Involvement of the intralaminar parafascicular nucleus in muscarinic-induced antinociception in rats

Abstract

The thalamic contribution to cholinergic-induced antinociception was examined by microinjecting the acetylcholine (ACh) agonist carbachol into the intralaminar nucleus parafascicularis (nPf) of rats. Pain behaviors organized at spinal (spinal motor reflexes), medullary (vocalizations during shock), and forebrain (vocalization afterdischarges, VADs) levels of the neuraxis were elicited by noxious tailshock. Carbachol (0.5, 1, and 2 μg/side) administered into nPf produced dose-dependent elevations of vocalization thresholds, but failed to elevate spinal motor reflex threshold. Injections of carbachol into adjacent sites dorsal or ventral to nPf failed to alter vocalization thresholds. Elevations in vocalization thresholds produced by intra-nPf carbachol were reversed in a dose-dependent manner by local administration of the muscarinic receptor antagonist atropine (30 and 60 μg/side). These results provide the first direct evidence supporting the involvement of the intralaminar thalamus in muscarinic-induced antinociception. Results are discussed in terms of the contribution of nPf to the processing of the affective dimension of pain.

Introduction

Involvement of the muscarinic cholinergic system in antinociception is well documented (for reviews, see [31], [32], [44], [50], [78]). Systemic, intrathecal, and ventricular administration of muscarinic agonists, acetylcholine (ACh) or cholinesterase inhibitors produces pronounced antinociceptive activity in several testing procedures. These antinociceptive effects are reversed by muscarinic, but not nicotinic or opiate receptor antagonists administered via similar routes [7], [50], [71], [77], [90], [92], [95]. Systemic administration of the muscarinic antagonist scopolamine blocks shock-induced and conditioned hypoalgesia, and increases pain reactivity to noxious stimulation [39]. Further, intracerebral microinjection of the broad spectrum cholinergic agonist carbachol into various brain areas, including the dorsomedial periaqueductal gray [40], pontine reticular formation [64], and rostral ventrolateral medulla [1] produces antinociceptive effects that are abolished by local administration of the non-selective muscarinic antagonist atropine.

Surprisingly, the thalamic contribution to muscarinic-induced antinociception has been relatively ignored despite its cholinergic hodology and considerable involvement in the processing and modulation of noxious stimulation. Converging anatomical, physiological, and behavioral evidence suggests that the intralaminar nucleus parafascicularis (nPf) is a thalamic site involved in muscarinic antinociception. Neuronal tracing techniques coupled with choline acetyltranferase (CHAT) immunohistochemistry identified cholinergic innervation of nPf arising from the pedunculopontine (PPTg) and laterodorsal (LDTg) tegmental nuclei in rat [10], [60], [105], [106], cat [74] and monkey [75]. Autoradiographic [67], [93], immunocytochemical [98], and in situ hybridization [22] studies localized muscarinic receptors in nPf, and accordingly, electrical stimulation of PPTg [24], or direct administration of muscarinic agonists into nPf [8] modulates the activity of nPf neurons.

Functionally, nPf participates in the generation of affective responses to noxious stimulation. The nPf receives nociceptive input via the spinothalamic [37] and spinopontothalamic [58] tracts, and provides nociceptive afferents to the anterior cingulate cortex (ACC) [47], [91], [101], [102], a cortical site involved in the processing of pain affect [4], [36], [51], [84], [97]. Noxious peripheral stimulation activates nociceptive-sensitive neurons [2], [3], [29], and increases c-fos-like protein expression [23] in nPf. Ablation of nPf relieves the emotional suffering associated with chronic pain in humans [66], [108], [109], and impairs avoidance and escape responses of animals to noxious stimulation [26], [54]. High frequency stimulation of nPf results in reports of intense pain and unpleasantness in humans [99] and aversive pain-like reactions in animals [53], [86]. We recently demonstrated that activation of μ-opiate or serotonin_1A/7 receptors in nPf preferentially suppresses affective responding of rats to noxious tailshock [41], [42], [43].

Given the involvement of nPf in the processing of noxious stimulation and findings that intraventricularly administered ACh reduced noxious evoked neural activity in nPf [27], the present study evaluated the behavioral antinociceptive action of intra-nPf injected carbachol. Muscarinic mediation of the antinociceptive action of carbachol was assessed by challenging its effects with intra-nPf administration of atropine. As the nPf contributes to elaboration of affective reactions to pain, it was predicted that intra-nPf administered carbachol would be especially effective in suppressing the affective reaction of rats to noxious stimulation.

Affective reactivity to noxious stimulation was assessed using the rodent vocalization test [25]. In this assay, the vocalizations of the rat that occur following application of noxious tailshock (vocalization afterdischarges; VADs) provide a validated measure of the affective reaction to noxious stimulation. Systemically administered drug treatments that preferentially suppress the affective reaction of humans to pain [38], [80], [81] also preferentially suppress production of VADs [17]. Generation of VADs is suppressed by damage of or drug treatments into forebrain sites that are known to contribute to the production of the emotional response of humans to clinical and experimental pain [14], [15], [43], [45], [66], [73], [96], [108], [109], and the capacity of noxious tailshock to support fear conditioning relies on its production of VADs [11], [13], [15]. Therefore, the intra-nPf injection of carbachol was predicted to preferentially elevate VAD thresholds when compared to other tailshock elicited responses that are organized at medullary (vocalizations during shock, VDS) or spinal (spinal motor reflexes, SMR) levels of the neuraxis.