Responses of diencephalic neurons to sensory stimulation in the goldfish, Carassius auratus

We studied the responses to sensory stimulation in two diencephalic areas, the central posterior nucleus of the dorsal thalamus (CP) and the anterior tuberal nucleus of the hypothalamus (TA). In both the CP and the TA, units sensitive to acoustic (500-Hz sound), hydrodynamic (25-Hz dipole stimulus), and visual (640-nm light flash) stimuli were found. In the CP, most units were unimodal and responded exclusively to visual stimulation. In contrast, in the TA, most units responded to more than one modality. The data suggest that the CP is primarily involved in the unimodal processing of sensory information, whereas the TA may be involved in multisensory integration.     

Distribution and properties of diencephalic neuronal responses to genital stimulation in the female cat

Responses of single units and unit clusters to vaginal stimulation were recorded from the thalamus, subthalamus, and hypothalamus of lightly sedated estrous and anestrous cats. Pronounced, short-latency neuronal responses were found to be distributed widely in the diencephalon generally, but with a relatively low density in the hypothalamus. Most of the unit responses to genital stimulation were accelerative changes in firing which outlasted the stimulus by many seconds or minutes. Units responding to vaginal stimulation were usually also responsive to a tail pinch or to tactile stimulation of the face, limbs, or trunk. The response properties of the diencephalic neurons were quite similar to those of genitally sensitive neurons in the midbrain and pons which have been described previously.     

Responses of caudal brain stem neurons to vaginal and somatosensory stimulation in the rat and evidence of genital-nociceptive interactions.

In rabbits lightly anesthetized with pentobarbital, the effects of injecting morphine and its antagonist, nalorphine, directly into the cerebral circulation via a carotid artery cannulation, on the jaw-opening reflex elicited by intradental stimulation, were studied. Using averaged electromyogram signals from the digastric muscle as the index, a single dose of morphine (3 mg/kg) was found to produce only a transient depression of the jaw-opening reflex. After a second, cumulative injection of morphine (3 mg/kg), the jaw-opening reflex was sufficiently inhibited for at least 30 min. The morphine-induced analgesia could be reversed by nalorphine, indicating that such action was truly pharmacological. The likelihood of morphine acting on some central sites and promoting the release of neurotransmitters which produce inhibition of transmission of nociceptive information from the dental pulp is discussed in the light of recent physiological and pharmacological linkage between morphine-triggered and central stimulation-induced analgesia.     

Effects of cerebellar and brain stem stimulation on hippocampal formation neurons in the monkey

Micro- and macroelectrode recording techniques were used to detect the presence of cerebellar influences on electrical activity in the hippocampal formation of the rhesus monkey. Only 2% of the cells studied responded to cerebellar stimulation. These responsive cells showed a decreased firing rate. No evoked potentials were detected after cerebellar stimulation in any of the monkeys. The effects of brain stem stimulation were tested in two monkeys. One electrode placement lateral to the central gray did not evoke any neuronal responses. The other, situated in the raphe nucleus near the border of the dorsal raphe and nucleus centralis superior, evoked inhibitory responses in 17% of the cells tested. These results (i) fail to provide support for the existence of an important pathway from the cerebellum to the hippocampal formation in monkeys, and (ii) preliminarily extend to a primate species previous results in rat and cat showing an inhibitory effect of raphe stimulation on hippocampal neurons.     

Responses of midbrain neurons to genital and somatosensory stimulation in estrous and anestrous cats

The midbrain was explored in cats for single-unit responses to probing of the urogenital sinus and vagina. The animals had been ovariectomized, and half of them were behaviorally estrous due to previous estradiol injections. While under ether anesthesia the cats were fixed in the stereotaxic by a skull-mounted adapter. During recording the animals were maintained in a sedated condition by administration of urethane. Cells displaying distinct, short-latency responses to vaginal probing (genitally-sensitive neurons) were found to be widespread in the midbrain, occupying the regions known to receive afferents from the ventrolateral funiculi of the spinal cord, and also lying in the ventromedial tegmentum. The unit responses to vaginal probing tended to be less tightly time-locked and distinctive than those previously described for genitally-sensitive neurons in the pons and medulla. In some instances, the presence or pattern of a unit response to genital stimulation was related to the characteristics of the cortical EEG or the unit's spontaneous discharge pattern at the time the stimulus was applied. Nearly all of the genitally-sensitive neurons responded to extragenital somatic or visceral stimuli, particularly those of nociceptive intensity. In the estrous animals, more genitally-sensitive neurons were responsive to innocuous somatosensory stimuli, and their receptive fields included more regions of the body surface than did those of comparable genitally-sensitive neurons in anestrous animals.     

Anatomical and physiological characteristics of vestibular neurons mediating the vertical vestibulo-ocular reflexes of the squirrel monkey

The morphology of 35 vestibular neurons whose firing rate was related to vertical eye movements was studied by injection of horseradish peroxidase intracellularly into physiologically identified vestibular axons in alert squirrel monkeys. The intracellularly injected cells were readily classified into four main groups. One group of cells, down position-vestibular-pause neurons (down PVPs; N = 12), increased their firing rate during downward eye positions, paused during saccades, and were located in the medial vestibular nucleus (MV) and the adjacent ventrolateral vestibular nucleus (VLV). They had axons that crossed the midline and ascended in the medial longitudinal fasciculus (MLF) to terminate in the trochlear nucleus, the lateral aspect of the caudal oculomotor nucleus, and the dorsal aspect of the rostral oculomotor nucleus. A second group of cells (N = 15) were also located in the MV and VLV, but increased their firing rate during upward eye positions, and paused during saccades. These cells had axons that crossed the midline and ascended in the contralateral MLF to terminate in the medial aspect of the oculomotor nucleus. A third group of cells (N = 4) were located in the superior vestibular nucleus, generated bursts of spikes during upward saccades, and increased their tonic firing rate during upward eye positions. These cells had axons that ascended laterally to the ipsilateral MLF to terminate in regions of the trochlear and oculomotor nuclei similar to those in which down PVPs terminated. A fourth group of cells (N = 4), located in the VLV, had axons that projected to the spinal cord, although they had firing rates that were significantly correlated with vertical eye position. Electrical stimulation of the vestibular nerve evoked spikes at monosynaptic latencies in each of the above classes of cells, six of which were injected with horseradish peroxidase. Each group of cells had collateral projections to other areas of the brainstem. Some of the neurons that projected to the contralateral trochlear and oculomotor nuclei had collaterals that crossed the midline to terminate in the oculomotor nucleus ipsilateral to the soma, and some gave rise to small collaterals that terminated in the abducens nucleus. Other areas of the brainstem that received collateral inputs from neurons projecting to oculomotor and trochlear nuclei included the interstitial nucleus of Cajal, the caudal part of the dorsal raphe nucleus, the nucleus raphe obscurus, Roller's nucleus, the intermediate and caudal interstitial nuclei of the MLF, and the nucleus prepositus.     

Anatomical and physiological characteristics of vestibular neurons mediating the horizontal vestibulo-ocular reflex of the squirrel monkey

The anatomical characteristics of vestibular neurons, which are involved in controlling the horizontal vestibulo-ocular reflex, were studied by injecting horseradish peroxidase (HRP) into neurons whose response during spontaneous eye movements had been characterized in alert squirrel monkeys. Most of the vestibular neurons injected with HRP that had axons projecting to the abducens nucleus or the medial rectus subdivision of the oculomotor nucleus had discharge rates related to eye position and eye velocity. Three morphological types of cells were injected whose firing rates were related to horizontal eye movements. Two of the cell types were located in the ventral lateral vestibular nucleus and the ventral part of the medial vestibular nucleus (MV). These vestibular neurons could be activated at monosynaptic latencies following electrical stimulation of the vestibular nerve; increased their firing rate when the eye moved in the direction contralateral to the soma; had tonic firing rates that increased when the eye was held in contralateral positions; and had a pause in their firing rate during saccadic eye movements in the ipsilateral or vertical directions. Eleven of the above cells had axons that arborized exclusively on the contralateral side of the brainstem, terminating in the contralateral abducens nucleus, the dorsal paramedian pontine reticular formation, the prepositus nucleus, medial vestibular nucleus, dorsal medullary reticular formation, caudal interstitial nucleus of the medial longitudinal fasciculus, and raphé obscurus. Eight of the cells had axons that projected rostrally in the ascending tract of Deiters and arborized exclusively on the ipsilateral side of the brainstem, terminating in the ipsilateral medial rectus subdivision of the oculomotor nucleus and, in some cases, the dorsal paramedian pontine reticular formation or the caudal interstitial nucleus of the medial longitudinal fasciculus. Two MV neurons were injected that had discharge rates related to ipsilateral eye position, generated bursts of spikes during saccades in the ipsilateral direction, and paused during saccades in the contralateral direction. The axons of those cells arborized ipsilaterally, and terminated in the ipsilateral abducens nucleus, MV, prepositus nucleus, and the dorsal medullary reticular formation. The morphology of vestibular neurons that projected to the abducens nucleus whose discharge rate was not related to eye movements, or was related primarily to vertical eye movements, is also briefly presented.     

Origin of brain stem and temporal cortical afferent fibers to the septal region in the squirrel monkey

The origins of the brain stem and temporal cortical projections to the septal region in the squirrel monkey were investigated with the horseradish peroxidase (HRP) retrograde axonal transport technique. After HRP injections placed into the septal region, labeled cells were observed in brain stem sites which generally correspond to regions which are associated with known monoamine cell groups previously identified in the primate. These structures include the nucleus locus ceruleus, dorsal tegmental nucleus of Gudden, nucleus reticularis tegmenti pontis, nucleus annularis, ventral tegmental region, and the medial aspect of the lateral hypothalamus. Temporal cortical efferent fibers to the septal region arise principally from layers II and III of the perirhinal region, suggesting the presence of a second-order olfactory innervation of this structure.     

Spinothalamocortical projections to the secondary somatosensory cortex (SII) in squirrel monkey

Anterograde labeling of the cervical spinothalamic tract was combined with retrograde labeling of thalamocortical cells projecting to the hand region of the second somatosensory cortex (hSII) to identify likely sites in the thalamus for processing and transmitting nociceptive information to hSII. Anterograde labeling of terminals was done with 2% WGA-HRP injections in the cervical enlargement; thalamocortical cells were retrogradely labeled with fluorescent tracers. In one experiment, the contralateral primary somatosensory cortex hand region (hSI) was injected to provide a direct comparison with hSII thalamic label. Both labeled cells and terminal-like structures were visualized in single thalamic sections and their numbers and positions quantitatively analyzed. The number of labeled cells within 100 microns from the STT terminals were counted as overlapping cells. Four thalamic nuclei, ventroposterior inferior (VPI), ventroposterior lateral (VPL), posterior nucleus (PO) and centrolateral nucleus (CL) combined to contain 86.5% of all hSII-projecting overlapping cells. Of all hSII-projecting thalamic overlapping cells, VPI contained the largest number (36.4% of the total) followed by the anterior portion of the posterior nuclear complex (POa; 20.4%), VPL (18.3%) and CL (11.4%). Results of the hSI injection show a different pattern of overlap in agreement with our earlier study. The relative distribution of overlapping cells was dependent on the antero-posterior position of the SII injections. The most anterior injections resulted in small numbers of labeled cells, with the majority of overlapping cells located in PO and CL. The more posterior injections resulted in overlapping cells mainly in VPI and VPL. The results indicate that, in the squirrel monkey, VPI, VPL, POa and CL relay nociceptive information from the spinal cord to the second somatosensory cortex.     

Occlusion of pressor responses to posterior diencephalic stimulation and muscular contraction

Although neural occlusion has been suggested to occur between the central and reflex mechanisms increasing arterial pressure, evidence consistent with this phenomenon is lacking. To assess the possibility of neural occlusion we recorded, in chloralose-anesthetized cats, the pressor responses to statically contracting the hindlimb muscles and to electrically stimulating histologically confirmed sites in the posterior hypothalamus and subthalamus. We also recorded the pressor responses to topical application of capsaicin onto the intestine and to stimulation of these diencephalic sites. The pressor responses to simultaneous static contraction and diencephalic stimulation were significantly smaller than the algebraic sum of the pressor responses to contraction and diencephalic stimulation evoked separately. Likewise, the pressor responses to simultaneous capsaicin application and diencephalic stimulation were significantly smaller than the algebraic sum of the responses evoked separately. High intensity stimulation of the L7 dorsal root or the diencephalic sites evoked pressor responses similar in magnitude to the algebraic sum of the two responses evoked separately; thus, the inability of the simultaneous maneuvers to evoke pressor responses that summed algebraically was not due to the fact that they caused a maximal effect. Our findings are consistent with the hypothesis that neural occlusion occurs during stimulation of the posterior diencephalon and static muscular contraction.     

Anatomical connections of the prepositus and abducens nuclei in the squirrel monkey

The primary goal of this investigation was to identify the areas of the brainstem and cerebellum that provide afferent projections to the nucleus prepositus hypoglossi in primates. After horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was injected into the prepositus in squirrel monkeys (Saimiri sciureus), the largest populations of retrogradely labeled neurons were found in the vestibular nuclei, the contralateral perihypoglossal nuclei, and the medullary and pontine reticular formation. Unlike the cat, the prepositus in Saimiri received substantial projections from the nucleus raphe dorsalis and the central mesencephalic reticular formation, whereas few or no labeled cells were found in the cerebellar cortex, the superior colliculus, or the nucleus reticularis tegmenti pontis. By comparing the afferents to the prepositus with those to the abducens nucleus, we found that all regions projecting to the abducens also projected to the prepositus, without exception. Anterogradely transported WGA-HRP showed that the major brainstem recipients of prepositus efferents were the vestibular and perihypoglossal nuclei, the inferior olive, the medullary reticular formation, and the extraocular motor nuclei. In the cerebellar cortex, the prepositus projected to restricted regions of crura I and II as well as the caudal vermis and vestibulocerebellum. The many parts of the oculomotor system receiving input from the prepositus and the parallel innervation of the prepositus and the abducens by a large number of premotor centers lend support to the hypothesis that the prepositus may distribute an efference copy of motor activity, and may also play an important role in the process of neural integration.     

Facilitation by estradiol of midbrain and pontine unit responses to vaginal and somatosensory stimulation in the squirrel monkey (Saimiri sciureus).

In six female squirrel monkeys anesthetized with a chloralose-urethane mixture, single midbrain and pontine units were tested for their responsiveness to somatosensory, vaginal, and rectal stimulation. Three monkeys (two ovariectomized, one intact) were given a series of estradiol injections that produced full cornification of vaginal smears. The other three animals, all ovariectomized, received only the oil vehicle and their smears remained uncornified throughout. Single-unit activity was sampled from comparable regions in both groups of monkeys. Units responding to tactile stimuli tended to have large receptive fields, and in the estrogen-treated monkeys they comprised a larger proportion of the sample than in the untreated monkeys (89 and 49%, respectively). Similarly, a greater proportion of the units responded to vaginal stimulation (65%) in the estrogen-treated animals than in those receiving the oil vehicle alone (40%). In contrast to the effect of estradiol treatment on neuronal responsiveness to vaginal and tactile cutaneous stimulation, units responding to nociceptive stimulation of the foot were found with equal frequency (48%) in both groups of monkeys. The finding that unit responses to tactile stimuli occurred with a greater frequency in estrogen-treated monkeys is consistent with results previously reported for midbrain and pontine units is estrous and anestrous cats. Unlike the cat, however, in the squirrel monkey there was no indication that the sizes of the peripheral receptive fields were altered by estradiol treatment.     

Effect of stimulation of prefrontal cortex and amygdala on diencephalic neurons.

Extracellular unit recordings were obtained from the nucleus medialis dorsalis (MD) and adjacent thalamic nuclei and from the hypothalamus of anesthetized cats. Electrical stimulation of the prefrontal cortex produces suppression of unit activity and rebound excitation in MD. In addition antidromic and short latency excitation was found which was followed by prolonged suppression of firing. This suppression was also followed by a strong postinihibitory activation of unit firing which was, in some cases, again followed by a second phase of inhibition. Amygdala stimulation produced similar results except that antidromic responses were found only in the intralaminar nuclei, and short-latency transynaptic excitation occurred somewhat later. The results are in consonance with the hypothesis that the same interneuronal mechanism is engaged by both afferent sources to MD. In the hypothalamus, fewer units responded to cortical and amygdalar stimulation and alternating sequences of excitation-inhibition-excitation were not found.     

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. I. Excitatory burst neurons

Saccadic burst neurons in the pontine reticular formation have been implicated in the generation of saccades in the horizontal plane on the basis of lesion and extracellular recording studies in the cat and monkey. In the present study, saccadic burst neurons were anatomically and physiologically characterized with intraaxonal recording and injection of horseradish peroxidase in the alert squirrel monkey. A population of burst neurons were found that appear analogous to the excitatory burst neurons (EBNs) described previously in the cat. All neurons are located in the caudal pontine reticular formation and have a major axonal projection to the ipsilateral abducens nucleus. Additional projections were found to the medial vestibular nucleus, the nucleus prepositus, and regions of the pontine and medullary reticular formation rostral, ventral, and caudal to the abducens. All neurons fire exclusively during saccades and have a discharge pattern similar to that of medium-lead burst neurons described previously in the cat and monkey. In most neurons the saccadic burst begins 5-15 msec before saccade onset. Linear relationships exist between burst duration and saccade duration, number of spikes in the burst and saccade amplitude, and firing frequency and instantaneous velocity. Physiological activity of each neuron shows the closest relationship with the amplitude of the saccade component in a particular direction. For all neurons, this on-direction is in the ipsilateral hemifield and is predominantly horizontal, but may have either an upward or downward vertical component. These results support a major role for the EBNs in the monkey in generating the saccadic burst in abducens motoneurons, as well as in contributing to the oculomotor activity in other classes of premotor neurons.     

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. II. Inhibitory burst neurons

Electrophysiological and intracellular labelling studies in the cat have identified a population of saccadic burst neurons in the medullary reticular formation that have an inhibitory, monosynaptic projection to the contralateral abducens nucleus. In the present study, intraaxonal recording and injection of horseradish peroxidase were used to identify and characterize the corresponding population of inhibitory burst neurons (IBNs) in the alert squirrel monkey. Squirrel monkey IBNs are located in the reticular formation ventral and caudal to the abducens nucleus and project contralaterally to the abducens. Additional contralateral projections are present to the vestibular nuclei, the nucleus prepositus, and the pontine and medullary reticular formation rostral and caudal to the abducens. All neurons fire a burst of spikes during saccades and are silent during fixation. In most neurons the burst begins 5-15 msec before saccade onset. The number of spikes in the saccadic burst is linearly related to the amplitude of the component of the saccade in the neuron's on-direction. Linear relationships also exist between burst duration and saccade duration and between firing frequency and instantaneous eye velocity. For all neurons, the on-direction is in the ipsilateral hemifield, with a vertical component that may be either upward or downward. Neurons with projections to the vertically related descending and superior vestibular nuclei tend to have on-directions with larger vertical components than neurons that lack these projections. These results, together with those on excitatory burst neurons reported in the preceding paper, demonstrate a reciprocal organization of burst neuron input to the abducens in the monkey similar to that found in the cat and indicate a major role for these neurons in generating the oculomotor activity in motoneurons as well as in other classes of premotor neurons.