Visual sampling after lesions of the superior colliculus in rats.

In 2 experiments with 20 male black-hooded rats, Ss with bilateral lesions of the superior colliculus showed significantly poorer relearning of a horizontal/vertical stripe discrimination than control Ss. In Exp I, all Ss showed disruption of performance when a stimulus–response (S–R) separation was introduced by raising the stimuli above the site of responding. However, colliculectomized Ss were much more disturbed by the S–R separation than were control Ss. In Exp II, all Ss showed lower performance levels when conflicting patterns were introduced into the upper portion of the stimulus doors, but this time Ss with collicular lesions were less disturbed than controls. It is suggested (a) that when the stimulus and response sites are discontinuous, rats must make an appropriate orienting response to effectively sample the visual stimuli and (b) that lesions of the superior colliculus alter performance by interfering with this orienting behavior. The impairment in relearning is attributed to the absence of preoperative overtraining on the discrimination task. (24 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Brain transections selectively alter ingestion and behavioral activation in neonatal rats.

Food-deprived neonatal rats actively ingest milk that is infused into their mouths through intraoral cannulas. This ingestion is accompanied by behavioral activation. The involvement of various brain regions in ingestion and activation was examined in 2-day-old Charles River pups by making transections along the neuraxis from the olfactory bulbs to the anterior pons. Following a 24-hr deprivation period, a series of oral milk infusions was given, and milk intake and activity were measured. Intake was severely reduced only in Ss with diencephalic transections. Cuts in front of or behind the diencephalon resulted in normal or slightly decreased intake. In contrast, activity tended to decline as the level of the transection became more caudal. Thus ingestion and its accompanying behavioral activation could be separated neuroanatomically. These results suggest that 2 brain mechanisms are involved in the ingestive response of the infant rat, one in the diencephalon and another caudal to the mesencephalon. However, behavioral activation appears less discretely organized, involving most of the neuraxis. (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A deficit in ambient visual guidance following superior colliculus lesions in rats.

21 male black-hooded rats were tested for their ability to locate a hidden platform in the Morris swimming pool, in which extrapool cues are required to guide locomotion. At the end of each trial, Ss were either removed immediately or allowed to remain on the platform for 60 sec. As in a previous experiment by the present authors (see record 1984-06177-001), bilateral lesions of the superior colliculus (SC) produced a severe deficit. Permitting the Ss to stay on the platform did not significantly affect performance in either Ss with SC lesions or sham-operated controls. Results indicate that the reduced orienting behavior on the platform observed in the rats with lesions in the previous experiment was not the cause of their navigational impairment. It is concluded that the impairment following SC lesions came about during the swimming itself; therefore, it may be attributed to a disturbance of, or a failure to utilize, ambient vision. (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

L-DOPA and quipazine elicit air-stepping in neonatal rats with spinal cord transections.

Acute mid-thoracic spinal cord transection eliminates hindlimb air-stepping in neonatal rats suspended in harnesses and administered {l}-DOPA. Because spinal cord transection eliminates all descending inputs to the hindlimb locomotor circuits, this experiment determined if coadministration of {l}-DOPA and quipazine (serotonin receptor agonist) would induce hindlimb air-stepping in rat pups 24 hrs after transection. Hindlimb steps of spinally transected pups that received {l}-DOPA or quipazine alone were infrequent and slow; hindlimb steps induced by {l}-DOPA?+?quipazine occurred more frequently and were faster than those elicited by either drug alone. These findings suggest that catecholaminergic and serotonergic systems both contribute to hindlimb stepping. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

In vivo whole-cell recording from neurons of the superior colliculus in fetal rats

The actions of halothane on serotonin-sensitive potassium channels (S K+ channels) were studied in sensory neurons of Aplysia. The normalized open probability of S K+ channels was increased by clinical concentrations of halothane in cell-attached and excised patches from neurons of the pleural ventrocaudal cluster. No voltage-dependence of channel activation by halothane was observed. Pre-treatment of neurons with 8-bromo-cAMP (8-Br-cAMP) or nordihydroguaiaretic acid (NDGA) had no effect on the relative level of channel activation by halothane. S K+ channels that were activated by arachidonic acid could also be activated by halothane and exhibited closely similar amplitude distributions of open channel current. Results from these experiments showed that S K+ channel activation by halothane did not depend on second messenger modulation of channel activity. We conclude that it is likely that halothane directly activates S K+ channels.     

Effects of neonatal enucleation on receptive-field properties of visual neurons in superior colliculus of the golden hamster

1. Monocular enucleation in infant hamsters results in a marked expansion of the normally very limited ipsilateral retinotectal projection (13). In 34 hamsters subjected to removal of one eye within 12 h of birth, the receptive-field characteristics of superior collicular neurons ipsilateral and contralateral to the remaining eye were investigated quantitatively and compared to those of normal animals. In six additional neonatal enucleates, the density of the expanded retinotectal projection was studied with the autoradiographic method and an attempt was made to relate the anatomical reorganization with the electrophysiological findings, 2. The response characteristics of visual cells in the colliculus contralateral to the remaining eye were not significantly different from those observed in normal animals. In the ipsilateral tectum, however, numerous changes were observed. Visual receptive fields were abnormally large. The incidence of directional selectivity was markedly reduced, as were the magnitudes of the discharges elicited by either flashed or moving stimuli. Fewer cells were activated by small flashed spots and most of the units that were responsive to such stimulation failed to exhibit the surround suppression typical for the majority of tectal neurons in normal hamsters. Most cells in the ipsilateral colliculus responded only to relatively low (less than 50 degrees/s) stimulus velocities and response decrements resulting from repeated stimulation also occurred much more readily for the neurons tested on this side. 3. The results of additional experiments in neonatal enucleates (n = 8), which were also subjected to acute bilateral removal of the visual cortex, demonstrated that such damage resulted in a marked reduction in the incidence of directional selectivity in the colliculus contralateral to the remaining eye but had no effect on the responses of cells innervated by the aberrant ipsilateral pathway. 4. A correlation between the relative density of the ipsilateral retinal projection at different points in the colliculus, as demonstrated by the autroradiography and the nature of the visual responses obtained in different portions of the structure, indicated that receptive-field size was negatively correlated with the density of the aberrant retinotectal projection and that absolute responsivity (number of impulses elicited by an optimal stimulus) was positively correlated with autoradiographic grain density. 5. These findings demonstrate that while the aberrant retinocollicular projection can, along with the other visual inputs to the tectum, result in the organization of normal response properties for a small number of tectal neurons, the majority of the visual cells innervated by this pathway have responses that are appreciably different from normal.     

Reinstatement of orienting behavior by d-amphetamine in rats with superior colliculus lesions.

Assessed the involvement of the nigrotectal pathway in the expression of visual orienting behavior by a combination of superior colliculus (SC) lesions and increased dopamine transmission produced by administration of dextroamphetamine (1 mg/kg, intraperitoneally [ip]), using 60 naive male Long-Evans rats. Orienting behavior elicited by apparently moving or stationary light displays, its habituation, and recovery were observed. In intact Ss, amphetamine injections had a small but reliable effect on the habituation of orienting behaviors. Ss with SC lesions did not orient to the lights. Amphetamine-injected Ss with SC lesions did orient, and the topography of their orienting behavior, rate of habituation, and recovery of orienting with changes in the light display were comparable to those of the intact S. Results suggest a view of SC-lesion-impaired orienting behavior as a disturbance of sensory attention and emphasize the interaction of the SC and other neural systems in processes mediating the direction of attention. (16 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mechanisms of within- and cross-modality suppression in the superior colliculus

The present studies were initiated to explore the basis for the response suppression that occurs in cat superior colliculus (SC) neurons when two spatially disparate stimuli are presented simultaneously or in close temporal proximity to one another. Of specific interest was examining the possibility that suppressive regions border the receptive fields (RFs) of unimodal and multisensory SC neurons and, when activated, degrade the neuron's responses to excitatory stimuli. Both within- and cross-modality effects were examined. An example of the former is when a response to a visual stimulus within its RF is suppressed by a second visual stimulus outside the RF. An example of the latter is when the response to a visual stimulus within the visual RF is suppressed when a stimulus from a different modality (e. g., auditory) is presented outside its (i.e., auditory) RF. Suppressive regions were found bordering visual, auditory, and somatosensory RFs. Despite significant modality-specific differences in the incidence and effectiveness of these regions, they were generally quite potent regardless of the modality. In the vast majority (85%) of cases, responses to the excitatory stimulus were degraded by >/=50% by simultaneously stimulating the suppressive region. Contrary to expectations and previous speculations, the effects of activating these suppressive regions often were quite specific. Thus powerful within-modality suppression could be demonstrated in many multisensory neurons in which cross-modality suppression could not be generated. However, the converse was not true. If an extra-RF stimulus inhibited center responses to stimuli of a different modality, it also would suppress center responses to stimuli of its own modality. Thus when cross-modality suppression was demonstrated, it was always accompanied by within-modality suppression. These observations suggest that separate mechanisms underlie within- and cross-modality suppression in the SC. Because some modality-specific tectopetal structures contain neurons with suppressive regions bordering their RFs, the within-modality suppression observed in the SC simply may reflect interactions taking place at the level of one input channel. However, the presence of modality-specific suppression at the level of one input channel would have no effect on the excitation initiated via another input channel. Given the modality-specificity of tectopetal inputs, it appears that cross-modality interactions require the convergence of two or more modality-specific inputs onto the same SC neuron and that the expression of these interactions depends on the internal circuitry of the SC. This allows a cross-modality suppressive signal to be nonspecific and to degrade any and all of the neuron's excitatory inputs.     

Air-stepping in neonatal rats: A comparison of L-dopa injection and olfactory stimulation.

The kinematic parameters of air-stepping induced by 2 methods known to elicit locomotion (olfactory stimulation vs. L-dopa injection) were compared in 3-day-old rats. In the 1st stage, suspended pups were induced to step with an olfactory stimulus of soiled shavings from the nest. In the 2nd stage, they received a subcutaneous injection of L-dopa. Their movements were faster, with a larger amplitude and a phase delay in ipsilateral coupling. Third, the olfactory stimulus was presented in conjunction with L-dopa. The characteristics of locomotion returned to the same level as with the olfactory stimulus alone. These results suggest that olfactory stimulation involves higher nerve centers able to modulate the dopaminergic pathways. They are discussed in relation to the neural structure involved in locomotion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Endothelin-1 in the superior colliculus of rats induces depressor responses due to peripheral hemodynamic changes

Microinjection of endothelin-1 (ET-1; 10 pmol) into the superficial layer of the superior colliculus caused systemic and regional hemodynamic changes, as measured by injection of radioactive microspheres at the peak of the hypotensive effect of endothelin-1. Endothelin-1 decreased total peripheral resistance by 39 +/- 2% (n=5); the vascular resistances were decreased in the spleen, the mesentery, the large intestine and the small intestine. Moreover, we found that in consequence of the increased fraction of cardiac output received by the above organs, decreases in vascular resistances were associated with increases in blood flows in them. Interestingly, ET-1 also decreased the vascular resistances and increased the total blood flows in the kidneys. The haemodynamic changes induced by injection of endothelin-1 to the superior colliculus were associated with significant decreases in the mean arterial blood pressure (37 +/- 4 mmHg, n=6) and no changes in heart rate. Exogenous ET-1, therefore, within the SC decreases blood pressure due to peripheral hemodynamic changes.     

Development of multisensory neurons and multisensory integration in cat superior colliculus

The development of multisensory neurons and multisensory integration was examined in the deep layers of the superior colliculus of kittens ranging in age from 3 to 135 d postnatal (dpn). Despite the high proportion of multisensory neurons in adult animals, no such neurons were found during the first 10 d of postnatal life. Rather, all sensory-responsive neurons were unimodal. The first multisensory neurons (somatosensory-auditory) were found at 12 dpn, and visually responsive multisensory neurons were not found until 20 dpn. Early multisensory neurons responded weakly to sensory stimuli, had long latencies, large receptive fields, and poorly developed response selectivities. Most surprising, however, was their inability to integrate combinations of sensory cues to produce significant response enhancement (or depression), a characteristic feature of the adult. Responses to combinations of sensory cues differed little from responses to their modality-specific components. At 28 dpn an abrupt physiological change was noted. Some multisensory neurons now integrated combinations of cross-modality cues and exhibited significant response enhancements when these cues were spatially coincident and response depressions when the cues were spatially disparate. During the next 2 months the incidence of multisensory neurons, and the proportion of these neurons capable of adult-like multisensory integration, gradually increased. Once multisensory integration appeared in a given neuron, its properties changed little with development. Even the youngest integrating neurons showed superadditive enhancements and spatial characteristics of multisensory integration that were indistinguishable from the adult. Nevertheless, neonatal and adult multisensory neurons differed in the manner in which they integrated temporally asynchronous stimuli, a distribution that may reflect the very different behavioral requirements at different ages. The possible maturational role of corticotectal projections in the abrupt gating of multisensory integration is discussed.     

Spatial disparity coding in the superior colliculus of the cat

We investigate the spatial and temporal aspects of firing patterns in a network of integrate-and-fire neurons arranged in a one-dimensional ring topology. The coupling is stochastic and shaped like a Mexican hat with local excitation and lateral inhibition. With perfect precision in the couplings, the attractors of activity in the network occur at every position in the ring. Inhomogeneities in the coupling break the translational invariance of localized attractors and lead to synchronization within highly active as well as weakly active clusters. The interspike interval variability is high, consistent with recent observations of spike time distributions in visual cortex. The robustness of our results is demonstrated with more realistic simulations on a network of McGregor neurons which model conductance changes and after-hyperpolarization potassium currents.     

A neural model of multimodal adaptive saccadic eye movement control by superior colliculus

How does the saccadic movement system select a target when visual, auditory, and planned movement commands differ? How do retinal, head-centered, and motor error coordinates interact during the selection process? Recent data on superior colliculus (SC) reveal a spreading wave of activation across buildup cells the peak activity of which covaries with the current gaze error. In contrast, the locus of peak activity remains constant at burst cells, whereas their activity level decays with residual gaze error. A neural model answers these questions and simulates burst and buildup responses in visual, overlap, memory, and gap tasks. The model also simulates data on multimodal enhancement and suppression of activity in the deeper SC layers and suggests a functional role for NMDA receptors in this region. In particular, the model suggests how auditory and planned saccadic target positions become aligned and compete with visually reactive target positions to select a movement command. For this to occur, a transformation between auditory and planned head-centered representations and a retinotopic target representation is learned. Burst cells in the model generate teaching signals to the spreading wave layer. Spreading waves are produced by corollary discharges that render planned and visually reactive targets dimensionally consistent and enable them to compete for attention to generate a movement command in motor error coordinates. The attentional selection process also helps to stabilize the map-learning process. The model functionally interprets cells in the superior colliculus, frontal eye field, parietal cortex, mesencephalic reticular formation, paramedian pontine reticular formation, and substantia nigra pars reticulata.     

Intrinsic circuitry in the deep layers of the cat superior colliculus

The mammalian superior colliculus is involved in the transformation of sensory signals into orienting behaviors. Sensory and motor signals are integrated in the colliculus to produce movements of the eyes, head, and neck. While there is a considerable amount of information available on the afferent and efferent connections of the colliculus, almost nothing is known about its intrinsic circuitry, particularly that of its deepest layers. It is likely that intrinsic connections in these deeper layers of the colliculus participate in the sensory-motor transformations leading to orienting movements. In this study, we used the neuroanatomical tracer biocytin to label small groups of neurons in the deeper layers of the cat superior colliculus and examine the distribution of their axons and terminals. We found a broadly distributed network of intrinsic projections throughout the deep layers of the superior colliculus. While the majority of terminals were found in a 1-2 mm radius around the injection site, labeled terminals were found throughout the deep layers of the colliculus up to 5 mm from the injection site. In addition, these injections sometimes labeled terminals in the superficial tectum. Extensive projections were demonstrated by the more superficial injections, but few terminals were found when injections were confined to the deepest layers of the colliculus. There was no evidence of anisotropy in the distribution of terminals from injections made at different rostrocaudal or mediolateral locations; neurons located in any one region in the colliculus could potentially influence any other region. This network of intrinsic connections in the cat superior colliculus could provide a means for deeper-layer efferent neurons to associate, and to modulate or coordinate their output. Interneurons could also provide a substrate for mutual inhibition between neurons at the rostral pole of the colliculus that are active during fixation, and more caudally located neurons whose activity is associated with saccadic eye movements.     

Comparative effects of neonatal and prepubertal castration on craniofacial growth in rats

The role of endogenous testosterone in the craniofacial growth of the young male rat was investigated. First, the effect of neonatal surgical castration was examined in a randomized, cross-sectional study in which male Wistar rats were allocated to be either castrated or sham-operated 4 h after birth. Then, the effect of prepubertal chemical castration was analysed in a second, randomized longitudinal study in which male Wistar rats were randomly allocated either to a control group or to two experimental groups, one injected with triptorelin at day 25 and the other injected on day 25 and on day 45. Every tenth day between 20 and 70 days of age for the first study, and between 30 and 110 days of age for the second, body length and weight were measured, cephalometric X-rays taken, and blood samples obtained. Neonatal and prepubertal castration resulted in decreased plasma concentrations of testosterone and in delayed growth of somatic and craniofacial components. The initiation, duration and magnitude of the effect was dependent on individual bones (cranial base, skull roof) and on the lower incisor, and related to the testosterone concentrations. These results suggest that testosterone effects participate in the process of normal craniofacial growth, particularly during puberty.     

Visuo-oculomotor properties of cells in the superior colliculus of the alert cat

Visual responses and eye movement (EM) -related activities were studied in single units of the superior colliculus (SC) of alert cats. Spontaneous EMs were encouraged by training. Throughout the SC (i.e., in intermediate and deep layers as well as in superficial layers), units were found to respond well to visual stimuli. Strong and consistent responses could be elicited by very dim, low-contrast stationary stimuli. Visual responses varied from phasic to tonic; some units responded tonically to stationary stimuli in the center of the receptive field, and phasically to peripheral stimuli. Many cells responded more vigorously to moving than to stationary stimuli, but very few responded exclusively to stimulus movement. The vast majority of cells were directionally selective. A small number of units were sensitive to the absolute, as well as the retinal, position of visual stimuli. These cells were activated by visual stimuli which fell in the receptive field only if the cat's gaze was fixated on one half of the screen. It seems that these cells must receive information about both eye position and the retinal (receptive field) position of the stimulus. It is possible that they reflect coding of target location within a head (or body) frame of reference. EM-related units were of two types: (1) about 20% of the sample responded prior to spontaneous or visually-triggered EMs, and (2) another 10% (or more) responded with, but not before, EMs. Some cells in the second group discharge almost synchronously with EMs and, thus, cannot plausibly be said to respond to the movement of images across the retina. All cells in the first group were directionally selective. The percentage of EM-related cells in the deep layers of SC is lower in cat than in monkey. Possible reasons for such differences are discussed.     

The visuo-motor pathway in the local circuit of the rat superior colliculus

Intrinsic circuit of the superior colliculus (SC), in particular the pathway from the optic tract (OT) to neurons in the intermediate layer (SGI), was investigated by whole-cell patch-clamp recording in slice preparations obtained from 17- to 24-d-old rats. Stimulation of the OT induced monosynaptic EPSPs in neurons in the superficial gray layer (SGS) and the optic layer (SO), and disynaptic or polysynaptic EPSPs in a majority of SGI neurons. Stimulation of the SGS induced monosynaptic or oligosynaptic EPSPs in the SGI neurons. Both the monosynaptic EPSPs induced in the SGS/SO neurons by stimulation of the OT and those induced in the SGI neurons by stimulation of the SGS were mediated by AMPA- and NMDA-type glutamate receptors. Thus, we have clarified the existence of the glutamatergic excitatory pathway from the OT to the SGI neurons via SGS and SO neurons. The EPSPs in the SGI neurons induced by stimulation of the OT or SGS were remarkably enhanced by bicuculline, suggesting that the signal transmission in this pathway is under strong suppression by the GABAergic system.     

Cellular morphology in the visual layer of the developing rat superior colliculus

Foldback sequences in nuclear DNA from cultured Hamster fibroblasts (BHK-21/C13 cells) have been characterized by electron microscopy. One half of the structures observed when denatured hamster DNA is allowed to anneal in the range O less than Cot1 less than 1 x 10(-4) M sec result from the annealing of inverted sequences forming foldback DNA. The remainder have a probable bimolecular origin. arising from rapidly-annealing sequences of satellite-like complexity. The average length of the inverted sequences in the foldback molecules is about 0.9 kilobases. There is estimated to be about 42,000 such sequences (21,000 pairs) in the hamster genome, approximately 45% of which form looped structures with a mean loop length of 1.74 kilobases. Contrary to previous reports, binding of the renatured duplex molecules to hydroxyapatite results in a poor recovery of structures containing identifiable foldback sequences, due to preferential enrichment of the bound fraction with duplexes formed by intermolecular annealing.     

Lateral inhibitory interactions in the intermediate layers of the monkey superior colliculus

The intermediate layers of the monkey superior colliculus (SC) contain neurons the discharges of which are modulated by visual fixation and saccadic eye movements. Fixation neurons, located in the rostral pole of the SC, discharge action potentials tonically during visual fixation and pause for most saccades. Saccade neurons, located throughout the remainder of the intermediate layers of the SC, discharge action potentials for saccades to a restricted region of the visual field. We defined the fixation zone as that region of the rostral SC containing fixation neurons and the saccade zone as the remainder of the SC. It recently has been hypothesized that a network of local inhibitory interneurons may help shape the reciprocal discharge pattern of fixation and saccade neurons. To test this hypothesis, we combined extracellular recording and microstimulation techniques in awake monkeys trained to perform oculomotor paradigms that enabled us to classify collicular fixation and saccade neurons. Microstimulation was used to electrically activate the fixation and saccade zones of the ipsilateral and contralateral SC to test for inhibitory and excitatory inputs onto fixation and saccade neurons. Saccade neurons were inhibited at short latencies following electrical stimulation of either the ipsilateral (1-5 ms) or contralateral (2-7 ms) fixation or saccade zones. Fixation neurons were inhibited 1-4 ms after electrical stimulation of the ipsilateral saccade zone. Stimulation of the contralateral saccade zone led to much weaker inhibition of fixation neurons. Stimulation of the contralateral fixation zone led to short-latency (1-2 ms) excitation of fixation neurons. Only a small percentage of saccade and fixation neurons were activated by the electrical stimulation (latency: 0.5-2.0 ms). These responses were confirmed as either orthodromic or antidromic responses using collision testing. The results suggest that a local network of inhibitory interneurons may help shape not only the reciprocal discharge pattern of fixation and saccade neurons but also permit lateral interactions between all regions of the ipsilateral and contralateral SC. These interactions therefore may be critical for maintaining stable visual fixation, suppressing unwanted saccades, and initiating saccadic eye movements to targets of interest.     

New growth of axons in the cochlear nucleus of adult chinchillas after acoustic trauma

This study determined the effect of acoustic overstimulation of the adult cochlea on axons in the cochlear nucleus. Chinchillas were exposed to an octave-band noise centered at 4 kHz at 108 dB sound pressure level for 1.75 h. One chinchilla was never exposed to the noise, and several others had one ear protected by an ear plug or prior removal of the malleus and incus. Exposure of unprotected ears caused loss of inner and outer hair cells and myelinated nerve fibers, mostly in the basal half of the cochlea. Cochlear nerve fiber degeneration, ipsilateral to the exposed ears, was traced to regions of the cochlear nucleus representing the damaged parts of the cochlea. In silver impregnations of a deafferented zone in the posteroventral cochlear nucleus, the concentration of axons decreased by 43% after 1 month and by 54% after 2 months. However, by 8 months, the concentration of thinner axons, with diameters of less than 0.46 microm, increased by 46-90% over that at 2 months. The concentration of axons with larger diameters did not change. Between 2 and 8 months small axonal endings appeared next to neuronal cell bodies. This later increase of thinner axons and endings is consistent with a reactive growth of new axons of relatively small diameter. The emergence of small perisomatic boutons suggests that the new axons formed synaptic endings, which might contribute to an abnormal reorganization of the central auditory system and to the pathological changes that accompany acoustic overstimulation.