Hyperactivity, aphagia and motor disturbance following lesions of superior colliculus and underlying tegmentum in rats

Medial orbitofrontal cortex and nucleus accumbens mediation in risk assessment behaviors in adolescents and adults

Risk assessment behaviors are necessary for gathering risk information and guiding decision-making. Risky decision-making heightens during adolescence, possibly as a result of low risk awareness and an increase in sensitivity to reward-associated cues and experiences. Higher adolescent engagement in high-risk behaviors may be, in part, due to developing circuits that contribute to risk assessment behaviors. Nucleus accumbens (NAc) activity is linked to risky decision-making and receives inputs carrying sensory and emotional information. Namely, the medial orbitofrontal cortex (MO) contributes to behavior guided by reward probability and sends direct projections to the NAc (MO→NAc), which may permit risk assessment in a mature circuit. Here, we evaluated risk assessment behaviors in adult and adolescent rats during elevated plus maze (EPM) exploration, including stretch and attend postures, head dips, and rears. We found that adolescents exhibited fewer EPM risk assessment behaviors than adults. We also quantified MO→NAc projections using a fluorescent anterograde tracer, Fluoro-Ruby, in both age groups. Labeled MO→NAc pathways exhibited greater total fluorescence in adults than in adolescents, indicating MO→NAc fibers increase over development. Using a disconnection approach to measure the contribution of the MO-NAc pathway in adults, we found that ipsilateral inactivation of the MO-NAc did not alter risk assessment behavior; however, MO-NAc disconnection reduced the number of stretch-and-attend postures. Together, this work suggests that the development of MO-NAc pathways can contribute to age-dependent differences in risk assessment.

Superior Colliculus Lesions Lead to Disrupted Responses to Light in Diurnal Grass Rats (Arvicanthis niloticus)

The circadian system regulates daily rhythms of physiology and behavior. Although extraordinary advances have been made to elucidate the brain mechanisms underlying the circadian system in nocturnal species, less is known in diurnal species. Recent studies have shown that retinorecipient brain areas such as the intergeniculate leaflet (IGL) and olivary pretectal nucleus (OPT) are critical for the display of normal patterns of daily activity in diurnal grass rats (Arvicanthis niloticus). Specifically, grass rats with IGL and OPT lesions respond to light in similar ways to intact nocturnal animals. Importantly, both the IGL and OPT project to one another in nocturnal species, and there is evidence that these 2 brain regions also project to the superior colliculus (SC). The SC receives direct retinal input, is involved in the triggering of rapid eye movement sleep in nocturnal rats, and is disproportionately large in the diurnal grass rat. The objective of the current study was to use diurnal grass rats to test the hypothesis that the SC is critical for the expression of diurnal behavior and physiology. We performed bilateral electrolytic lesions of the SC in female grass rats to examine behavioral patterns and acute responses to light. Most grass rats with SC lesions expressed significantly reduced activity in the presence of light. Exposing these grass rats to constant darkness reinstated activity levels during the subjective day, suggesting that light masks their ability to display a diurnal activity profile in 12:12 LD. Altogether, our data suggest that the SC is critical for maintaining normal responses to light in female grass rats.

Jacobian Maps Reveal Under-reported Brain Regions Sensitive to Extreme Binge Ethanol Intoxication in the Rat

Individuals aged 12-20 years drink 11% of all alcohol consumed in the United States with more than 90% consumed in the form of binge drinking. Early onset alcohol use is a strong predictor of future alcohol dependence. The study of the effects of excessive alcohol use on the human brain is hampered by limited information regarding the quantity and frequency of exposure to alcohol. Animal models can control for age at alcohol exposure onset and enable isolation of neural substrates of exposure to different patterns and quantities of ethanol (EtOH). As with humans, a frequently used binge exposure model is thought to produce dependence and affect predominantly corticolimbic brain regions. in vivo neuroimaging enables animals models to be examined longitudinally, allowing for each animal to serve as its own control. Accordingly, we conducted 3 magnetic resonance imaging (MRI) sessions (baseline, binge, recovery) to track structure throughout the brains of wild type Wistar rats to test the hypothesis that binge EtOH exposure affects specific brain regions in addition to corticolimbic circuitry. Voxel-based comparisons of 13 EtOH- vs. 12 water- exposed animals identified significant thalamic shrinkage and lateral ventricular enlargement as occurring with EtOH exposure, but recovering with a week of abstinence. By contrast, pretectal nuclei and superior and inferior colliculi shrank in response to binge EtOH treatment but did not recover with abstinence. These results identify brainstem structures that have been relatively underreported but are relevant for localizing neurocircuitry relevant to the dynamic course of alcoholism.

Relationships between the superior colliculus and hippocampus: Neural and behavioral considerations

Theories of superior collicular and hippocampal function have remarkable similarities. Both structures have been repeatedly implicated in spatial and attentional behaviour and in inhibitory control of locomotion. Moreover, they share certain electrophysiological properties in their single unit responses and in the synchronous appearance and disappearance of slow wave activity. Both are phylogenetically old and the colliculus projects strongly to brainstem nuclei instrumental in the generation of theta rhythm in the hippocampal EEC

On the other hand, close inspection of behavioural and electrophysiological data reveals disparities. In particular, hippocampal processing mainly concerns stimulus ambiguity, contextual significance, and spatial relations or other subtle, higher order characteristics. This requires the use of largely preprocessed sensory information and mediation of poststimulus investigation. Although collicular activity must also be integrated with that of “higher” centres (probably to a varying degree, depending on the nature of stimuli being processed and the task requirements), its primary role in attention is more “peripheral” and specific in controlling orienting/localisation via eye and body movements toward egocentrically labelled spatial positions. In addition, the colliculus may exert a nonspecific influence in alerting higher centres to the imminence of information potentially worthy of focal attention. Nevertheless, it is noteworthy that collicular and hippocampal lesions produce deficits on similar tasks, although the type of deficit is usually different (often opposite) in each case. Functional overlap between hippocampus and colliculus (i.e., strategically synchronised or mutually interdependent activity) is virtually certain vis-à-vis stimulus sampling, for example in the acquisition of information via vibrissal movements and visual scanning. In addition, insofar as stimulus significance is a factor in collicular orienting mechanisms, the hippocampus — cingulate – cortex — colliculus pathway may play a significant role, modulating collicular responsiveness and thus ensuring an attentional strategy appropriate to current requirements (stimulus familiarity, stage of learning). A tentative “reciprocal loop” model is proposed which bridges physiological and behavioural levels of analysis and which would account for the observed degree and nature of functional overlap between the superior colliculus and hippocampus.

Effects of d-amphetamine on arousal

This article is a review of studies conducted on the effects of amphetamine on behavior in animals by the late Walter Isaac and his associates. The principle findings and hypotheses developed over the 20 or more years of research are outlined and discussed. d-Amphetamine is thought to affect an organism's behavior by influencing the arousal level, mainly via the visual system.

Role of the mesopontine area in the circling behavior induced by apomorphine in rats bearing unilateral lesion of the entopeduncular nucleus

The role of the mesopontine area and superior colliculus in turning behavior induced by systemic administration of apomorphine was studied in rats bearing a unilateral entopeduncular lesion. Bilateral electrolytic damage of the superior colliculus resulted in an enhancement of the ipsilateral circling response to apomorphine, perhaps as a consequence of an increased locomotor drive in such animals. Bilateral electrolytic lesions of the mesopontine area decreased apomorphine-induced turning in entopeduncular rats, while a bilateral kainic acid lesion of the same region was ineffective. It was concluded that the pedunculopontine nucleus and adjacent reticular formation are not an essential link for the striopallidal complex output mediating circling in this model. Fibers running through this region could be implicated in the expression of the behavior under study. Since a unilateral electrolytic lesion of the mesopontine area contralateral to the damaged entopeduncular nucleus reduced drug-induced turning, we propose that an uncrossed pathway from the intact striopallidal complex mediates circling in our rats.

The effects of d-amphetamine on the electrophysiological activity of the superior colliculus in the rat

The superior colliculus (SC) is thought to have an inhibitory effect on arousal (and hence activity) in rats when light is present. d-Amphetamine is believed to suppress this inhibition of the SC on arousal. The present study investigated the electrophysiological activity of the SC of the rat under the influence of d-amphetamine in both light and dark conditions. A single numerical arousal index value (AI) for the electrophysiological data obtained by dividing the frequency of the waves by the voltage over a specified interval was used. Results showed that light conditions decreased the AI of the SC. The AI was, however, not affected by d-amphetamine. Also, the electrophysiological activity of the SC showed spontaneous emissions of high-voltage (600 microV) alpha waves which generally occurred in two- to four-second bursts. Occurrence of this high-voltage activity was reduced by increasing doses of d-amphetamine.

The effects of superior colliculus lesions on reactions to novelty in the hamster

The aim of this experiment was to examine reactions to novelty of hamsters with large bilateral collicular lesions in an open field containing large conspicuous objects. A video-actographic system was used to quantify the contacts with the objects, the speed of the displacements and the angular head movements. Due to different evolutions of the scores in each group, most of the differences were found at the end of the experiment. Collicular animals, unlike controls, make contacts with the objects but do not habituate, which suggests that these contacts are not investigatory but fortuitous. This possibility is supported by the fact that collicular animals display hyperactive locomotion with stereotyped patterns by the end of the experiment. It is proposed that the depth of lesions and the salience of the stimulus are two modulating factors of the reaction to novelty in collicular animals.

Contralateral head movements produced by microinjection of glutamate into superior colliculus of rats: evidence for mediation by multiple output pathways

One of the major efferent pathways of the superior colliculus crosses midline to run caudally in the contralateral predorsal bundle, innervating targets in the brain stem and eventually reaching the cervical spinal cord. A variety of evidence suggests that this tecto-reticulo-spinal pathway may mediate the orienting movements that can be evoked by tectal stimulation. However, we have recently found that orienting head movements can still be obtained in rats after section of the tecto-reticulo-spinal pathway, implying that additional pathways are also involved. The present study sought to test this implication, by taking advantage of the fact that in rats the cells of origin of the tecto-reticulo-spinal pathway are largely segregated within the lateral part of the stratum album intermediate. It is thus possible to find out whether orienting head movements can be produced by a cell-excitant from tectal regions that contain few cells of origin of the tecto-reticulo-spinal pathway. Hooded rats in an open field were filmed during microinjections of sodium L-glutamate (50 mM, 200 nl) into the superior colliculus, and the films analysed for the appearance of contralaterally directed movements of the head and body. Subsequent histological reconstruction of the injection sites indicated that such movements could be obtained from widespread areas within the superior colliculus, including not only lateral stratum album intermediale but also the deep layers, and parts of the medial superficial and intermediate layers. Moreover, sites in or close to lateral stratum album intermediate often gave circling movements with downward pointing head, whereas some sites outside lateral stratum album intermediale gave sustained immobility with the head pointing contralaterally and upwards. This evidence supports the view that tectal efferent pathways besides the tecto-reticulo-spinal pathway are involved in the control of head movement. In addition, at least some of these pathways are not collaterals of the tecto-reticulo-spinal pathway, since the movements were obtained from collicular regions with few tecto-reticulo-spinal pathway cells. Finally, the results are consistent with the view that different collicular output pathways mediate movements that have different functions.

A functional analysis of the collicular output pathways: a dissociation of deficits following lesions of the dorsal tegmental decussation and the ipsilateral collicular efferent bundle in the Mongolian gerbil

Mongolian gerbils received either lesions of the superior colliculus, small lesions of the uncrossed efferents of the superior colliculus, knifecuts of the dorsal tegmental decussation, or sham operations. The animals were subsequently tested for avoidance of overhead visual threat, orientation and locomotion toward small targets, and negotiation of a large barrier in order to reach a small target. Animals with collicular lesions showed no responses to overhead threat and had severe deficits in orienting to small targets. Animals with lesions of the uncrossed tectal efferents showed diminished responses to overhead threat but had no deficits in orienting to targets. Animals with lesions of the dorsal tegmental decussation showed only slight reductions in responses to overhead threats but clear impairments in the orientation tasks. The impairments in orientation, however, were less severe than those seen in collicular animals. Animals in all groups were able to negotiate barriers efficiently. These results suggest that separate output pathways of the superior colliculus mediate different types of visuomotor behaviours. The results further suggest that visual orientation to small targets does not depend completely on output through the predorsal bundle, but must also involve other collicular outputs.

Visual cortical lesions abolish the use of motion parallax in the Mongolian gerbil

Mongolian gerbils received lesions of either the visual cortex, pretectal nuclei, superior colliculus or a sham operation. Visual distance estimation was tested by means of a jumping task on which gerbils have previously been shown to employ motion parallax information generated by head movements. Videotaped jumps were analyzed to determine latency to jump, jump distance, and head movement frequency. While all of the lesion groups showed some changes in performance, the most severe deficits in jump accuracy were seen after the visual cortical lesions. In none of the lesion groups, however, were head movements abolished. The results suggest that the visual cortex subserves a critical aspect of dynamic distance estimation but that the motor program for generating head movements is located elsewhere.

Head and body movements produced by electrical stimulation of superior colliculus in rats: effects of interruption of crossed tectoreticulospinal pathway

Stimulation of the superior colliculus in rats produces movements of the head and body that resemble either orientation and approach towards a contralateral stimulus, or avoidance of, or escape from, such a stimulus. A variety of evidence indicates that the crossed descending pathway, which runs in the contralateral predorsal bundle to the pontomedullary reticular formation and the spinal cord, is involved in orienting movements. The nature of this involvement was investigated, by assessing the effects on tectally-elicited movements of midbrain knife-cuts intended to section the pathway as it crosses midline in the dorsal tegmental decussation. As expected, ipsilateral movements resembling avoidance or escape were little affected by dorsal tegmental decussation section, whereas contralateral circling movements of the body were almost abolished. However, contralateral movements of the head in response to electrical stimulation were not eliminated, nor were orienting head movements to visual or tactile stimuli. There was some suggestion that section of the dorsal tegmental decussation increased the latency of head movements from electrical stimulation at lateral sites, and decreased the accuracy of orienting movements to sensory stimuli. These results support the view that the crossed tectoreticulospinal system is concerned with approach rather than avoidance movements. However, it appears that other, as yet unidentified, tectal efferent systems are also involved in orienting head movements. It is possible that this division of labour may reflect functional differences between various kinds of apparently similar orienting responses. One suggestion is that the tectoreticulospinal system is concerned less in open-loop orienting responses (that are initiated but not subsequently guided by sensory stimuli), than in following or pursuit movements.

Tonic desynchronisation of cortical electroencephalogram by electrical and chemical stimulation of superior colliculus and surrounding structures in urethane-anaesthetised rats

Damage to the superior colliculus in rats impairs desynchronisation of the cortical electroencephalogram in response to light flashes. However, it is unclear which elements within the superior colliculus, and which efferent collicular pathways, might be involved in alerting cerebral cortex to visual stimuli. To investigate this problem, the superior colliculus and surrounding structures were stimulated either electrically (3 s trains of 0.2 ms 100 Hz cathodal pulses), or chemically (200 nl of 5 mM sodium L-glutamate), in rats anaesthetised with urethane. The cortical electroencephalogram was recorded bilaterally from frontal cortex. At each site tested with electrical stimulation the threshold current (up to 60 microA) required to produce tonic desynchronisation (outlasting stimulation-offset by at least 10 s) was determined. Comparison of the effects of electrical and chemical stimulation suggested the following: (1) stimulation of cells in the deep layers of the superior colliculus can desynchronise the cortical electroencephalogram. There may also be an additional effective area in the rostral part of the superficial layers, but this needs to be confirmed in unanaesthetised animals. (2) Stimulation of fibres in the deep white layers of caudal superior colliculus, and of cells in a wide area of caudal midbrain reticular formation, are also effective at desynchronising the cortical electroencephalogram. It is therefore possible that the ipsilateral descending pathway, that runs from the superior colliculus to terminate in the parabigeminal and cuneiform nuclei and surrounding reticular formation, is involved in mediating cortical desynchronisation initiated by the superior colliculus. Evidence from other studies indicates that some sites in this pathway may be part of a "defence arousal system". (3) Sites on the ascending pathways from the superior colliculus, to structures including dorsal thalamus, pretectum, zona incerta and rostral midbrain reticular formation, were relatively ineffective at tonically desynchronising the cortex. However, some of these pathways might mediate phasic, movement-related arousal of collicular origin.

Feeding induced by injections of muscimol into the substantia nigra of rats: unaffected by haloperidol but abolished by large lesions of the superior colliculus

Intense activation of central dopamine systems has been associated with oral stereotyped behaviour, whereas less intense stimulation of these systems can increase feeding in non-deprived animals. There are several lines of evidence which suggest that the gamma-aminobutyric acid-containing striatonigral and nigrotectal projections are essential pathways mediating dopamine-related oral stereotypy. The present series of experiments was conducted to examine whether the same output route also mediates dopamine-related feeding. In the first experiment it was shown that bilateral injections of a sub-stereotypic dose of muscimol (0.05 nM) into the substantia nigra increased feeding of non-deprived rats. In Experiment II the feeding response was further characterised by demonstrating that food intake was initially suppressed for 30 min after which it was potentiated for 90 min. In Experiment III it was shown that a single dose of haloperidol (0.4 mg/kg), which was adequate to suppress overall food intake, was ineffective in preventing the increase in feeding produced by intranigral muscimol (0.05 nM). In contrast, it was demonstrated in Experiment IV that large lesions of the superior colliculus completely abolished the muscimol-induced increase in feeding. These results suggest that the striatonigral and nigrotectal projections may be important efferent pathways for both the oral stereotypy and the feeding responses linked with central dopamine transmission.

Reduced locomotor activity as an acute effect of damage to superior colliculus in rats

Rats with either electrolytic or radiofrequency lesions of the superior colliculus were tested in an open-field within 24 h of operation. They crossed significantly fewer squares and spent more time motionless then control animals, an effect that disappeared upon retesting 13 days later. Previously reported locomotor hyperactivity thus appears to be a chronic but not an acute effect of collicular damage in rats.

Dissociation of stimulation-bound feeding and apomorphine-induced gnawing by lesions of superior colliculus

The intense stereotyped gnawing induced by high doses of apomorphine is almost abolished by large bilateral lesions of the superior colliculus. It has been argued that the feeding produced by electrical stimulation of the lateral hypothalamic area is closely related to dopamine-mediated oral stereotypies; if so, it might be expected that lesions of the superior colliculus would also disrupt stimulation-bound feeding. Feeding was obtained from 14 hypothalamic electrodes in 8 hooded Lister rats. Subsequent electrolytic lesions of the superior colliculus had no overall effect on this behaviour from 13 of the 14 electrodes, even though the lesions reduced the gnawing induced by 8-20 mg/kg apomorphine to less than 10% of its preoperative intensity. It is concluded that stimulation-bound feeding and apomorphine-induced gnawing are not dependent on identical neural circuitry, and therefore stimulation-bound feeding is probably not mediated by the nigrotectal pathway.

Suppression of apomorphine-induced oral stereotypy in rats by microinjection of muscimol into midbrain

Rats with large lesions of the superior colliculus do not display the oral stereotypy normally induced by high systemic doses of dopamine-agonists. It has been suggested that collicular lesions have such an effect because they destroy the GABAergic nigrotectal pathway. This suggestion was investigated by observing the effects of bilateral microinjections of the GABA-agonist muscimol into midbrain sites in rats given 8 mg/kg subcutaneous apomorphine. A low dose of muscimol (25 ng in 0.5 ul saline/side) injected into regions of the superior colliculus with nigrotectal innervation almost abolished apomorphine-induced licking and gnawing. Control microinjections of saline into the superior colliculus, or of muscimol into overlying cerebral cortex, were ineffective. This result is consistent with the GABAergic nigrotectal projection being important for the expression of dopamine-related oral stereotypy. It was also found, however, that 25 ng of muscimol suppressed oral stereotypy when microinjected into the mesencephalic reticular formation underlying the superior colliculus. The anatomical basis of this latter effect is uncertain.

Locomotor activity of rats in open field after microinjection of procaine into superior colliculus or underlying reticular formation

Whereas large lesions of the superior colliculus in rats increase locomotor activity in the open field, bilateral collicular microinjections of muscimol (an agonist of the inhibitory neurotransmitter GABA) have been reported to reduce open-field activity. This difference might be due to muscimol's acting on a subpopulation of collicular neurones, or to some feature of the microinjection technique. The issue was investigated by observing open-field behavior after reversible lesions produced by bilateral microinjections of the local anaesthetic procaine (10-300 micrograms in 0.5 microliter) into midbrain sites. Injections of procaine into the superior colliculus produced effects similar to those reported after muscimol injections: both locomotor activity and other exploratory responses were suppressed, with the rats spending much of their time motionless in an alert posture. In contrast, animals with injections of procaine into the mesencephalic reticular formation (MRF) ventral to the superior colliculus resembled rats given large collicular lesions: they showed very striking increases in locomotor activity, while their rearing and exploratory head movements were reduced. It is suggested that in some experiments large collicular lesions may have increased locomotor activity in the open field because they invaded underlying MRF. However, it is also possible that in rodents the acute effects of collicular inactivation, as assessed by microinjection of muscimol or procaine, are different from the chronic effects that are observed in experiments with electrolytic or radiofrequency lesions.

Lesions of the superior colliculus in the rat differentiate between nigrostriatal and mesolimbic dopamine systems

Bilateral electrolytic lesions of the superior colliculus in rats increased spontaneous locomotor activity, enhanced amphetamine-induced hyperactivity and attenuated apomorphine-induced biting. These lesions were associated with an increased rate of turnover of dopamine in the nucleus accumbens, but not in the striatum. Similarly concentrations of the dopamine metabolites homovanillic acid and 3,4-dihydroxyphenylacetic acid were elevated in accumbens tissue but not in striatum in rats with bilateral collicular lesions. The results indicate that lesions of the superior colliculus cause differentiation between hyperactivity and stereotypy, and that this may be related to blockade of a nigrostriatal outflow, and relief of inhibition on mesolimbic systems.

Toward a neuroethology of mammalian vision: ecology and anatomy of rodent visuomotor behavior

The great diversity of the niches inhabited by rodents, and the variety of the visual demands of these niches, present an excellent prospect for a comprehensive neuroethological analysis of rodent visuomotor behavior. To this end, rodent taxonomy is reviewed, with special attention to the multiple independent invasions of arboreal, terrestrial, fossorial and aquatic niches by distantly related rodent species. Current work on rat, gerbil and hamster is reviewed with emphasis on visual contributions to naturalistic behaviors such as exploration, foraging, predator detection and conspecific recognition.

Visual discrimination learning in rats with lesions of superior colliculus: door-push and approach errors in modified jumping stand

It has been suggested that, for some species, lesions of the superior colliculus affect visual discrimination learning, but only in certain conditions: (a) when problems are first learnt only after operation, or (b) when discriminanda require detailed scanning, or (c) when “approach” responses to the discriminanda are measured, rather than the response of actually touching them. These suggestions were examined in rats learning visual discriminations in a modified jumping-stand apparatus, after sustaining large lesions of the superior colliculus (and in some cases also of the pretectum). The lesions produced open-field hyperactivity and reduced exploration, indicating effective tectal damage, but the rats learnt a series of difficult discriminations in a door-push task as fast as normal rats, and they did not make more approach errors. Their main abnormality in the discrimination apparatus was that they looked less often between the stimulus doors before stepping across to one of them from the central platform. It is suggested that in rats, as in other animals, lesions of the superior colliculus disrupt the control of scanning head and eye movements; in rats, however, such disruption need not affect discrimination learning (at least in some kinds of apparatus), possibly because the retina of the rat has a relatively poorly developed area centralis.

Grating detection and visual acuity after lesions of striate cortex in hooded rats

The ability of rats to detect high-contrast square-wave gratings over a range of spatial frequencies was measured before and after ablation of striate cortex. The animals relearnt to detect low-frequency gratings very quickly after operation, and their acuity was reduced from 1.0 c/deg to about 0.7 c/deg. These effects were in striking contrast to those produced by larger posterior cortical ablations, which included both striate and prestriate cortex (Dean 1978); after the larger lesions, rats required many weeks of retraining to detect even low-frequency gratings and their acuity was reduced to 0.3 c/deg. The difference in the effects of the two lesions suggested that the rats with striate ablation were using information about spatial contrast that was relayed either by spared remnants of the geniculo-cortical pathway, or by the pathway from superior colliculus to prestriate cortex via the lateral posterior nucleus. To try and distinguish between these possibilities, the destriate rats were given a further lesion of the superior colliculus. This second lesion severely disrupted contrast detection: the animals made about as many errors as rats with large posterior cortical removal in relearning to detect a low-frequency grating, which is about 20 to 30 times as many as after either striate cortex or superior colliculus lesions alone. This result suggests that rats, like other mammals, can use spatial information conveyed in the tectocortical path when striate cortex has been destroyed.

Gnawing and changes in reactivity produced by microinjections of picrotoxin into the superior colliculus of rats

Lesions of the superior colliculus in rats attenute the oral stereotypy produced by systemic administration of dopamine agonists. Current evidence suggest that such drugs affect the superior colliculus by reducing transmission in the inhibitory GABAergic nigrotectal pathway. To investigate whether the superior colliculus plays a direct role in producing stereotyped oral movements, the present experiment therefore examined the effects of collicular microinjections of the GABA antagonist picrotoxin on the behaviour of rats observed in an open-field. Gnawing was observed after injections of picrotoxin (25 ng) into sites in the intermediate and deep layers of the superior colliculus, consistent with the superior colliculus playing a direct part in producing the stereotyped gnawing seen after systemic administration of dopamine agonists. However, gnawing was only observed after a period in which the animal showed strong avoidance reactions to stimuli that normally evoked orienting or little reaction. This change in reactivity was observed after injections of picrotoxin into all parts of the colliculus, but the most sensitive (responding to doses as low as 12.5 ng) were mainly in the superficial and intermediate layers. It appears that there may be more than one GABAergic system within the superior colliculus.

Collicular picrotoxin alleviates akinesia but not sensory neglect in rats with bilateral 6-hydroxydopamine lesions of ventral midbrain

Anatomical and biochemical investigations have suggested that GABA transmission in the superior colliculus consequent upon activity of the nigrotectal pathway is increased following 6-hydroxydopamine (6-OHDA) lesions of the ascending dopamine systems. Moreover, it has been proposed that this increase in inhibitory activity within the colliculus may be responsible for the sensory neglect commonly observed after dopamine denervation. The present experiment sought to test this proposal by examining the effects of injections of the GABA antagonist picrotoxin into the superior colliculus of 6-OHDA lesioned rats, in the hope that the neglect caused by the 6-OHDA would be reversed. However, in 33 of 36 cases studied intracollicular microinjections of picrotoxin produced no detectable improvement in orientation to sensory stimuli, although a wide range of other behavioural effects was observed. These included stereotyped exploratory movements (e.g. head waving, walking, sniffing and rearing) similar to those produced in the 6-OHDA treated rats by systemic injection of apomorphine (0.1 mg/kg). These data indicate that 6-OHDA lesions of substantia nigra and ventral midbrain areas do not produce sensory neglect simply by increasing GABA transmission within the nigrotectal pathway. Instead, such an increase in nigrotectal activity may impair production of particular kinds of movement, possibly related to exploratory behaviour.

Dissociation of d-amphetamine-induced locomotor activity and stereotype behaviour by lesions of the superior colliculus

To investigate the role of the nigrotectal pathway in the expression of the behavioral effects of increased dopamine transmission, the behavior of rats with lesions of the superior colliculus was studied in open-field and hole-board after systemic administration of d-amphetamine. The rats with collicular lesions had higher locomotor activity scores than controls after saline injections, and after all doses of amphetamine studied (0.5-24 mg/kg): the difference was particularly marked at the highest doses used (16 and 24 mg/kg). Stereotyped behavior, on the other hand, was less pronounced in the rats with collicular lesions, who unlike controls rarely showed stereotyped sniffing, head waving, or forepaw padding while moving about in either apparatus, and never licked or gnawed in the hole-board. This evidence suggests that the nigrotectal pathway may play a role in the expression of some of the stereotyped behaviors observed after the administration of dopamine agonists, but that it is not necessary for the increased locomotor activity produced by such drugs.