Role of sensory stimuli in the formation of cortical effects on hypothalamic structures

Unit discharges were recorded in the ventromedial and posterior hypothalamic nuclei of immobilized cats during electrical stimulation of the ipsilateral and contralateral nerves of the brachial plexus, somatosensory areas I and II (SI and SII) of the visual cortex, and the mesencephalic reticular formation. Reversible cold block of areas SI and SII did not change the effects of nerve stimulation. Stimulation of the posteroventral thalamic nucleus did not change spontaneous activity during stimulation of SII. Cortical influences on hypothalamic structures arising during stimulation of nerves and the posteroventral thalamic nucleus are thus mild in degree.     

Mechanisms of brain-stem activation of the neocortex and motor reflexes in chronic premesencephalic cats

To study the role of brain-stem structures in activation of the neocortex and motor reflexes chronic experiments were carried out on 18 cats. After premesencephalic section of the brain stem neocortical activation arises both spontaneously and in response to stimulation of the posterior hypothalamus or skin of the forelimb. After bilateral destruction of the posterior hypothalamus of these animals neocortical activation developed neither spontaneously nor in response to stimulation of the skin of the forelimb. Excitation of the mesencephalic reticular formation was accompanied by activation of motor reflexes. It is concluded that the posterior hypothalamus, independently of the mesencephalic reticular formation, can activate the neocortex, and that the mesencephalic reticular formation, independently of structures located rostrally, can activate motor reflexes.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 62, No. 1, pp. 22–28, January, 1976.     

Immunocytochemical localization of corticotropin-releasing factor in the brain of the turtle, Mauremys caspica

Brain sections of the turtle, Mauremys caspica were studied by means of an antiserum against rat corticotropin-releasing factor. Immunoreactive neurons were identified in telencephalic, diencephalic and mesencephalic areas such as the cortex, nucleus caudatus, nucleus accumbens, amygdala, subfornical organ, paraventricular nucleus, hypothalamic dorsolateral aggregation, nucleus of the paraventricular organ, infundibular nucleus, pretectal nucleus, periventricular grey, reticular formation and nucleus of the raphe. Many immunoreactive cells located near the ependyma were bipolar, having an apical dendrite that contacted the cerebrospinal fluid. Immunoreactive fibers were seen in these locations and in the lamina terminalis, lateral forebrain bundle, supraoptic nucleus, median eminence, neurohypophysis, tectum opticum, torus semicircularis and deep mesencephalic nucleus. Parvocellular bipolar immunoreactive neurons from the paraventricular and infundibular nuclei projected axons that joined the hypothalamo-hypophysial tract and reached the outer zone of median eminence, and the neural lobe of the hypophysis where immunoreactive fibers terminated close to intermediate lobe cells. From these results it can be concluded that, as in other vertebrates, corticotropin-releasing factor in the turtle may act as a releasing factor and, centrally, as a neurotransmitter or neuromodulator.     

Differential effects of medial forebrain bundle lesions on adrenocortical responses following limbic stimulation

Adult male rats had electrolytic lesions placed bilaterally in the medial forebrain bundle and were subsequently implanted with stimulating electrodes in one of the following limbic regions: (1) dorsal hippocampus; (2) ventral hippocampus; (3) medial septal nucleus; (4) basolateral amygdala; (5) mesencephalic reticular formation. Following electrical stimulation, blood was drawn by acute venesection, under either, for plasma corticosterone determinations. In non-lesioned animals, electrical stimulation in all of the limbic regions led to elevated plasma corticosterone levels. In rats with lesions in the medial forebrain bundles, the adrenocortical response to stimulation in the dorsal hippocampus, the basolateral amygdala or the reticular formation was markedly attenuated. On the contrary, the same lesions were without effect upon the corticosterone secretory response to medial septal stimulation, and had only a slight inhibitory effect upon the response to electrical stimulation in the ventral hippocampus. The results demonstrate that the medial forebrain bundle plays a major role in the transmission of impulses to the mediobasal hypothalamus, originating in the dorsal hippocampus, basolateral amygdala or mesencephalic reticular formation, which activate adrenocortical secretion; its role in the transmission of cues arising in the ventral hippocampus or medial septum is, however, minor.     

Experimental model of a secondary pacemaker center of feeding motivation

The possibility of simulating the functions of the feeding motivation pacemaker by various structures of the limbico-reticular complex was studied in chronic experiments on rabbits. In response to combined stimulation of the feeding center of the lateral hypopthalamus and various formations of the limbico-reticular complex, a secondary pacemaker of feeding motivation excitation was shown to be formed only in the mesencephalic reticular formation.P. K. Anokhin Research Institute of Normal Physiology, Academy of Medical Sciences of the USSR. Department of Normal Physiology, I. M. Sechenov First Moscow Medical Institute. Institute of Experimental Medicine, Academy of Medical Sciences of the USSR, Moscow. Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 86, No. 11, pp. 515–517, November, 1978.     

Afferent and efferent connections of brainstem locomotor regions: study by means of horseradish peroxidase transport technique

Afferent and efferent connections of the hypothalamic and mesencephalic locomotor regions and also the bulbar locomotor strip were studied in cat using retrograde (horseradish peroxidase) transport technique. To study the sources of afferent projections, the enzyme microinjections were performed exactly into the same brain sites eliciting treadmill locomotion by means of electrical stimulation. When studying efferent projections horseradish peroxidase labeled neurons were revealed within locomotor regions after enzyme microinjections into different brain structures. Experimental data have shown that the hypothalamic and mesencephalic locomotor regions have mutual afferent and efferent projections with numerous brain areas including interconnections. Apart from the entopeduncular nucleus, the great number of different sensory nuclei are noted: among the sources of afferent projections are the nucleus tractus spinalis nervi trigemini, nucleus cuneatus, nucleus tractus solitarius and vestibular nuclei. In addition, after horseradish peroxidase injection into the mesencephalic locomotor region labeled neurons were found in the cochlear nuclei. Direct descending neuronal projections of the hypothalamic and mesencephalic locomotor regions are distributed mainly in the ipsilateral brainstem. Only a few of them reach the lumbar spinal cord. The most marked efferent projections of given regions are those to the brainstem reticular formation. After horseradish peroxidase injection into a functionally identified bulbar locomotor strip, labeled neurons were revealed in different stem regions mainly caudal to the enzyme injection site. The existence of a locomotor strip as an independent structural formation is called into question. When studying the locomotor region connections, the structural heterogeneity of these regions is revealed. Transitory fibers of ascending tracts are presumably within their limits side by side with neurons. The role of these fibers in stepping initiation by electrical stimulation of locomotor regions remains uncertain.     

Afferent subcortical connections into the motor cortical larynx area in the rhesus monkey

In three rhesus monkeys (Macaca mulatta), the inferior motor cortex was explored by electrical stimulation for sites yielding vocal fold adduction. The retrograde tracer wheat germ-agglutinin-conjugated horseradish peroxidase was injected into the effective sites. Within the forebrain, retrogradely labeled cells were found in the claustrum, basal nucleus of Meynert, substantia innominata, extended amygdala, lateral and posterior hypothalamic area, field H of Forel, and a number of thalamic nuclei with the strongest labeling in the nuclei ventralis lateralis, ventralis posteromedialis, including its parvocellular part, medialis dorsalis and centrum medianum, and weaker labeling in the nuclei ventralis anterior, ventralis posterolateralis, intermediodorsalis, paracentralis, parafascicularis and pulvinaris anterior. In the midbrain, labeling was found in the deep mesencephalic nucleus, ventral tegmental area, and substantia nigra. In the lower brainstem, labeled cells were found in the pontine reticular formation, median and dorsal raphe nuclei, medial parabrachial nucleus, and locus coeruleus. The findings are discussed in terms of the possible role of these structures in voluntary vocal control.     

Influence of the posterior hypothalamus on the visual cortex in various states of the reticular formation

It has been demonstrated in chronic experiments on wakeful rabbits that the stimulation of the posterior hypothalamus by a single electrical stimulus leads to the formation in the visual cortex of a short-latency response which exerts a substantial influence on the formation of the reaction to light stimulation. Depending upon the intervals between the hypothalamic and light stimuli, an initial suppression of the response is observed (1–15 msec), a subsequent selective facilitation of its positive component in the presence of the simultaneous suppression of the negative (20–100 msec), and the complete recovery of the response (200–300 msec). Aminazine and amizil do not alter the directionality of the influence of the stimulation of the posterior hypothalamus on the responses of the visual cortex; however, they do significantly attenuate the degree of expressivity and the dynamics of this influence. Experiments involving local foci in the mesencephalic reticular formation (strychnine, KCl) attest to the inhibitory influence of the latter on the activity of the hypothalamocortical input. The role of the phasic mechanism of hypothalamic control in the realization of the perceptual function of the visual cortex is considered.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 76, No. 8, pp. 1001–1009, August, 1990.     

Afferent and efferent connections of the ventrolateral tegmental area in the rat

 The present study examined the organization of afferent and efferent connections of the rat ventrolateral tegmental area (VLTg) by employing the retrograde and anterograde axonal transport of Fluorogold and Phaseolus vulgaris-leucoagglutinin, respectively. Our interest was focused on whether the anatomical connections of the VLTg would provide evidence as to the involvement of this reticular area in audiomotor behavior. Our retrograde experiments revealed that minor inputs to the VLTg arise in various telencephalic structures, including the cerebral cortex. Stronger projections originate in the lateral preoptic area, the zona incerta, the nucleus of the posterior commissure and some other thalamic areas, the lateral substantia nigra, the deep layers of the superior colliculus, the dorsal and lateral central gray, the deep mesencephalic nucleus, the paralemniscal zone, the intercollicular nucleus, the external cortex of the inferior colliculus, the oral and caudal pontine reticular nucleus, the deep cerebellar nuclei, the gigantocellular and lateral paragigantocellular reticular nuclei, the prepositus hypoglossal nucleus, the spinal trigeminal nuclei, and the intermediate layers of the spinal cord. Most importantly, we disclosed strong auditory afferents arising in the dorsal and ventral cochlear nuclei and in the cochlear root nucleus. The efferent projections of the VLTg were found to be less widespread. Telencephalic structures do not receive any input from the VLTg. Moderate projections were seen to diencephalic reticular areas, the zona incerta, the nucleus of the posterior commissure, and to various other thalamic areas. The major VLTg projections terminate in the deep layers of the superior colliculus, the deep mesencephalic nucleus, the intercollicular nucleus and external cortex of the inferior colliculus, the oral and caudal pontine reticular nucleus, the gigantocellular and lateral paragigantocellular reticular nuclei, and in the medial column of the facial nucleus. From our data, we conclude that the VLTg might play a role in sensorimotor behavior. Accepted: 3 April 1997     

New locomotor regions of the brainstem revealed by means of electrical stimulation

New sites in the brainstem eliciting treadmill locomotion have been revealed in decerebrated cats by electrical stimulation. These are the cochlear nuclei, cuneate nucleus, spinocerebellar tracts, and substantia grisea centralis at the level of red nuclei which lie outside the known locomotor regions. Participation of neurons and fibers forming ascending sensory tracts (medial and lateral lemniscus) is of special interest in initiation of locomotion. Collaterals of these tracts pass through the hypothalamic and mesencephalic locomotor regions and may contribute largely to initiation of locomotor generators. Hypotheses about the leading role of non-specific afferent activation of the brainstem reticular formation in initiation of locomotion are put forward.     

Somatosensory systems and the milk-ejection reflex in the rat. I. Lesions of the mesencephalic lateral tegmentum disrupt the reflex and damage mesencephalic somatosensory connections

Bilateral electrolytic lesions and unilateral tracer injections were performed in lactating rats in order to study the participation of the mesencephalic lateral tegmentum in the milk-ejection reflex. The release of oxytocin was detected as a rise in intramammary pressure during each milk ejection. In animals with lesions, the lateral part of the deep grey layers of the superior colliculus, the intercollicular area and the rostromedial portion of the external nucleus of the inferior colliculus were destroyed. The mesencephalic lateral tegmentum of animals in which the milk-ejection reflex was blocked sustained a larger damage than in rats where the frequency of the milk-ejection response was only slowed down. Solutions of True Blue, horseradish peroxidase or horseradish peroxidase coupled to wheat germ agglutinin were injected in the mesencephalic lateral tegmentum of rats with and without lesions. Retrogradely labelled cells were found in several nuclei of the somatosensory pathways: the principal sensory and spinal parts of the trigeminal complex, the cuneate and gracile nuclei, the lateral cervical nucleus and the nucleus proprius of the spinal cord. Labelled cells were also found in the ventral nucleus of the lateral lemniscus, the ventral parabrachial nucleus, the gigantocellular reticular nucleus, the lateral nucleus of the substantia nigra, the prerubral nucleus of the thalamus, the hypothalamic ventromedial nucleus, the zona incerta and in the anterior and lateral hypothalamic areas. Labelled fibres and "terminal-like" labelling were found in the anterior pretectal area, in the thalamic parafascicular nucleus, in the posterior nucleus and the ventroposterior complex, in the zona incerta and in the fields of Forel, but none were observed in the supraoptic or paraventricular nuclei. Injections made in the area of the lateral cervical nucleus and in the cuneate and gracile nuclei labelled fibres and "terminal-like" fields in the external nucleus of the inferior colliculus, the intercollicular area, the deep grey layers of the superior colliculus and in the mesencephalic lateral tegmentum. After injections in the posterior nucleus and ventroposterior complex of the thalamus, retrogradely labelled cells were found in the lateral tegmentum, the intercollicular area and the external nucleus of the inferior colliculus. These results indicate that bilateral lesioning of the mesencephalic lateral tegmentum, which disrupts the milk-ejection response, could damage somatosensory projections originating from the dorsal horn of the spinal cord, the lateral cervical nucleus, the dorsal column nuclei and the sensory and spinal trigeminal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Changes in the accessory group of hypothalamic neurosecretory cells after stimulation of the supraoptic nucleus

Changes in the accessory group of neurosecretory cells in cats were studied in acute experiments under chloralose anesthesia. Unilateral stimulation of the supraoptic nucleus, leading to vasoconstriction, caused activation of cells of the accessory group on the side of stimulation and an increase in size of the nucleolus. Cells of the paraventricular nucleus were unchanged whereas cells of the supraoptic nucleus were activated. The uniformity of response of the accessory group of neurosecretory cells and of the supraoptic nucleus, together with histochemical data in the literature, suggests that the two groups of cells produce vasopressin, some of which accumulates in the posterior lobe of the pituitary and some acts as a mediator outside the hypothalamus.Laboratory of Physiology of the Cerebral Circulation, A. L. Polenov Leningrad Neurosurgical Research Institute. (Presented by Academician V. N. Chernigovskii.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 83, No. 2, pp. 236–238, February, 1977.     

Eye movements evoked by electrical microstimulation of the mesencephalic reticular formation in goldfish

Anatomical studies in goldfish show that the tectofugal axons provide a large number of boutons within the mesencephalic reticular formation. Electrical stimulation, reversible inactivation and cell recording in the primate central mesencephalic reticular formation have suggested that it participates in the control of rapid eye movements (saccades). Moreover, the role of this tecto-recipient area in the generation of saccadic eye movements in fish is unknown. In this study we show that the electrical microstimulation of the mesencephalic reticular formation of goldfish evoked short latency saccadic eye movements in any direction (contraversive or ipsiversive, upward or downward). Movements of the eyes were usually disjunctive. Based on the location of the sites from which eye movements were evoked and the preferred saccade direction, eye movements were divided into different groups: pure vertical saccades were mainly elicited from the rostral mesencephalic reticular formation, while oblique and pure horizontal were largely evoked from middle and caudal mesencephalic reticular formation zones. The direction and amplitude of pure vertical and horizontal saccades were unaffected by initial eye position. However the amplitude, but not the direction of most oblique saccades was systematically modified by initial eye position. At the same time, the amplitude of elicited saccades did not vary in any consistent manner along either the anteroposterior, dorsoventral or mediolateral axes (i.e. there was no topographic organization of the mesencephalic reticular formation with respect to amplitude). In addition to these groups of movements, we found convergent and goal-directed saccades evoked primarily from the anterior and posterior mesencephalic reticular formation, respectively. Finally, the metric and kinetic characteristics of saccades could be manipulated by changes in the stimulation parameters. We conclude that the mesencephalic reticular formation in goldfish shares physiological functions that correspond closely with those found in mammals.     

Blood pressure reflexes to stimulation of tibial nerve a fibers in mesencephalic and bulbar cats

The role of the various subgroups of A fibers of the tibial nerve (pulse frequency of electrical stimulation 10/sec) in the formation of reflex changes in blood pressure (BP) was investigated in unanesthetized cats with total transection of the brain stem at the level of the pontomedullary junction (bulbar animals) or at the rostral border of the mesencephalon (mesencaphalic animals), and also in anesthetized cats with an intact brain. The lowest thresholds for the reflexes were found in anesthetized animals with an intact brain, the highest in bulbar cats. Excitation of A fibers in anesthetized cats with an intact brain evoked only depressor reflexes. In some bulbar and mesencephalic animals only pressor reflexes appeared. In the experiments of this group excitation of fibers with a conduction velocity of over 15 m/sec in mesencephalic cats evoked reflexes of near maximal strengths, whereas in bulbar cats excitation of thinner A fibers also was necessary. In unanesthetized animals disconnection of the suprabulbar structures thus lowers the sensitivity of the central mechanisms of vasomotor regulation to impulses in lowthreshold A fibers. No such effect was found in another group of experiments in which depressor reflexes appeard in response to stimulation of fast-conducting A fibers only. In these experiments, if slower A fibers also were stimulated, the reflexes became pressor but the difference between their magnitude in the bulbar and mesencephalic cats was not signicant.Laboratory of Biophysics and Pathophysiology of the Circulation, Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. M. Chernukh.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 84, No. 10, pp. 393–396, October, 1977.     

Effect of adrenocorticotropic hormone and calcitrine on the lung surfactant system by action on lateral hypothalamus and amygdala

It is shown on rats that cobalt activation of the hyperactivity focus in the lateral hypothalamic area causes a decrease of the phospholipid and cholesterol content and a reduced blood supply in the lungs but an increase of these factors in the basolateral amygdala area. Intracranial microinjection of adrenocorticotropic hormone, combined administration of adrenocorticotropic hormone+calcitrine, and administration of calcitrine alone cause a marked increase of the alveolar phospholipid content and normalize blood filling for action on the lateral hypothalamic area. Action on the basolateral amygdala induces an increase of the phospholipid and cholesterol concentration and augmentation of lung blood supply, but to a lesser degree. The role of peptidergic mechanisms in the realization of hypothalamic and amygdala influences on lung surfactant and hemodynamics is described. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 119, N ^o 2, pp. 133–135, February, 1995 Presented by G. N. Kryzhanovskii, Member of the Russian Academy of Medical Sciences     

Angiotensin II as a factor inhibiting the fear response

Angiotensin II, if injected into the lateral ventricles of rabbits in doses of 0.015–0.15 g, has an inhibitory action on the fear response evoked by electrical stimulation of the ventromedial hypothalamic nucleus, but in doses of 1–10 ng it blocks the inborn behavioral fear responses in rats. On microionophoretic application of angiotensin II to single neurons in the cerebral cortex and parafascicular complex of the thalamus, predominantly activation responses were observed. Predominance of inhibitory neuronal responses were noted in structures of the hypothalamus and mesencephalic reticular formation to angiotensin II. Responses of cortical and subcortical neurons to angiotensin II are potentiated after stimulation of the fear center in the ventromedial hypothalamus. The hypothesis was put forward that depression of the fear response after administration of angiotensin II is connected with changes in cortico-subcortical relations, during which ascending activating influences of the mesencephalic reticular formation on the cerebral cortex are abolished due to descending influences of cortical and thalamic neurons.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 72, No. 6, pp. 713–722, June, 1986.     

Subcortical projections to the lateral geniculate body in the rat

Summary The subcortical projections to the lateral geniculate body (LGB) in the rat were studied by means of discrete HRP iontophoretic deposits in the dorsal or the ventral LGB; the labelling was compared to that resulting from HRP deposits in neighboring nuclei.After injecting HRP in the dorsal LGB, labelled cells appeared bilaterally in the ventral LGB, pretectum, superior colliculus, lateral groups of the dorsal raphe nucleus and locus coeruleus. Ipsilaterally, labelled cells were found in the lateral posterior thalamus, nucleus of the posterior commissure and deep mesencephalic reticular nucleus.After injecting HRP into the ventral LGB, labelled cells were observed bilaterally in the pretectum, superior colliculus and dorsal raphe nucleus (lateral groups). Contralateral labelling appeared in the ventral LGB and parabigeminal nucleus. Ipsilateral labelling was found in the zona incerta, lateral posterior thalamus, lateral and medial mesencephalic reticular formation, vestibular and dorsal tegmental nuclei.These findings provide evidence of subcortical projections to the LGB arising in visually-related areas as well as extravisual areas, which might be related to the LGB boutons that survive complete cortical and retinal ablations.     

The cerebellar projection from the reticular formation of the brain stem in the rabbit

Summary By retrograde transport of horseradish peroxidase the reticulocerebellar projections were examined in twenty-six rabbits.After injections in the cerebellum retrogradely labeled neurons were more numerous in the caudal reticular formation (ventral and gigantocellular reticular nuclei) than in its rostral part (caudal and oral pontine reticular nuclei). The labeled cells were of all sizes, large, medium-sized and small. Giant cells were labeled only after injections in the posterior lobe vermis.After injections in the anterior lobe, the posterior vermis, the fastigial nucleus and the flocculus, retrogradely labeled neurons were found bilaterally in the ventral reticular nucleus, the gigantocellular reticular nucleus and the caudal pontine reticular nucleus. Some cases with posterior vermal and fastigial injections in addition showed labeled neurons bilaterally in the oral pontine reticular nucleus. There were no major side differences. The cases with injections in the anterior part of the paramedian lobule gave rise to only a few labeled cells in the gigantocellular reticular nucleus.Negative findings were consistently made in the mesencephalic reticular formation.     

Role of the posterior hypothalamus in activity of the ascending activating system

In chronic experiments on cats after premesencephalic section of the brain stem electrical stimulation of the posterior hypothalamus evoked desynchronization of neocortical electrical activity. After isolated injury to the posterior hypothalamus, moderate electrical stimulation of the medial part of the mesencephalic reticular formation did not evoke any marked activation of the neocortex. The results point to the important role of the hypothalamus in the activity of the ascending activating system.     

Afferent projections to the rat nuclei raphe magnus, raphe pallidus and reticularis gigantocellularis pars α demonstrated by iontophoretic application of choleratoxin (subunit b)

The aim of the present study was to identify the specific afferent projections to the rostral and caudal nucleus raphe magnus, the gigantocellular reticular nucleus pars α and the rostral nucleus raphe pallidus. For this purpose, small iontophoretic injections of the sensitive retrograde tracer choleratoxin (subunit b) were made in each of these structures. In agreement with previous retrograde studies, after all injection sites, a substantial to large number of labeled neurons were observed in the dorsal hypothalamic area and dorsolateral and ventrolateral parts of the periaqueductal gray, and a small to moderate number were found in the lateral preoptic area, bed nucleus of the stria terminalis, paraventricular hypothalamic nucleus, central nucleus of the amygdala, lateral hypothalamic area, parafascicular area, parabrachial nuclei, subcoeruleus area and parvocellular reticular nucleus. In addition, depending on the nucleus injected, we observed a variable number of retrogradely labeled cells in other regions. After injections in the rostral nucleus raphe magnus, a large number of labeled cells were seen in the prelimbic, infralimbic, medial and lateral precentral cortices and the dorsal part of the periaqueductal gray. In contrast, after injections in the other nuclei, fewer cells were localized in these structures. Following raphe pallidus injections, a substantial to large number of labeled cells were observed in the medial preoptic area, median preoptic nucleus, ventromedial part of the periaqueductal gray, Kölliker-Fuse and lateral paragigantocellular reticular nuclei. Following injections in the other areas, a small to moderate number of cells appeared. After gigantocellular reticular pars α injections, a very large and substantial number of labeled neurons were found in the deep mesencephalic reticular formation and oral pontine reticular nucleus, respectively. After the other injections, fewer cells were seen. Following rostral raphe magnus or raphe pallidus injections, a substantial number of labeled cells were observed in the insular and perirhinal cortices. Following caudal raphe magnus or gigantocellular reticular pars α injections, fewer cells were found. After raphe magnus or gigantocellular reticular pars α injections, a moderate to substantial number of cells were localized in the fields of Forel, lateral habenular nucleus and ventral caudal pontine reticular nucleus. Following raphe pallidus injections, only a small number of cells were seen. Our data indicate that the rostral and caudal parts of the nucleus raphe magnus, the gigantocellular reticular nucleus pars α and the nucleus raphe pallidus receive afferents of comparable strength from a large number of structures. In addition, a number of other afferents give rise to stronger inputs to one or two of the four nuclei studied. Such differential inputs might be directed to populations of neurons with different physiological roles previously recorded specifically in these nuclei.