Oral behaviour induced by intranigral muscimol is unaffected by haloperidol but abolished by large lesions of superior colliculus

Pathways from the Superior Colliculus to the Basal Ganglia

The present work aims to review the structural organization of the mammalian superior colliculus (SC), the putative pathways connecting the SC and the basal ganglia, and their role in organizing complex behavioral output. First, we review how the complex intrinsic connections between the SC's laminae projections allow for the construction of spatially aligned, visual-multisensory maps of the surrounding environment. Moreover, we present a summary of the sensory-motor inputs of the SC, including a description of the integration of multi-sensory inputs relevant to behavioral control. We further examine the major descending outputs toward the brainstem and spinal cord. As the central piece of this review, we provide a thorough analysis covering the putative interactions between the SC and the basal ganglia. To this end, we explore the diverse thalamic routes by which information from the SC may reach the striatum, including the pathways through the lateral posterior, parafascicular, and rostral intralaminar thalamic nuclei. We also examine the interactions between the SC and subthalamic nucleus, representing an additional pathway for the tectal modulation of the basal ganglia. Moreover, we discuss how information from the SC might also be relayed to the basal ganglia through midbrain tectonigral and tectotegmental projections directed at the substantia nigra compacta and ventrotegmental area, respectively, influencing the dopaminergic outflow to the dorsal and ventral striatum. We highlight the vast interplay between the SC and the basal ganglia and raise several missing points that warrant being addressed in future studies.

The Superior Colliculus: Cell Types, Connectivity, and Behavior

The superior colliculus (SC), one of the most well-characterized midbrain sensorimotor structures where visual, auditory, and somatosensory information are integrated to initiate motor commands, is highly conserved across vertebrate evolution. Moreover, cell-type-specific SC neurons integrate afferent signals within local networks to generate defined output related to innate and cognitive behaviors. This review focuses on the recent progress in understanding of phenotypic diversity amongst SC neurons and their intrinsic circuits and long-projection targets. We further describe relevant neural circuits and specific cell types in relation to behavioral outputs and cognitive functions. The systematic delineation of SC organization, cell types, and neural connections is further put into context across species as these depend upon laminar architecture. Moreover, we focus on SC neural circuitry involving saccadic eye movement, and cognitive and innate behaviors. Overall, the review provides insight into SC functioning and represents a basis for further understanding of the pathology associated with SC dysfunction.

Physiological oculo-auricular-facial-mandibular synkinesis elicited in humans by gaze deviations

Integrated motor behaviors involving ocular motion-associated movements of the head, neck, pinna, and parts of the face are commonly seen in animals orienting to a visual target. A number of coordinated movements have also been observed in humans making rapid gaze shifts to horizontal extremes, which may be vestiges of these. Since such integrated mechanisms point to a non-pathological co-activation of several anatomically separate cranial circuits in humans, it is important to see how the different pairs of integrative motor behaviors with a common trigger (i.e., ocular motion) manifest in relation to one another. Here, we systematically examined the pattern of eye movement-induced recruitment of multiple cranial muscles in humans. Simultaneous video-oculography and bilateral surface electromyograms of transverse auricular, temporalis, frontalis, and masseter muscles were recorded in 15 healthy subjects (8 females; 29.3±5.2 years) while they made head-fixed, horizontal saccadic, pursuit and optokinetic eye movements. Potential chin laterotrusion linked to contractions of masticator muscles was captured with a yaw-fixed accelerometer. Our findings objectively show an orchestrated aural-facial-masticatory muscle response to a range of horizontal eye movements (prevalence of 21-93%). These responses were most prominent during eccentric saccades. We further reveal distinctions between the various observed activation patterns in terms of their profile (transient or sustained), laterality (with respect to direction of gaze) and timing (with respect to saccade onset). Possible underlying neural substrates, their atavistic behavioral significance, and potential clinical applications for monitoring sensory attention and designing attention-directed hearing aids in the future are discussed.

A Dynamic Circuit Hypothesis for the Pathogenesis of Blepharospasm

Blepharospasm (sometimes called “benign essential blepharospasm,” BEB) is one of the most common focal dystonias. It involves involuntary eyelid spasms, eye closure, and increased blinking. Despite the success of botulinum toxin injections and, in some cases, pharmacologic or surgical interventions, BEB treatments are not completely efficacious and only symptomatic. We could develop principled strategies for preventing and reversing the disease if we knew the pathogenesis of primary BEB. The objective of this study was to develop a conceptual framework and dynamic circuit hypothesis for the pathogenesis of BEB. The framework extends our overarching theory for the multifactorial pathogenesis of focal dystonias (Peterson et al., 2010) to incorporate a two-hit rodent model specifically of BEB (Schicatano et al., 1997). We incorporate in the framework three features critical to cranial motor control: (1) the joint influence of motor cortical regions and direct descending projections from one of the basal ganglia output nuclei, the substantia nigra pars reticulata, on brainstem motor nuclei, (2) nested loops composed of the trigeminal blink reflex arc and the long sensorimotor loop from trigeminal nucleus through thalamus to somatosensory cortex back through basal ganglia to the same brainstem nuclei modulating the reflex arc, and (3) abnormalities in the basal ganglia dopamine system that provide a sensorimotor learning substrate which, when combined with patterns of increased blinking, leads to abnormal sensorimotor mappings manifest as BEB. The framework explains experimental data on the trigeminal reflex blink excitability (TRBE) from Schicatano et al. and makes predictions that can be tested in new experimental animal models based on emerging genetics in dystonia, including the recently characterized striatal-specific D1R dopamine transduction alterations caused by the GNAL mutation. More broadly, the model will provide a guide for future efforts to mechanistically link multiple factors in the pathogenesis of BEB and facilitate simulations of how exogenous manipulations of the pathogenic factors could ultimately be used to prevent and reverse the disorder.

A GABAergic nigrotectal pathway for coordination of drinking behavior

The contribution of basal ganglia outputs to consummatory behavior remains poorly understood. We recorded from the substantia nigra pars reticulata (SNR), the major basal ganglia output nucleus, during self-initiated drinking. The firing rates of many lateral SNR neurons were time-locked to individual licks. These neurons send GABAergic projections to the deep layers of the orofacial region of the lateral tectum (superior colliculus, SC). Many tectal neurons are also time-locked to licking, but their activity is usually antiphase to that of SNR neurons, suggesting inhibitory nigrotectal projections. We used optogenetics to selectively activate the GABAergic nigrotectal afferents in the deep layers of the SC. Photo-stimulation of the nigrotectal projections transiently inhibited the activity of the lick-related tectal neurons, disrupted their licking-related oscillatory pattern, and suppressed self-initiated drinking. These results demonstrate that GABAergic nigrotectal projections play a crucial role in coordinating drinking behavior.

Elevated dopamine alters consummatory pattern generation and increases behavioral variability during learning

The role of dopamine in controlling behavior remains poorly understood. In this study we examined licking behavior in an established hyperdopaminergic mouse model—dopamine transporter knockout (DAT KO) mice. DAT KO mice showed higher rates of licking, which is due to increased perseveration of licking in a bout. By contrast, they showed increased individual lick durations, and reduced inter-lick intervals. During extinction, both KO and control mice transiently increased variability in lick pattern generation while reducing licking rate, yet they showed very different behavioral patterns. Control mice gradually increased lick duration as well as variability. By contrast, DAT KO mice exhibited more immediate (within 10 licks) adjustments—an immediate increase in lick duration variability, as well as more rapid extinction. These results suggest that the level of dopamine can modulate the persistence and pattern generation of a highly stereotyped consummatory behavior like licking, as well as new learning in response to changes in environmental feedback. Increased dopamine in DAT KO mice not only increased perseveration of bouts and individual lick duration, but also increased the behavioral variability in response to the extinction contingency and the rate of extinction.

Superior colliculus mediates cervical dystonia evoked by inhibition of the substantia nigra pars reticulata

Cervical dystonia (CD; spasmodic torticollis) can be evoked by inhibition of substantia nigra pars reticulata (SNpr) in the nonhuman primate (Burbaud et al., 1998; Dybdal et al., 2012). Suppression of GABAergic neurons that project from SNpr results in the disinhibition of the targets to which these neurons project. It therefore should be possible to prevent CD by inhibition of the appropriate nigral target region(s). Here we tested the hypothesis that the deep and intermediate layers of the superior colliculus (DLSC), a key target of nigral projections, are required for the emergence of CD. To test this hypothesis, we pretreated the DLSC of four macaques with the GABA(A) agonist muscimol to determine whether this treatment would prevent CD evoked by muscimol infusions in SNpr. Our data supported this hypothesis: inhibition of DLSC attenuated CD evoked by muscimol in SNpr in all four animals. In two of the four subjects, quadrupedal rotations were evoked by muscimol application into SNpr sites that were distinct from those that induced dystonia. We found that inhibition of DLSC did not significantly alter quadrupedal rotations, suggesting that this response is dissociable from the SNpr-evoked CD. Our results are the first to demonstrate a role of DLSC in mediating the expression of CD. Furthermore, these data reveal a functional relationship between SNpr and DLSC in regulating posture and movement in the nonhuman primate, raising the possibility that the nigrotectal pathway has potential as a target for therapeutic interventions for CD.

Behavioral pharmacology of orofacial movement disorders

Dysfunction in orofacial movement is evident in patients with schizophrenia, Parkinson's disease and Huntington's disease. In animal studies on orofacial dyskinesia, these neurological disorders have been considered as a starting point to examine the pathophysiology and mechanisms underlying the symptoms. There is circumstantial evidence that orofacial dyskinesia in humans might be the consequence of hyperfunctioning mesolimbic-pallidal circuitry, in which the mesolimbic region occupies a central role, in contrast to typical Parkinson-like symptoms which involve hypofunction in the nigrostriato-nigral circuity. Studies in animals suffer from technical difficulties concerning the assessment of orofacial behaviors. There are some experimental designs that provide detailed information on the amplitude and the frequency of the jaw movements. By using such methods, the involvement of neurotransmitter systems and functional neural connections within the basal ganglia has been studied in rat rhythmical jaw movements. Regarding neurotransmitter systems, dopaminergic, cholinergic, γ-aminobutyric acid (GABA)ergic and glutamaterigic systems have been shown to be involved in rat rhythmical jaw movements. The involved neural connections have also been investigated, focusing on the differential role between the dorsal and ventral part of the striatum, the shell and core of the nucleus accumbens and the output pathways from the striatum and the nucleus accumbens. Taking available clinical and experimental evidence, the orofacial dyskinesias are thought to arise when hierarchically lower order output stations of the mesolimbic region start to dysfunction as a consequence of the arrival of distorted information sent by the mesolimbic region. This review seeks to provide an overview of prior and recent findings across several orofacial movement disorders and interpret new insights in the context of the limitations of behavioral pharmacology and prior knowledge of the regulation of behavior by dopamine receptors and other related neuronal systems.

The superior colliculus contains a discrete region involved in the control of jaw movements: role of GABAA receptors

The role of GABA(A) receptors in the superior colliculus in the production of rat repetitive jaw movements was examined, as this nucleus receives tonic GABAergic inhibitory inputs from the dorsolateral part of the substantia nigra pars reticulata and the entopeduncular nucleus. Both regions are also connected with the ventrolateral striatum where stimulation of either dopamine or acetylcholine receptors has been found to elicit distinct types of jaw movements in rats. The GABA(A) receptor antagonist bicuculline (50 and 150 ng/0.2 microl per side) dose-dependently produced repetitive jaw movements only when injected bilaterally into a circumscribed region (A 3.0) of the lateral deeper layers of the superior colliculus; this region is known to receive input predominantly from the dorsolateral part of the substantia nigra pars reticulata. The effects of bicuculline were GABA(A) receptor specific because the effects were abolished by muscimol, a GABA(A) receptor agonist, given into the same site. The bicuculline-induced jaw movements differed qualitatively from those elicited by injection of a mixture of (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 microg) and quinpirole (10 microg), agonist at dopamine D1 and D2 receptors, respectively, or carbachol (2.5 microg), an acetylcholine receptor agonist, into the ventrolateral striatum. Nevertheless, injection of muscimol into the lateral deeper layers of the superior colliculus (A 3.0) inhibited jaw movements evoked by the dopamine D1/D2 receptor stimulation. Conversely, the jaw movements evoked by acetylcholine receptor stimulation were enhanced by injection of muscimol into the superior colliculus. In conclusion, GABA(A) receptor blockade in a circumscribed region (A 3.0) of the lateral deeper layers of the superior colliculus elicits characteristic repetitive jaw movements, and the GABA(A) receptors in that region modulate the dopamine D1/D2 receptor-mediated and acetylcholine receptor-mediated jaw movements in an opposite manner.

The role of nigral and thalamic output pathways in the expression of oral stereotypies induced by amphetamine injections into the striatum

Microinjections of amphetamine into the ventrolateral striatum (VLS) elicit a striking behavioral syndrome characterized by compulsive oral and forelimb motor stereotypies. The neural pathways that mediate these behavioral responses downstream from the striatum have not yet been identified. In a series of experiments, we investigated the involvement of the substantia nigra pars reticulata (SNr) and the ventromedial nucleus of the thalamus (VMT) in the mediation of this behavioral syndrome. We demonstrated that lidocaine-induced reversible inactivation of the SNr reduced amphetamine-induced stereotyped biting and gnawing behaviors, suggesting that the nigral output pathway plays a significant role in the expression of these behavioral responses. In turn, injections of lidocaine into the VMT only transiently reduced amphetamine-stimulated biting and increased stereotyped gnawing and paw nibbling, suggesting that the expression of oral stereotypies induced by amphetamine injections into the VLS is not dependent on thalamocortical feedback.

Microinjections of muscimol into lateral superior colliculus disrupt orienting and oral movements in the formalin model of pain

An important reaction in rodent models of persistent pain is for the animal to turn and bite/lick the source of discomfort (autotomy). Comparatively little is known about the supraspinal pathways which mediate this reaction. Since autotomy requires co-ordinated control of the head and mouth, it is possible that basal ganglia output via the superior colliculus may be involved; previously this projection has been implicated in the control of orienting and oral behaviour. The purpose of the present study was therefore, to test whether the striato-nigro-tectal projection plays a significant role in oral responses elicited by subcutaneous injections of formalin. Behavioural output from this system is normally associated with the release of collicular projection neurons from tonic inhibitory input from substantia nigra pars reticulata. Therefore, in the present study normal disinhibitory signals from the basal ganglia were blocked by injecting the GABA agonist muscimol into different regions of the rat superior colliculus. c-Fos immunohistochemistry was used routinely to provide regional estimates of the suppressive effects of muscimol on neuronal activity. Biting and licking directed to the site of a subcutaneous injection of formalin (50 microliters of 4%) into the hind-paw were suppressed in a dose-related manner by bilateral microinjections of muscimol into the lateral superior colliculus (10-50 ng; 0.5 microliter/side); injections into the medial superior colliculus had little effect. Bilateral injections of muscimol 20 ng into lateral colliculus caused formalin-treated animals to re-direct their attention and activity from lower to upper regions of space. Muscimol injected unilaterally into lateral superior colliculus elicited ipsilateral turning irrespective of which hind-paw was injected with formalin. Oral behaviour was blocked when the muscimol and formalin injections were contralaterally opposed; this was also true for formalin injections into the front foot. Interestingly, when formalin was injected into the perioral region, injections of muscimol into the lateral superior colliculus had no effect on the ability of animals to make appropriate contralaterally directed head and body movements to facilitate localization of the injected area with either front- or hind-paw. These findings suggest that basal ganglia output via the lateral superior colliculus is critical for responses to noxious stimuli which entail the mouth moving to and acting on the foot, but not when the foot is the active agent applied to the mouth. The data also suggest that pain produces a spatially non-specific facilitation of units throughout collicular maps, which can be converted into a spatially inappropriate signal by locally suppressing parts of the map with the muscimol.

Animal model explains the origins of the cranial dystonia benign essential blepharospasm

The current study demonstrates that combining two mild alterations to the rat trigeminal reflex blink system reproduces the symptoms of benign essential blepharospasm, a cranial dystonia characterized by uncontrollable spasms of blinking. The first modification, a small striatal dopamine depletion, reduces the tonic inhibition of trigeminal reflex blink circuits. The second alteration, a slight weakening of the lid-closing orbicularis oculi muscle, begins an adaptive increase in the drive on trigeminal sensory-motor blink circuits that initiates blepharospasm. By themselves, neither of these modifications causes spasms of lid closure, but combined, they induce bilateral forceful blinking and spasms of lid closure. A two-factor model based on these rodent experiments may explain the development of benign essential blepharospasm in humans. The first factor, a subclinical loss of striatal dopamine, creates a permissive environment within the trigeminal blink circuits. The second factor, an external ophthalmic insult, precipitates benign essential blepharospasm. This two-factor model may also be applicable to the genesis of other cranial dystonias.

Regional distribution of the anticonvulsant and behavioural effects of muscimol injected into the substantia nigra of rats

Previous anatomical investigations have reported a direct projection from substantia nigra pars lateralis to the dorsal midbrain anticonvulsant zone. The present study tested the hypothesis that the anticonvulsant properties of nigral inhibition previously attributed to substantia nigra pars reticulata were, in fact, due to the suppression of neural activity in the adjacent pars lateralis. Using the electroshock model of epilepsy, a systematic map of the anticonvulsant effects of bilateral injections of muscimol (60 ng/0.5 mu l per side) into different parts of substantia nigra was constructed. Electroshock (1 s of 40 mA 50 Hz AC) was administered via ear-clip electrodes 5 or 60 min following injections of muscimol, or 60 min after control injections of saline. To provide insight into the functional mechanisms whereby nigral inhibition might suppress tonic seizures the behavioural effects elicited by muscimol were also noted. No evidence supporting the experimental prediction was found. The most sensitive region of substantia nigra for suppressing tonic hindlimb extension was caudal pars reticulata. These data indicate a serious mismatch between the results of microinjection mapping studies and underlying patterns of anatomical connectivity. The behavioural reaction most closely associated with tonic seizure suppression was stereotyped locomotion; both were obtained maximally from caudal pars reticulata. Rostral substantia nigra was associated more with oral stereotypy, while a raised head position was observed at lateral injection sites and a lowered positioning of the head at medial locations. These data suggest that the rat substantia nigra may contain a functional organization based on a form of somatomotor topography. This organization may influence which part of the substantia nigra is most effective in suppressing seizures expressed by different muscle groups of the body.

Mapping of limbic seizure progressions utilizing the electrogenic status epilepticus model and the 14C-2-deoxyglucose method

We have previously described a model of limbic status epilepticus in which chronic prolonged seizure states of immobile, exploratory, minor convulsive or clonic convulsive behavior are induced by intracerebral electrical stimulation; these states appear to belong to the same behavioral progression as kindled seizures. We postulated that the underlying seizure substrates, as mapped by the 14C-2-deoxyglucose method, should reflect a corresponding anatomic progression of discharge spread. Status epilepticus was induced in rat by pulsed-train current delivered for up to 90 min to one of several subcortical areas. Autoradiographs revealed that most of the observed patterns of seizure-induced metabolic activation comprised a hierarchical sequence, such that progressively more extensive patterns subsumed anatomic territories activated in less extensive patterns, thus allowing inferences as to the progression of discharge spread. In this sequence, the basolateral amygdala ipsilateral to the induction electrode was among the first structures to be activated. In successively larger activation patterns a small unilateral network related to basolateral amygdala was involved; this evolved through a transitional state to a unilateral extensive limbic pattern; which in turn was succeeded by bilateral extensive limbic activation. This hierarchical sequence culminated in a neocortical activation pattern, in which most of the forebrain was involved in intense seizure-induced activation. Seizure behaviors increased in severity in correspondence with the underlying seizure-activated anatomic substrate. In contrast, patterns of seizure activation were observed which did not fit within the early stages of the above sequence, although analysis indicates that the later stages of spread may be shared. The study of these patterns and those reported in the literature indicates that although limbic seizure networks may be anatomically distinct at their origination, further expansion is characterized by overlap; upon assumption of extensive patterns of activation the number of nuclei participating is so vast that the identity of the limbic originator is lost and common convulsive manifestations occur.

Descending projections from the superior colliculus to the reticular formation around the motor trigeminal nucleus and the parvicellular reticular formation of the medulla oblongata in the rat

We observed by the anterograde and retrograde tracing techniques in the rat that the lateral part of the superior colliculus (SC), where the nigrotectal fibers from the dorsolateral part of the substantia nigra pars reticulata (SNr) terminated, sent projection fibers to the reticular region around the motor trigeminal nucleus (RFmt) and parvicellular reticular formation (RFp) of the medulla oblongata, where many premotor neurons for the orofacial motor nuclei were known to be distributed. The SC neurons sending their axons to the RFmt and RFp were mainly located in the stratum griseum intermedium, and additionally in the stratum griseum profundum. Our results suggest that neuronal signals conveyed through the nigro-tecto-bulbar pathway to the RFmt and RFp may exert control influences upon oral behavior.

Synaptic organization of GABAergic inputs from the striatum and the globus pallidus onto neurons in the substantia nigra and retrorubral field which project to the medullary reticular formation

Anatomical tract-tracing and immunohistochemical techniques involving correlated light and electron microscopy were used to determine whether the descending striatal and pallidal afferents to the substantia nigra pars reticulata converge onto individual neurons projecting to the pontomedullary and medullary reticular formation in the rat. Injections of biocytin into the ventrolateral region of the striatum and Phaseolus vulgaris-leucoagglutinin into the ventrolateral and caudal regions of the globus pallidus led to overlapping anterogradely labelled terminal fields within the dorsolateral substantia nigra pars reticulata. These terminal fields were punctuated by neurons which had been retrogradely labelled following injections of wheatgerm agglutinin conjugated to horseradish peroxidase into the lateral pontomedullary reticular formation. The anterogradely labelled striatal and pallidal terminals displayed different morphological characteristics; the striatal terminals were small and diffusely distributed throughout the neuropil without any particular neuronal association whereas the pallidal terminals were large and formed pericellular baskets around the perikarya of retrogradely and non-retrogradely labelled nigral neurons. In areas of the substantia nigra where there was an overlap between the two terminal fields, individual retrogradely labelled nigroreticular neurons were found to be apposed by both sets of anterogradely labelled terminals. Electron microscopic analysis revealed that the striatonigral and pallidonigral terminals displayed different ultrastructural features, the striatal terminals were small, contained few mitochondria and formed symmetric synaptic contacts predominantly with the distal dendrites of nigroreticular neurons whereas the pallidal terminals were large, contained numerous mitochondria and formed symmetric synaptic contacts preferentially with perikarya and proximal dendrites of nigroreticular neurons. Post-embedding immunohistochemical staining revealed that both striatonigral and pallidonigral terminals, some which formed synaptic contact with nigroreticular neurons, displayed GABA immunoreactivity. Examination of twelve retrogradely labelled neurons in the electron microscope revealed that all received synaptic inputs from both sets of anterogradely labelled terminals. In addition to the substantia nigra pars reticulata, neurons of the retrorubral field were also retrogradely labelled following injections of wheatgerm agglutinin conjugated to horseradish peroxidase into pontomedullary reticular formation. These retrorubroreticular neurons were part of a continuum of labelled cells which extended from the dorsolateral substantia nigra pars reticulata caudally into the retrorubral field. When combined with anterograde tracing methods it was found that the retrorubroreticular neurons received synaptic inputs from pallidal terminals which were morphologically similar to the pallidonigral terminals and formed symmetric synapses with the neuronal somata and proximal dendrites. In contrast to nigroreticular neurons, the stratonigral terminals were not seen in contact with retrorubroreticular cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographical organization of the nigrotectal projection in rat: evidence for segregated channels

Recent evidence suggests that projections from the superior colliculus to the brainstem in rat are organized into a series of anatomically segregated output channels. To understand how collicular function may be modified by the basal ganglia it is important to know whether particular output modules of the superior colliculus can be selectively influenced by input from substantia nigra. The purpose of the present study was, therefore, to examine in more detail topography within the nigrotectal system in the rat. Small injections (10-50 nl) of a 1% solution of wheatgerm agglutinin conjugated with horseradish peroxidase were made at different locations within substantia nigra and surrounding structures. A discontinuous puff-like pattern of anterogradely transported label was found in medial and caudal parts of the ipsilateral intermediate layers of the superior colliculus. In contrast, the rostrolateral enlargement of the intermediate layers contained a greater density of more evenly distributed terminal label. Injection sites associated with this dense pattern of laterally located label were concentrated in lateral pars reticulata, while the puff-like pattern was produced by injections into ventromedial pars reticulata. Retrograde tracing experiments with the fluorescent dyes True Blue and Fast Blue revealed that injections involving the rostrolateral intermediate layers were consistently associated with a restricted column of labelled cells in the dorsolateral part of ipsilateral pars reticulata. Comparable injections into medial and caudal regions of the superior colliculus produced retrograde labelling in ventral and medial parts of the rostral two-thirds of pars reticulata. Both anterograde and retrograde tracing data indicated that contralateral nigrotectal projections arise from cells located in ventral and medial pars reticulata. The present results suggest that the main ipsilateral projection from substantia nigra pars reticulata to the superior colliculus comprises two main components characterized by regionally segregated populations of output cells and spatially separated zones of termination. Of particular interest is the apparent close alignment between terminal zones of the nigrotectal channels and previously defined populations of crossed descending output cells in the superior colliculus. Thus, the rostrolateral intermediate layers contain a concentration of terminals specifically from dorsolateral pars reticulata and output cells which project to the contralateral caudal medulla and spinal cord. Conversely, the medial and caudal intermediate layers receive terminals from ventral and medial pars reticulata and contain cells which project specifically to contralateral regions of the paramedian pontine and medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Anticonvulsant role of nigrotectal projection in the maximal electroshock model of epilepsy--I. Mapping of dorsal midbrain with bicuculline

Previous work has indicated that the anticonvulsant effect of nigral inactivation on the maximal electroshock model of generalized seizures is mediated by the projection from substantia nigra to superior colliculus. In accordance with this idea, and with the GABAergic nature of the nigrotectal pathway, microinjections of the GABAA antagonist bicuculline methiodide into the superior colliculus have been reported to block tonic hindlimb extension induced by maximal electroshock. To characterize the relevant circuitry more precisely, the present study sought to determine which region of the superior colliculus was important for the anticonvulsant effect of bicuculline by systematic mapping in the rat. Bilateral injections of bicuculline methiodide (50 pmol in 400 nl/side) were most effective in the caudal deep layers of the superior colliculus and adjoining midbrain reticular formation. These results suggest that the well-known projection from substantia nigra pars reticulata to the superior colliculus may not be involved in the anticonvulsant effect of nigral inactivation in the electroshock model, because this pathway terminates primarily in the intermediate layers of the superior colliculus throughout its rostrocaudal extent. Instead, some other pathway from ventral midbrain to a dorsal midbrain anticonvulsant zone appears to be part of the brain's anticonvulsant circuitry. The following paper [Redgrave et al. (1991) Neuroscience 46, 391-406] describes an anatomical study to characterize this pathway.

Complex motor and sensorimotor functions of striatal and accumbens dopamine: involvement in instrumental behavior processes

The suggestions that dopamine (DA) systems are involved in "motor control" and "reward" represent the classic working hypotheses on the behavioral functions of these systems. The research generated by these hypotheses has yielded results that are far more complicated than the simplest form of either hypothesis would indicate. Pharmacological or lesion-induced interference with DA function does not suppress all aspects of movement control, nor all aspects of reward, nor all aspects of motivation. The deficits produced by interference with DA systems are selective and dissociative in nature, affecting some aspects of motor or motivational function, but leaving others basically intact. In some sense the hypotheses that DA is involved in "motor" or "reward" or "motivational" processes are partly correct, but the processes to which these terms refer are too broad to offer an accurate and detailed description of the behavioral functions of brain DA. A review of the literature on the behavioral pharmacology of DA suggests that the behaviors most easily disrupted by DA antagonists are highly activated and complex learned instrumental responses that are elicited or supported by mild conditioned stimuli, and maintained for considerable periods of time. It is proposed that DA in accumbens and striatum modulates the ability of neocortical and limbic areas involved in sensory, associative, and affective processes to influence complex aspects of motor function, and also modulates the execution of complex motor acts organized by the neocortex. Thus, interference with DA systems produces a "subcortical apraxia", which dissociates complex stimulus processes from complex motor processes, but leaves aspects of those processes intact.

Animal models of Lesch-Nyhan syndrome

In humans, deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) is associated with a disorder known as Lesch-Nyhan syndrome which includes severe neurobehavioral abnormalities. Several animal models which have been developed to examine the neurobiologic substrates of this disorder have suggested a role for abnormal function in purine/dopamine neurotransmission, but the relationship between HPRT-deficiency and these abnormalities remains unknown. Recently, HPRT-deficient mice have been produced which appear to have similar, though more subtle changes in brain dopamine function. These mice will be useful in elucidating the relationship between HPRT-deficiency and the neurological deficits observed in patients with this disorder.

Behavioral demonstration of a reciprocal interaction between dopamine receptor subtypes in the mouse striatum: possible involvement of the striato-nigral pathway

It is well known that stimulation of the D-2 dopamine receptor in vitro inhibits the increased efflux of cyclic adenosine monophosphate caused by D-1 receptor agonists. Furthermore, behavioral data suggest that the striato-nigral pathway is more involved with the dopamine agonist-induced expression of oral behaviors, which are, in turn, mediated by stimulation of the D-1 receptor. We examined an in vivo model to determine whether this D-1/D-2 reciprocal interaction is detectable at a behavioral level. First, mice were pretreated with wide range of doses of the D-2 antagonist, spiperone, and then injected with a behaviorally active dose of apomorphine (a nonspecific direct dopamine agonist) and were observed for incidence of oral behavior and rated for stereotypic behavior. A biphasic effect of spiperone pretreatment was observed, at some low doses both stereotypy and oral behavior were enhanced, while at high doses, both agonist-induced behaviors were progressively inhibited. To test the specificity of this effect for the striato-nigral pathway, mice were administered discrete electrolytic lesions in the ventral portion of the internal capsule in one hemisphere. The animals that responded to apomorphine by rotating ipsilaterally to the lesion were used in two, five-point apomorphine dose-response curves, one with, and one without, pretreatment with the dose of spiperone which most enhanced stereotypic behavior and incidence of oral behavior. The spiperone pretreatment caused a clear increase in the maximum rotational response to apomorphine without affecting the ED50. These data suggest that behavior associated with the striato-nigral efferent from striatum is marked by the opposition of D-1 and D-2 receptors.

Organization of the crossed tecto-reticulo-spinal projection in rat--II. Electrophysiological evidence for separate output channels to the periabducens area and caudal medulla

The previous paper (Redgrave et al., Neuroscience 37, 571-584, 1990) presented anatomical evidence indicating there are at least two largely segregated components of the crossed tecto-reticulo-spinal pathway which project to the periabducens area and caudal medulla. An immediate question arising from this finding is whether tectal cells which project either to the periabducens area or to the caudal medulla have different electrophysiological response properties. An answer to this question would be relevant to the issue of whether different components of the tecto-reticulo-spinal system are specialized for the production of different classes of orienting movement. Accordingly, extracellularly recorded units in the superior colliculus of urethane anaesthetized rats were tested for antidromic activity following electrical stimulation of the periabducens area or the caudal medulla. When antidromic potentials were successfully recorded the sensory properties of the units were tested with a range of unimodal visual, somatosensory and auditory stimuli. The following results were obtained. (i) Tectal cells antidromically activated by stimulation of the caudal medulla were preferentially sensitive to somatosensory stimuli from the perioral region, while cells activated from the periabducens area were more frequently responsive to auditory stimuli. (ii) Tectal fibres activated by stimulation of the caudal medulla had significantly higher conduction velocities than the fibres activated by electrodes in the periabducens region. (iii) More than 90% of antidromically activated cells were located in stratum album intermediale or dorsal stratum profundum. These electrophysiological findings confirm and extend previous anatomical observations which indicate that components of the crossed descending projection of the colliculus may be functionally specialized for the production of different classes of orienting movements.

Event or emergency? Two response systems in the mammalian superior colliculus

Recent studies of the effects of stimulating the superior colliculus (SC) in rodents suggest that this structure mediates at least two classes of response to novel sensory stimuli. One class contains the familiar orienting response, together with movements resembling tracking or pursuit, and appears appropriate for undefined sensory 'events'. The second class contains defensive movements such as avoidance or flight, together with cardiovascular changes, that would be appropriate for a sudden emergency such as the appearance of a predator, or of an object on collision course. The two response systems appear to depend on separate output projections, and are probably subject to different sensory and forebrain influences. These findings (1) suggest an explanation for the complex anatomical organization of the SC, with multiple output pathways differentially accessed by a very wide variety of inputs, (2) emphasize the similarities between the SC and the optic tectum in non-mammalian species, and (3) suggest that the SC may be useful as a model for studying both the sensory control of defensive responses, and how intelligent decisions can be taken about relatively simple sensory inputs.

Oral movements induced by interference with nigral GABA neurotransmission: relationship to tardive dyskinesias

Previous studies have shown that the emergence of spontaneous dyskinetic behaviors, such as vacuous chewing movements, following several months of neuroleptic treatment in the rat, is correlated with depletion of nigral GABA. To explore the specificity of this relationship, we acutely interfered with nigral GABA transmission pharmacologically, by microinfusing either the GABA receptor antagonist, bicuculline, or the GABA-depleting agent, isoniazid, bilaterally into substantia nigra. We found that both acute treatments induced vacuous chewing movements in rats. Moreover, the time to onset of action of each of these drugs corresponded to the onsets of their respective effects on GABA transmission. In addition, we found that the application of muscimol into the target field of the nigrotegmental projection, which has been shown to block gnawing elicited by nigral GABA receptor stimulation, completely abolished elicitation of vacuous chewing movements by intranigral isoniazid. In contrast, bilateral microinfusions of muscimol into the nigrocollicular target region, in the deep layers of superior colliculus, blocked elicitation of gnawing by intranigral muscimol, but completely spared elicitation of vacuous chewing movements by intranigral isoniazid. We conclude that qualitatively different dyskinetic syndromes can be produced by bidirectional perturbations of nigral GABA function and are differentially mediated by nigrotegmental and nigrotectal projections. These syndromes may represent animal models of distinct components of extrapyramidal side effects of chronic neuroleptic administration.

GABAergic supersensitivity within the pars reticulata of the rat substantia nigra following chronic haloperidol administration

These studies were performed to evaluate the effects of chronic haloperidol administration on the responsiveness of the pars reticulata neurons of the substantia nigra (SNR) to microiontophoretically applied gamma-aminobutyric acid (GABA) and glycine. Rats were administered haloperidol in their feed (CHAL treatment) for 30 days in increasing concentrations. All experiments were conducted two days after termination of the CHAL treatment. GABA receptor binding and neuronal responsiveness to GABA were significantly increased within the SNR following CHAL treatment. The mean EC50 value for GABA was significantly decreased for SNR neurons in CHAL-treated rats, but there was no change in the EC50 for glycine. Specific [3H]GABA binding (4 nM) was elevated by more than 50% within the SNR. Scatchard analysis of [3H]muscimol binding data revealed that the binding capacities (Bmax) of both high and low affinity GABA binding sites within the SNR were significantly increased without a change in the apparent dissociation constants (Kd) of either site. Although no regional difference in responsiveness to GABA was detected within the SNR of CHAL-treated rats, the spontaneous activity of neurons located within the rostral two-thirds of the nucleus was reduced; however, there was no change in spontaneous activity of neurons located within the caudal one-third. These data provide direct physiological evidence to support the conclusion that the increase in GABA binding within the SNR following CHAL treatment reflects a neuronal supersensitivity to GABA.

Substantia nigra-mediated anticonvulsant actions: role of nigral output pathways

Bilateral ablation of the superior colliculus in rats abolished the anticonvulsant effect of muscimol, a gamma-aminobutyric acid (GABA) agonist, infused into the substantia nigra. Our data indicate that the ability of intranigral muscimol to protect against maximal electroshock convulsions requires the integrity of the nigrotectal pathway. We present evidence that the nigrotegmental, nigrothalamic, and nigrostriatal projections do not appear to contribute to the nigral-evoked attenuation of electroshock convulsions. Inasmuch as the nigrotectal pathway utilizes the inhibitory neurotransmitter GABA, we hypothesize that the anticonvulsant effect of nigral manipulations in the electroshock test is a result of the disinhibition of neurons in the superior colliculus, the firing of which must therefore be capable of suppressing or disrupting electroshock-induced convulsions.

An investigation of the role played by the superior colliculus and ventromedial thalamus in self-injurious behavior produced by intranigral microinjection of muscimol

Bilateral injection of muscimol (30 or 60 ng) into the substantia nigra (pars reticulata) of rats produced a variety of stereotyped acts, self-injurious behavior (SIB), and antinociception. Bilateral electrolytic lesions of the superior colliculus strongly suppressed SIB without reducing the antinociceptive effects of intranigral muscimol. Electrolytic lesions of the ventromedial thalamus had no effect on behavioral responses to intranigral muscimol. These studies suggest that the SIB produced by intranigral muscimol is mediated by neuronal pathways that terminate in or pass through the superior colliculus. The ventromedial thalamus does not appear to play a role in mediating behavioral responses to intranigral muscimol.

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.

Involvement of the midbrain reticular formation in self-injurious behavior, stereotyped behavior, and analgesia induced by intranigral microinjection of muscimol

Bilateral microinjection of muscimol (60 ng), a gamma-aminobutyric acid (GABA) agonist, into the central region of the substantia nigra (pars reticulata) produced self-injurious behavior (SIB), stereotyped behavior and analgesic-like effects in rats. Bilateral electrolytic lesions of the midbrain reticular formation ventrolateral to the periaqueductal gray matter completely blocked the SIB but had little effect on stereotyped behavior produced by intranigral muscimol. Lesions of the midbrain reticular formation reduced the antinociceptive effect of intranigral muscimol on the tail-flick but not on the hot-plate test. Bilateral microinjection of muscimol (10-100 ng) into the midbrain reticular formation produced intense stereotyped behavior and had an analgesic-like effect on the hot-plate test but not on the tail-flick test. Stereotyped behavior appeared to interfere with the paw-lick response on the hot-plate test. These data suggest that the antinociceptive effect of intranigral muscimol on the tail-flick test is mediated by fibers that project to or pass through the midbrain reticular formation and that analgesia may play an important role in muscimol-induced SIB. The midbrain reticular formation does not appear to be involved in the stereotyped behavior produced by intranigral muscimol.

Superior colliculus and visual neglect in rat and hamster. III. Functional implications

In comparison with the geniculostriate pathway, the retinotectal projection in rat and hamsters appears to emphasize information concerning localized transient stimuli, particularly in the periphery of the visual field. An important question is whether the superior colliculus merely relays this information elsewhere, or instead takes part in its analysis. This question is broken down into two parts. First, what decisions do rats and hamsters have to take concerning localized transient visual stimuli in the periphery? It is suggested that the following decisions are taken: (a) does the stimulus require any response? If the transient is self-produced, or is known on the basis of past experience to predict no important consequence, then it may be ignored; and (b) does the stimulus convey enough information to determine a response, either unlearnt (e.g. attack, flee, freeze) or learnt? If the stimulus appears to warrant some response, but it is not clear which, then it requires investigation. Second, what evidence is there that the superior colliculus participates in any of these decisions? It is argued on general grounds that the involvement of the superior colliculus in investigative orienting necessitates its knowing about the other decisions, since a useful orienting device cannot respond promiscuously to uninteresting or dangerous stimuli. This argument is supported by evidence from stimulation and recording studies, which in addition suggest that the superior colliculus is directly involved in producing a number of responses appropriate to peripheral transients, besides orienting. Thus, one function of the superior colliculus may be to help analyze and take decisions about localized transients in the periphery of the field.(ABSTRACT TRUNCATED AT 250 WORDS)

Self-injurious behavior in rats produced by intranigral microinjection of GABA agonists

Bilateral injection of the GABA agonist muscimol (10-300 ng) into the caudal substantia nigra (pars reticulata) of rats produced dose-dependent stereotyped gnawing and self-biting. Limiting the opportunity to gnaw on inanimate objects shifted the dose-response curve for muscimol-induced self-injurious behavior (SIB) to the left and increased the maximum incidence of SIB. Microinjection of muscimol (30 ng) into the rostral and caudal regions of the substantia nigra were equally effective in producing SIB, though the incidence of SIB decreased sharply when muscimol was injected 1 mm rostral or caudal to the substantia nigra. Bilateral intranigral injection of THIP (100-1000 ng) and (+/-)baclofen (100-1000 ng) induced a low incidence of SIB. However, neither IP administration of picrotoxin (5 mg/kg) or simultaneous microinjection of (+)bicuculline methiodide (BMI; 300 or 1000 ng) along with muscimol (30 ng) blocked muscimol-induced SIB. In fact, (+)BMI increased the occurrence of self-biting and reduced the latency to onset of SIB. The involvement of GABAergic mechanisms in muscimol-induced SIB is discussed.

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.

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.

Rotational behavior following cholinergic stimulation of the superior colliculus in rats

Rate which received microinjections of carbachol into the superior colliculus exhibited pronounced dose-dependent rotational behavior contralateral to the site of injection (Experiment 1). Wet dog shakes were also observed in some animals. Similar injections in the midbrain reticular formation produced immobility with slight contralateral flexion of the neck. Convulsions were observed in some rats after injections into either anatomical location. In Experiment 2, circling induced by carbachol in the superior colliculus was blocked by prior injection of either the muscarinic receptor antagonist scopolamine or the nicotinic receptor antagonist mecamylamine, suggesting that both nicotinic and muscarinic receptors are involved in the effect. In Experiment 3 contralateral rotational behavior was induced by intracollicular microinjections of the combination of acetylcholine chloride and physostigmine. The results suggest that collicular mediation of contralateral rotational behavior, and perhaps orientation, might involve cholinergic receptors.

Evidence that the nigrotegmental GABAergic projection mediates stereotypy induced by apomorphine and intranigral muscimol

The substantia nigra plays a pivotal role in the relay of output from the striatum. One neural pathway from substantia nigra projects GABAergic fibers to the caudal mesencephalic tegmentum, terminating in the vicinity of the pedunculopontine nucleus (PPN). To evaluate the functional importance of this projection in the mediation of stereotyped behaviors of striatal and nigral origin, we microinjected low doses of the GABA agonist, muscimol, bilaterally into the vicinity of the PPN. This muscimol treatment resulted in a total blockade of all stereotyped behaviors normally elicited by systemic apomorphine or by intranigral muscimol. Blockade was not observed in animals microinjected with muscimol into the dorsal reticular formation, 1 mm above the level of the PPN. Our results indicate that the nigrotegmental projection may play a crucial role in the expression of stereotyped and dyskinetic behaviors of basal ganglia origin.

Unilateral injection of GABA agonists in the superior colliculus: asymmetry to tactile stimulation

A unilateral microinjection of each one of three different GABA agonists (Muscimol: 1.4 nmoles; Baclofen: 0.8 nmole; THIP: 10.7 nmoles) into the superior colliculus was found to result in a reversible asymmetry in the rat's responsiveness to tactile stimulation. The rat was hyporeactive to stimulations applied contralaterally and hyperreactive to stimulations applied ipsilaterally to the infusion site. Furthermore, the rat showed ipsiversive turning in response to tactile stimulation applied either ipsi- or contralaterally to the infusion site. The results are discussed in relation with motor and sensory asymmetry produced by unilateral manipulations affecting the striato-nigro tectal system.

A quantitative analysis of stereotyped gnawing induced by apomorphine

An apparatus was designed and constructed to enable a quantitative analysis of the stereotyped gnawing produced by the dopamine agonist apomorphine. Using this apparatus it was discovered that increasing the subcutaneous dose of apomorphine increased (1) the number of animals that gnawed, and (2) the duration of gnawing in those animals that gnawed at all doses. Other aspects of apomorphine-induced gnawing, in particular the latency to respond and the frequency and duration of individual gnaws, were relatively unaffected. Likely properties of the system responsible for the organization of sterotyped gnawing are discussed.