Pharmacological differentiation between responses of rat medullary dorsal horn neurons to noxious mechanical and noxious thermal cutaneous stimuli

Functional involvement of nucleus tractus solitarii neurons projecting to the parabrachial nucleus in trigeminal neuropathic pain

Peripheral nerve injury can induce neuroplastic changes in the central nervous system and result in neuropathic pain. This study investigated functional involvement in dorsal paratrigeminal nucleus (dPa5) and nucleus tractus solitarii (NTS) neurons projecting to the parabrachial nucleus (PBN) after trigeminal nerve injury. Anatomical quantification was performed based on phosphorylated extracellular signal-regulated kinase (pERK) expression underlying orofacial neuropathic pain associated with infraorbital nerve chronic constriction injury (ION-CCI) in rats. ION-CCI rats exhibited heat and mechanical hypersensitivity in the ipsilateral upper lip. After injection of retrograde tracer fluorogold (FG) into the contralateral PBN, ION-CCI rats received capsaicin or noxious mechanical stimulation to the upper lip. The total number of FG-labeled neurons in dPa5 and NTS did not change after ION-CCI, and pERK expression in dPa5 did not differ between sham and ION-CCI rats. In the NTS contralateral to ION-CCI, the number of pERK-immunoreactive neurons and percentage of pERK-immunoreactive FG-labeled PBN projection neurons were increased after capsaicin stimulation in ION-CCI rats. The present findings suggest that enhanced noxious inputs from the NTS to the PBN after trigeminal nerve injury modulates PBN neuron activity, which accompanies the affective components of orofacial neuropathic pain.

Intracisternal NMDA produces analgesia in the orofacial formalin test of freely moving rats

The present study was performed to investigate the antinociceptive response to the intracisternal administration of NMDA in the orofacial area. To achieve this purpose, the effects of NMDA injected intracisternally on the orofacial formalin test were monitored in freely moving rats. We also investigated underlying the mechanisms of NMDA-induced antinociceptive response. Experiments were carried out on 80 male SD rats and surgical procedures were performed under pentobarbital sodium (40 mg/kg, i.p.). Fifty microliters of 5% formalin was applied subcutaneously to the vibrissa pad without any restraining of the animals. For each animal, the number of noxious behavioral responses and the time spent grooming, rubbing, and/or scratching the facial region proximal to the injection site were recorded for nine successive 5-min intervals. The orofacial formalin responses showed two distinct phases separated by a time of relative inactivity. Intracisternal administration of NMDA produced intense scratching behavioral responses with dose related manner. NMDA injected intracisternally 30 min prior to formalin injection, however, inhibited noxious behavioral responses produced by a formalin injection significantly. Pretreatment with naloxone 20 min prior to NMDA injection abolished the inhibition of number of scratches and the duration of scratching produced by the intracisternal injection of NMDA in the late phase. Pretreatment with L-NAME, NO synthesis inhibitor, however, did not affect the antinociceptive response produced by NMDA injected intracisternally. These results suggest that NMDA injected intracisternally produces brief pain behavioral responses and also produces delayed antinociceptive effects in the orofacial formalin test. The opioid pathway seems to be involved in the NMDA-induced antinociception in the orofacial area.

Differential efficacy of intrathecal NMDA receptor antagonists on inflammatory mechanical and thermal hyperalgesia in rats

Spinal cord dorsal horn N-methyl-D-aspartate (NMDA) receptors have been implicated in central sensitization, enhanced responsiveness to peripheral stimuli following peripheral injury. Since hyperalgesia is a behavioral consequence of central sensitization, it should be attenuated at the level of the dorsal horn with NMDA receptor antagonists. However, responsiveness to thermal and mechanical hyperalgesia may be distinct, and have thus far not been directly compared in chronic inflammatory pain models. In the present study, inflammation was induced with complete Freund's adjuvant (CFA) injected into the rat hind paw and NMDA receptor antagonists dizocilpine (MK-801) or 2-amino-5-phosphonovaleric acid (AP-5) were intrathecally injected in rats to determine the effects on both mechanical and thermal hyperalgesia. Locomotor tests and reflexes were also conducted to evaluate potential motor side effects. The NMDA receptor antagonists dose-dependently ameliorated mechanical hyperalgesia, but had marginal effects on thermal hyperalgesia. In ranges near antihyperalgesic doses, significant disruption of motor coordination was observed for both antagonists. These results suggest that, depending on the stimulus, NMDA receptors may have variable significance for central sensitization-mediated hyperalgesia, and that NMDA receptor antagonists may have therapeutic potential for some, but not all components in the clinical manifestation of inflammatory pain.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Blockade and reversal of spinal morphine tolerance by peptide and non-peptide calcitonin gene-related peptide receptor antagonists

This study examined the effects of the peptide CGRP receptor antagonist CGRP(8-37) and the newly-developed non-peptide CGRP receptor antagonist BIBN4096BS for their potential to both inhibit the development and reverse tolerance to the antinociceptive action of morphine. Repeated administration of intrathecal morphine (15 microg), once daily, produced a progressive decline of antinociceptive effect and an increase in the ED(50) value in the tailflick and paw pressure tests. Co-administration of CGRP(8-37) (4 microg) or BIBN4096BS (0.05, 0.1 microg) with morphine (15 microg) prevented the decline of antinociceptive effect and increase in ED(50) value in the tailflick test. Intrathecal administration of the CGRP receptor antagonists did not alter the baseline responses in either tests. Acute CGRP(8-37) also did not potentiate the acute actions of spinal morphine. In animals rendered tolerant to intrathecal morphine, subsequent administration of CGRP(8-37) (4 microg) with morphine (15 microg) partially restored the antinociceptive effect and ED(50) value of acute morphine, reflecting the reversal of tolerance. Animals tolerant to intrathecal morphine expressed increased CGRP and substance P-like immunostaining in the dorsal horn of the spinal cord. The increase in CGRP, but not substance P-like immunostaining, was blocked by a co-treatment with CGRP(8-37) (4 microg). In animals already tolerant to morphine, the increase in CGRP but not substance P-like immunostaining was partially reversed by CGRP(8-37) (4 microg). These data suggest that activation of spinal CGRP receptors contributes to both the development and expression of spinal opioid tolerance. CGRP receptor antagonists may represent a useful therapeutic approach for preventing as well as reversing opioid tolerance.

Morphine and NMDA receptor antagonism reduce c-fos expression in spinal trigeminal nucleus produced by acute injury to the TMJ region

Pain management in temporomandibular disorders (TMDs) often involves pharmacotherapy; however, the site of action for drugs that reduce TMD pain is not known. To determine possible central neural targets of analgesic drugs relevant in TMD pain, morphine or the N-methyl-D-aspartate receptor antagonist, MK-801, was given alone or in combination prior to TMJ injury. The number of neurons expressing the immediate early gene, c-fos, was quantified in the lower brainstem and upper cervical spinal cord as an index of neural activation. It was hypothesized that those neuronal groups most necessary for the sensory-discriminative aspects of acute TMJ injury should display the greatest reduction in c-fos expression after drug treatment. Barbiturate-anesthetized male rats were given morphine or MK-801 15 min prior to injection of mustard oil into the TMJ region. Morphine given centrally (i.c.v.) or peripherally (i.v.) caused a marked dose-related reduction in Fos-like immunoreactivity (Fos-LI) in laminae I-II at the middle portions of subnucleus caudalis (mid-Vc) and at the subnucleus caudalis/upper cervical spinal cord (Vc/C2) transition. Higher doses of morphine also reduced Fos-LI in the dorsal paratrigeminal region (dPa5) and at the subnucleus interpolaris/subnucleus caudalis (Vi/Vc-vl) transition. MK-801 given i.v. reduced Fos-LI only in laminae I-II at the Vc/C2 transition. Combined subthreshold doses of morphine and MK-801 reduced c-fos expression in the dPa5, mid-Vc, and the Vc/C2 transition region, below that predicted from the effects of either drug alone. These results suggest that neurons in laminae I-II of the mid-Vc and Vc/C2 transition and, to a lesser extent, in the dPa5 region play a critical role in mediating the sensory and/or reflex aspects of pain after acute injury to the TMJ region.

Combined systemic administration of the glycine/NMDA receptor antagonist, (+)-HA966 and morphine attenuates pain-related behaviour in a rat model of trigeminal neuropathic pain

Chronic constriction injury to the infraorbital nerve (CCI-ION) by loose ligatures may represent a useful model for some trigeminal neuropathic pain disorders. Activation of the N-methyl-D-aspartate (NMDA) receptor is involved in the induction and maintenance of neuropathic pain and may contribute to the poor opioid sensitivity of this syndrome. We evaluated the effect of combined systemic administration of the functional antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, (+)-(1-Hydroxy-3-aminopyrrolodine-2-one) ((+)-HA966) with morphine on mechanical allodynia-like behaviour in CCI-ION rats. Two weeks after surgery rats with a CCI-ION displayed mechanical hyperresponsiveness to von Frey filament stimulation of the vibrissal pad with a median at 0.217 g (95% confidence limits, 0. 217-0.224) versus > or = 12.5 g pre-operative. Administration of either (+)-HA966 (2.5 mg/kg s.c.) alone or morphine (1 mg/kg i.v.) alone was devoid of effects on the mechanical hyperresponsiveness. By contrast, combined administration of (+)-HA966 and morphine (0.25, 0. 5 and 1 mg/kg i.v.) dose-dependently increased the mechanical response thresholds (peak-effects 0.745 g (0.745-0.745), 4.64 (3.3-8. 7) and 12.5 g (8.4-12.5), respectively). This effect was prevented and reversed by naloxone (0.1 mg/kg i.v.). The drug combination produced no motor deficits in animals using the rotarod test. The present results indicate that combination therapy with NMDA/glycine receptor antagonists and morphine may be a useful approach for the clinical management of trigeminal neuropathic pain disorders.

Acute pain mechanisms

The systems activated by tissue-injuring stimuli are complex. The nociceptive primary afferents have little spontaneous activity under normal conditions; however, after tissue injury, they display longlasting, ongoing activity. This results, in part, because the injury elicits the release of active factors that sensitize or excite the peripheral nerve terminal. A threshold that is lowered to the extent that body temperature and the pressure of edema are adequate stimuli results in spontaneous pain. This phenomenon is mediated by a variety of blood-borne active factors released during plasma extravasation, by agents released from local inflammatory cells, and by neurotransmitters released from the peripheral terminals of the primary afferent fibers themselves. Well-defined projections into the dorsal horn convey the "pain message" to at least two well-defined populations of neurons: those that are nociceptive specific and those that display an intensity-linked discharge over a range of stimuli from innocuous to noxious. Convergence from various fiber types, modalities, and end organs permits the encoding of afferent traffic with respect to intensity and location. The convergence of axons from somatic and visceral structures reflects the mechanism for the so-called "referred pain state." Most importantly, these dorsal horn systems have a dynamic component in addition to the hard-wiring; their output can be regulated both up and down. The up-regulation provides the basis for much of the facilitated processing that is believed to account for a significant percentage of the postinjury pain state. The facilitated state has a unique pharmacology, with the underlying mechanisms reflecting a cascade of actions that starts with the NMDA receptor and proceeds through the spinal release of intermediaries, such as prostaglandins and nitric oxide. Conversely, the ability to down-regulate the dorsal horn stimulus response function accounts for the powerful control exerted by a wide variety of diverse factors, including the spinal delivery of opioid and nonopioid analgesics and the "endogenous analgesia system." These linkages reflect the complexity of the encoding mechanisms that transduce the tissue injury into the behavioral sequela known as pain. This article also emphasizes that, although considerable progress has been made in the past decade, the current pace of research promises greater insights.

Excitatory amino release within spinal trigeminal nucleus after mustard oil injection into the temporomandibular joint region of the rat

Inflammation of the temporomandibular joint (TMJ) region evokes pain and hyperalgesia as well as causing persistent changes in the response properties of central trigeminal neurons. To determine if excitatory amino acids have a role in TMJ-induced responses, extracellular concentrations were measured in microdialysate samples from probes positioned in the spinal trigeminal nucleus (Vsp) near the transition region between subnucleus interpolaris and subnucleus caudalis (Vi/Vc) in chloralose-anesthetized rats. Injection of the selective small fiber excitant, mustard oil (20 microliters, 20% solution), into the ipsilateral TMJ region caused a transient (by 10 min) increase in glutamate (from 0.48 +/- 0.16 to 1.94 +/- 0.78 microM, P < 0.005) and aspartate (from 0.29 +/- 0.11 to 1.78 +/- 0.82 microM, P < 0.025) among sites located at the ventrolateral pole of the Vi/Vc transition region (n = 6). Samples from probes located within the ventral Vsp, but outside this Vi/Vc transition region (n = 9), did not show significant changes in amino acid concentrations. Glutamate and aspartate also increased after mustard oil injections into the contralateral TMJ region. Dialysate concentrations of serine and taurine did not change significantly after mustard oil injections. Addition of high potassium (150 mM) to the perfusate solution caused increases in glutamate and aspartate regardless of probe location. The transient and selective release of glutamate and aspartate within the Vi/Vc transition after acute irritation of the TMJ region is consistent with a proposed role for excitatory amino acids in mediating noxious sensory input from deep orofacial structures. Together with previous reports of c-fos expression, these results suggest that neurons within the ventrolateral portion of the Vi/Vc transition may serve as a relay site for the integration of sensory or reflex responses to acute inflammation of the TMJ region.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonism reduces Fos-like immunoreactivity in central trigeminal neurons after corneal stimulation in the rat

The role of glutamate receptors in processing noxious sensory input from the cornea was assessed in barbiturate-anesthetized rats. Animals were treated with selective antagonists for N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor subtypes prior to application of mustard oil to the corneal surface. Neural activation was estimated from the number of neurons that produced Fos, the protein product of the immediate early gene, c-fos, as detected by immunocytochemistry. Fos-positive neurons were found at two distinct regions of the spinal trigeminal nucleus: the subnucleus interpolaris/caudalis transition and the subnucleus caudalis/upper cervical cord transition. The number of Fos-positive neurons was reduced dose-dependently by the competitive N-methyl-D-aspartate receptor antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (0.08-8 nmol, i.c.v.), or by the non-N-methyl-D-aspartate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (2.5-250 nmol, i.c.v.). The greatest reduction in Fos-positive cells was seen at the subnucleus caudalis/upper cervical cord transition after blockade of either receptor subtype. Combined blockade of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors did not cause a further reduction in the number of Fos-positive neurons than was seen after the highest dose of either antagonist alone. Peripheral or central administration of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester, had no effect on the number of Fos-positive neurons after corneal stimulation. These results suggest that corneal input to neurons at the subnucleus caudalis/upper cervical cord transition, and to a lesser extent, at the subnucleus interpolaris/subnucleus caudalis transition depends on excitatory amino acid transmission. Both N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptor subtypes, but not the formation of nitric oxide, contribute to the processing of acute corneal stimuli by central trigeminal neurons.

Opioids modulate N-methyl-D-aspartic acid (NMDA)-evoked responses of neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis)

Extracellular single unit recordings were made from 74 neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis). N-methyl-D-aspartic acid (NMDA) excited nociceptive as well as non-nociceptive neurons. NMDA receptor antagonist, DL-2-Amino-5-Phosphonovaleric acid (AP-5), blocked the NMDA-evoked excitation. Microiontophoretic application of a selective mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO), reduced the NMDA-evoked responses of 100% of nociceptive specific (NS), 93% of wide dynamic range (WDR) and 86% of low threshold (LT) neurons in the superficial and deeper dorsal horn of the medulla. In contrast, application of a selective delta 1-opioid receptor agonist, [D-Pen2,5]enkephalin (DPDPE), reduced the NMDA-evoked responses of 90% of NS neurons, 72% of WDR neurons and 67% of LT neurons in the superficial and deeper dorsal horn of the medulla. DPDPE also produced excitatory or biphasic effects. The inhibitory actions of DAMGO and DPDPE were reversed by naloxone and/or 7-benzylidenenaltrexone (BNTX), mu- and delta 1-receptor antagonists. It is concluded that mu- and delta-opioid receptor agonists produce a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive and non-nociceptive neurons in the medullary dorsal horn.

A pharmacological study of the modulation of neuronal and behavioural nociceptive responses in the rat trigeminal region

Electrical stimulation of the brain, particularly in the periventricular grey areas, caused long-lasting increases in behavioural escape thresholds to heating and mechanical stimuli applied to the facial region of the rat. The brain stimulation selectively suppressed responses to noxious stimuli. Responses to non-noxious stimuli, evoked by low threshold brush, were unaffected. The same animals that were studied in the behavioural tests were then anaesthetized with urethane and the inhibitory effect of the same brain stimulation was studied in single neurones recorded in the caudal trigeminal nucleus. A clear correlation (rs = 0.63) emerged between degree of behavioural antinociception and the amount of inhibition seen in nociceptive neurones. In addition the mean duration of the inhibition (6 min) was similar to the mean duration of the antinociceptive effect (7.3 min). Other classes of non-nociceptive neurones were unaffected by the stimulation. The neurones were also studied using iontophoretically applied monoamine candidates for the inhibitory neurotransmitter noradrenaline (NA) and 5-hydroxytryptamine (5-HT). The profile of the effects of NA most closely fitted that of the inhibitory neurotransmitter. This profile was expressed in terms of depression and excitation of different classes of neurones, and by the duration of effects. The depressants effects could be antagonized by iontophoretic idazoxan. In addition clonidine induced long-lasting depression of firing. 5-HT was more likely than NA to excite nociceptive neurones and to depress non-nociceptive neurons. Only NA consistently elevated thermal response thresholds in a similar manner to that produced by brain stimulation. These results provide some support for the hypothesis that selective descending inhibition of nociceptive responses in neurones of the rat caudal trigeminal nucleus is mediated by NA, possibly by an action at alpha2-adrenoceptors.

Infusion of substance P or neurokinin A by microdialysis alters responses of primate spinothalamic tract neurons to cutaneous stimuli and to iontophoretically released excitatory amino acids

The responses of 25 spinothalamic tract (STT) neurons to mechanical and thermal stimulation of the skin, as well as to a battery of iontophoretically applied excitatory amino acids (EAAs), were tested before and then during microdialysis of substance P (SP) or neurokinin A (NKA) into the dorsal horn of anesthetized monkeys. Neither peptide had significant effects on the background activity or the responses to mechanical or thermal stimulation of the skin. However, each peptide produced significant increases in the responses to iontophoretic application of one or more EAAs. In addition, following combined application of the EAAs and either SP or NKA, the responses of the cells to mechanical stimulation of the skin increased. Combined application of SP and NKA failed to produce an increase in responses to either the EAAs or to cutaneous stimuli that was greater than that produced by either peptide alone. It is concluded that SP and NKA produce an increase in the responses of STT cells to iontophoretic applications of EAAs and the combined effects of these compounds produce sustained increases in responses to mechanical stimulation of the skin. These changes mimic those observed when STT cells are sensitized by peripheral noxious stimuli, suggesting that the mechanism of induction and expression of sensitization involves the facilitation of dorsal horn neuron responses to EAAs by tachykinins.

Modality-specific hypersensitivity of dorsal horn convergent neurones during reperfusion of their receptive fields on the rat's tail

In rats anaesthetised with enflurane, we examined the responses of convergent neurones in the dorsal horn of the spinal cord to noxious thermal and mechanical stimulation and to innocuous brushing, during reperfusion of their receptive fields on the tail, following transient ischaemia. Neurones were included if they responded, before induction of ischaemia, to both pinching and brushing of receptive fields restricted to the tail. Ischaemia was induced by occluding the blood supply to the tail for 30 min using a tourniquet. Compared to their own responses before ischaemia, during reperfusion almost all the neurones (17 of 20) exhibited significantly increased activity to noxious pinching and innocuous brushing of their receptive fields, following 30 and 60 min of reperfusion. Receptive field size increased markedly in 16 of 20 of the neurones tested. Only 13 of 35 of the neurones responded to noxious thermal stimulation of the tail before induction of ischaemia, and of these only two exhibited enhanced sensitivity to thermal stimulation during reperfusion. Our results indicate that there is a population of convergent neurones that demonstrates hypersensitivity to mechanical stimulation of the rat's tail, but not to noxious thermal stimulation, during reperfusion of their receptive fields following transient ischaemia.

Microinjections of glutamate within trigeminal subnucleus interpolaris alters adrenal and autonomic function in the cat

The influence of rostral portions of the trigeminal sensory complex on adrenal and autonomic function was assessed by microinjections of L-glutamate (500 or 5 mM, 100 nl) directed at subnucleus interpolaris (Vi) or at the nucleus principalis/subnucleus oralis level (Vp/Vo) in chloralose-anesthetized cats. Microinjections of glutamate (500 mM) within Vi evoked prompt (by +1 min) dose-related increases in the adrenal secretion of epinephrine (+11.4 +/- 2.5 ng/min, P < 0.001), adrenal blood flow (+0.19 +/- 0.06 ml/min, P < 0.05), mean arterial pressure (+6.6 +/- 3.0 mmHg, P < 0.025) and heart rate (+8.0 +/- 2.7 beats/min, P < 0.01, n = 16). Microinjections of lower doses of L-glutamate (5 mM, n = 7) within Vi had no effect. Microinjections of 500 mM glutamate within VP/Vo (n = 15) or within the spinal trigeminal tract (n = 13) had no consistent effect on adrenal or autonomic function. Plasma concentrations of ACTH were not altered significantly by glutamate regardless of dose or of the site of injection. The results suggest that local release of glutamate within Vi, but not within Vp/Vo, influences adrenal and autonomic function. Together with previous results obtained after injections of glutamate within subnucleus caudalis, these data indicate that glutaminergic input to both Vi and to more caudal portions of the spinal trigeminal nucleus contribute to the control of autonomic function such as that which often accompanies trigeminal nociception.

Excitatory amino acid receptor mediation of sensory inputs to functionally identified dorsal horn neurons in cat spinal cord

As excitatory amino acid receptors have been implicated in nociceptive sensory transmission, the principal objective of the present study was to investigate the effects of various excitatory amino acid antagonists on naturally evoked responses in spinal dorsal horn neurons. Extracellular single unit activity was recorded from functionally identified, spinal dorsal horn neurons in unanesthetized, decerebrated cats and in alpha-chloralose-anesthetized cats. The tests included iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV), the non-N-methyl-D-aspartate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and kynurenate, and also the intravenous administration of the N-methyl-D-aspartate receptor antagonist, ketamine. In addition, attempts were made to determine the effects on these neurons of iontophoretic application of the excitatory amino acid agonists, L-glutamate, N-methyl-D-aspartate, quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate. Marked differences were noted in the actions of agonists and antagonists between the responses observed in the unanesthetized, decerebrated and the anesthetized animals. In decerebrated cats, responses to hair afferent stimulation were blocked by kynurenate, 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were attenuated by 2-amino-5-phosphonovaleric acid and in one unit also by ketamine. Neither 6-cyano-7-nitroquinoxaline-2,3-dione nor kynurenate affected the responses to noxious thermal stimulation. The proportion of cells responding to the agonists were: N-methyl-D-aspartate 24/27 (89%), quisqualate 12/13 (92%) and domoate 6/7 (86%). In chloralose-anesthetized cats, responses to hair afferent stimulation were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and kynurenate but not by 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were not affected by any of these antagonists, while the response to non-noxious thermal stimulation was blocked by 2-amino-5-phosphonovaleric acid, ketamine and kynurenate in the one neuron studied. The proportion of cells excited by the agonists differed from those observed in decerebrated cats: N-methyl-D-aspartate 9/32 (28%), quisqualate 50/54 (93%), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate 19/23 (83%) and domoate 17/38 (45%). Application of the putative endogenous excitatory amino acid precursor N-acetyl-aspartyl-glutamate (NAAG) did not elicit a response in any of the neurons studied.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo release of calcitonin gene-related peptide-like material from the cervicotrigeminal area in the rat. Effects of electrical and noxious stimulations of the muzzle

The continuous perfusion with an artificial cerebrospinal fluid of the cervicotrigeminal area of the spinal cord in halothane-anaesthetized rats allowed the collection of calcitonin gene-related peptide-like material with the same immunological and chromatographic characteristics as authentic rat alpha-calcitonin gene-related peptide. The spinal release of calcitonin gene-related peptide-like material could be significantly increased by the local application of 60 mM K+ (approximately +100%), high-intensity percutaneous electrical stimulation (approximately +200%) and noxious heat (by immersion in water at 52 degrees C; approximately +150%) applied to the muzzle. By contrast, noxious mechanical (pinches) and chemical (subcutaneous formalin injection) stimulations and deep cooling (by immersion in water at 0 degrees C) of the muzzle did not alter the spinal release of calcitonin gene-related peptide-like material. In addition, low-intensity electrical stimulation, recruiting only the A alpha/beta primary afferent fibres, significantly reduced (approximately -30%) the release of calcitonin gene-related peptide-like material from the cervicotrigeminal area. These data suggest that among the various types of natural noxious stimuli, noxious heat may selectively excite calcitonin gene-related peptide-containing A delta and C primary afferent fibres projecting within the dorsal horn of the spinal cord, and that activation of A alpha/beta fibres reduces spontaneous calcitonin gene-related peptide-like material release possibly through an inhibitory presynaptic control of calcitonin gene-related peptide-containing A delta/C fibres.

Excitatory amino acid binding sites in the trigeminal principal sensory and spinal trigeminal nuclei of the rat

Quantitative autoradiography was used to examine the density and distribution of excitatory amino acid (EAA) binding site subtypes in the principal sensory and spinal trigeminal nuclei of the rat trigeminal complex. The highest densities of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate and metabotropic receptors were found in the superficial laminae (I and II) of subnucleus caudalis, a region known to be densely innervated by primary afferent nociceptive terminals. Lower densities of EAA binding sites were observed in spinal subnuclei interpolaris and oralis and within the principal sensory nucleus. These results are consistent with the hypothesis that EAAs are involved in primary afferent nociceptive neurotransmission.

Neural changes in acute arthritis in monkeys. III. Changes in substance P, calcitonin gene-related peptide and glutamate in the dorsal horn of the spinal cord

The effects of an experimentally induced arthritis on immunoreactivity of putative primary afferents neurotransmitter/neuromodulators were examined. Immunoreactive staining for substance P (SP), calcitonin gene-related peptide (CGRP) and glutamate (Glu) in the monkey dorsal horn was examined following inflammation of one knee joint induced by injection of 5% kaolin and 5% carrageenan. Spinal cords were examined at different time periods after induction of arthritis (2.5, 4, 6 and 8 h). Side to side differences in immunoreactivity were determined by a computer assisted quantitation system. A significant overall decrease in immunoreactivity of the lumbar versus the cervical dorsal horn was found for SP. The decrease for SP showed maximal changes of 68.3% at 4 h and 54.7% at 6 h. Immunoreactivity for CGRP was decreased 31.5% at 8 h and variable at other time points. Immunoreactivity for Glu, showed an ipsilateral increase of 31.4% at 4 h, 33.7% at 6 h, 39.9% at 8 h and a significant effect for lumbar versus cervical. Repetitive peripheral stimulation of the joint was shown to be important for changes in SP and Glu immunoreactivity. Without frequent peripheral stimulation in the early stages of the development of arthritis, SP showed no quantitative side to side differences. Increases in Glu immunoreactivity were present but not as prominent with minimal joint manipulation. These studies suggest that Glu may be involved in the aching pain of inflammation at rest whereas SP, CGRP and Glu may mediate pain induced by joint movement.

Enhancement of spinothalamic neuron responses to chemical and mechanical stimuli following combined micro-iontophoretic application of N-methyl-D-aspartic acid and substance P

A role for sensitization of nociceptors in the generation of primary hyperalgesia is well documented. More recent work has begun to define a role of an increased excitability of neurons within the spinal cord in the generation of secondary hyperalgesia. The present study demonstrates increased responses of primate spinothalamic neurons following co-administration of N-methyl-D-aspartic acid (NMDA) and substance P (SP) by micro-iontophoresis. Wide dynamic range and high threshold STT neurons in laminae I-VI showed an increased frequency of discharges following application of NMDA which was characterized by a slow onset to peak discharge rate and a slow return to background levels of discharge. Combined application of NMDA with SP resulted in an enhancement of responses to NMDA that often long outlasted the administration of SP. This increase in response of the cells to NMDA was not produced by repeated application of NMDA alone or following combined application of NMDA with an SP analog. NMDA responses were reduced or prevented in all cases by co-application of an NMDA-receptor antagonist. Finally, long-lasting potentiation of NMDA responses by SP was paralleled by enhanced responses to mechanical stimulation of skin. It is proposed that a mechanism involving the combined synaptic release of excitatory amino acids and peptides leads to secondary hyperalgesia.

Rat tooth pulp projections to spinal trigeminal subnucleus caudalis are glutamate-like immunoreactive

It has been shown that glutamate-like immunoreactive axon terminals are present within the spinal trigeminal nucleus, including subnucleus caudalis. The morphology of many of these terminations is consistent with their identification as primary afferents. To establish whether primary afferent projections to subnucleus caudalis are glutamate-like immunoreactive, we injected an anterograde tract tracer into rat incisor tooth pulp, histochemically visualized this tracer within subnucleus caudalis, and then used an immunocytochemical technique to label glutamate-like immunoreactive profiles within these same sections. The anterograde tract tracer used, the B subunit of cholera toxin conjugated to horseradish peroxidase (B-HRP), is transported transganglionically and can be used to localize tooth pulp projection fibers in the spinal trigeminal nucleus. A majority of B-HRP projection fibers from rat lower incisors terminated ipsilaterally in axon terminals in the dorsal region of subnucleus caudalis. Labeled axon terminals were both scallop-shaped and smooth in profile. Small numbers of fibers containing B-HRP extended into laminae I-III caudally and were present in both the border zone between laminae IV and V and the most lateral region of lamina V rostrally. Approximately 75% of the B-HRP-labeled projection fibers were glutamate-like immunoreactive, providing evidence that the excitatory amino acid glutamate functions as a neurotransmitter in a subpopulation of these fibers. Terminals reactive for both B-HRP and glutamate-like immunoreactivity contained small, spherical round vesicles, formed asymmetric synapses, and participated in axoaxonic and axodendritic synaptic junctions. These results support the hypothesis that glutamate may be a transmitter of A delta and C fibers involved in relaying nociceptive information from the tooth pulp.

Controlled noxious chemical stimulation: responses of rat trigeminal brainstem neurones to CO2 pulses applied to the nasal mucosa

The nasal mucosa of halothane-anesthetized rats was stimulated with defined CO2 pulses. Recordings were performed from single trigeminal brainstem neurons to characterize their responses to this controlled chemical irritation. All cells examined with this stimulus displayed graded discharges to increasing concentrations of CO2. Enhanced responses were obtained in a group of neurons when the duration of the interstimulus interval was increased. The application of chemical irritants, notably mustard oil or acetic acid induced vigorous ongoing discharges in all cells tested. The CO2 stimulation method described here thus provides an ideal model for the quantitative physiological and pharmacological examination of chemically induced nociception.

Modification of the responses of primate spinothalamic neurons to mechanical stimulation by excitatory amino acids and an N-methyl-D-aspartate antagonist

Excitatory amino acids (EAAs) are likely to play a key role in sensory transmission in the spinal cord. In the present study, the microiontophoresis technique was used to investigate the effects of L-glutamate (GLUT), N-methyl-D-aspartate (NMDA), and quisqualate (QUIS), as well as an NMDA receptor antagonist, AP-7, on the discharges evoked in nociceptive primate spinothalamic tract (STT) neurons by mechanical stimulation of the skin. Responses to innocuous brushing of the skin were facilitated by GLUT and NMDA (and in some neurons by QUIS) and sometimes reduced by AP-7. GLUT also facilitated the responses to noxious mechanical stimuli. The results are consistent with anatomical evidence for the presence of synapses that contain EAAs on primate STT cells and with the proposal that the co-release of EAAs and neuropeptides may contribute to hyperalgesia.

Sensory responses of caudal trigeminal neurons to thermal and mechanical stimuli and their behavioural correlates in the rat

Behavioural experiments in the freely moving rat were carried out to determine thermal and mechanical nociceptive thresholds to ramp stimuli applied to the face. The mean thermal escape threshold was 43.5 degrees C, and the mean mechanical escape threshold was 179.2 g/mm2. In a parallel set of experiments recordings were made from single neurons in the caudal trigeminal nucleus of the anaesthetized rat. Neurons were classified according to their responses to a range of thermal and mechanical stimuli applied to the face. Three classes of neuron responded exclusively to mechanical stimuli and four classes responded to thermal stimuli (usually in addition to responding to mechanical stimuli). The mean thermal threshold of neurons responsive to warming stimulation was 44.4 degrees C. Neurons responsive to innocuous warming were located in deeper laminae. Many of the neurons responsive to noxious heat appeared to show an exponential relation between temperature and firing rate. An argument is made for a direct role of exponentially responding neurons in thermal nociception. The distribution of all neuronal response thresholds was left-skewed compared with a normal distribution, whereas the behavioural escape thresholds approximated a normal distribution.

Segmental release of Met-enkephalin-like material from the spinal cord of rats, elicited by noxious thermal stimuli

In order to investigate possible changes induced by noxious thermal stimuli in the activity of enkephalinergic neurones at various levels of the spinal cord, either the whole cord, the cervicotrigeminal area or the lumbar area were perifused with artificial CSF at a rate of 0.1 ml/min in halothane anaesthetized rats, and Met-enkephalin-like material (MELM) was measured in 0.5 ml fractions collected from the perifusates. Immersing the muzzle of intact rats in water at 52 degrees C produced a significant enhancement of MELM content in perifusates from both the whole spinal cord and the cervicotrigeminal area but not from the lumbar area. Heating the tail resulted in an increase in MELM release from the whole spinal cord of intact as well as of cervically transected rats. It is concluded that noxious thermal stimuli can induce a segmental release of MELM, i.e., only within spinal zones receiving the nociceptive inputs.

CCK 8 analgesia and hyperalgesia after intrathecal administration in the rat: comparison with CCK-related peptides

The C-terminal octapeptide of cholecystokinin (CCK 8) was administered intrathecally to rats. Doses in the nanogram range produced weak but significant antinociception in the paw pressure test five minutes after injection whereas microgram doses of CCK 8 produced hyperalgesia. The CCK 8-induced analgesia or hyperalgesia was not seen in the tail flick test and was not associated with motor incapacitation or any other noticeable side effects. The C-terminal tetrapeptide of CCK (CCK 4) and pentagastrin were found to be ineffective in all tests but caerulein and molluscan cardioexcitatory neuropeptide (FMRF-amide), like CCK 8, produced antinociception in the paw pressure test.

The role of N-methylaspartate receptors in mediating responses of rat and cat spinal neurones to defined sensory stimuli

1. Single-cell recordings were made from neurones in various spinal laminae in anaesthetized or decerebrated, spinalized or intact rats and cats. Cells were activated by controlled peripheral sensory stimuli which mimicked natural conditions and with some cells also by micro-electrophoretically administered excitatory amino acid analogues. Such responses were tested with amino acid antagonists administered both micro-electrophoretically and intravenously. 2. With cells in the dorsal horn, the dissociative anaesthetic ketamine, administered either micro-electrophoretically or intravenously at doses which selectively reduce responses to N-methylaspartate, had no consistent effect on any of the sensory responses examined. 3. The non-selective amino acid antagonist cis-2,3-piperidine dicarboxylate was somewhat more effective at reducing sensory responses. 4. With motoneurones, intravenous N-methylaspartate-blocking doses of ketamine consistently reduced nociceptive responses. Non-nociceptive responses were less affected. 5. With ventral horn interneurones, intravenous but not micro-electrophoretic ketamine reduced nociceptive responses on about half the cells tested. 6. These results are interpreted in terms of the physiological role of the N-methylaspartate class of excitatory amino acid receptor in mediating responses in the ventral but not dorsal horn of the spinal cord to peripheral somatic stimuli.

Cutting rostral trigeminal nuclear complex projections preferentially affects perioral nociception in the rat

Sagittal knife cuts were made medial to rostral trigeminal brainstem nuclei in 11 rats. These cuts resulted in significant group deficits in facial thermal nociception at only two of the 5 ipsilateral sites tested. The affected sites were close to the vibrissa and oral cavity, suggesting that rostral trigeminal projections signal anterior rather than more caudal facial nociception. Three rats were also tested for EEG arousal responsiveness to innocuous air puffs before and after the cuts at the same facial sites. Only one of the rats demonstrated a significant deficit at one of the 5 sites tested, suggesting that the rostral projections interrupted in this study are not necessary for innocuous facial sensation.

Co-localization of fixative-modified glutamate and glutaminase in neurons of the spinal trigeminal nucleus of the rat: an immunohistochemical and immunoradiochemical analysis

The spinal trigeminal nucleus (STN) is involved in processing orofacial sensory information, including tactile, thermal and nociceptive input, and relaying this information to higher brain centers, such as the thalamus. Very little information is available regarding the major excitatory neurotransmitters of this nucleus. The amino acid glutamate has been proposed as a major excitatory neurotransmitter in the central nervous system. In the present study, a novel monoclonal antibody, specific for fixative-modified glutamate, was utilized in conjunction with polyclonal antisera against glutaminase and aspartate aminotransferase (AATase) in an attempt to identify and map the locations of possible glutamatergic neurons in the STN. Co-localization experiments were performed by radiolabeling our monoclonal antibody and using this antibody in conjunction with the polyclonal antisera against glutaminase and AATase to evaluate the possible coexistence of glutamate with glutaminase or AATase in STN neurons. In all three subnuclei of the STN, immunohistochemically labeled neuronal profiles were observed with both of the polyclonal antisera and with the monoclonal antibody. Subnucleus caudalis contained the greatest number of labeled profiles per coronal section followed by subnucleus interpolaris and subnucleus oralis. The number and the distribution of immunoreactive profiles observed after the use of the glutaminase antiserum was comparable to that obtained with the monoclonal antibody. Co-localization experiments demonstrated that all glutaminase-like immunoreactive neurons also contained fixative-modified glutamate-like immunoradioactivity. These results suggest that glutamatergic neurons are present in the spinal trigeminal nucleus. The AATase antiserum labeled more neuronal profiles in each of the three subnuclei than did the glutaminase antiserum or the monoclonal antibody. In addition, co-localization experiments indicated that glutamate-like immunoreactivity was present in only two-thirds of AATase-like immunoreactive neuronal profiles. These findings suggest that glutaminase may be a more reliable marker of glutamatergic function than AATase.

Effects of trigeminal tractotomy on facial thermal nociception in the rat

Wire knife-cuts lesioning the descending trigeminal tract and trigeminal sensory nuclear complex at the level of subnucleus interpolaris significantly elevated response latencies to noxious thermal stimuli applied to one perioral locus of facial skin. Group analyses of four other ipsilateral sites did not reveal significant effects. Medial cuts, extensively damaging the sensory nuclear complex as well as the tract, were more likely to produce significant elevations than were lateral cuts. These results suggest that tractotomy does not result in complete facial analgesia in the rat, and that the effectiveness of tractotomy may in part be due to interruption of intranuclear trigeminal projections.

The action of K-agonists on the nociceptive responses of neurones in the medullary dorsal horn of the anaesthetized rat

Responses of medullary dorsal horn neurones to both mechanical and thermal noxious stimuli were recorded in urethane anaesthetized rats. Opiates with reported activity at K receptors (tifluadom, BL 5572M, and U50488) were found to reduce responses to both noxious stimuli, and in this respect, were indistinguishable from the mu agonist fentanyl. These observations are in contrast to the behavioural antinociceptive effects of K agonists as these substances are active in tests using mechanical noxious stimuli but in those using thermal stimuli have little effect. It is therefore possible that the modality decoding seen in behavioural experiments occurs at a supraspinal level.