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

Glial-Neuronal Interactions in Pathogenesis and Treatment of Spinal Cord Injury

Traumatic spinal cord injury (SCI) elicits an acute inflammatory response which comprises numerous cell populations. It is driven by the immediate response of macrophages and microglia, which triggers activation of genes responsible for the dysregulated microenvironment within the lesion site and in the spinal cord parenchyma immediately adjacent to the lesion. Recently published data indicate that microglia induces astrocyte activation and determines the fate of astrocytes. Conversely, astrocytes have the potency to trigger microglial activation and control their cellular functions. Here we review current information about the release of diverse signaling molecules (pro-inflammatory vs. anti-inflammatory) in individual cell phenotypes (microglia, astrocytes, blood inflammatory cells) in acute and subacute SCI stages, and how they contribute to delayed neuronal death in the surrounding spinal cord tissue which is spared and functional but reactive. In addition, temporal correlation in progressive degeneration of neurons and astrocytes and their functional interactions after SCI are discussed. Finally, the review highlights the time-dependent transformation of reactive microglia and astrocytes into their neuroprotective phenotypes (M2a, M2c and A2) which are crucial for spontaneous post-SCI locomotor recovery. We also provide suggestions on how to modulate the inflammation and discuss key therapeutic approaches leading to better functional outcome after SCI.

The Parabrachial Nucleus Directly Channels Spinal Nociceptive Signals to the Intralaminar Thalamic Nuclei, but Not the Amygdala

The parabrachial nucleus (PBN) is one of the major targets of spinal projection neurons and plays important roles in pain. However, the architecture of the spinoparabrachial pathway underlying its functional role in nociceptive information processing remains elusive. Here, we report that the PBN directly relays nociceptive signals from the spinal cord to the intralaminar thalamic nuclei (ILN). We demonstrate that the spinal cord connects with the PBN in a bilateral manner and that the ipsilateral spinoparabrachial pathway is critical for nocifensive behavior. We identify Tacr1-expressing neurons as the major neuronal subtype in the PBN that receives direct spinal input and show that these neurons are critical for processing nociceptive information. Furthermore, PBN neurons receiving spinal input form functional monosynaptic excitatory connections with neurons in the ILN, but not the amygdala. Together, our results delineate the neural circuit underlying nocifensive behavior, providing crucial insight into the circuit mechanism underlying nociceptive information processing.

Separating analgesia from reward within the ventral tegmental area

Activation of the dopaminergic mesolimbic reward circuit that originates in the ventral tegmental area (VTA) is postulated to preferentially suppress emotional responses to noxious stimuli, and presumably contributes to the addictive liability of strong analgesics. VTA dopamine neurons are activated via cholinergic afferents and microinjection of carbachol (cholinergic agonist) into VTA is rewarding. Here, we evaluated regional differences within VTA in the capacity of carbachol to suppress rats' affective response to pain (vocalization afterdischarges, VADs) and to support conditioned place preference (CPP) learning. As carbachol is a non-specific agonist, muscarinic and nicotinic receptor involvement was assessed by administering atropine (muscarinic antagonist) and mecamylamine (nicotinic antagonist) into VTA prior to carbachol treatment. Unilateral injections of carbachol (4μg) into anterior VTA (aVTA) and posterior VTA (pVTA) suppressed VADs and supported CPP; whereas, injections into midVTA failed to effect either VADs or CPP. These findings corroborate the hypothesis that the neural substrates underlying affective analgesia and reward overlap. However, the extent of the overlap was only partial. Whereas both nicotinic and muscarinic receptors contributed to carbachol-induced affective analgesia in aVTA, only muscarinic receptors mediated the analgesic action of carbachol in pVTA. The rewarding effects of carbachol are mediated by the activation of both nicotinic and muscarinic receptors in both aVTA and pVTA. The results indicate that analgesia and reward are mediated by separate cholinergic mechanisms within pVTA. Nicotinic receptor antagonism within pVTA failed to attenuate carbachol-induced analgesia, but prevented carbachol-induced reward. As addictive liability of analgesics stem from their rewarding properties, the present findings suggest that these processes can be neuropharmacologically separated within pVTA.

Alleviation of chronic pain following rat spinal cord compression injury with multimodal actions of huperzine A

Diverse mechanisms including activation of NMDA receptors, microglial activation, reactive astrogliosis, loss of descending inhibition, and spasticity are responsible for ∼40% of cases of intractable neuropathic pain after spinal cord injury (SCI). Because conventional treatments blocking individual mechanisms elicit only short-term effectiveness, a multimodal approach with simultaneous actions against major pain-related pathways may have value for clinical management of chronic pain. We hypothesize that [-]-huperzine A (HUP-A), an alkaloid isolated from the club moss Huperzia serrata, that is a potent reversible inhibitor of acetylcholinesterase and NMDA receptors, could mitigate pain without invoking drug tolerance or dependence by stimulating cholinergic interneurons to impede pain signaling, inhibiting inflammation via microglial cholinergic activation, and blocking NMDA-mediated central hypersensitization. We tested our hypothesis by administering HUP-A i.p. or intrathecally to female Sprague-Dawley rats (200-235 g body weight) after moderate static compression (35 g for 5 min) of T10 spinal cord. Compared with controls, HUP-A treatment demonstrates significant analgesic effects in both regimens. SCI rats manifested no drug tolerance following repeated bolus i.p. or chronic intrathecal HUP-A dosing. The pain-ameliorating effect of HUP-A is cholinergic dependent. Relative to vehicle treatment, HUP-A administration also reduced neural inflammation, retained higher numbers of calcium-impermeable GluR2-containing AMPA receptors, and prevented Homer1a up-regulation in dorsal horn sensory neurons. Therefore, HUP-A may provide safe and effective management for chronic postneurotrauma pain by reestablishing homeostasis of sensory circuits.

NMDA or non-NMDA receptor antagonism within the amygdaloid central nucleus suppresses the affective dimension of pain in rats: evidence for hemispheric synergy

The amygdala contributes to generation of affective behaviors to threats. The prototypical threat to an individual is exposure to a noxious stimulus and the amygdaloid central nucleus (CeA) receives nociceptive input that is mediated by glutamatergic neurotransmission. The present study evaluated the contribution of glutamate receptors in CeA to generation of the affective response to acute pain in rats. Vocalizations that occur following a brief noxious tail shock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by bilateral injection into CeA of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (AP5, 1 μg, 2 μg, or 4 μg) or the non-NMDA receptor antagonist 6-Cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX, .25 μg, .5 μg, 1 μg, or 2 μg). Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of AP5 or CNQX into CeA. Unilateral administration of AP5 or CNQX into CeA of either hemisphere also selectively elevated vocalization thresholds. Bilateral administration of AP5 or CNQX produced greater increases in vocalization thresholds than the same doses of antagonists administered unilaterality into either hemisphere indicating synergistic hemispheric interactions.

PERSPECTIVE

The amygdala contributes to production of emotional responses to environmental threats. Blocking glutamate neurotransmission within the central nucleus of the amygdala suppressed rats' emotional response to acute painful stimulation. Understanding the neurobiology underlying emotional responses to pain will provide insights into new treatments for pain and its associated affective disorders.

Role of central muscarinic cholinergic receptors in the formalin-induced pain in rats

Objectives:

In the present study, central effects of physostigmine and atropine have investigated in the formalin-induced pain in rats.

Materials and Methods:

In conscious rats implanted with an intracerebroventricular (i.c.v.) cannula, the effects of i.c.v. injection of physostigmine and atropine were investigated on the formalin test in the rat. Formalin test was induced by subcutaneous (s.c.) injection of formalin (50 μl, 1%) in ventral surface of left hind paw, and durations of licking and biting of the injected paw were measured in 5-min blocks for 1 h.

Results:

Formalin produced a biphasic response (first phase: 0–5 and second phase: 15–40 min) in durations of licking and biting of the injected paw. Physostigmine at doses of 2.5, 5 and 10 ug significantly (P < 0.05) attenuated both first and second phases of pain response. Atropine (5 and 10 ug), used alone, produced no significant effect on pain, but pretreatment with atropine (10 ug) significantly (P < 0.05) blocked antinociception induced by physostigmine (5 ug).

Conclusion:

These results indicate that i.c.v. physostigmine can affect both neurogenic and inflammatory phases of formalin-induced pain through a mechanism in which the muscarinic cholinergic receptors are involved.

Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The medial thalamic parafascicular nucleus (PF) and the rostral anterior cingulate cortex (rACC) are implicated in the processing and suppression of the affective dimension of pain. The present study evaluated the functional interaction between PF and rACC in mediating the suppression of pain affect in rats following administration of morphine or carbachol (acetylcholine agonist) into PF. Vocalizations that occur following a brief noxious tailshock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by injection of morphine or carbachol into PF. Vocalizations that occur during tailshock were suppressed to a lesser degree, whereas, spinal motor reflexes (tail flick and hindlimb movements) were only slightly suppressed by injection of carbachol into PF and unaffected by injection of morphine into PF. Blocking glutamate receptors in rACC (NMDA and non-NMDA) by injecting D-2-amino-5-phosphonovalerate (AP-5) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. Carbachol-induced increases in vocalization thresholds were not affected by injection of either glutamate receptor antagonist into rACC. The results demonstrate that glutamate receptors in the rACC contribute to the suppression of pain affect produced by injection of morphine into PF, but not to the suppression of pain affect generated by intra-PF injection of carbachol.

L-364,718 potentiates electroacupuncture analgesia through cck-a receptor of pain-related neurons in the nucleus parafascicularis

Electroacupuncture (EA) has been successfully used to alleviate pain produced by various noxious stimulus. Cholecystokinin-8 (CCK-8) is a neuropeptide involved in the mediation of pain. We have previously shown that CCK-8 could antagonize the analgesic effects of EA on pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the nucleus parafascicularis (nPf). However, its mechanism of action is not clear. In the present study, we applied behavioral and neuroelectrophysiological methods to determine whether the mechanisms of CCK-8 antagonism to EA analgesia are mediated through the CCK-A receptors of PENs and PINs in the nPf of rats. We found that focusing radiant heat on the tail of rats caused a simultaneous increase in the evoked discharge of PENs or a decrease in the evoked discharge of PINs in the nPf and the tail-flick reflex. This showed that radiant heat could induce pain. EA stimulation at the bilateral ST 36 acupoints in rats for 15 min resulted in an inhibition of the electrical activity of PEN, potentiation of the electrical activity of PIN, and prolongation in tail-flick latency (TFL), i.e. EA stimulation produced an analgesic effect. The analgesic effect of EA was antagonized when CCK-8 was injected into the intracerebral ventricle of rats. The antagonistic effect of CCK-8 on EA analgesia was reversed by an injection of CCK-A receptor antagonist L-364,718 (100 ng/μl) into the nPf of rats. Our results suggest that the pain-related neurons in the nPf have an important role in mediating EA analgesia. L-364,718 potentiates EA analgesia through the CCK-A receptor of PENs and PINs in the nPf.

Cobratoxin inhibits pain-evoked discharge of neurons in thalamic parafascicular nucleus in rats: involvement of cholinergic and serotonergic systems

The present study investigated the inhibitory effect of cobratoxin (CTX) on pain-evoked discharge of neurons in thalamic parafascicular nucleus (Pf) of rats and analyzed some of the mechanisms involved in this effect. Intracerebroventricular injection (icv) of CTX at 0.56, 1.12 and 4.50 microg/kg resulted in a dose-dependent inhibitory effect on the pain-evoked discharges of Pf neurons. The inhibition of pain-evoked discharges of Pf neurons by CTX at high dose (4.50 microg/kg) persisted at least for 2h, while the inhibitory effect of morphine (40 microg) persisted no longer than 30 min. The inhibitory effect of CTX was reversed by pretreatment with atropine (icv, 5 microg). In contrast, icv injection of naloxone (4 microg) had no effect on CTX-induced inhibition. Furthermore, pretreatment with parachlorophenylalanine, a specific inhibitor of tryptophan hydroxylase, also significantly attenuated the inhibitory effect of CTX. The results suggested that: (a) CTX has a dose-dependent inhibitory effect on pain-evoked discharges of Pf neurons, confirming electrophysiologically the antinociceptive action of CTX; (b) the inhibitory effect of CTX has a longer duration compared to that of morphine; (c) central cholinergic and serotonergic systems, but not opioidergic system, are involved in the inhibitory effect of CTX.

Contribution of the periaqueductal gray to the suppression of pain affect produced by administration of morphine into the intralaminar thalamus of rat

The parafascicular nucleus (nPf) of the intralaminar thalamus is implicated in the processing of pain affect in both animals and humans. Administration of morphine into nPf results in preferential suppression of the affective reaction to noxious tail shock in rats. The involvement of the ventrolateral periaqueductal gray in mediating the antinociceptive action of morphine injected into nPf was evaluated. Vocalizations that occur after tail shock offset (vocalization afterdischarges) are a validated rodent model of pain affect and were preferentially suppressed by injection of morphine into nPf. Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of morphine into nPf. Inactivation of the vPAG via the microinjection of muscimol (GABA(A) agonist) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. The results demonstrate that a functional link between the nPf and vPAG in generating the antinociceptive action of morphine injected into nPf.

PERSPECTIVE

Microinjection of morphine into nucleus parafascicular preferentially suppressed rats' affective reaction to noxious stimulation. This affective analgesia was reversed by inactivation of the ventrolateral periaqueductal gray. Understanding the neurobiology underlying the suppression of pain affect will provide insights into new treatments for pain and its associated affective disorders.

An essential role for Frizzled5 in neuronal survival in the parafascicular nucleus of the thalamus

Frizzled5 (Fz5), a putative Wnt receptor, is expressed in the retina, hypothalamus, and the parafascicular nucleus (PFN) of the thalamus. By constructing Fz5 alleles in which beta-galactosidase replaces Fz5 or in which Cre-mediated recombination replaces Fz5 with alkaline phosphatase, we observe that Fz5 is required continuously and in a cell autonomous manner for the survival of adult PFN neurons, but is not required for proliferation, migration, or axonal growth and targeting of developing PFN neurons. A motor phenotype associated with loss of Fz5 establishes a role for the PFN in sensorimotor coordination. Transcripts coding for Wnt9b, the likely Fz5 ligand in vivo, and beta-catenin, a mediator of canonical Wnt signaling, are both downregulated in the Fz5(-/-) PFN, implying a positive feedback mechanism in which Wnt signaling is required to maintain the expression of Wnt signaling components. These data suggest that defects in Wnt-Frizzled signaling could be the cause of neuronal loss in degenerative CNS diseases.

Examining the role of the medial thalamus in modulating the affective dimension of pain

The purpose of this project was to explore the role of the medial thalamus (MT), including the medial dorsal thalamus (MD) and associated midline nuclei in pain processing. Experiment 1 explored the role of electrolytic lesions to the MT in the formalin test. It was hypothesized that animals with electrolytic lesions to the MT would have attenuated paw licking behavior during the second phase of the formalin tests as compared to sham lesion controls. This hypothesis was based on evidence of projections from the MD to the ACC, and previous research demonstrating attenuation of paw licking behavior in the second phase of the formalin test in animals with ACC lesions. Experiment 2 tested the effects of electrolytic MT lesions on mechanical paw withdrawal thresholds in the L5 nerve ligation model. It was hypothesized that lesions of the MT would not alter mechanical paw withdrawal thresholds. Experiment 3 tested the effects of electrolytic MT lesions on escape/avoidance behavior in the place escape avoidance paradigm. For experiment 1, animals with MT lesions were found to have slightly elevated paw licking behavior, but only across two time points. No differences in mechanical paw withdrawal thresholds and in escape/avoidance behavior were detected as compared to the sham lesion group. These results indicate a limited role for the medial thalamic nuclei in coding for pain intensity and the affective dimension of pain. Additional research is needed to explore the role of individual medial nuclei in pain processing.

Both endogenous and exogenous ACh plays antinociceptive role in the hippocampus CA1 of rats

The present study examines the effect of acetylcholine (ACh), muscarinic acetylcholine receptors (mAChRs) agonist pilocarpine and mAChRs antagonist atropine on the pain-evoked response of pain-excited neurons (PEN) and pain-inhibited neurons (PIN) in the hippocampal CA1 of rats. The trains of electric impulses applied to the sciatic nerve were used as noxious stimulation. The discharges of PEN and PIN in the hippocampal CA1 were recorded by glass microelectrode. The results showed that intrahippocampal microinjection of ACh (2 microg/1 microl) or pilocarpine (2 microg/1 microl) decreased the frequency of discharge of PEN, and increased the frequency of discharge of PIN evoked by the noxious stimulation in the hippocampal CA1, while intrahippocampal administration of atropine (0.5 microg/1 microl) produced opposite response. On the basis of the above findings, we can deduce that ACh and mAChRs are involved in the modulation of nociceptive information transmission in the hippocampal CA1.

Pharmacological Characterization of the Muscarinic Agonist (3R,4R)-3-(3-Hexylsulfanyl-pyrazin-2-yloxy)-1-aza-bicyclo[2.2.1]heptane (WAY-132983) in in Vitro and in Vivo Models of Chronic Pain

Here, we have investigated the in vitro pharmacology of a muscarinic agonist, (3R,4R)-3-(3-hexylsulfanyl-pyrazin-2-yloxy)-1-aza-bicyclo[2.2.1]heptane (WAY-132983), and we demonstrated its activity in several models of pain. WAY-132983 had a similar affinity for the five muscarinic receptors (9.4-29.0 nM); however, in calcium mobilization studies it demonstrated moderate selectivity for M(1) (IC(50) = 6.6 nM; E(max) = 65% of 10 muM carbachol-stimulation) over the M(3) (IC(50) = 23 nM; E(max) = 41%) and M(5) receptors (IC(50) = 300 nM; E(max) = 18%). WAY-132983 also activated the M(4) receptor, fully inhibiting forskolin-induced increase in cAMP levels (IC(50) = 10.5 nM); at the M(2) receptor its potency was reduced by 5-fold (IC(50) = 49.8 nM). In vivo, WAY-132983 demonstrated good systemic bioavailability and high brain penetration (>20-fold over plasma levels). In addition, WAY-1329823 produced potent and efficacious antihyperalgesic and antiallodynic effects in rodent models of chemical irritant, chronic inflammatory, neuropathic, and incisional pain. It is noteworthy that efficacy in these models was observed at doses that did not produce analgesia or ataxia. Furthermore, a series of antagonist studies demonstrated that the in vivo activity of WAY-132983 is mediated through activation of muscarinic receptors primarily through the M(4) receptor. The data presented herein suggest that muscarinic agonists, such as WAY-132983, may have a broad therapeutic efficacy for the treatment of pain.

Targeting the cholinergic system as a therapeutic strategy for the treatment of pain

Acetylcholine mediates its effects through both the nicotinic acetylcholine receptors (ligand-gated ion channels) and the G protein-coupled muscarinic receptors. It plays pivotal roles in a diverse array of physiological processes and its activity is controlled through enzymatic degradation by acetylcholinesterase. The effects of receptor agonists and enzyme inhibitors, collectively termed cholinomimetics, in antinociception/analgesia are well established. These compounds successfully inhibit pain signaling in both humans and animals and are efficacious in a number of different preclinical and clinical pain models, suggesting a broad therapeutic potential. In this review we examine and discuss the evidence for the therapeutic exploitation of the cholinergic system as an approach to treat pain.

Alterations in behavioral responses to a cholinergic agonist in post-pubertal rats with neonatal ventral hippocampal lesions: relationship to changes in muscarinic receptor levels

Excitotoxic neonatal ventral hippocampal (NVH) lesion in rats is considered as a putative animal model of schizophrenia as lesioned animals show characteristic post-pubertal emergence of neurochemical and behavioral abnormalities analogous to some of those seen in this disease. Converging evidence points to the involvement of central cholinergic system in this neuropsychiatric disorder, and our previous studies have suggested that cholinergic neurotransmission may be altered in post-pubertal NVH lesioned rats. We investigated here muscarinic receptor reactivity in NVH lesioned animals by measuring the effects of the muscarinic receptor agonist oxotremorine on physiological responses known to be modulated by these receptors such as body temperature, salivation, tremor, pain, and prepulse inhibition of the acoustic startle (PPI). Quantitative receptor autoradiography revealed that post-pubertal NVH lesioned animals display increased levels of [3H]pirenzepine/M1-like and [3H]AFDX-384/M2-like receptor binding sites in the striatum, nucleus accumbens, and in subareas of the dorsal hippocampus. Moreover, in response to the systemic administration of oxotremorine (0.25 mg/kg), post-pubertal NVH lesioned rats exhibited increases in salivation and tremor, and a greater reduction in body temperature compared to sham control animals. Increases in the hot-plate latency were also observed suggesting enhanced antinociceptive effects of oxotremorine in post-pubertal NVH lesioned animals. Finally, oxotremorine (0.1 and 0.25 mg/kg) disrupted PPI in post-pubertal sham control rats while the muscarinic receptor antagonist biperiden (0.5 and 1.0 mg/kg) normalized this behavior in NVH lesioned rats. Taken together, these findings reveal that post-pubertal NVH lesioned rats display enhanced muscarinic receptor responsiveness, which may relate to some behavioral abnormalities reported in this animal model.

Activation of 5-HT1A and 5-HT7 receptors in the parafascicular nucleus suppresses the affective reaction of rats to noxious stimulation

The antinociceptive effects of the serotonin (5-HT)1A/7 receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) administered into the medial thalamus were evaluated. Pain behaviors organized at spinal (spinal motor reflexes, SMRs), medullary (vocalizations during shock, VDSs), and forebrain (vocalization after discharges, VADs) levels of the neuraxis were elicited by tailshock. Administration of 8-OH-DPAT (5, 10, and 20 microg/side) into nucleus parafascicularis (nPf) produced dose-dependent increases in VDS and VAD thresholds, but failed to elevate SMR threshold. The increase in VAD threshold was significantly greater than that of VDS threshold. Similar effects were observed with administration of 8-OH-DPAT (20 microg/side) into the rostral portion of the central lateral thalamic nucleus. The bilateral or unilateral administration of 8-OH-DPAT (20 microg) into other thalamic nuclei, or into sites dorsal to nPf, did not elevate vocalization thresholds. Increases in vocalization thresholds produced by nPf-administered 8-OH-DPAT were mediated by both 5-HT1A and 5-HT7 receptors. Intra-nPf administration of the 5-HT1A receptor antagonist WAY-100635 (0.05 or 0.5 microg/side), or the 5-HT7 receptor antagonist SB-269970 (1 or 2 microg/side), but not the dopamine D2 receptor antagonist raclopride (10 microg/side), reversed 8-OH-DPAT induced elevations in vocalization thresholds. These results provide the first reported evidence of behavioral antinociception following the administration of a 5-HT agonist into the medial thalamus.