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

Processing of visually evoked innate fear by a non-canonical thalamic pathway

The ability of animals to respond to life-threatening stimuli is essential for survival. Although vision provides one of the major sensory inputs for detecting threats across animal species, the circuitry underlying defensive responses to visual stimuli remains poorly defined. Here, we investigate the circuitry underlying innate defensive behaviours elicited by predator-like visual stimuli in mice. Our results demonstrate that neurons in the superior colliculus (SC) are essential for a variety of acute and persistent defensive responses to overhead looming stimuli. Optogenetic mapping revealed that SC projections to the lateral posterior nucleus (LP) of the thalamus, a non-canonical polymodal sensory relay, are sufficient to mimic visually evoked fear responses. In vivo electrophysiology experiments identified a di-synaptic circuit from SC through LP to the lateral amygdale (Amg), and lesions of the Amg blocked the full range of visually evoked defensive responses. Our results reveal a novel collicular-thalamic-Amg circuit important for innate defensive responses to visual threats.

The hippocampus and contextual memory retrieval in Pavlovian conditioning

Several theories of hippocampal function have suggested a role for the hippocampus in contextual memory retrieval. However, these theories are based on studies using permanent pretraining lesions, which confound the role of the hippocampus in learning with its role in retrieval. We argue that an alternative methodology is required to assess the role of the hippocampus in memory retrieval processes. This alternative methodology involves temporary inactivation of the hippocampus in Pavlovian paradigms that lend themselves to an examination of retrieval. An example of this approach is considered in a Pavlovian fear-conditioning paradigm. We examined the influence of hippocampal inactivation on the context-specific expression of latent inhibition, a response decrement displayed when excitatory conditioning is preceded by non-reinforced presentations of a to-be-conditioned stimulus. Reversible inactivation of the dorsal hippocampus prior to retrieval testing eliminated the context-specific expression of latent inhibition. The nature of the hippocampal role in retrieval processes is discussed in the light of these data.

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.

C-Fos immunohistochemical localization of neurons in the mesencephalic locomotor region in the rat brain

The projection from the limbic system via the subpallidal region to the mesencephalic locomotor region is implicated in limbic-motor integration. The goal of this study was to visualize neurons of the mesencephalic locomotor region which are active during locomotor activity induced by the disinhibition of the subpallidal region. The subpallidal region was disinhibited by picrotoxin, which antagonizes the effects of GABA. The unilateral injection of picrotoxin into the subpallidal region caused a significant increase in locomotor activity. Active tegmental neurons were subsequently visualized by immunocytochemical staining of c-Fos protein. There were significantly more immunostained neurons in the picrotoxin-injected animals than in the saline-treated rats. Heavily stained neuronal nuclei, prevailing on the brain side ipsilateral to the injection of picrotoxin, were localized within a narrow strip of tissue which stretched from the ventrolateral periaqueductal gray (including the dorsal raphe), the cuneiform nucleus, through the region of the dorsal tegmental bundle to the pedunculopontine nucleus. There were 3.5 times more immunostained neurons in the cuneiform/pedunculopontine region and 2.5 times more stained neurons in the periaqueductal region of the picrotoxin-injected rats, as compared to the saline group. This strip of immunostained cells represents neurons which are involved in the initiation and maintenance of locomotor activity due to subpallidal activation (predominantly pedunculopontine and cuneiform nuclei), as well as neurons possibly involved in the inhibition of locomotor activity (ventrolateral periaqueductal gray) and other feedback regulations. This study will help identify the neuronal pool involved in coupling the motivational commands with the locomotor system for execution of behaviour.

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

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

Responses resembling defensive behaviour produced by microinjection of glutamate into superior colliculus of rats

Electrical stimulation of the superior colliculus in rats elicits not only orienting movements, as it does in other mammals, but also behaviours resembling such natural defensive responses as prolonged freezing, cringing, shying, and fast running and jumping. To investigate the location of the cells mediating these behaviours, the superior colliculus was systematically mapped with microinjections of sodium L-glutamate (50 mM, 200 nl), and the resultant behavioural changes as assessed in an open field were analysed for defence-like responses. The main regions that gave defensive behaviour were (i) rostromedial superior colliculus (all layers), and (ii) both medial and lateral parts of the caudal deep layers. Cells in these areas project into the ipsilateral descending pathway. However, the cells of origin of this pathway are also found in collicular regions, such as rostral intermediate gray and parts of far caudal colliculus, that did not give defensive movements in response to glutamate stimulation. It is unclear whether this is because only parts of the ipsilateral pathway mediate defensive behaviours, or because glutamate is a relatively inefficient stimulating agent for these systems. An unexpected feature of the results was that at a number of collicular sites the nature of the defensive response changed with successive (up to three) injections of glutamate, often appearing to become more intense. Whether the mechanism underlying this potentiation is related to the conditioning of natural defensive behaviour is unknown.

Association of the mesencephalic locomotor region with locomotor activity induced by injections of amphetamine into the nucleus accumbens

Injections of amphetamine into the nucleus accumbens increased locomotor activity of rats. Subsequent injections of procaine into the midbrain, in the region of the pedunculopontine nucleus, significantly reduced the amphetamine-induced locomotor activity. Control experiments showed that procaine injections into the contralateral pedunculopontine nucleus had little or no effect, as well as ipsilateral injections dorsal and ventral to the pedunculopontine nucleus. These findings suggest that release of dopamine from amphetamine injections into the accumbens gives rise to ipsilateral descending influences on the region of the pedunculopontine nucleus, a major component of the mesencephalic locomotor region. Descending influences from the nucleus accumbens to mesencephalic locomotor region may serve as a link for limbic-motor integration in behavioral response initiation.

Dorsal-ventral differences in the midbrain distribution of single neurons with head movement-correlated and locomotion-correlated firing in the golden hamster

Movement-correlated firing was studied in 111 midbrain neurons recorded in freely behaving hamsters. For 42% of these cells, most of which lay in the deeper laminae of the superior colliculus, firing occurred principally or exclusively in association with head movements. In 38% of the neurons, most of which were in subtectal regions, activity was greatest during locomotion, but also occurred in correlation with other types of movement.

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)

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

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

Microinjection of procaine or GABA into the nucleus basalis magnocellularis affects cue-elicited unit responses in the rat frontal cortex

Male rats were chronically implanted for recording of single units in the frontal cortex during a cue-event paradigm. The rats were sedated and restrained during the experiments. Units were selected which had large-amplitude, clearly isolated action potentials. The animals were first trained to associated a 2-s tone cue with rewarding medial forebrain bundle stimulation. After training, units responded to the cue by an increase or decrease in discharge rate. Cumulative histograms of the unit response to the cue were obtained and then either procaine hydrochloride or GABA was microinjected into the nucleus basalis magnocellularis (nBM). Immediately after drug administration another histogram was obtained to ascertain the drug effect. Procaine microinjections to the nBM suppressed the frontal cortex unit responses in 9 of 10 units that had previously responded with an increase in firing rate and 10 of 12 units that had decreased their firing rate before drug administration. GABA microinjections antagonized the response in 15 of 19 excited units and 2 of 2 inhibited units. Recovery was obtained in 23 units. Other units did not remain isolated long enough to obtain complete recovery. The nBM supplies the frontal cortex with as much as 70% of its cholinergic innervation. Lesions of the region do not significantly alter the amounts of neurotransmitters other than acetylcholine in the frontal cortex. These results indicate that neurons in the nucleus basalis magnocellularis are involved in the cue-elicited changes in the rate of discharge of units in the rat frontal cortex.

Role of the ventromedial nucleus of the thalamus in motor behaviour--I. Effects of focal injections of drugs

An assortment of drugs was injected into one or both ventromedial nuclei of the thalamus, to see how these influenced stereotypy, locomotion and posture in spontaneously behaving and actively rotating rats. Unilateral intrathalamic muscimol promoted weak ipsiversive circling, while bilateral treatment gave catalepsy. Similar injections of 4-amino-hex-5-enoic acid, which inhibits gamma-aminobutyrate metabolism, raised gamma-aminobutyrate levels in the ventromedial nuclei more than three-fold yet had none of these behavioural effects. The indirectly acting gamma-aminobutyrate agonists flurazepam and cis-1,3-aminocyclohexane carboxylic acid had little effect on posture and locomotion and, like muscimol and 4-amino-hex-5-enoic acid, elicited only very weak stereotypies. Procaine behaved like the gamma-aminobutyrate antagonist bicuculline, provoking vigorous locomotor hyperactivity and teeth chattering if given uni- or bilaterally. Pretreatment of one ventromedial nucleus with muscimol or 4-amino-hex-5-enoic acid, and to a lesser extent flurazepam or cis- 1,3-aminocyclohexane carboxylic acid, gave rise to pronounced ipsilateral asymmetries when combined with a large systemic dose of apomorphine. Contraversive rotations were initiated by unilateral stereotaxic injection of muscimol into the substantia nigra pars reticulata, or with apomorphine from the supersensitive striatum in unilaterally 6-hydroxydopamine lesioned rats. Drug treatments in the ipsilateral ventromedial nucleus showed a similar rank order of potency at inhibiting these circling behaviours, seemingly by reducing apomorphine-induced posture and muscimol-induced hypermotility. The suppression of circling by muscimol in these tests was highlighted by introducing the compound into the ventromedial nucleus at the height of circling activity. Both types of circling stimulus lost the capacity to increase locomotion, but still caused head turning and stereotypy in rats made cataleptic with bilateral ventromedial muscimol. Treating one ventromedial thalamus with muscimol greatly intensified any pre-existing posture directed towards that side, and vice versa. These data suggest that the ventromedial nucleus is not involved with the expression of stereotyped behaviours, but can profoundly influence posture and locomotion, especially in the presence of some other motor stimulus. The recovery of circus movements in rats with impaired ventromedial nucleus function implies this nucleus is not essential for the execution of circling in these models.

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.

Interactions between dopamine and gamma-aminobutyrate in the substantia nigra: implications for the striatonigral output hypothesis

Experiments employing a rodent circling model were conducted to test the predictive capacity of the theory which states that striatonigral gamma-aminobutyrate neurones transmit striatal information influencing the animal's locomotion and orientation. In agreement with this proposal, blocking nerve conduction in one substantia nigra with procaine, or nigral gamma-aminobutyrate receptors with bicuculline administered stereotaxically, frequently forced rats to move ipsiversively to systemic apomorphine, as though the treatment had impaired striatonigral transmission on that side of the brain. Attempts to reverse the direction of apomorphine circling by stimulating gamma-aminobutyrate receptors with muscimol, by facilitating the amino acid's action with flurazepam, or by increasing its synaptic concentration either with a breakdown inhibitor (ethanolamine O-sulphate or 4-amino-hex-5-enoic acid) or an uptake blocker (cis-1,3-aminocyclohexane carboxylic acid) in one nigra, proved unsuccessful. In fact, ethanolamine O-sulphate, flurazepam and muscimol all gave the appearance of hindering rather than enhancing the passage of striatal-derived motor information through the nigra. Broadly speaking, these drugs gave predictable behavioral responses from the ventromedial thalamus, suggesting they were acting in accordance with known mechanisms. The anomalous behaviour with ethanolamine O-sulphate may be attributed to its elevating gamma-aminobutyrate levels in other brain areas, since similar ipsiversive rotations occurred if gamma-aminobutyrate catabolism was prevented at a wide variety of extranigral sites. A simple explanation for the paradoxical ipsiversive behaviours produced by intranigral flurazepam or muscimol in combination with systemic or intracerebral injection of dopamine agonists, is that they act via presynaptic receptors to inhibit the release of endogenous gamma-aminobutyrate and thereby impede striatonigral outflow ipsilaterally.