Functional anatomy of nociceptive neurones in rat superior colliculus

A vibrissa pathway that activates the limbic system

Vibrissa sensory inputs play a central role in driving rodent behavior. These inputs transit through the sensory trigeminal nuclei, which give rise to the ascending lemniscal and paralemniscal pathways. While lemniscal projections are somatotopically mapped from brainstem to cortex, those of the paralemniscal pathway are more widely distributed. Yet the extent and topography of paralemniscal projections are unknown, along with the potential role of these projections in controlling behavior. Here, we used viral tracers to map paralemniscal projections. We find that this pathway broadcasts vibrissa-based sensory signals to brainstem regions that are involved in the regulation of autonomic functions and to forebrain regions that are involved in the expression of emotional reactions. We further provide evidence that GABAergic cells of the Kölliker-Fuse nucleus gate trigeminal sensory input in the paralemniscal pathway via a mechanism of presynaptic or extrasynaptic inhibition.

Neural correlates of active avoidance behavior in superior colliculus

Active avoidance of harmful situations seems highly adaptive, but the underlying neural mechanisms are unknown. Rats can effectively use the superior colliculus during active avoidance to detect a salient whisker conditioned stimulus (WCS) that signals an aversive event. Here, we recorded unit and field potential activity in the intermediate layers of the superior colliculus of rats during active avoidance behavior. During the period preceding the onset of the WCS, avoids are associated with a higher firing rate than escapes (unsuccessful avoids), indicating that a prepared superior colliculus is more likely to detect the WCS and lead to an avoid. Moreover, during the WCS, a robust ramping up of the overall firing rate is observed for trials leading to avoids. The firing rate ramping is not caused by shuttling and may serve to drive downstream circuits to avoid. Therefore, a robust neural correlate of active avoidance behavior is found in the superior colliculus, emphasizing its role in the detection of salient sensory signals that require immediate action.

Vibrissa sensation in superior colliculus: wide-field sensitivity and state-dependent cortical feedback

Rodents use their vibrissae (whiskers) to sense and navigate the environment. A main target of this sensory information is the superior colliculus in the midbrain, which rats can use to detect meaningful whisker stimuli in behavioral contexts. Here, we used field potential, single-unit, and intracellular recordings to show that, although cells in the intermediate layers of the superior colliculus respond relatively effectively to single whiskers, the cells respond much more robustly to simultaneous, or nearly simultaneous, wide-field (multiwhisker) stimuli. The enhanced multiwhisker response is temporally stereotyped, consisting of two short latency peaks caused by convergent trigeminal synaptic inputs and cortical feedback, respectively. The cells are highly sensitive to the degree of temporal dispersion and contact order of multiwhisker stimuli, which makes them excellent detectors of initial multiwhisker contact. In addition, their output is most robust during quiescent states because of the dependence of cortical feedback on forebrain activation, and this may serve as an alerting signal to drive orienting responses.

Fear-potentiated startle in rats is mediated by neurons in the deep layers of the superior colliculus/deep mesencephalic nucleus of the rostral midbrain through the glutamate non-NMDA receptors

The amygdala sends heavy and broad projections to the rostral midbrain including the periaqueductal gray (PAG), the deep layers of the superior colliculus/deep mesencephalic nucleus (deep SC/DpMe), and the lateral mesencephalic reticular formation (MRF) that in turn project to the nucleus reticularis pontis caudalis (PnC), an obligatory relay in the primary acoustic startle circuit. Chemical lesions or inactivation of these areas blocked fear-potentiated startle, suggesting that these areas serve as a relay between the amygdala and the PnC. In the present study, we tried to determine more precisely which of these sites were critical for fear-potentiated startle and the role of glutamate receptors in this site in mediating fear-potentiated startle. Local infusion of the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)-quinoxaline (NBQX) dose-dependently blocked fear-potentiated startle when infused into the deep SC/DpMe before testing but had no effect on baseline startle amplitude. NBQX did not block fear-potentiated startle when infused before training. The same dose of NBQX infused into the dorsal/lateral PAG, the lateral MRF, or the superficial layers of the SC did not affect fear-potentiated startle. However, NBQX tended to reduce contextual freezing when infused into the dorsal/lateral PAG. These findings suggest that the deep SC/DpMe is the site that serves as a critical output relay between the amygdala and the PnC in mediating fear-potentiated startle and that glutamatergic transmission is required for this action.

Covariant maturation of nocifensive oral behaviour and c-fos expression in rat superior colliculus

Injections of formalin into the rodent paw elicit a rapid orientation of the head and mouth to the source of discomfort, followed by licking and biting the injected area. Previous work has shown this response is dependent on the integrity of the midbrain superior colliculus. The present experiments were initiated to examine the ontogeny of this oral nocifensive reaction and to determine whether it is correlated with the functional maturation of collicular responses to noxious stimuli (as indicated by c-fos immunohistochemistry). Rat pups at various postnatal ages received formalin injections in either the hindpaw or perioral regions. Behaviour was videotaped, and after 120 min, animals were killed and the brain and spinal cord processed for Fos-like immunoreactivity. Uninjected controls were treated identically. Formalin-induced oral responses following injections into the hindpaw and the expression of Fos in the superior colliculus were virtually absent until 10 days postnatal, despite the presence of Fos-like immunoreactivity in many other structures (e.g. spinal cord, parabrachial area, periaqueductal grey). In contrast, animals from day 1 were able to use limbs to localise the perioral injection site. From day 10 onward, there was a progressive increase in oral nocifensive behaviours and Fos expression in the superior colliculus. Our observations are consistent with the hypothesis that the normal elaboration of pain-induced oral behaviour is initiated only after a functionally active superior colliculus has developed, and support previous observations that link the colliculus particularly with oral nocifensive behaviours.

Parallel analyses of nociceptive neurones in rat superior colliculus by using c-fos immunohistochemistry and electrophysiology under different conditions of anaesthesia

Multiple sensory inputs to the superior colliculus (SC) play an important role in guiding head and eye movements toward or away from biologically significant stimuli. Much is now known about the visual, auditory, and somatosensory response properties of SC neurones that mediate these behavioural reactions. Rather less is known about the responses of SC neurones to noxious stimuli, and thus far, most of this information has been obtained in anaesthetised animals. Therefore, the purpose of the present study was to use the c-fos immunohistochemical technique and standard extracellular electrophysiology as parallel measures of nociceptive activity in the SC under different conditions of anaesthesia. In unanaesthetised animals, experimental and control treatments induced a qualitatively similar pattern of Fos-like immunoreactivity (FLI) in the SC, which was quantitatively related to the severity or biologic salience of the treatment; thus, baseline control < control injections of saline < a nonpainful stressor (immobilisation) < noxious injections of formalin. Compared with baseline levels, urethane and avertin anaesthesia induced FLI expression in the SC intermediate layers, although the FLI response to both noxious stimulation and control conditions was differentially suppressed in different layers of the SC by anaesthesia. Parallel electrophysiologic recordings found that anaesthesia was associated with high levels of spontaneous activity in the SC intermediate layers, often in neurones which were also nociceptive. High rates of background spike activity were also induced in the SC intermediate layers by noxious stimulation in chronically recorded awake animals. Although these results point to some differences between the nociceptive responses of SC neurones in anaesthetised and unanaesthetised animals, both data sets support the view that there are different populations of nociceptive neurones in the rodent SC that may be related to different adaptive functions of pain.

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.

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.

Common patterns of increased and decreased fos expression in midbrain and pons evoked by noxious deep somatic and noxious visceral manipulations in the rat

Immunohistochemical detection of the protein product (Fos) of the c-fos immediate early gene was used to study neuronal activation in the rostral pons and midbrain of halothane-anesthetised rats following noxious deep somatic or noxious visceral stimulation. In animals exposed only to halothane anesthesia, Fos-like immunoreactive (IR) neurons were located in the midbrain periaqueductal gray matter, tectum, and parabrachial nucleus. Following noxious stimulation of hindlimb muscle, knee joint, vagal cardiopulmonary, or peritoneal nociceptors, there was, compared to halothane-only animals, a significant increase in the numbers of Fos-like (IR) cells in the caudal ventrolateral periaqueductal gray and the intermediate gray lamina of the superior colliculus. Given the general agreement that increased Fos expression is a consequence of increased neuronal activity, the finding that a range of noxious deep somatic and noxious visceral stimuli evoked increased neuronal activity in a discrete, caudal ventrolateral periaqueductal gray region is consistent with previous suggestions that this region is an integrator of deep noxious evoked reactions. The noxious deep somatic and noxious visceral manipulations also evoked, compared to halothane-only animals, reductions in the numbers of Fos-like IR cells in the stratum opticum of the superior colliculus and the unlaminated portion of the external subnucleus of the inferior colliculus. To our knowledge this is the first report of reductions in Fos-expression in the tectum evoked by noxious stimulation. In separate experiments, the effects of noxious deep somatic and noxious visceral manipulations on arterial pressure and heart rate were measured. The noxious visceral manipulations evoked substantial and sustained falls in arterial pressure (15-45 mmHg), and heart rate (75-100 bpm), whereas the depressor and bradycardiac effects of the noxious deep somatic manipulations were weaker, not as sustained, or entirely absent. As similar distributions and numbers of both increased and decreased Fos-like IR cells were observed after each of the deep noxious manipulations, it follows that the deep noxious evoked increases and decreases in Fos expression were not secondary to the evoked depressor or bradycardiac effects.