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

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.

Functional Organization of the Sympathetic Pathways Controlling the Pupil: Light-Inhibited and Light-Stimulated Pathways

Pupil dilation is mediated by a sympathetic output acting in opposition to parasympathetically mediated pupil constriction. While light stimulates the parasympathetic output, giving rise to the light reflex, it can both inhibit and stimulate the sympathetic output. Light-inhibited sympathetic pathways originate in retina-receptive neurones of the pretectum and the suprachiasmatic nucleus (SCN): by attenuating sympathetic activity, they allow unimpeded operation of the light reflex. Light stimulates the noradrenergic and serotonergic pathways. The hub of the noradrenergic pathway is the locus coeruleus (LC) containing both excitatory sympathetic premotor neurones (SympPN) projecting to preganglionic neurones in the spinal cord, and inhibitory parasympathetic premotor neurones (ParaPN) projecting to preganglionic neurones in the Edinger-Westphal nucleus (EWN). SympPN receive inputs from the SCN via the dorsomedial hypothalamus, orexinergic neurones of the latero-posterior hypothalamus, wake- and sleep-promoting neurones of the hypothalamus and brain stem, nociceptive collaterals of the spinothalamic tract, whereas ParaPN receive inputs from the amygdala, sleep/arousal network, nociceptive spinothalamic collaterals. The activity of LC neurones is regulated by inhibitory α₂-adrenoceptors. There is a species difference in the function of the preautonomic LC. In diurnal animals, the α₂-adrenoceptor agonist clonidine stimulates mainly autoreceptors on SymPN, causing miosis, whereas in nocturnal animals it stimulates postsynaptic α₂-arenoceptors in the EWN, causing mydriasis. Noxious stimulation activates SympPN in diurnal animals and ParaPN in nocturnal animals, leading to pupil dilation via sympathoexcitation and parasympathetic inhibition, respectively. These differences may be attributed to increased activity of excitatory LC neurones due to stimulation by light in diurnal animals. This may also underlie the wake-promoting effect of light in diurnal animals, in contrast to its sleep-promoting effect in nocturnal species. The hub of the serotonergic pathway is the dorsal raphe nucleus that is light-sensitive, both directly and indirectly (via an orexinergic input). The light-stimulated pathways mediate a latent mydriatic effect of light on the pupil that can be unmasked by drugs that either inhibit or stimulate SympPN in these pathways. The noradrenergic pathway has widespread connections to neural networks controlling a variety of functions, such as sleep/arousal, pain, and fear/anxiety. Many physiological and psychological variables modulate pupil function via this pathway.

Anatomical pathways involved in generating and sensing rhythmic whisker movements

The rodent whisker system is widely used as a model system for investigating sensorimotor integration, neural mechanisms of complex cognitive tasks, neural development, and robotics. The whisker pathways to the barrel cortex have received considerable attention. However, many subcortical structures are paramount to the whisker system. They contribute to important processes, like filtering out salient features, integration with other senses, and adaptation of the whisker system to the general behavioral state of the animal. We present here an overview of the brain regions and their connections involved in the whisker system. We do not only describe the anatomy and functional roles of the cerebral cortex, but also those of subcortical structures like the striatum, superior colliculus, cerebellum, pontomedullary reticular formation, zona incerta, and anterior pretectal nucleus as well as those of level setting systems like the cholinergic, histaminergic, serotonergic, and noradrenergic pathways. We conclude by discussing how these brain regions may affect each other and how they together may control the precise timing of whisker movements and coordinate whisker perception.

Electroacupuncture analgesia in rat ankle sprain pain model: neural mechanisms

OBJECTIVES

Acupuncture, an alternative medical therapy with a long history, is appealing because it can activate endogenous analgesic mechanisms by minimally invasive means. The mechanisms of acupuncture, however, are not well understood yet. The following sentence was removed from our original manuscript. One of the major problems impeding understanding of the acupuncture mechanism is lack of experimental models that mimic various forms of persistent pain that respond to acupuncture in humans.

METHODS

In this review, we summarize and discuss previous and recent findings regarding electroacupuncture-induced analgesia in an ankle sprain pain model and the potential underlying mechanisms of acupuncture.

RESULTS

A novel model of ankle sprain pain is introduced recently and the mechanism of electroacupuncture-induced analgesia in this model has been explored. The following sentence was removed from our original manuscript. This model provides a reproducible and quantifiable index of persistent pain at the ankle joint in rats. Acupuncture at a remote site produces long-lasting and powerful analgesia. The consistent analgesic effect of acupuncture in this model has allowed us to pursue the underlying neural mechanisms.

CONCLUSIONS

These studies provide insight into the mechanisms of acupuncture analgesia in one particular form of persistent pain, and hopefully will allow us to expand our knowledge to other painful conditions.

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.

The effect of a thoracic spinal block on fos expression in the lumbar spinal cord of the rat induced by a noxious electrical stimulus at the hindpaw

BACKGROUND

Fos expression in the lumbar spinal cord, resulting from a noxious electrical stimulus at the hindpaw, is hypothesized to originate from three sources: direct sensory input of the noxious stimulus, local interactions in the spinal cord, and input of modulating signals from supraspinal regions. Our aim in this study was to discriminate among these sources by eliminating the supraspinal input.

METHODS

Therefore, a spinal block was administered in male Wistar rats by administering a local anesthetic (bupivacaine) through an intrathecal catheter at the mid-thoracic level. This thoracic spinal block completely suppressed the noxious stimulation-induced withdrawal reflex that is normally elicited by electrical stimulus. Fos immunoreactivity (Fos-IR) was quantified in all laminae of the L4 segment of the spinal cord.

RESULTS

Noxious stimulation resulted in a general and strong increase in Fos-IR in the ipsilateral dorsal horn, mainly in Laminae I, II, and V. Thoracic spinal block caused a remarkable increase in the amount of Fos-IR in Lamina V, but had no significant effect on the Fos-IR in Laminae I and II.

CONCLUSIONS

The increase in Fos-IR in Lamina V may have resulted from the interruption of a pain-modulating descending mechanism from the brain. A known modulating descending mechanism is the serotonergic system, controlled by the periaqueductal gray. This system inhibits the neurons in the superficial laminae. Another nonserotonergic system originates in the anterior pretectal nucleus. The latter facilitates neurons in the superficial laminae, while neurons in Lamina V are inhibited. We conclude that both systems are probably involved in the observed effects of the peripheral noxious stimulation given in the present model.

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.

Small animal, whole brain fMRI: innocuous and nociceptive forepaw stimulation

Supra-spinal pain processing involves a number of extensive networks. An examination of these networks using small animal functional magnetic resonance imaging (fMRI) is difficult. While prior studies have successfully delineated regions consistent with known pain processing pathways, they have been restricted to acquisitions of limited spatial extent with coarse in-plane resolution to achieve a high temporal resolution. An isotropic, whole brain fMRI protocol has been developed for the examination of the supra-spinal consequences of innocuous and nociceptive electrical stimulation of the rat forepaw. Innocuous electrical stimulation of the rat forepaw delineated BOLD contrast responses consistent with known somatosensory processing pathways (contralateral primary somatosensory cortex (S1), a region consistent with secondary somatosensory cortex, the ventral posterolateral thalamic nucleus and ipsilateral cuneate nucleus), providing face validity for the technique. The putative noxious stimulus delineated additional regions consistent with the classical lateral and medial pain systems as well as secondarily associated areas: the aversion and descending inhibition systems. These included the ipsilateral inferior colliculus, anterior pretectal nucleus, mediodorsal thalamic nucleus, with regions in the pre-frontal, cingulated, ventral orbital and infra-limbic cortices, nucleus accumbens all exhibiting negative BOLD changes. Such regions are in agreement with, and extend, those previously reported. Acquisition, post-processing and analysis methodologies undertaken in this study constitute a marked extension of previous fMRI in the rat, enabling whole brain coverage at a spatial resolution sufficient to delineate regional changes in BOLD contrast consistent with somatosensory and nociceptive networks.

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.

Participation of brainstem nuclei in the pronociceptive effect of lesion or neural block of the anterior pretectal nucleus in a rat model of incisional pain

The anterior pretectal nucleus (APtN) participates in nociceptive process and controls spinal nociceptive inputs, and its integrity reduces the severity of the responses to persistent injury. In this study we examined whether the pedunculopontine tegmental nucleus (PPTg) or the gigantocellularis nucleus pars alpha (GiA), stations that relay APtN inputs to the spinal cord, can control the persistent pain induced by a hind paw incision in rats with disrupted APtN. The withdrawal threshold to mechanical stimulation of the incised paw measured with von Frey filaments was significantly reduced in rats with contralateral APtN lesion or neural block of this nucleus with 2% lidocaine. Intrathecal xylamine, an inhibitor of noradrenaline uptake, inhibited the neural block of the APtN-induced increase in the incisional pain. Injection of glutamate into the contralateral PPTg or ipsilateral GiA reduced the incisional pain. Neural block of the PPTg or GiA reduced the threshold, mainly in APtN-disrupted rats. We conclude that persistent noxious stimulation activates descending pathways involving the contralateral APtN and PPTg, and ipsilateral GiA. Disruption of the APtN allows the activation of alternative circuitry involving at least the PPTg and GiA as intermediary stations that might maintain the control of nociceptive inputs in the spinal cord, probably involving noradrenergic mechanisms.

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.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

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.