Opposing excitatory and inhibitory influences from the cerebellum and basal ganglia converge on the superior colliculus: an electrophysiological investigation in the rat

Basal Ganglia Circuits for Action Specification

Behavior is readily classified into patterns of movements with inferred common goals-actions. Goals may be discrete; movements are continuous. Through the careful study of isolated movements in laboratory settings, or via introspection, it has become clear that animals can exhibit exquisite graded specification to their movements. Moreover, graded control can be as fundamental to success as the selection of which action to perform under many naturalistic scenarios: a predator adjusting its speed to intercept moving prey, or a tool-user exerting the perfect amount of force to complete a delicate task. The basal ganglia are a collection of nuclei in vertebrates that extend from the forebrain (telencephalon) to the midbrain (mesencephalon), constituting a major descending extrapyramidal pathway for control over midbrain and brainstem premotor structures. Here we discuss how this pathway contributes to the continuous specification of movements that endows our voluntary actions with vigor and grace.

Thalamic interactions of cerebellum and basal ganglia

Cerebellum and basal ganglia are reciprocally interconnected with the neocortex via oligosynaptic loops. The signal pathways of these loops predominantly converge in motor areas of the frontal cortex and are mainly segregated on subcortical level. Recent evidence, however, indicates subcortical interaction of these systems. We have reviewed literature that addresses the question whether, and to what extent, projections of main output nuclei of basal ganglia (reticular part of the substantia nigra, internal segment of the globus pallidus) and cerebellum (deep cerebellar nuclei) interact with each other in the thalamus. To this end, we compiled data from electrophysiological and anatomical studies in rats, cats, dogs, and non-human primates. Evidence suggests the existence of convergence of thalamic projections originating in basal ganglia and cerebellum, albeit sparse and restricted to certain regions. Four regions come into question to contain converging inputs: (1) lateral parts of medial dorsal nucleus (MD); (2) parts of anterior intralaminar nuclei and centromedian and parafascicular nuclei (CM/Pf); (3) ventromedial nucleus (VM); and (4) border regions of cerebellar and ganglia terminal territories in ventral anterior and ventral lateral nuclei (VA-VL). The amount of convergences was found to exhibit marked interspecies differences. To explain the rather sparse convergences of projection territories and to estimate their physiological relevance, we present two conceivable principles of anatomical organization: (1) a "core-and-shell" organization, in which a central core is exclusive to one projection system, while peripheral shell regions intermingle and occasionally converge with other projection systems and (2) convergences that are characteristic to distinct functional networks. The physiological relevance of these convergences is not yet clear. An oculomotor network proposed in this work is an interesting candidate to examine potential ganglia and cerebellar subcortical interactions.

Circuits for Action and Cognition: A View from the Superior Colliculus

The superior colliculus is one of the most well-studied structures in the brain, and with each new report, its proposed role in behavior seems to increase in complexity. Forty years of evidence show that the colliculus is critical for reorienting an organism toward objects of interest. In monkeys, this involves saccadic eye movements. Recent work in the monkey colliculus and in the homologous optic tectum of the bird extends our understanding of the role of the colliculus in higher mental functions, such as attention and decision making. In this review, we highlight some of these recent results, as well as those capitalizing on circuit-based methodologies using transgenic mice models, to understand the contribution of the colliculus to attention and decision making. The wealth of information we have about the colliculus, together with new tools, provides a unique opportunity to obtain a detailed accounting of the neurons, circuits, and computations that underlie complex behavior.

An integrative role for the superior colliculus in selecting targets for movements

A fundamental goal of systems neuroscience is to understand the neural mechanisms underlying decision making. The midbrain superior colliculus (SC) is known to be central to the selection of one among many potential spatial targets for movements, which represents an important form of decision making that is tractable to rigorous experimental investigation. In this review, we first discuss data from mammalian models-including primates, cats, and rodents-that inform our understanding of how neural activity in the SC underlies the selection of targets for movements. We then examine the anatomy and physiology of inputs to the SC from three key regions that are themselves implicated in motor decisions-the basal ganglia, parabrachial region, and neocortex-and discuss how they may influence SC activity related to target selection. Finally, we discuss the potential for methodological advances to further our understanding of the neural bases of target selection. Our overarching goal is to synthesize what is known about how the SC and its inputs act together to mediate the selection of targets for movements, to highlight open questions about this process, and to spur future studies addressing these questions.

Physiological evidence for a trans-basal ganglia pathway linking extrastriate visual cortex and the superior colliculus

Visually responsive regions along the cat's lateral suprasylvian (LS) sulcus provide excitatory inputs to the deep layers of the superior colliculus (SC). It is via this direct cortico-collicular route that LS cortex is thought to enhance the visual activity of SC output neurons and thereby facilitate SC-mediated orientation behaviours. However, it has long been suggested that LS also might influence the SC via an 'indirect' route through the basal ganglia. Such a multi-synaptic route would ultimately modulate SC activity via basal ganglia output neurons in substantia nigra, pars reticulata. Using cortical electrical stimulation, the present experiments in the anaesthetized cat provide a physiological confirmation of this indirect route. Moreover, the patterns of activity evoked in antidromically identified nigro-collicular neurons indicate the involvement of multiple trans-basal ganglia pathways. The most complex evoked patterns consisted of a variable period of inhibition preceded and followed by periods of excitation. Although many neurons displayed only components of this triphasic response, these electrically evoked responses generally matched the characteristics of their responses to natural visual stimuli. Cortical stimulation evoked excitation in all of crossed nigro-collicular neurons and inhibition in the majority of uncrossed nigro-collicular neurons. These data suggest that LS activity accesses multiple trans-basal ganglia circuits that shape nigro-collicular responses that are appropriate for their SC targets. In this way, visual stimuli in one hemifield can be selected as targets for SC-mediated orientation, while simultaneously inhibiting activity in the opposite SC that might generate responses to competing targets.

Reorganization of functional brain maps after exercise training: Importance of cerebellar-thalamic-cortical pathway

Exercise training (ET) causes functional and morphologic changes in normal and injured brain. While studies have examined effects of short-term (same day) training on functional brain activation, less work has evaluated effects of long-term training, in particular treadmill running. An improved understanding is relevant as changes in neural reorganization typically require days to weeks, and treadmill training is a component of many neurorehabilitation programs. Adult, male rats (n=10) trained to run for 40 min/day, 5 days/week on a Rotarod treadmill at 11.5 cm/s, while control animals (n=10) walked for 1 min/day at 1.2 cm/s. Six weeks later, [(14)C]-iodoantipyrine was injected intravenously during treadmill walking. Regional cerebral blood flow-related tissue radioactivity was quantified by autoradiography and analyzed in the three-dimensionally reconstructed brain by statistical parametric mapping. Exercised compared to nonexercised rats demonstrated increased influence of the cerebellar-thalamic-cortical (CbTC) circuit, with relative increases in perfusion in deep cerebellar nuclei (medial, interposed, lateral), thalamus (ventrolateral, midline, intralaminar), and paravermis, but with decreases in the vermis. In the basal ganglia-thalamic-cortical circuit, significant decreases were noted in sensorimotor cortex and striatum, with associated increases in the globus pallidus. Additional significant changes were noted in the ventral pallidum, superior colliculus, dentate gyrus (increases), and red nucleus (decreases). Following ET, the new dynamic equilibrium of the brain is characterized by increases in the efficiency of neural processing (sensorimotor cortex, striatum, vermis) and an increased influence of the CbTC circuit. Cerebral regions demonstrating changes in neural activation may point to alternate circuits, which may be mobilized during neurorehabilitation.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

A direct projection from superior colliculus to substantia nigra pars compacta in the cat

Dopaminergic neurons exhibit a short-latency, phasic response to unexpected, biologically salient stimuli. The midbrain superior colliculus also is sensitive to such stimuli, exhibits sensory responses with latencies reliably less than those of dopaminergic neurons, and, in rat, has been shown to send direct projections to regions of the substantia nigra and ventral tegmental area containing dopaminergic neurons (e.g. pars compacta). Recent electrophysiological and electrochemical evidence also suggests that tectonigral connections may be critical for relaying short-latency (<100 ms) visual information to midbrain dopaminergic neurons. By investigating the tectonigral projection in the cat, the present study sought to establish whether this pathway is a specialization of the rodent, or whether it may be a more general feature of mammalian neuroanatomy. Anterogradely and retrogradely transported anatomical tracers were injected into the superior colliculus and substantia nigra pars compacta, respectively, of adult cats. In the anterograde experiments, abundant fibers and terminals labeled with either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin were seen in close association with tyrosine hydroxylase-positive (dopaminergic) somata and processes in substantia nigra pars compacta and the ventral tegmental area. In the retrograde experiments, injections of biotinylated dextran amine into substantia nigra produced significant retrograde labeling of tectonigral neurons of origin in the intermediate and deep layers of the ipsilateral superior colliculus. Approximately half of these biotinylated dextran amine-labeled neurons were, in each case, shown to be immunopositive for the calcium binding proteins, parvalbumin or calbindin. Significantly, virtually no retrogradely labeled neurons were found either in the superficial layers of the superior colliculus or among the large tecto-reticulospinal output neurons. Taken in conjunction with recent data in the rat, the results of this study suggest that the tectonigral projection may be a common feature of mammalian midbrain architecture. As such, it may represent an additional route by which short-latency sensory information can influence basal ganglia function.

Superior colliculus firing changes after lesion or electrical stimulation of the subthalamic nucleus in the rat

Recent data have suggested a critical role for the basal ganglia in the remote control of epileptic seizures. In particular, it has been shown that inhibition of either substantia nigra pars reticulata or subthalamic nucleus as well as activation of the superior colliculus suppresses generalized seizures in several animal models. It was previously shown that high frequency stimulation of the subthalamic nucleus, thought to act as functional inhibition, stopped ongoing non-convulsive generalized seizures in rats. In order to determine whether high frequency stimulation of the subthalamic nucleus involved an activation of superior colliculus neurons, we examined the effects of subthalamic nucleus manipulation, by either high frequency stimulation or chemical lesion, on the spontaneous electrical activity of superior colliculus neurons. Acute high frequency stimulation of the subthalamic nucleus (frequency 130 Hz) induced an immediate increase of unitary activity in 70% of responding cells, mainly located within the deep layers, whereas a reduction was observed in the remaining 30%. The latter responses are dependent on the intensity and frequency of the stimulation. Unilateral excitotoxic lesion of the subthalamic nucleus induced a delayed and transient decrease of superior colliculus activity. Our data suggest that high frequency stimulation of the subthalamic nucleus suppresses generalised epileptic seizures through superior colliculus activation.

Antipsychotics and single-cell activity in the rat superior colliculus

Schizophrenic patients have problems with saccadic eye movements that can be characterized as a loss of control over the saccadic system. Preliminary clinical results suggest that antipsychotics can either disrupt or improve saccadic performance. The brain mechanism through which antipsychotics might affect saccades is the subject of study. The superior colliculus (SC) is crucially involved in the generation of saccades. Previous experimental studies showed that the substantia nigra reticulata (SNr), a structure with profound inhibitory influence on the SC, is differentially affected by classical and atypical antipsychotics. In this study, we investigated the potential effects of atypical antipsychotics (clozapine, olanzapine, and risperidone) and a classical antipsychotic (haloperidol) on the firing rate of SC cells in the rat. In anesthetized rats, we performed extracellular recordings on spontaneous active neurons in the intermediate and deep layers of the SC. After subcutaneous injection of an antipsychotic drug, changes in firing rate were compared with responses upon saline injection. Olanzapine (1.0 mg/kg), risperidone (0.3 and 1.0 mg/kg), and haloperidol (0.5 mg/kg) did not significantly alter cell activity, but clozapine (10.0 mg/kg) induced a short-lasting but significant decrease. Except for clozapine, the effects of antipsychotics on the SC were nonsignificant and therefore independent of the effects in the SNr. Our results support the notion that clozapine is different from the other atypical antipsychotics.

The fine organization of nigro-collicular channels with additional observations of their relationships with acetylcholinesterase in the rat

The nigro-collicular pathway that links the basal ganglia to the sensorimotor layers of superior colliculus plays a crucial role in promoting orienting behaviors. This connection originating in the pars reticulata and lateralis of the substantia nigra has been shown in rat and cat to be topographically organized. In rat, a functional compartmentalization of the substantia nigra has also been shown reflecting that of the striatum. In light of this, we reinvestigated the topographical arrangement of the nigro-collicular pathway by examining the innervation of each nigral functional zone. We performed small injections of either biocytin or wheatgerm agglutinin conjugated with horseradish peroxidase restricted to identified somatic, visual and auditory nigral zones. Frontally cut sections showed that innervations provided by the three main nigral zones form a mosaic of complementary domains stratified from the stratum opticum to the ventral part of the intermediate collicular layers, with the somatic afferents sandwiched between the visual and the auditory ones. When reconstructed from semi-horizontal sections, nigral innervations organized in the form of a honeycomb-like array composed of 100 cylindrical modules covering three-quarters of the collicular surface. Such a modular architecture is reminiscent of the acetylcholinesterase lattice we previously described in rat intermediate collicular layers. In the enzyme lattice, the surroundings of the cylindrical modules are composed of a mosaic of dense and diffuse enzyme subdomains. Thus, we compared the distribution of the overall nigral projection and of its constituent channels with the acetylcholinesterase lattice. The procedure combined axonal labelling with histochemistry on single sections for acetylcholinesterase activity. The results demonstrate that the overall nigral projection overlaps the acetylcholinesterase lattice and its constituent channels converge with either the dense or the diffuse enzyme subdomains. The stereometric arrangement of the nigro-collicular pathway is suggestive of an architecture promoting the selection of collicular motor programs for different classes of orienting behavior.

Gaze shifts evoked by electrical stimulation of the superior colliculus in the head-unrestrained cat. II. Effect of muscimol inactivation of the caudal fastigial nucleus

The medioposterior cerebellum [vermian lobules VI and VII and caudal fastigial nucleus (cFN)] is known to play a major role in the control of saccadic gaze shifts toward a visual target. To determine the relative contribution of the cFN efferent pathways to the brainstem reticular formation and to the superior colliculus (SC), we recorded in the head-unrestrained cat the effects of cFN unilateral inactivation on gaze shifts evoked by electrical microstimulation of the deeper SC layers. Gaze shifts evoked after muscimol injection still exhibited the typical qualitative features of normal saccadic gaze shifts. Nevertheless, consistent modifications in amplitude and latency were observed. For ipsiversive movements (evoked by the SC contralateral to the inactivated cFN), these changes depended on the locus of stimulation on the motor map: for the anterior 2/3 of the SC, amplitude increased and latency tended to decrease; for the posterior 1/3 of the SC, amplitude decreased and latency increased. For the contraversive direction, amplitude moderately decreased and latency tended to increase for all but the caudal-most stimulated SC site. These modifications of SC-evoked gaze shifts during cFN inactivation differed from the ipsiversive hypermetria/contraversive hypometria pattern observed for visually triggered gaze shifts recorded during the same recording sessions. We conclude that (i) the topographical organization of gaze shift amplitude in the deeper SC layers is influenced by the cerebellum and is either severely distorted or demonstrates an amplitude reduction during inactivation of the contralateral or ipsilateral cFN, respectively; (ii) gaze shifts evoked by SC microstimulation and visually triggered gaze shifts either rely on distinct cerebellar-dependent control processes or differ by the location of the caudal-most active SC population. We present a functional scheme providing several predictions regarding the modulatory influence of the cerebellum on SC neuronal activities and on the topographical organization of fastigial-SC projections.

Honeycomb-like structure of the intermediate layers of the rat superior colliculus: afferent and efferent connections

There is increasing evidence that acetylcholinesterase is organised in a lattice-like fashion in the intermediate layers of the mammalian superior colliculus. In a recent study, we described this organisation in rat by showing that it comprises a well formed honeycomb-like lattice with about 100 cylindrical compartments or modules occupying both the intermediate collicular layers. Considering this enzyme domain as a reference marker for comparing the organisation of collicular input-output systems, the present study investigates whether the principal sensori-motor systems in intermediate layers also have honeycomb-like arrangements. In 33 animals, the distributions of afferents (visual from extrastriate cortex; somatic from the primary somatosensory cortex, the trigeminal nucleus and the cervical spinal cord) and efferents (cells of origin of the crossed descending bulbospinal tract and uncrossed pathway to the pontine gray, the ascending system to the medial dorsal thalamus) were examined in a tangential plane following applications of horseradish peroxidase-wheatgerm agglutinin conjugate (used as an anterograde and retrograde tracer). In 22 of the 33 rats, axonal tracing was made within single tangential sections also stained for cholinesterasic activity in order to compare the neuron profiles with the cholinesterasic lattice.The results show that these afferent and efferent systems are also organised in honeycomb-like networks. Moreover, those related to the cortical, trigeminal and some of the spinal afferents are aligned with the cholinesterasic lattice. Likewise most of colliculo-pontine, colliculo-bulbospinal and half of colliculo-diencephalic projecting cells also tend to be in spatial register with the enzyme lattice. This indicates that the honeycomb-like arrangement is a basic architectural plan in the superior colliculus for the organisation of both acetylcholinesterase and major sensori-motor systems for orientation.

Cerebellar output exerts spatially organized influence on neural responses in the rat superior colliculus

The deep cerebellar nuclei project to largely segregated target regions in the contralateral superior colliculus. Single-unit recordings have previously shown that nuclear inactivation normally suppresses spontaneously active collicular target neurons. However, facilitation of activity has also been found in a proportion of collicular units. In the present study we tested the hypothesis that the type of effect is related to the cerebellotectal topography. We recorded simultaneously in the deep cerebellar nuclei and superior colliculus of 53 anaesthetized rats. GABA microinjections produced a complete, reversible, arrest of activity in the deep cerebellar nuclei. We investigated the effect of this inactivation on 292 sensory and non-sensory cells in the collicular intermediate and deep layers. Of these, 29% showed a reduced response to their preferred sensory stimulus or decreased their spontaneous firing rate in the case of non-sensory cells. However, 15% increased their sensory responsiveness and/or spontaneous firing rate following cerebellar inactivation. No effect was seen in the remaining 56% of cells. The distribution of these different effects was highly significantly related to the topography of the cerebellotectal terminal fields. Thus, 68% of the suppressive effects were obtained from cells lying in the terminal fields of the deep cerebellar nucleus inactivated. Conversely, 86% of the excitatory effects and 66% of the cells showing no effect were obtained from cells falling outside the terminal field. The results support the view that the superior colliculus is an important site for the functional integration of primary sensory information, not only with cortical and basal ganglia afferents, but also with cerebellar information. The contrasting physiological responses observed within the terminal cerebellotectal topography appear to map closely on to the known distribution of the cells of origin of the two major descending output pathways of the superior colliculus and are possibly mediated by intrinsic inhibitory connections within its intermediate and deep layers. These results provide evidence for a neural architecture in the superior colliculus whose function is the selection of appropriate actions in response to novel stimuli and the suppression of competing motor programmes.

The synaptic pharmacology underlying sensory processing in the superior colliculus

The superior colliculus (SC) is one of the most ancient regions of the vertebrate central sensory system. In this hub afferents from several sensory pathways converge, and an extensive range of neural circuits enable primary sensory processing, multi-sensory integration and the generation of motor commands for orientation behaviours. The SC has a laminar structure and is usually considered in two parts; the superficial visual layers and the deep multi-modal/motor layers. Neurones in the superficial layers integrate visual information from the retina, cortex and other sources, while the deep layers draw together data from many cortical and sub-cortical sensory areas, including the superficial layers, to generate motor commands. Functional studies in anaesthetized subjects and in slice preparations have used pharmacological tools to probe some of the SC's interacting circuits. The studies reviewed here reveal important roles for ionotropic glutamate receptors in the mediation of sensory inputs to the SC and in transmission between the superficial and deep layers. N-methyl-D-aspartate receptors appear to have special responsibility for the temporal matching of retinal and cortical activity in the superficial layers and for the integration of multiple sensory data-streams in the deep layers. Sensory responses are shaped by intrinsic inhibitory mechanisms mediated by GABA(A) and GABA(B) receptors and influenced by nicotinic acetylcholine receptors. These sensory and motor-command activities of SC neurones are modulated by levels of arousal through extrinsic connections containing GABA, serotonin and other transmitters. It is possible to naturally stimulate many of the SC's sensory and non-sensory inputs either independently or simultaneously and this brain area is an ideal location in which to study: (a) interactions between inputs from the same sensory system; (b) the integration of inputs from several sensory systems; and (c) the influence of non-sensory systems on sensory processing.

Spatial variation in the effects of inactivation of substantia nigra on neuronal activity in rat superior colliculus

Superior colliculus (SC)-mediated behaviours are under the disinhibitory control of the striato-nigro-collicular projection. We systematically investigated the homogeneity of substantia nigra pars reticulata (SNr) influence on different populations of SC neurons by recording the effects of intranigral GABA microinjections on 149 cells at different locations in the rat SC. Suppression of the tonic activity of SNr resulted in both the facilitation and paradoxical inhibition of spatially-segregated SC target neurons. These dual influences were found to broadly map onto the SC origins of the descending projections known to support approach and avoidance/defensive behaviours. These findings are consistent with an organisation which promotes contrasting processes for the selection of a behaviour and simultaneous suppression of competing motor programs.

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.

A floating microwire technique for multichannel chronic neural recording and stimulation in the awake freely moving rat

We describe a cheap, and relatively simple, method for the construction and implantation of bundles of six fully-floating 25 microm microwire electrodes in the rat central nervous system (CNS). Wires are stiffened for implantation by temporarily attaching them to a micropipette guide with sucrose which subsequently dissolves in the brain. The associated headpiece connector mates with a plug-in FET which gives high quality recordings, free of movement artefacts, even when used with 3 m of unscreened cable. Different electrode configurations are easily selected and sufficient space is available on the headpiece to accommodate injection guide cannulae. The electrode performance was evaluated in 42 implanted rats where the system was used successfully for long term recording of superior colliculus (SC) deep layer neurons and behavioural responses evoked by electrical stimulation of the same wires. We obtained neural recordings on 81% of the 252 implanted wires, with 79% of these providing good, reliably discriminable, single unit responses following post-operative recovery. During a five-week testing period on a subsample of the 42 'best' electrodes (one per animal), we found the same sensory or motor response 1 week after initial testing in 91% wires, 67% after 2 weeks and 24% after 5 weeks. Using waveform templating techniques we were able to show that 62% of the electrodes still working at 5 weeks were reliably recording the same cell.

Nociceptive neurones in rat superior colliculus. I. Antidromic activation from the contralateral predorsal bundle

Accumulating evidence suggests that the rodent superior colliculus (SC) plays as important a role in avoidance and defensive behaviours as it does in orientation and approach. These two complementary behaviours are associated with two anatomically segregated tectofugal output pathways, such that orientation and approach are mediated by the crossed descending projection, whereas avoidance and defence are subserved via the uncrossed projection. Because nociceptive neurones in the SC have been presumed to participate in withdrawal or defensive behaviours, it has been proposed that they have direct access only to the uncrossed efferent pathway. However, in certain behavioural situations, the most adaptive response to injury, or to a painful object in prolonged contact with the skin, is to orient towards the source of discomfort so that the skin can be licked and/or the offending object removed. Presumably then, nociceptive as well as low-threshold neurones would have access to the crossed descending pathway in order to initiate such behaviours. Determining whether or not this is the case was the objective of the present study. Both nociceptive-specific (82%) and wide-dynamic-range (18%) SC neurones were identified using long-duration (up to 6 s), frankly noxious mechanical and thermal stimuli in urethane-anaesthetised Long-Evans hooded rats. The majority (85.7%) of the nociceptive neurones encountered were located within the intermediate layers, which corresponds with the location of the cells-of-origin of the crossed descending projection. Nearly half (44.9%) were activated antidromically from electrical stimulation of the crossed descending pathway at a site in the brainstem below its decussation. The mean conduction velocity of these nociceptive output neurones was 9.02 m/s, which corresponds well to previous estimates of conduction velocity in the crossed tecto-reticulo-spinal tract. These data demonstrate that a significant proportion of nociceptive neurones in the rat SC have axons that project to the contralateral brainstem via the crossed descending projection. Nociceptive neurones could, therefore, effect orientation responses to noxious stimuli via similar output pathways that low-threshold neurones utilize to initiate orientation to innocuous stimuli.