Central antiaversive and antinociceptive effects of anterior pretectal nucleus stimulation: attenuation of autonomic and aversive effects of medial hypothalamic stimulation

The role of the anterior pretectal nucleus in pain modulation: A comprehensive review

Descending pain modulation involves multiple encephalic sites and pathways that range from the cerebral cortex to the spinal cord. Behavioral studies conducted in the 1980s revealed that electrical stimulation of the pretectal area causes antinociception dissociation from aversive responses. Anatomical and physiological studies identified the anterior pretectal nucleus and its descending projections to several midbrain, pontine, and medullary structures. The anterior pretectal nucleus is morphologically divided into a dorsal part that contains a dense neuron population (pars compacta) and a ventral part that contains a dense fiber band network (pars reticulata). Connections of the two anterior pretectal nucleus parts are broad and include prominent projections to and from major encephalic systems associated with somatosensory processes. Since the first observation that acute or chronic noxious stimuli activate the anterior pretectal nucleus, it has been established that numerous mediators participate in this response through distinct pathways. Recent studies have confirmed that at least two pain inhibitory pathways are activated from the anterior pretectal nucleus. This review focuses on rodent anatomical, behavioral, molecular, and neurochemical data that have helped to identify mediators of the anterior pretectal nucleus and pathways related to its role in pain modulation.

Panic-like defensive behavior but not fear-induced antinociception is differently organized by dorsomedial and posterior hypothalamic nuclei of Rattus norvegicus (Rodentia, Muridae)

The hypothalamus is a forebrain structure critically involved in the organization of defensive responses to aversive stimuli. Gamma-aminobutyric acid (GABA)ergic dysfunction in dorsomedial and posterior hypothalamic nuclei is implicated in the origin of panic-like defensive behavior, as well as in pain modulation. The present study was conducted to test the difference between these two hypothalamic nuclei regarding defensive and antinociceptive mechanisms. Thus, the GABA(A) antagonist bicuculline (40 ng/0.2 µL) or saline (0.9% NaCl) was microinjected into the dorsomedial or posterior hypothalamus in independent groups. Innate fear-induced responses characterized by defensive attention, defensive immobility and elaborate escape behavior were evoked by hypothalamic blockade of GABA(A) receptors. Fear-induced defensive behavior organized by the posterior hypothalamus was more intense than that organized by dorsomedial hypothalamic nuclei. Escape behavior elicited by GABA(A) receptor blockade in both the dorsomedial and posterior hypothalamus was followed by an increase in nociceptive threshold. Interestingly, there was no difference in the intensity or in the duration of fear-induced antinociception shown by each hypothalamic division presently investigated. The present study showed that GABAergic dysfunction in nuclei of both the dorsomedial and posterior hypothalamus elicit panic attack-like defensive responses followed by fear-induced antinociception, although the innate fear-induced behavior originates differently in the posterior hypothalamus in comparison to the activity of medial hypothalamic subdivisions.

Abnormal anterior pretectal nucleus activity contributes to central pain syndrome

Central pain syndrome (CPS) is a debilitating condition that affects a large number of patients with a primary lesion or dysfunction in the CNS, most commonly due to spinal cord injury, stroke, and multiple sclerosis lesions. The pathophysiological processes underlying the development and maintenance of CPS are poorly understood. We have recently shown, in an animal model of CPS, that neurons in the posterior thalamic nucleus (PO) have increased spontaneous and evoked activity. We also demonstrated that these changes are due to suppressed inhibitory inputs from the zona incerta (ZI). The anterior pretectal nucleus (APT) is a diencephalic nucleus that projects on both the PO and ZI, suggesting that it might be involved in the pathophysiology of CPS. Here we test the hypothesis that CPS is associated with abnormal APT activity by recording single units from APT in anesthetized rats with CPS resulting from spinal cord lesions. The firing rate of APT neurons was increased in spinal-lesioned animals, compared with sham-operated controls. This increase was due to a selective increase in firing of tonic neurons that project to and inhibit ZI and an increase in bursts in fast bursting and slow rhythmic neurons. We also show that, in normal animals, suppressing APT results in increased PO spontaneous activity and evoked responses in a subpopulation of PO neurons. Taken together, these findings suggest that APT regulates ZI inputs to PO and that enhanced APT activity during CPS contributes to the abnormally high activity of PO neurons in CPS.

GABA(A) receptor blockade in dorsomedial and ventromedial nuclei of the hypothalamus evokes panic-like elaborated defensive behaviour followed by innate fear-induced antinociception

Dysfunction in the hypothalamic GABAergic system has been implicated in panic syndrome in humans. Furthermore, several studies have implicated the hypothalamus in the elaboration of pain modulation. Panic-prone states are able to be experimentally induced in laboratory animals to study this phenomenon. The aim of the present work was to investigate the involvement of medial hypothalamic nuclei in the organization of panic-like behaviour and the innate fear-induced oscillations of nociceptive thresholds. The blockade of GABA(A) receptors in the neuronal substrates of the ventromedial or dorsomedial hypothalamus was followed by elaborated defensive panic-like reactions. Moreover, innate fear-induced antinociception was consistently elicited after the escape behaviour. The escape responses organized by the dorsomedial and ventromedial hypothalamic nuclei were characteristically more elaborated, and a remarkable exploratory behaviour was recorded during GABA(A) receptor blockade in the medial hypothalamus. The motor characteristic of the elaborated defensive escape behaviour and the patterns of defensive alertness and defensive immobility induced by microinjection of the bicuculline either into the dorsomedial or into the ventromedial hypothalamus were very similar. This was followed by the same pattern of innate fear-induced antinociceptive response that lasted approximately 40 min after the elaborated defensive escape reaction in both cases. These findings suggest that dysfunction of the GABA-mediated neuronal system in the medial hypothalamus causes panic-like responses in laboratory animals, and that the elaborated escape behaviour organized in both dorsomedial and ventromedial hypothalamic nuclei are followed by significant innate-fear-induced antinociception. Our findings indicate that the GABA(A) receptor of dorsomedial and ventromedial hypothalamic nuclei are critically involved in the modulation of panic-like behaviour.

Heterogeneous output pathways link the anterior pretectal nucleus with the zona incerta and the thalamus in rat

The anterior pretectal nucleus (APT) and the zona incerta (ZI) are diencephalic nuclei that exert a strong inhibitory influence selectively in higher order thalamic relays. The APT is also known to project to the ZI as well as the thalamus, but anatomical details of the APT-ZI projection have not been described. In the present study, the efferent pathways of the APT were examined in the APT-ZI-thalamus network by using anterograde and retrograde tracing in combination with pre- and postembedding immunocytochemical stainings and in situ hybridization. The vast majority of APT fibers selectively innervated the parvalbumin-positive, ventral part of the ZI, which contains ZI neurons with axons projecting to higher order thalamic nuclei. The APT-ZI pathway consisted of both gamma-aminobutyric acid (GABA)-negative and GABA-positive components; 38.2% of the terminals in the ZI contained GABA, and 8.6% of the projecting somata in the APT were glutamic acid decarboxylase 67 (GAD67) mRNA positive. The combination of parvalbumin immunostaining with retrograde tracing showed that strongly and weakly parvalbumin-positive as well as parvalbumin-negative neurons were all among the population of APT cells projecting to the ZI. Similar heterogeneity was found among the APT cells projecting to the thalamus. Double retrograde tracing from higher order thalamic nuclei and their topographically matched ZI regions revealed hardly any APT neuron with dual projections. Our data suggest that both ZI and the higher order thalamic relays are innervated by distinct, physiologically heterogeneous APT neurons. These various efferent pathways probably interact via the rich recurrent collaterals of the projecting APT cells. Therefore, the powerful, GABAergic APT and ZI outputs to the thalamus are apparently co-modulated in a synergistic manner via dual excitatory and inhibitory APT-ZI connections.

Carbachol injections into the nucleus accumbens disrupt acquisition and expression of fear-potentiated startle and freezing in rats

The nucleus accumbens is involved in different types of emotional learning, ranging from appetitive instrumental learning to Pavlovian fear conditioning. In previous studies, we found that temporary inactivation of the nucleus accumbens blocked both the acquisition and expression of conditioned fear. This was not due to altered dopaminergic activity as we have also found that intra-nucleus accumbens infusions of the dopamine agonist amphetamine do not affect either the acquisition or the expression of conditioned fear. Therefore, in the present study we examined whether cholinergic activity in the nucleus accumbens is involved in the acquisition and expression of conditioned fear. Specifically, the effect of intra-nucleus accumbens infusions of the unselective cholinergic agonist carbachol on the acquisition and expression of conditioned fear was assessed. Across several experiments, we measured fear to visual and acoustic conditioned stimuli and to the experimental context. Further, two different measures of conditioned fear were recorded: fear potentiation of startle and freezing. Intra-nucleus accumbens carbachol infusions disrupted acquisition as well as expression of conditioned fear, regardless of the modality of the fear-eliciting stimulus or of the specific measure of conditioned fear. This disruption of conditioned fear was not simply a by-product of enhanced motor activity which also occurred after intra-nucleus accumbens carbachol infusions. Interestingly, despite the substantial effect of intra-nucleus accumbens carbachol on expression of conditioned fear, the results of the final experiment suggest that these rats extinguish similarly to control rats. Taken together, the present results indicate that acetylcholine within the nucleus accumbens is important for the learning and retrieval of conditioned fear.

Amphetamine injections into the nucleus accumbens affect neither acquisition/expression of conditioned fear nor baseline startle response

The acoustic startle response is enhanced during states of fear and attenuated during pleasant ones. Our question was whether pharmacological stimulation of the reward system disrupts the learning and retrieval of conditioned fear as measured by fear-potentiated startle. We therefore injected the dopamine agonist amphetamine into the nucleus accumbens (NAC) immediately before either acquisition or expression of conditioned fear and measured the effect of these injections on fear-potentiated startle and baseline startle response. This study clearly showed that amphetamine injections into the NAC had no effect on baseline startle amplitude and acquisition/expression of conditioned fear. In contrast, amphetamine injections into the nucleus accumbens clearly enhanced spontaneous motor activity. These results suggest that dopamine within the NAC is not involved in modulation of fear-potentiated startle and baseline startle.

Antinociception induced by stimulating the anterior pretectal nucleus in two models of pain in rats

1. This study examined whether different parts of the rat anterior pretectal nucleus (APtN) may be involved in the spinal control of brief (tail flick test) or persistent (surgical incision of the plantar aspect of a hind paw) noxious inputs via activation of descending pathways. 2. We have confirmed that stimulation of the dorsal APtN produces a strong antinociceptive effect in the tail flick test, as opposed to a very weak effect obtained from the ventral APtN. Stimulation at the ventral APtN was the most effective part of the nucleus against a persistent incisional pain. 3. The incisional pain was significantly increased following injection of 1 or 2% lignocaine (0.25 microL) into the nucleus, but the effect was more intense after neural block of the ventral rather than the dorsal APtN. Injection of 2% lignocaine (0.10 microL) into the ventral, but not dorsal, APtN significantly increased the perception of the incisional pain. 4. We conclude that the effect of stimulating the APtN depends on the site of stimulation and model of pain used. Sustained noxious stimuli activate pathways from the ventral APtN to reduce further noxious spinal inputs. The noxious stimulation produced during the tail flick test may be not enough to activate the same circuitry, but electrical stimulation at the dorsal APtN is very effective in inhibiting brief thermal noxious inputs at the spinal level.

Pharmacological and neuroanatomical evidence for the involvement of the anterior pretectal nucleus in the antinociception induced by stimulation of the dorsal raphe nucleus in rats

Several studies have shown that the anterior pretectal nucleus (APtN) is involved in descending inhibitory pathways that control noxious inputs to the spinal cord and that it may participate in the normal physiological response to noxious stimulation. Among other brain regions known to send inputs to the APtN, the dorsal column nuclei (DCN), pedunculopontine tegmental nucleus (PPTg), deep mesencephalon (DpMe), and dorsal raphe nucleus (DRN) are structures also known to be involved in antinociception. In the present study, the effects of stimulating these structures on the latency of the tail withdrawal reflex from noxious heating of the skin (tail flick test) were examined in rats in which saline or hyperbaric lidocaine (5%) was previously microinjected into the APtN. Brief stimulation of the PPTg, DpMe or DRN, but not the DCN, strongly depressed the tail flick reflex. The antinociceptive effect of stimulating the DRN, but not the PPTg or DpMe was significantly reduced, but not abolished, by the prior administration of the local anaesthetic into the APtN. The antinociception induced by stimulation of the PPTg or DpMe, therefore, is unlikely to depend on connections between these structures and the APtN. Similar inhibition of the effect of stimulating the DRN was obtained from rats previously microinjected with naloxone (2.7 nmol) or methysergide (2 nmol) into the APtN. Strongly labelled cells were identified in the DRN following microinjection of the fluorescent tracer Fast Blue into the APtN. These results indicate that the APtN may participate as a relay station through which the DRN partly modulates spinal nociceptive messages. In addition, they also indicate that endogenous opioid and serotonin can participate as neuromodulators of the DRN-APtN connection.

Reciprocal connections between the zona incerta and the pretectum and superior colliculus of the cat

The goal of the present experiments was to examine the relationships of the zona incerta with two structures associated with visuomotor behavior, the superior colliculus and pretectum. The experiments were carried out in the cat, a species commonly used in studies of visuomotor integration, and utilized wheat germ agglutinin horseradish peroxidase and biocytin as retrograde and anterograde neuronal tracers. Retrograde axonal transport demonstrated that most cells in the ventral subdivision of the zona incerta project to the superior colliculus. Anterograde tracers demonstrated that the incertotectal terminal field is most dense in the intermediate gray layer, which is the primary source of the descending pathway from the superior colliculus to brainstem gaze centers. Further experiments showed that scattered cells within the intermediate gray layer give rise to a reciprocal pathway that terminates in both the dorsal and ventral subdivisions of the zona incerta. The distribution of both labeled incertotectal cells and tectoincertal terminals extends dorsolateral to the zona incerta proper, between the reticular thalamic nucleus and the external medullary lamina. Electron microscopic examination of labeled tectoincertal terminals demonstrated that they contain mainly spherical vesicles and have slightly asymmetric to symmetric synaptic densities. Labeled terminals were observed contacting labeled cells in the zona incerta, suggesting that the reciprocal pathway may be monosynaptic. The zona incerta is also reciprocally interconnected with the pretectum. The anterior pretectal nucleus provides a dense projection to the ventral part of the zona incerta and receives a sparse reciprocal projection. The posterior pretectal nucleus and nucleus of the optic tract may also project to the zona incerta. The pretectoincertal fibers form terminals that contain primarily spherical vesicles and make distinctly asymmetric synaptic contacts. In summary, these results indicate that the deep layers of the superior colliculus, which are important for controlling saccades, are the target of a projection from the ventral subdivision of the zona incerta. Like the substantia nigra, the zona incerta may play a permissive role in the tectal initiation of saccadic eye movements. The incertotectal terminal field in the cat is less dense than that observed previously in the rat, suggesting species differences in the development of this pathway. An additional finding of this study is that one of the main sources of input to these incertotectal cells is the anterior pretectal nucleus. This pretectal incertal tectal pathway is likely to play a role in the guidance of tectally initiated saccades by somatosensory stimuli.

Efferent connections from the anterior pretectal nucleus to the diencephalon and mesencephalon in the rat

The anterior pretectal nucleus has been described as part of the visual pretectal complex. However, several electrophysiological and behavioural studies showed that this area is involved in somatosensory modulation, more specifically, antinociception. The efferents of the anterior pretectal nucleus have not been identified taking into account the different function of this nucleus in relation to the rest of the pretectal complex. In the study herein described, a sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin was used to trace the mesencephalic and diencephalic efferents of the anterior pretectal nucleus in the rat. The majority of the connections were ipsilateral. Fibres with varicosities were observed in discrete areas of the thalamus (central lateral, posterior complex), hypothalamus (lateral, posterior and ventromedial), zona incerta, parvocellular red nucleus, intermediate and deep layers of the superior colliculus, central grey, deep mesencephalon, pontine parabrachial region, and pontine nuclei. Fibres en passant were detected in the medial lemniscus, from the level of the injection site to rostral medullary levels. Some labelled axons were seen coursing to the contralateral side through the posterior commissure and the decussation of the superior cerebellar peduncle. These results show that the anterior pretectal nucleus projects principally to areas involved in somatosensory and motor control in a manner that permits sensory modulation at higher and lower levels of the brain. These connections may explain the antinociceptive and antiaversive effects of stimulating the anterior pretectal nucleus in freely moving animals.

Orthograde axonal transport studies of projections from the zona incerta and pretectum to the basilar pontine nuclei in the rat

This study employed orthogradely transported axonal tracers to demonstrate, in the rat, projections that reach the basilar pontine nuclei from the zona incerta or pretectal nuclei. Except for the most rostral levels, all subdivisions of the zona incerta give rise to substantive basilar pontine projections. Although some topographic differences exist among the temination patterns of various subdivisions, no clear somatotopically organized scheme is apparent. Most incertopontine axons descend to the basilar pons in association with fibers of the medial lemniscus or crus cerebri and reach ipsilateral ventral and medial pontine gray regions. A sparse number of terminals are evident in the contralateral medial pontine gray. The anterior pretectal axons also descend with the medial lemniscus and crus cerebri to enter exclusively the ipsilateral basilar pons where they terminate most densely in ventral and medial regions. Dual orthograde labeling experiments indicate that some pretectal terminal fields in the pontine gray are shared with incertopontine projections and with afferents from the dorsal column nuclei. This potential convergence of basilar pontine afferent projections is significant in light of 1) the known somatosensory input to the zona incerta and pretectum and 2), the fractured somatotopy of peripheral cutaneous inputs that arrive in the cerebellar cortex via mossy fibers. The present studies also employed electron microscopy to identify synaptic boutons formed by incerto- and pretectopontine axons, and they proved to be remarkably similar. Each is a medium to small-sized bouton that contains spheroidal synaptic vesicles and forms asymmetric membrane specializations. Most incerto- and pretectopontine boutons participate in glomerular synaptic complexes that include a single, centrally located bouton contacted on its perimeter by several types of dendritic profiles including shafts and spine-like appendages. A relatively small number of labeled boutons of either type contacts single, isolated dendritic elements in the neuropil. Taken together, these findings suggest that some basilar pontine neurons might receive convergent inputs from the zona incerta and pretectum as well as other somatosensory-related systems such as the dorsal column nuclei and sensorimotor cortex.

Direct projections from the anterior pretectal nucleus to the ventral medulla oblongata in rats

The anterior pretectal nucleus has recently been implicated in the descending modulation of nociception. Electrical stimulation of the nucleus was found to reduce the nociceptive responses of deep dorsal horn neurons and to inhibit spinally integrated withdrawal reflexes. It is believed that at least part of the descending inhibitory effects of the anterior pretectal nucleus are mediated by reticulospinal cells of the ventrolateral medulla. The purpose of the present study was to trace the direct medullary projections of the anterior pretectal nucleus, to describe their topographical organization and to reveal the chemical nature of some of their putative target cells. The connections were studied using anterograde tract-tracing with Phaseolus vulgaris leucoagglutinin. Direct projections from the anterior pretectal nucleus to the ipsilateral rostral ventral medulla were found in all cases. A dense innervation of the dorsal inferior olive, the gigantocellular reticular nucleus pars ventralis and pars alpha and the ventral pontine reticular nucleus was found from all aspects of the anterior pretectal nucleus. Descending labelled terminals were also observed in the gigantocellular reticular nucleus proper and, laterally, in the lateral paragigantocellular nucleus and in the region of the A5 noradrenergic cell group. A relatively lower density of labelled terminals was noted in the medullary raphe nuclei and in the rostroventrolateral reticular nucleus. Following tract-tracer injections into five distinct subregions of the anterior pretectal nucleus, the topographical organization of the projection was examined and the relatively highest density and most widespread projection was found to originate from the caudoventral part of the anterior pretectal nucleus. A combined tract-tracing and immunolabelling study revealed that some of the descending, labelled terminals were in close proximity of tyrosine hydroxylase-immunoreactive dendrites in the C1 and A5 cell groups. Some labelled fibres were also noted among the serotonin-immunoreactive cells in the lateral extension of the B3 cell population. The existence of direct projections to the ventral medulla and pons correlates well with physiological data which showed that the descending, antinociceptive effects of the anterior pretectal nucleus are relayed via the rostral ventrolateral medulla. The data are also in keeping with pharmacological studies that suggested the role of catecholaminergic cells in the mediation of these descending effects. It is proposed that the rostral ventral medullary projections provide a path through which antinociceptive effects of the anterior pretectal nucleus are mediated to the spinal cord.

The anterior pretectal nucleus: a proposed role in sensory processing

Four nuclei of the pretectal complex, the olivary pretectal nucleus, the medial pretectal nucleus, the nucleus of the optic tract and the posterior pretectal nucleus, all have a demonstrated role in visual function. In contrast, the anterior pretectal nucleus (APtN) has no inputs from retina and has few outputs to visual accessory nuclei. The APtN has connections with areas associated with sensory functions and it has been suggested that this nucleus may have a role to play in somatosensory processing. An increasing number of behavioural and electrophysiological studies support this view. Brief low-intensity electrical or chemical stimulation of the APtN causes antinociception in the tail flick test in both unanaesthetised and anaesthetised animals. This inhibition of the tail flick response is attenuated by naloxone, alpha-adrenoceptor antagonists and muscarinic cholinergic receptor antagonists. Electrical stimulation of the APtN is similarly effective in the paw pressure and formalin tests. APtN stimulation also causes a brief inhibition of the tooth pulp-evoked jaw opening reflex. studies with [C14]2-deoxyglucose indicate that peripheral noxious stimuli will cause an increase in metabolic activity within the APtN. Animals with electrodes placed in the APtN will self-administer electrical stimulation and this can reduce the aversive and autonomic effects of stimulating the ventromedial hypothalamus. Part of the antinociceptive effects of stimulating the APtN are due to a descending inhibition of spinal dorsal horn projection neurones. Multireceptive neurones deep in the dorsal horn are inhibited by APtN stimulation. In contrast, superficial projection neurones that respond to intense cutaneous stimuli are excited by APtN stimulation. The APtN receives an excitatory input from low-threshold afferents via the dorsal column pathway and a high-threshold excitatory drive from superficial cells projecting through the dorsolateral funiculus. The excitatory input from the dorsal columns may well participate in the long-term inhibition of spinal projection neurones evoked by dorsal column stimulation. These ascending excitatory pathways may also be important to the long-term activation of descending inhibition from the APtN.

The antinociception evoked by anterior pretectal nucleus stimulation is partially dependent upon ventrolateral medullary neurones

Electrical stimulation (35 microA rms/15 s) of the anterior pretectal nucleus (APtN) inhibits the spinal reflex of the tail-flick (TF) to noxious heat in unanaesthetised rats. APtN stimulation also reduces the nociceptive response of spinal dorsal horn neurones in halothane-anaesthetised rats. This study determined if the antinociceptive effects of APtN stimulation depended on neurones in the ventral medulla. Bilateral electrolytic lesions of the ventrolateral medulla, but not the nucleus raphe magnus, reduced by 70% the antinociceptive effect of APtN stimulation in the TF test. In rats anaesthetised with halothane, electrical stimulation of the APtN (single square wave 0.1 msec pulses, 2-20 microA, 1 Hz) excited cells in the ventrolateral medulla. These data suggest a connection between both areas. This connection is further confirmed by neuroanatomical tract tracing studies in which the retrograde dye Fast Blue was injected into the ventrolateral medulla. Fluorescent cell bodies were found in the APtN. We therefore conclude that the ventrolateral medulla is part of a descending antinociceptive pathway from the APtN.