Ventrobasal Thalamus Research Articles

Electrical activity of the reticular formation during aversive and appetitive conditioning in rats

Evoked potentials (EP) to a conditioned stimulus (CS) were recorded from several parts of the reticular formation (RF), the midbrain central gray (CG), the occipital cortex (OC), the basolateral amygdala and the ventrobasal thalamus during appetitive and aversive classical conditionings in rats. In addition, single unit activity to rewarding CS (+) and punishing CS (-) in a single discriminative classical conditioning was recorded from RF, OC, CG and the tectum. In the EP study, peak-to-peak (p-p) amplitudes, especially of the pontine RF, increased in the initial phase of pre-conditioning, in aversive conditionin, and in appetitive conditioning but not in aversive pseudo-conditioning. The p-p amplitude in the pontine RF might reflect an amount of stimulus value the animal received. Slow negative potentials after CS presentation in RF was a reflection on body movements, indicating heavy involvement of RF in motor functions. In the single unit studies, cells which selectively responded to the CS(-) were more frequently found in the medial tegmental area close to the median raphe nucleus as compared to other areas. These results were considered to indicate high sensitivity of RF to the significance of the stimulus, and also the involvement of the medial tegmental area close to the median raphe nucleus in aversive emotional processes. Although many studies employing stimulation or lesion techniques have reported the steady relation between aversive emotional responses and CG, no supporting result was obtained in the present experiments.

Anatomical Studies of the Spinocervical Tract of the Rat

The retrograde transport of horseradish peroxidase (HRP) was used to identify and examine the cells of origin of the spinocervical tract (SCt) in the rat. Initially, precise data on the boundaries of the rat lateral cervical nucleus (LCn) were gathered after injecting HRP into the ventrobasal thalamus. These data indicated that the LCn of the rat is restricted to a region on the extreme lateral edge of the dorsalmost portion of the lateral funiculus (DLf) within spinal segment C2. Following small iontophoretic injections of HRP that were restricted to this area, labeled SCt neurons were found in the ipsilateral nucleus proprius at all levels of the spinal cord but were most numerous in the cervical enlargement. Lesion studies indicated that the overwhelming majority of SCt axons ascend to the LCn within the DLf. In an attempt to determine whether our injection techniques labeled a significant number of cells through axons of passage, HRP injections were made in the DLf ventral to the LCn. Such injections labeled, presumably through axons of passage, cells in several areas of the spinal cord gray matter, including a large number in the contralateral marginal zone. Injections in areas immediately rostral to the LCn labeled 20% or less of the total number of cells within the enlargements that were labeled by injections into the LCn. Thus, the majority of cells labeled by injections of HRP into the LCn were labeled through preterminal fibers or terminals themselves. The cells of origin of the SCt in the rat are similar in location to those in the cat but far fewer in number.

Interhemispheric nigrothalamic projections and behavioral recovery following turning behavior induced by unilateral peripheral sensory and motor restriction

We investigated the relationship between unilateral peripheral lesions plus partial sensory deprivation and interhemispheric nigrothalamic projections. Sensory-motor asymmetries were induced in rats by unilateral ligature of sensory and motor nerves of the forelimbs and hind limbs and partial sensory deprivation (cutting the vibrissae unilaterally and suturing one eyelid). This operation resulted in spontaneous turning which was partially compensated for within the first 5 postoperative days. Sixteen days after the surgery, animals were implanted with horseradish peroxidase in the ventrobasal thalamus of one hemisphere. Brains were processed by the tetramethylbenzidine procedure. Microscopical analysis revealed interhemispheric nigrothalamic projections from the substantia nigra situated ipsilateral but not contralateral to the denervated limbs. The results are interpreted in terms of a reorganization (sprouting) of interhemispheric nigrothalamic projections in response to unilateral peripheral lesions. The lesion-induced behavioral asymmetry and its partial compensation may be one of the determinants of the observed neuronal plasticity.

Diencephalic neuronal activity during acquisition and ingestion of food in sheep

Single unit recordings were made in the lateral hypothalamic region and ventrobasal thalamus in conscious sheep during presentation of food and feeding. Unit responses to all aspects of testing were observed between the two areas. However, analysis of the data revealed a functional division of unit types between the two regions. Unit responses specific to the sight of food, to the approach of food to the mouth, and during ingestion of food were observed in the lateral hypothalamic area, but not the ventrobasal thalamus. These responses were not associated with specific or generalized movements, non-specific arousal, with olfactory stimulation, or with other oral stimuli. Units responding in association with specific mouth movements or somatosensory stimulation of the face or mouth-parts were observed in the ventrobasal thalamus but not in the lateral hypothalamic area. In the case of movement-related activity, the response was independent of the stimulus inducing the movement, food and non-food stimuli being equally effective. This was also true of the somatosensory responses observed. Two possibilities in relation to this functional division are discussed. Firstly, that the ventrobasal thalamus receives oral sensory afferents which may be associated with the thalamic area for taste. Secondly, that the activity of hypothalamic cells recorded in the sheep is consistent with a role for this area of the brain in the guidance and control of ingestive behaviour in ruminant as well as non-ruminant species.

Acetylcholinesterase in the ventrobasal thalamus: Transience and patterning during ontogenesis

In this study, maturational alterations in acetylcholinesterase-dependent staining of the thalamic ventrobasal complex of rat and mouse were examined. The study was undertaken to address the question of whether this nucleus exhibits transient acetylcholinesterase positivity during its development and whether the enzyme is likely to be synthesized by its immature intrinsic neurons. Also, the patterning due to acetylcholinesterase staining of cells and fibers, and the developmental changes in these patterns have not been described in earlier work. In contrast to surrounding thalamic nuclei, the ventrobasal complex is acetylcholinesterase-positive at birth. In rat, acetylcholinesterase staining of the ventrobasal thalamus is still more intense than in adjacent nuclei at the end of the first week postnatally. Virtually all somata in the nucleus are filled with dense reaction product at this time. Ultrastructurally, reaction product is associated with the granular endoplasmic reticulum. At this stage, there is a marked difference in intensity of staining between the medial and lateral subdivisions of the nucleus, and patterned clustering of somata within each subdivision is readily appreciated in acetylcholinesterase-stained material. In the second postnatal week, intrinsic acetylcholinesterase activity is progressively lost. By the end of the third postnatal week, the nucleus is quite pale except for one area. In the posterior portion of the lateral subdivision, adjacent to the nucleus reticularis, interconnecting bundles of acetylcholinesterase-positve fibers enter the nucleus. They course medially in the lateral subdivision and break up into a plexus of fine fibers. The development of acetylcholinesterase-dependent staining patterns in the mouse is quite similar except that histochemically detectable levels of enzyme are substantially lower in the neonatal period. It is concluded that the ventrobasal complex can be distinguished from other thalamic nuclei in regard to earlier onset and/or transience of acetylcholinesterase staining. Ultrastructural observations suggest that virtually all immature ventrobasal neurons are synthesizing acetylcholinesterase. It is suggested that the transient staining for enzyme is due primarily to alteration in synthesis and/or turnover in neurons of the ventrobasal complex. In addition, the acetylcholinesterase staining reveals a patterning of fibers and cells that also undergoes developmental alteration. Evidence is discussed suggesting that axons in the barrels of somatosensory cortex (SmI) are derived from these transiently acetylcholinesterase-positive somata. Consequently, the loss of acetylcholinesterase fiber staining in the barrels, during the third postnatal week (noted previously), may be related to a decrease in synthesis of enzyme in the neuronal somata of the ventrobasal complex.

Effect of baclofen on sensory transmission through the ventrobasal thalamic nucleus of the rat

Previous work suggests that after administration of baclofen there is a reduction in excitatory cuneothalamic transmission, while preliminary experiments in this laboratory had shown that baclofen produces lowered level of cortical activity. Since much of the corticothalamic output is inhibitory, the consequences of possible reductions in excitatory corticopetal and inhibitory corticofugal flow into the ventrobasal thalamus required investigation. Baclofen produced increased latency and decreased probability of discharge of the short-latency discharge of single ventrobasal thalamic units to peripheral stimulation. It is suggested that these observations are compatible with a decreased excitatory cuneothalamic volley. Accompanying the decline in the early discharge was a development of later rhythmic discharges at 80–100 msec intervals, lasting up to 900 msec. This type of response is also seen in decorticate rats and has the same temporal pattern as the inhibition of the response to the second of a pair of peripheral stimuli shown in rats with an intact cerebral cortex. Since baclofen produced a decline in cortical activity (N 1 wave of evoked potential, synchronisation of the ECoG), is is suggested that this emergence of rhythmic activity is produced by a reduction of corticothalamic inhibition.

Reorganisation of nigro-thalamic projections and compensation of behavioral asymmetries following unilateral paralysis and unilateral sensory deprivation

We investigated the relationship between unilateral peripheral lesions plus partial sensory deprivation and interhemispheric nigrothalamic projections. Sensory-motor asymmetries were induced in rats by unilateral ligature of sensory and motor nerves of the forelimbs and hind limbs and partial sensory deprivation (cutting the vibrissae unilaterally and suturing one eyelid). This operation resulted in spontaneous turning which was partially compensated for within the first 5 postoperative days. Sixteen days after the surgery, animals were implanted with horseradish peroxidase in the ventrobasal thalamus of one hemisphere. Brains were processed by the tetramethylbenzidine procedure. Microscopical analysis revealed interhemispheric nigrothalamic projections from the substantia nigra situated ipsilateral but not contralateral to the denervated limbs. The results are interpreted in terms of a reorganization (sprouting) of interhemispheric nigrothalamic projections in response to unilateral peripheral lesions. The lesion-induced behavioral asymmetry and its partial compensation may be one of the determinants of the observed neuronal plasticity.

Spinal and early scalp-recorded components of the somatosensory evoked potential following stimulation of the posterior tibial nerve

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the posterior tibial nerve (PTN) in 12 normal adults. Recording using both cephalic and non-cephalic references were obtained from multiple electrodes placed over the spine and scalp. Following PTN stimulation, the fastest recorded potentials of the afferent sensory volley proceeds up the spinal cord at constant velocity. After arrival of the volley at cervical cord levels, 3 widely distributed waves, P 28, P 31 and N 34, are recorded from scalp electrodes. These ‘far-field’ potentials are followed by a localized positivity (P 38) which has a peak voltage either at the vertex or just laterally toward the side of stimulation. A contralateral negativity (N 38) was present in most individuals. We propose that P 28 arises from medial lemniscus; that P 31 is generated by ventrobasal thalamus; and that N 34 is probably the result of further activity in thalamus and/or thalamocortical radiations. The P 38/N 38 complex represents the primary cortical response to PTN stimulation. Its most consistent characteristic is a positivity at the vertex or immediately adjacent scalp areas ipsilateral to the stimulated leg. The topography of the P 38/N 38 potential varies slightly from individual to individual in a manner consistent with a functional dipole situated in the leg and foot area on the mesial aspect of the post-central gyrus, whose exact location and orientation changes in accordance with known variations in the location of the leg area.

Quantitative histological study on the thalamic ventro-basal complex of the cat. Numerical aspects of the transfer from medial lemniscal fibers to cortical relay.

A quantitative analysis was carried out on the thalamic ventrobasal (VB) complex of the cat. The following numerical and metrical parameters of the neuronal elements (cells and fibers) were determined: 1. Volume of nucleus: 27.38 mm3 2. Total number of neurons: 243,000 3. Total number of fibers of medial lemniscus: 26,000 4. Volume of arborization space of one lemniscal fiber: 2.26 X 10(6) micrometers 3. Numerical data of relay neurons and lemniscal fibers and their relations as basic factors in the estimation of the degree of divergence and convergence of lemniscal input were calculated and compared. It was found that the probable degree of convergence is four-fold (1-4) and of divergence 27-fold (1-27) with regard to the relationship of fibers and cells in the VB. The quantitative data obtained in the VB and our considerations on convergence and divergence were compared with analogous values obtained for the lateral geniculate body LGB. The differences between the two sensory relay nuclei reflect differences in their modes of impulse transmission.

Diffuse noxious inhibitory controls (DNIC) involve trigeminothalamic and spinothalamic neurones in the rat.

Fifty-eight lumbar dorsal horn and trigeminal nucleus caudalis neurones which could be activated by both innocuous and noxious peripheral stimuli have been recorded in the anaesthetized rat. Using transcutaneous electrical stimulation to produce A and C fibre activity in these neurones from the hindpaw or facial receptive fields the ability of a distant noxious (mechanical or thermal) stimulus applied to the nose, tail, ears and paws to inhibit the neuronal activity was demonstrated. These effects have been termed diffuse noxious inhibitory controls (DNIC). DNIC produced powerful long-lasting inhibitions on all units studied in accordance with our previous results. Approximately 40% of these convergent neurones could be antidromically activated from the contralateral ventrobasal thalamus. Similar neuronal characteristics, effects of DNIC and proportions of projection cells were found in both the dorsal horn and trigeminal complex. However, the spinothalamic tract cells conducted more rapidly than the trigeminothalamic neurones. These results indicate that DNIC can produce comparable effects on the thalamic representation of the efferent activity of these spinal cord and trigeminal neurones. The possible role of DNIC in nociception is discussed.

Effects of morphine and indomethacin on single unit responses in the ventrobasal (VB) thalamus in adjuvant arthritic rats

Rats rendered polyarthritic by injection of Mycobacterium butyricum into the tail were used as a model of chronic pain. (1) The activities of 154 neurones in the VB thalamus have been recorded in adjuvant arthritic rats anaesthetized with urethane (1200mg/kg i.p.). The nature of neurones which had receptive fields located in inflamed areas of hind paw (115/154 in 48 rats) were studied. 16 neurones were activated only by non-noxious stimuli (nN neurone) such as brushing or tapping and displayed the classical properties of leminiscal responses. 92 neurones were activated only by noxious stimuli (N neurone) such as tibial joint movement induced by electrical stimulation (1 Hz, 5msec, 0.5-8mA and train 30) to inflamed areas of hind paw. A relatively large number of N neurones (28/92) displayed spontaneous paroxysmal discharge. 8 neurones responded to both noxious and non-noxious stimuli (NnN neurone). All these neurones displayed long lasting afterdischarge (2-12 times of duration of stimulation). 12 N neurones were stimulated with the intensity of 0.5 to 8mA. In 8/12 neurones, these discharge rates increased linearly with intensity, although the rates of the remaining 4/12 reached a plateau at an intensity of 4 to 6mA. (2) The anti-nociceptive action of morphine and indomethacin on evoked neuronal responses of the 62 VB thalamus N neurones were investigated. Morphine lmg/kg, i.v., indomethacin lmg/kg i.v. and indomethacin 3ug/rat i.vent., produced marked depressive actions on the VB neuronal responses to joint movement induced by electrical stimulation. Spontaneous activity was not significantly modified by these drugs.

A quantitative study of the thalamocortical and other synapses in layer IV of pyramidal cells projecting from mouse SmI cortex to the caudate-putamen nucleus.

The thalamocortical and other synapses of the apical dendrites of corticostriatal projection neurons in mouse primary somatosensory cortex (SmI) were examined by combining anterograde degeneration with the retrograde transport of horseradish peroxidase (HRP). Electrolytic lesions were made in the ventrobasal thalamus, followed 3 days later by injections of 40% HRP into the ipsilateral caudate-putamen nucleus. The next day, the mice were perfused and the SmI cortex ipsilateral to the lesion and injection sites was chopped at 125-micrometer and reacted for HRP using a CoCl2-DAB method. HRP-labeled corticostriatal cells in SmI cortex were medium-sized pyramidal cells, having somata located in the superficial portion of layer V and apical dendrites extending into layer I. Seven corticostriatal cells were serially thin sectioned and the layer IV portions of their apical dendrites were reconstructed. Each apical dendrite formed only one or two thalamocortical synapses (0.3 to 0.9% of their synapses in layer IV) indicating that corticostriatal neurons may be minimally responsive to direct synaptic input from the specific thalamic nuclei. Each apical dendrite formed about 12.6 asymmetrical synapses for every symmetrical synapse, suggesting that the relative numbers of excitatory and inhibitory synapses impinging on apical dendrites belonging to an individual class of neurons may be specified.

Effects of systemic naloxone upon ventrobasal thalamus neuronal responses in arthritic rats

This study deals with the effect of various doses of systemic naloxone (10 μg, 300 μg, 1 mg/kg) upon activities of 21 ventrobasa thalamus neurons recorded in 20 rats rendered arthritic by injection of Freund's adjuvant into the tail. These neurons presented reproducible responses to movement and/or mild lateral pressure on a joint and a joint and were recorded for at least 30 min after naloxone administration. Several neurons (5) were tested with two doses. After intravenous injection of naloxone at the dose of 10 μg/kg (10 cases) there was a rapid decrease of the responses. The maximum effect occurred at 15 min when the mean value expressed as a percentage of the control was 46.20 ± 8.51% (n= 10, P < 0.001). Recovery could be considered as complete at 30 min. At the dose of 300 μg/kg (9 cases), the decrease in the responses was less important, variable from one neuron to another but significant between 5 and 20 min (mean= 67.43 ± 9.00%at20min, n= 7, P < 0.01). At the dose of 1 mg/kg (7 cases), there was no significant modification of the response. Spontaneous firing rate of the neurons was slightly significantly increased after injection of the two highest doses and unmodified after the lowest. The relationship between the depressive effect produced by low doses of naloxone upon the neuronal responses, and the ‘bi-directional’ analgesic-hyperalgesic action of the drug, demonstrated in these suffering rats, is discussed.

Excitatory effect of acetylcholine on different types of neurons in the first somatosensory neocortex of the rat: Laminar distribution and pharmacological characteristics

In rats anaesthetized with either urethane, pentobarbital or fluothane the effects of acetylcholine, cholinergic agonists and antagonists (applied by iontophoresis) were studied on single cortical neurons of first somatosensory region. The laminar distribution of the neurons excited by acetylcholine was determined by the reconstruction of each electrode track based on a dye deposit made at the last recording site. Neurons were identified using antidromic stimulation of the pyramidal tract, the ventrobasal thalamus and the corpus callosum. Neurons excited by acetylcholine could be segregated into two groups: one encompassing layer Vb and the upper part of layer VI, the other more deeply located at the limit between the cerebral cortex and the subjacent white matter. Neuronal responses to glutamate and nicotine, unlike those to acetylcholine were evenly distributed in the cortex. Pyramidal tract neurons and corticothalamic neurons were frequently excited by acetylcholine and were shown to be located with the first group of acetylcholine sensitive neurons. Commissural neurons were rarely excited by acetylcholine and were not restricted to either group. The analysis of neuronal responses to acetylcholine and various agonists (carbachol, nicotine, acetyl-β-methylcholine, carbamyl-β-methylcholine, butyrylcholine) and antagonists (atropine, mecamylamine) revealed a prominent but not exclusive muscarinic character. It is concluded (i) that cortical neurons of first somatosensory cortex which are excited by acetylcholine belong to two populations, one consisting, at least in part, of projection neurons (upper group) and the other of interneurons (lower group); (ii) that cortical acetylcholine receptors are of a ‘mixed’ type strongly weighted toward the muscarinic side.

Aspirin clearly depresses responses of ventrobasal thalamus neurons to joint stimuli in arthritic rats.

This study dealt with the effect of aspirin upon activities of 17 ventrobasal thalamic neurons recorded in 17 rats rendered arthritic by injection of Freund's adjuvant into the tail. These neurons presented reproducible responses to mobilization and/or mild lateral pressure on a joint and were recorded for at least 40 min after aspirin administration. After intravenous injection of aspirin at the dose of 50 mg/kg (13 neurons tested), there was a progressive decrease in the number of spikes in the discharges. The maximum effect occurred at 30 min where the mean value of the response expressed as a percentage of the control was m = 34.62 +/- 7.5% (n = 13, p less than or equal to 0.001). Recovery was progressive and could be considered as complete at 60 min. By contrast, no significant modification of the spontaneous firing has been observed. With lower doses of aspirin (12.5 or 25 mg/kg tested with 4 neurons) there was respectively no clear depressive effect or only a transient decrease of the response.

The effects of nucleus raphe magnus lesions on an ascending thermal pathway in the rat.

1. In the thalamus and hypothalamus of rats, anaesthetized with Urethane, single unit recordings have been made from cells which respond to small innocuous changes in scrotal skin temperature applied with a water-perfused brass thermode. 2. Once a scrotal temperature-sensitive neurone had been isolated the brain stem was electrolytically lesioned through implanted tungsten electrodes to determine whether the input from the scrotal skin temperature sensors ascends through the brain stem lemniscal pathways or the mid-line raphe nuclei. All recordings sites and lesions were identified histologically. 3. Thirty-six neurones have been studied of which half were located in the ventrobasal thalamus, six were located in the anterior thalamic nuclei and the remainder were in the medial hypothalamus. 4. The nucleus raphe magnus was lesioned on eighteen separate occasions; in each case the temperature-responsive activity of the thalamic or hypothalamic neurone was abolished. 5. Extensive brain-stem lesions which spared only the mid-line nucleus raphe magnus had no discernible effect on the responses of the thalamic or hypothalamic neurones to scrotal skin temperature. 6. The ascending pathway from the thermal sensors of the rat scrotal skin must pass through, or relay in, the nucleus raphe magnus.

Special senses are really special: Evidence for a reciprocal, bilateral pathway between insular cortex and nucleus parabrachialis

Nucleus parabrachialis (NPB) is implicated in gustation and a wide range of visceral and homeostatic functions. The nucleus was recently shown to be the major relay for second order gustatory and general visceral afferent (GVA) pathways from the nucleus of the solitary tract (NTS) to the gustatory-GVA division of the ventrobasal thalamus (VBcm) via the parabrachial nucleus (NPB). VBcm projects to the primary cortical sensory area for these modalities in the insular cortex (IC). In the present report anterograde and retrograde transport of either HRP or wheat germ agglutinin conjugated to HRP (WGA-HRP) revealed a strong, direct, reciprocal connection between NPB and IC. Inputs from and outputs to the contralaterai NPB were also seen. Thus, in addition to the thalamo-cortical (VBcm → IC) ascending gustatory-GVA pathway, there is a more direct route to primary cortex from third order neurons in NPB. With the exception of olfaction, all other ascending lemniscal sensory systems have an obligatory synaptic relay in the thalamus en route to their primary cortex. The present study demonstrates that the gustatory-GVA system has both; information from these modalities may reach their cortical area via a thalamic relay and via direct projections from NPB. Moreover, both olfactory and gustatory-GVA pathways impinge upon limbic structures without a thalamo-cortical relay. In these regards, the special senses—olfaction, gustation and general visceral afference—are really special. The strong, reciprocal projection from IC to NPB compliments our previous description [40] of a bilateral pathway from this same cortical area to NTS and several brainstem visceromotor nuclei. Taken together, these findings suggest that there is a well-organized cortical substrate for gustatory and visceral afference which may have a functionally important descending influence on both the afferent and motoneuronal aspects of gustatory-autonomic brainstem mechanisms.

A quantitative study of thalamocortical and other synapses involving the apical dendrites of corticothalamic projection cells in mouse SmI cortex.

Lesion-induced degeneration was combined with the retrograde transport of horseradish peroxidase (HRP) to examine the thalamocortical and other synaptical relationships of corticothalamic projection cells in mouse SmI cortex. Injections containing 40% HRP were placed in the ventrobasal thalamus and the next day, electrolytic lesions were made of the injection site. About four days later, the animals were perfused with aldehydes and SmI cortex ipsilateral to the injection and lesion sites was tissue chopped and reacted for HRP. The somata of HRP-filled corticothalamic cells in SmI cortex had diameters of about 10 micrometers and occurred in the upper half of layer VI and in the lower half of layer V; their apical dendrites usually terminated within or just below layer IV, but in some instances, extended nearly to the pial surface. Reconstructions of serial thin sections through the layer IV portions of the apical dendrites of seven well-labelled corticothalamic cells showed them to form 7 to 20% of their synapses with degenerating thalamocortical axon terminals. In contrast, the proportion of thalamocortical synapses formed by the layer IV dendrites of corticostriatal (Hersch & White, in preparation) and corticocortical (White & Hersch, 1981) projection cells ranges from 0.3 to 0.9% and from 1.5 to 6.8%, respectively. We interpret these findings to indicate that pyramidal cell dendrites are specified to form characteristic proportions of thalamocortical synapses.

The origin of the acetylcholinesterase-rich afferents to layer IV of infant somatosensory cortex. A histochemical analysis following lesions.

In this histochemical study, possible origins of the acetylcholinesterase-rich inputs to the barrel field of somatosensory cortex (SmI) were examined. Rats were studied between 6 and 16 days of age when acetylcholinesterase (AChE) positivity is readily detectable in the barrel centers. The experiments involved making lesions in either the ventrobasal thalamus or midbrain and noting the effects on AChE staining in SmI. Also, the results of destroying the large mystacial vibrissae at birth and 6 days of age were histochemically evaluated. Thalamic lesions eliminated most of the AChE positivity in layer IV but midbrain hemisection had no detectable effect. Destruction of the contralateral vibrissae, at birth, altered the pattern of AChE staining of the barrel field. Vibrissal damage at 6 days of age did not result in a noticeable alteration in staining of the barrel centers for AChE. We conclude that the AChE-positive barrel afferents are likely to originate from the ventrobasal complex.

Visceral afferent and efferent connections in the brain.

Behavioral and physiological studies have revealed increasing evidence in thirst, hunger, energy and water metabolism for the importance of visceral input to the brain as well as central modulation of visceral events. Recent work in our laboratory has now revealed some new information about the central course of these afferent and efferent systems. From our work and others there is now good evidence that the bed of the hepatic-portal system contains sodium and osmoreceptors. Our electrophysiological and histochemical studies show that one branch overlaps the gustatory pathway. That is, involves the vagus nerve, nucleus tractus solitarius, parabrachial nucleus and ventro-basal thalamus (VBC). Another branch goes not to VBC but to the supra-optic nucleus. At the one level so far investigated clear evidence was obtained for convergence on single cells of visceral and gustatory inputs. It may well occur at other levels. A parallel and perhaps interacting efferent system more related to energy regulation has also been explored. The major parasympathetic output to the viscera are the axons of the brain stem dorsal-motor nucleus (DMV) forming the vagus nerves. We have shown that the paraventricular nucleus, which was known to project to DMV, is the only structure above the brain stem that has a direct input to DMV. However, since the nucleus of the solitary tract and reticular formation send fibers to the DMV the neural substrate exists for considerable indirect input. The significance of these results is that it provides a central neural substrate of the 2-way pathways linking brain and viscera. Thus, we now know more about neural control mechanisms involved in several motivational and metabolic systems. The close contiguity and even convergence of gustatory and visceral afferents may help to explain taste-aversion learning and alterations in the hedonics of taste under different nutritional states. The unique place of the paraventricular nucleus in relation to parasympathetic efferents will doubtless lead to new knowledge of the role of the brain in control of energy metabolism.