of the lateral geniculate bodies. Responses were recorded from the tegmentum, tectum, and ventromedial thalamus (medial pulvinar). Areas from which responses were obtained were contiguous. A variety of sensory, motor and autonomic responses have been obtained when these same areas have been stimulated.

Neural pathways mediating drinking and feeding in rats

Drinking and feeding were elicited by stimulation of topographically differentiated sites in the rat brain. When stimulated with low current via small electrodes, drinking was elicited from the zona incerta (ZI) feeding from the lateral hypothalamus (LH), and both responses from a restricted portion of the LH dorsolateral to the fornix. With stimulation of sites outside these regions consummatory responses occurred unreliably. Discrete lesions were made at sites of stimulation and resulting axonal degeneration was stained by the Fink and Heimer technique. Although similar connections existed, the main degeneration from drinking sites in the ZI differed from that arising from feeding sites in LH; degeneration from drinking sites coursed in the ZI and in other structures including the nucleus reuniens, ventromedial thalamus, the region ventral to the globus pallidus and the region near the subcommissural organ; degeneration from feeding sites in the LH coursed in the medial forebrain bundle and in the pathways that extend from it. Degeneration from sites dorsolateral to the fornix where both feeding and drinking were elicited was similar to that from feeding sites. These anatomical differences and similarities suggest the existence of two neural systems for drinking, one being closely related to the system for feeding and the other being independent.