Synergistic effect of combining dual enkephalinase inhibitor PL37 and sumatriptan in a preclinical model of migraine

OBJECTIVE

The aim of this study was to test whether a combination of sumatriptan with dual enkephalinase inhibitor PL37 would result in an additive or a synergistic effect.

BACKGROUND

Combination treatment is frequently used to improve the therapeutic efficacy of drugs. The co-administration of two drugs may result in efficacy at lower doses than those needed for either drug alone, thus minimizing side effects. Here, we tested the effect of the co-administration of two drugs on cutaneous mechanical hypersensitivity (MH), a symptom often affecting cephalic regions in patients with migraine: dual enkephalinase inhibitor PL37, a small molecule that protects enkephalins from rapid degradation, and sumatriptan, a serotonin 5-HT1B/1D receptor agonist.

METHODS

We investigated the effects of oral administrations of sumatriptan, PL37, or their combination on changes in cutaneous mechanical sensitivity induced by a single intraperitoneal administration of the nitric oxide donor, isosorbide dinitrate (ISDN) in male rats. Mechanical sensitivity was assessed using von Frey filaments applied to the face of animals to determine pain thresholds. Isobolographic analysis was performed to determine the nature of the interaction between sumatriptan and PL37.

RESULTS

Sumatriptan as well as PL37 each produced a dose-dependent inhibition of ISDN-induced cephalic MH. Median effective dose (ED50 ) values were 0.3 and 1.1 mg/kg for sumatriptan and PL37, respectively. An isobolographic analysis of the effect of combined doses of sumatriptan and PL37 based on their calculated ED50 values demonstrated a synergistic effect of the combination on cephalic MH, with an interaction index of 0.14 ± 0.04.

CONCLUSION

These results suggest that PL37 acts synergistically with sumatriptan to produce an anti-allodynic effect in a rat model of migraine. Thus, combining PL37 and sumatriptan may be a useful therapeutic strategy in the management of migraine.

PLAIN LANGUAGE SUMMARY

There have been many advances in migraine treatment, but we still need more options that are effective and have few side effects. Sumatriptan is one available drug for acute treatment of migraine, but it does not work for every patient and is not suitable for some people. We tested a new drug called PL37 (that blocks enkephalinases) together with sumatriptan and the combination minimized side effects and allowed lower doses of the drugs for effective migraine treatment in an animal model.

Dual enkephalinase inhibitor PL37 as a potential novel treatment of migraine: evidence from a rat model

The dual enkephalinase inhibitor PL37, a small molecule that protects enkephalins from their rapid degradation, has demonstrated analgesic properties in animal pain models and in early human clinical trials. This study tested the antimigraine potential of PL37 on cutaneous mechanical hypersensitivity affecting cephalic regions in migraineurs. Using behavioral testing and c-Fos immunoreactivity in male rats, we investigated the effects of single (oral or intravenous) and repeated oral administration of PL37 on changes in cutaneous mechanical sensitivity and sensitization of the trigeminocervical complex induced by repeated administration of the nitric oxide donor, isosorbide dinitrate. In naive rats, single or repeated administration of PL37 or vehicle had no effect on cephalic mechanical sensitivity. However, single oral PL37 treatment effectively inhibited isosorbide dinitrate-induced acute cephalic mechanical hypersensitivity. Single intravenous but not oral PL37 administration inhibited chronic cephalic mechanical hypersensitivity. Daily oral administration of PL37 prevented cephalic mechanical hypersensitivity and decreased touch-induced c-Fos expression in trigeminocervical complex following repeated isosorbide dinitrate administration. These data reveal the therapeutic potential of the dual enkephalinase inhibitor PL37 as an acute and prophylactic treatment for migraine. Protecting enkephalins from their degrading enzymes therefore appears as an innovative approach to treat migraine.

Model-based signal processing enables bidirectional inferring between local field potential and spikes evoked by noxious stimulation

BACKGROUND

Recording spontaneous and evoked activities by means of unitary extracellular recordings and local field potential (LFP) are key understanding the mechanisms of neural coding. The LFP is one of the most popular and easy methods to measure the activity of a population of neurons. LFP is also a composite signal known to be difficult to interpret and model. There is a growing need to highlight the relationship between spiking activity and LFP. Here, we hypothesized that LFP could be inferred from spikes under evoked noxious conditions.

METHOD

Recording was performed from the medullary dorsal horn (MDH) in deeply anesthetized rats. We detail a process to highlight the C-fiber (nociceptive) evoked activity, by removing the A-fiber evoked activity using a model-based approach. Then, we applied the convolution kernel theory and optimization algorithms to infer the C-fiber LFP from the single cell spikes. Finally, we used a probability density function and an optimization algorithm to infer the spikes distribution from the LFP.

RESULTS

We successfully extracted C-fiber LFP in all data recordings. We observed that C-fibers spikes preceded the C-fiber LFP and were rather correlated to the LFP derivative. Finally, we inferred LFP from spikes with excellent correlation coefficient (r = 0.9) and reverse generated the spikes distribution from LFP with good correlation coefficients (r = 0.7) on spikes number.

CONCLUSION

We introduced the kernel convolution theory to successfully infer the LFP from spikes, and we demonstrated that we could generate the spikes distribution from the LFP.

Recurrent administration of the nitric oxide donor, isosorbide dinitrate, induces a persistent cephalic cutaneous hypersensitivity: A model for migraine progression

Background

A subgroup of migraineurs experience an increase in attack frequency leading to chronic migraine.

Methods

We assessed in rats the roles of dose and repeat administration of systemic isosorbide dinitrate (ISDN), a nitric oxide donor, on the occurrence and development of cephalic/face and extracephalic/hindpaw mechanical allodynia as a surrogate of migraine pain, and the effect of acute systemic sumatriptan and olcegepant and chronic systemic propranolol on these behavioral changes.

Results

A single high (H-ISDN) but not low (L-ISDN) dose of ISDN induces a reversible cephalic and extracephalic mechanical allodynia. However, with repeat administration, L-ISDN produces reversible cephalic but never extracephalic allodynia, whereas H-ISDN induces cephalic and extracephalic allodynia that are both potentiated. H-ISDN-induced cephalic allodynia thus gains persistency. Sumatriptan and olcegepant block single H-ISDN-induced behavioral changes, but only olcegepant reduces these acute changes when potentiated by repeat administration. Neither sumatriptan nor olcegepant prevent chronic cephalic hypersensitivity. Conversely, propranolol blocks repeat H-ISDN-induced chronic, but not acute, behavioral changes.

Conclusions

Repeated ISDN administration appears to be a naturalistic rat model for migraine progression, suitable for screening acute and preventive migraine therapies. It suggests frequent and severe migraine attacks associated with allodynia may be a risk factor for disease progression.

The nucleus raphe magnus OFF-cells are involved in diffuse noxious inhibitory controls

Diffuse noxious inhibitory controls (DNIC) are very powerful long-lasting descending inhibitory controls which are pivotal in modulating the activity of spinal and trigeminal nociceptive neurons. DNIC are subserved by a loop involving supraspinal structures such as the lateral parabrachial nucleus and the subnucleus reticularis dorsalis. Surprisingly, though, whether the nucleus raphe magnus (NRM), another supraspinal area which is long known to be important in pain modulation, is involved in DNIC is still a matter of discussion. Here, we reassessed the role of the NRM neurons in DNIC by electrophysiologically recording from wide dynamic range (WDR) neurons in the trigeminal subnucleus oralis and pharmacologically manipulating the NRM OFF- and ON-cells. In control conditions, C-fiber-evoked responses in trigeminal WDR neurons are inhibited by a conditioning noxious heat stimulation applied to the hindpaw. We show that inactivating the NRM by microinjecting the GABAA receptor agonist, muscimol, both facilitates C-fiber-evoked responses of trigeminal WDR neurons and strongly attenuates their inhibition by heat applied to the hindpaw. Interestingly, selective blockade of ON-cells by microinjecting the broad-spectrum excitatory amino acid antagonist, kynurenate, into the NRM neither affects C-fiber-evoked responses nor attenuates DNIC of trigeminal WDR neurons. These results indicate that the NRM tonically inhibits trigeminal nociceptive inputs and is involved in the neuronal network underlying DNIC. Moreover, within NRM, OFF-cells might be more specifically involved in both the tonic and phasic descending inhibitory controls of trigeminal nociception.

Bilateral descending hypothalamic projections to the spinal trigeminal nucleus caudalis in rats

Several lines of evidence suggest that the hypothalamus is involved in trigeminal pain processing. However, the organization of descending hypothalamic projections to the spinal trigeminal nucleus caudalis (Sp5C) remains poorly understood. Microinjections of the retrograde tracer, fluorogold (FG), into the Sp5C, in rats, reveal that five hypothalamic nuclei project to the Sp5C: the paraventricular nucleus, the lateral hypothalamic area, the perifornical hypothalamic area, the A11 nucleus and the retrochiasmatic area. Descending hypothalamic projections to the Sp5C are bilateral, except those from the paraventricular nucleus which exhibit a clear ipsilateral predominance. Moreover, the density of retrogradely FG-labeled neurons in the hypothalamus varies according to the dorso-ventral localization of the Sp5C injection site. There are much more labeled neurons after injections into the ventrolateral part of the Sp5C (where ophthalmic afferents project) than after injections into its dorsomedial or intermediate parts (where mandibular and maxillary afferents, respectively, project). These results demonstrate that the organization of descending hypothalamic projections to the spinal dorsal horn and Sp5C are different. Whereas the former are ipsilateral, the latter are bilateral. Moreover, hypothalamic projections to the Sp5C display somatotopy, suggesting that these projections are preferentially involved in the processing of meningeal and cutaneous inputs from the ophthalmic branch of the trigeminal nerve in rats. Therefore, our results suggest that the control of trigeminal and spinal dorsal horn processing of nociceptive information by hypothalamic neurons is different and raise the question of the role of bilateral, rather than unilateral, hypothalamic control.

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.

The Orofacial Formalin Test in the Mouse: A Behavioral Model for Studying Physiology and Modulation of Trigeminal Nociception

The aim of the current study was to adapt the orofacial formalin pain model previously developed in rats for use in mice and to characterize as fully as possible the behavioral changes in this species. The effects of subcutaneous injection of different formalin concentrations (.5%, 1%, 2%, 4%, and 8%) were examined on the face-rubbing response. In mice, formalin injection into the upper lip induced sustained face-rubbing episodes with vigorous face-wash strokes directed to the perinasal area. A positive linear relationship between formalin concentration and amplitude of the rubbing activity was observed during the first and second phase of the test with concentration up to 4%. With the highest concentration used (8%), the amplitude of both phases had plateaued. Systemic administration of morphine and paracetamol induced a dose-dependent inhibition of the rubbing behavior during the second phase. Although both paracetamol and morphine inhibited the first phase, a dose-dependent inhibition was found only for morphine. The ED50 value (95% confidence interval) for suppressing the rubbing response during the first phase was 2.45 mg/kg (1.90-3.08 mg/kg) for morphine. The ED50 values for suppressing the rubbing response during the second phase were 3.52 mg/kg (2.85-4.63 mg/kg) for morphine and 100.66 mg/kg (77.98-139.05 mg/kg) for paracetamol. Heterosegmental nociceptive stimulation evoked by subcutaneous injection of capsaicin into the back of the animal 10 min before the formalin test produced a dose-dependent inhibition of the second phase of the rubbing response. The ED50 values for suppressing the rubbing response during the first and second phases were 9.04 microg (1.36-65.13 microg) and 0.92 microg (0.28-2.99 microg), respectively. In conclusion, the mouse orofacial formalin test appears to be a reliable model for studying the behavioral encoding of the intensity of nociceptive orofacial stimulation and the counter-irritation phenomenon and for testing analgesic drugs.

PERSPECTIVE

To further exploit the new opportunities of investigating nociceptive processing at the molecular level with the transgenic "knockout" approach, we require suitable behavioral models in mice. The presented mouse orofacial formalin test appears to be a reliable model for studying the behavioral encoding of the intensity of nociceptive stimulation and the counter-irritation phenomenon and for testing analgesic drugs.

Bidirectional modulation of windup by NMDA receptors in the rat spinal trigeminal nucleus

Activation of afferent nociceptive pathways is subject to activity-dependent plasticity, which may manifest as windup, a progressive increase in the response of dorsal horn nociceptive neurons to repeated stimuli. At the cellular level, N-methyl-d-aspartate (NMDA) receptor activation by glutamate released from nociceptive C-afferent terminals is currently thought to generate windup. Most of the wide dynamic range nociceptive neurons that display windup, however, do not receive direct C-fibre input. It is thus unknown where the NMDA mechanisms for windup operate. Here, using the Sprague-Dawley rat trigeminal system as a model, we anatomically identify a subpopulation of interneurons that relay nociceptive information from the superficial dorsal horn where C-fibres terminate, to downstream wide dynamic range nociceptive neurons. Using in vivo electrophysiological recordings, we show that at the end of this pathway, windup was reduced (24 +/- 6%, n = 7) by the NMDA receptor antagonist AP-5 (2.0 fmol) and enhanced (62 +/- 19%, n = 12) by NMDA (1 nmol). In contrast, microinjections of AP-5 (1.0 fmol) within the superficial laminae increased windup (83 +/- 44%, n = 9), whereas NMDA dose dependently decreased windup (n = 19). These results indicate that NMDA receptor function at the segmental level depends on their precise location in nociceptive neural networks. While some NMDA receptors actually amplify pain information, the new evidence for NMDA dependent inhibition of windup we show here indicates that, simultaneously, others act in the opposite direction. Working together, the two mechanisms may provide a fine tuning of gain in pain.

Synergistic antinociceptive effect of amitriptyline and morphine in the rat orofacial formalin test

BACKGROUND

Combination therapy is often used to increase the clinical utility of analgesic agents. The coadministration of two compounds may achieve analgesia at doses lower than those required for either compound alone, leading to enhanced pain relief and reduction of adverse effects. Herein, the authors describe the effect of coadministration of morphine and amitriptyline on cutaneous orofacial inflammatory pain in rats.

METHODS

Amitriptyline, morphine, or the combination of amitriptyline and morphine was administered systemically to rats, and antinociceptive effects were determined by means of the rat orofacial formalin test. Isobolographic analysis was used to define the nature of the interactions between morphine and amitriptyline.

RESULTS

Amitriptyline as well as morphine produced a dose-related inhibition in the first phase and the second phase of rubbing activity. ED50 values against rubbing behavior were 14.6 mg/kg (95% confidence interval, 10.2-33.5 mg/kg) and 1.3 mg/kg (95% confidence interval, 1.0-1.7 mg/kg) for amitriptyline and morphine, respectively. Combinations of increasing fractional increments of amitriptyline and morphine ED50 doses produced a synergistic effect against rubbing behavior, as revealed by isobolographic analysis.

CONCLUSIONS

The current study suggests that systemic amitriptyline and morphine synergistically inhibit cutaneous orofacial inflammatory pain in rats.

Contribution of neurokinin 1 receptors in the cutaneous orofacial inflammatory pain

This study investigated the role of neurokinin 1 receptors (NK1R) in inflammatory cutaneous orofacial pain. The effects of subcutaneous and intracisternal administration of the NK1R antagonist SR140333 on the face rubbing response provoked by injection of 50 micro l of 1.5% formalin into the vibrissa pad were examined. Subcutaneous administration of SR140333 (5, 15, 30 mg/kg) induced a dose-related depressant effect on both the first and second phases of the formalin test. In contrast, intracisternal administration of SR140333 (10, 30, 60, 90 microg) produced a decrease only on the second phase with an apparent ceiling effect at approximately 50%. These data suggest that persistent nociceptive effects associated with orofacial cutaneous inflammation depend at least in part on the activation of NK1R.

Low doses of N-methyl-D-aspartate antagonists in superficial laminae of medulla oblongata facilitate wind-up of convergent neurones

In this study, a trigeminal model was used in which high threshold C-fibre-evoked activities of convergent neurones located in the subnucleus oralis of the trigeminal complex are modulated through the superficial part, the substantia gelatinosa, of the subnucleus caudalis. The two subnuclei are located 3 mm apart, therefore, it was possible to inject dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, into either the superficial or the deep parts of subnucleus caudalis without interfering with ongoing recording of convergent neurones in subnucleus oralis. A differential NMDA-dependent modulation of wind-up was observed according to the dose and the injection target. (1) The injections of small non-diffusible doses (0.12 microg) of dizocilpine into the superficial part of subnucleus caudalis facilitated wind-up. The effect peaked at 25 min with a mean increase above control of 173+/-31%. Injection (0.5 microg) of either the less active enantiomer dizocilpine or saline into superficial subnucleus caudalis had no significant effect on subnucleus oralis convergent neurones. This suggests that NMDA-dependent interneurones, probably located in substantia gelatinosa of subnucleus caudalis, exert, in normal conditions, an inhibitory control on wind-up of convergent subnucleus oralis neurones. (2) The injection of larger doses (0.5 microg) into the superficial part of subnucleus caudalis induced a predominant inhibitory effect on wind-up. The mean peak effect at 15 min was 46+/-7% compared to control (100%). Small and large doses of dizocilpine injected into the deep part of subnucleus caudalis had a predominant inhibitory effect. The inhibition of wind-up of subnucleus oralis neurones after injection of NMDA receptor antagonists in superficial or deep subnucleus caudalis indicates that wind-up may be due, at least in part, to NMDA activation at synapses that do not involve the recorded convergent neurones.

Superficial and deep convergent nociceptive neurons are differentially affected by N-methyl-D-aspartate applied on the brainstem surface of the rat medullary dorsal horn

The activation of N-methyl-D-aspartate receptors is implicated in the spinal and trigeminal processing of nociceptive information conveyed by convergent (wide dynamic range) neurons and particularly in C-fiber-evoked responses elicited by repetitive and high-intensity electrical stimulation of the neuronal receptive field. In this study, the effects of intrathecal NMDA application on the electrically evoked nociceptive responses of trigeminal subnucleus caudalis convergent neurons have been investigated. The total C-fiber-evoked activity triggered by 30 successive stimuli was divided into two components: the C-fiber input response and the 'wind-up' response. Application of 0.1 microg (in 50 microl) of NMDA evoked a bi-directional effect on the total C-fiber-evoked activity of 19 neurons tested. A significant increase in the total C-fiber-evoked activity was observed 15-25 min after the NMDA application for nine neurons located in superficial laminae II and III. In contrast, a significant decrease in the total C-fiber-evoked activity was observed 5-25 min after the NMDA application for 10 neurons located more deeply, in lamina V. The NMDA-induced modifications seen in the total C-fiber-evoked activity were likely a reflection of a significant increase or decrease in neuronal activity evoked by the C-fiber input rather than wind-up of the responses since the latter was not significantly modified by the NMDA application. These results provide evidence for a possible inhibitory role for NMDA-dependent interneurons of the superficial laminae of the medullary dorsal horn on the nociceptive activity of deep convergent neurons.

Stimulus-function, wind-up and modulation by diffuse noxious inhibitory controls of responses of convergent neurons of the spinal trigeminal nucleus oralis

Extracellular unitary recordings were made from 53 spinal trigeminal nucleus oralis (Sp5O) convergent neurons in halothane-anaesthetized rats. The neurons had an ipsilateral receptive field including mainly oral or perioral regions. They responded to percutaneous electrical stimulation with two peaks of activation. The first had a short latency (4.3 +/- 0.3 ms) and low threshold (0.35 +/- 0.04 mA), whereas the second had a longer latency (68.1 +/- 3.4 ms) and higher threshold (7.3 +/- 0.5 mA). Intracutaneous injection of capsaicin (0.1%) produced a strong and rapid reduction of the long-latency responses of Sp5O convergent neurons with little effect on the short-latency responses. In most cases (73%), the long-latency responses exhibited a wind-up phenomenon during repetitive (0.66 Hz) suprathreshold electrical stimulation. These results suggest that C-fibres mediate the long-latency response of Sp5O convergent neurons. Regarding the C-fibre-evoked responses, a linear relationship between the intensity of the applied current and the magnitude of the response was found within the one to three times threshold range. The Sp5O convergent neurons also encoded the intensity of mechanical stimuli applied to the skin or mucosa in the 5-50 g ranges. The evoked activity of Sp5O convergent neurons could be suppressed by noxious heat applied to the tail (52 degrees C) and long-lasting poststimulus effects followed this. These findings show that convergent neurons in the Sp5O resemble those in the deep laminae of the spinal dorsal horn and spinal trigeminal nucleus caudalis, and further support that the Sp5O plays a part in the processing of nociceptive information from the orofacial region.

Effects of lesions in the trigeminal oralis and caudalis subnuclei on different orofacial nociceptive responses in the rat

Behavioural responses to two different orofacial noxious stimulations were analysed following lesion of spinal trigeminal subnucleus oralis (Sp5O) in the rat. Lesions were obtained by intranuclear microinjections of quinolinic acid (0.4 microliter of 60 nmol/microliter solution). The control groups received microinjection of saline. Noxious stimulation was a subcutaneous injection of formalin into the upper lip or electrical stimulation of the tooth pulp. The measured behavioural responses were duration of rubbing induced by the formalin injection and thresholds of the jaw-opening reflex (JOR), head rotation (HR) and face rubbing (FR) evoked by the pulp stimulation. In addition, formalin injection was also performed in two groups of rats that had received intranuclear injection of quinolinic acid or saline into rostral subnucleus caudalis (Sp5C). Rubbing duration was not significantly modified by the lesion of Sp5O, whereas a significant decrease occurred after the lesion of rostral Sp5C. After the lesion of Sp5O, an increase in the threshold of JOR was observed whereas the thresholds of HR and FR were not significantly modified. These results suggest that Sp5O is not necessary for the processing and relay of nociceptive inputs triggered by intense stimulations of oral and perioral areas. However further experiments are needed to reconcile these results with the relevant data obtained from cell recording experiments which indicate the existence, in Sp5O, of neuronal activities related to the sensory discriminative aspect of intense nociception.

Effect of an NMDA receptor antagonist on the wind-up of neurons in the trigeminal oralis subnucleus

Extracellular recordings of convergent neurons of the oralis subnucleus of the trigeminal sensory complex were performed in paralysed rats under halothane-N2O-O2 anesthesia using glass micropipettes. The effects of MK801, a noncompetitive NMDA receptor antagonist, were observed on the increased cell activity (wind-up), triggered by the repetition, at a low frequency (0.66 Hz) and high intensity (3 times the threshold of C-fiber response), of electrical stimulation of the receptive field. Successive cumulative doses (up to 1 mg/kg) of MK801 i.v. resulted in a dose-dependent decrease in the responses related to C-fiber input (11 cells). A single dose of 1 mg/kg applied in four cells had effects similar to the 1 mg/kg dose given cumulatively. Three units were either weakly or not modified by MK801. In a second experiment, recordings were performed in 12 cells for 80 min after an injection of a small dose of MK801 (0.15 mg/kg). C input was not significantly modified by the antagonist. The effects of MK801 on the first part of the wind-up response (wind-up proper) peaked between 15 and 50 min and returned to control values at about 80 min. The effects on the postdischarge followed approximately the same time course. It is concluded that despite being devoid of substantia gelatinosa, the oralis subnucleus contains neurons that display an NMDA receptor-linked wind-up similar to the phenomenon described in the dorsal horn of the spinal cord.

Effects of morphine microinjections into the trigeminal sensory complex on the formalin test in the rat

The aim of this study was to find out whether morphine locally applied into the different subnuclei of the spinal trigeminal nucleus could impair the behavioral response evoked by a tonic nociceptive stimulus. Microinjections of morphine were performed unilaterally in rats through a chronically implanted tube. The duration of the formalin-induced biphasic rubbing activity was used as a measure of nociception. Morphine, 0, 2, 7, or 14 microg, in 0.2 microl of saline were microinjected 10 min before a subcutaneous injection of formalin (1.5%) in the ipsilateral upper lip. Morphine microinjections into subnucleus oralis (Sp5O) and subnucleus caudalis (Sp5C) induced a significant decrease in rubbing duration of either the early or late phase of the response or both. The early response to formalin was depressed only after morphine injection into Sp5O. The late response was depressed by microinjections into both Sp5O and Sp5C. No significant effect was observed in subnucleus interpolaris (Sp5I). These results are further evidence for the role played by Sp5C in orofacial nociception. They also give support to the involvement of the Sp5O in perioral nociceptive mechanisms of short duration.

Effects of systemic morphine on the activity of convergent neurons of spinal trigeminal nucleus oralis in the rat

The spinal trigeminal nucleus oralis has been shown to relay nociceptive inputs mainly from the oral and perioral regions. In this study, we examined the effects of intravenous administration of morphine on C-fiber-evoked activities of spinal trigeminal nucleus oralis convergent neurons in halothane-anesthetized rats. Morphine depressed the C-fiber-evoked responses of spinal trigeminal nucleus oralis convergent neurons in a dose-related (3-12 mg/kg range) and naloxone-reversible fashion. The ED50 was 6.1 mg/kg, a dose similar to that found in the spinal horn. The observed strong depressive action of morphine on noxious-evoked activities of spinal trigeminal nucleus oralis neurons is consistent with our previous statement, based on electrophysiological studies, that this region plays an important role in the transmission of trigeminal nociceptive information. The effect of morphine on the spinal trigeminal nucleus oralis neurons is discussed in relation to its possible site and mechanism of action.

Antinociceptive effect of morphine microinjections into the spinal trigeminal subnucleus oralis

We analysed the effects of morphine microinjections (0.2 microliter) into the subnucleus oralis (SNO), i.e. the rostral division of the spinal trigeminal nucleus, on the formalin test adapted for orofacial pain. Duration of rubbing following an injection (50 microliters) of formalin solution (1.5%) in the right upper lip of the rat was measured. Compared to microinjections of saline (NaCl, 0.9%) used as control, 90 and 180 nmol microliter-1 of morphine solution in the SNO significantly curtailed rubbing. This effect was dose-dependent, reversed by naloxone, and also site-specific to the SNO since no effect was observed after microinjection of morphine into areas adjacent to the SNO. These results are further evidence for the involvement of the SNO in perioral nociceptive mechanisms.

GABA receptors in Deiters nucleus modulate posturokinetic responses to cortical stimulation in the cat

The early component of the postural responses which accompany the limb flexion during unilateral stimulation of the motor cortex in the cat is not of reflex origin, but results from a central command. These postural adjustments are characterized by a decreased force under the limb diagonally opposite to the moving one and an increased force under the other two. Since the lateral vestibular nucleus (LVN) exerts an excitatory influence on ipsilateral limb extensor motoneurons, experiments were performed in cats to establish whether the cortical-induced postural changes were mediated through the LVN. This structure is tonically inhibited by GABAergic synapses originating from Purkinje cells of the cerebellar vermis, so that local microinjection into the LVN of GABA agonists or antagonists should either decrease or increase the spontaneous discharge of their neurons. Unilateral microinjection of 0.25 microliters of the GABA-A agonist muscimol or the GABA-B agonist baclofen (at 2-4 micrograms/microliters saline) into the LVN produced a short-lasting episode of ipsilateral postural hypotonia and contralateral hypertonia, during which the cats were unable to stand on the measurement platform. When, shortly after, some recovery of the postural activity appeared, no changes in threshold, latency or amplitude of the cortical-induced flexion movement were observed; however, the early component of the postural responses decreased in the other three limbs. Moreover, the slope of the response curve of the moving limb remained unmodified, while that of the early component of the postural responses, which involved the remaining limbs, decreased following stimulation of the motor cortex at different stimulus intensities. These effects started a few min after the injection and lasted for about 2-3 h. The effects described above were dose-dependent. Moreover, histological controls indicated that the structure responsible for these postural changes corresponded to the middle part of the LVN. The specificity of the results was shown by the fact that unilateral microinjection of 0.25 microliters of the GABA-A antagonist bicuculline or the GABA-B antagonist phaclofen (at 5-8 micrograms/microliter saline) into the LVN produced a postural asymmetry opposite in sign to that elicited in the same experiments by the corresponding agonists. These injections did not modify the amplitude of the cortical-induced limb flexion, but rather enhanced the amplitude of the early component of the postural responses in the other three limbs.(ABSTRACT TRUNCATED AT 400 WORDS)

Injection of a cholinergic agonist in the dorsolateral pontine tegmentum of cats affects the posturokinetic responses to cortical stimulation

Microinjection into the dorsolateral pontine tegmentum of the muscarinic agonist bethanechol, leading to activation of cholinoceptive pontine reticular formation (pRF) neurons and the related medullary inhibitory reticulospinal system, did not modify the threshold, latency and amplitude of the forelimb flexion elicited by unilateral stimulation of the corresponding motor cortex. However, the remaining limbs which displayed a diagonal pattern of postural adjustments showed a dissociation of their postural responses in 2 components: the early component of central origin greatly decreased in amplitude, while the late component attributed to reflex mechanism triggered by the unbalance brought about by the flexion movement increased. Further evidence indicated that the pRF system intervenes in the gain regulation of the early postural responses during the cortically induced limb movement.

Cholinoceptive pontine reticular structures modify the postural adjustments during the limb movements induced by cortical stimulation

1. Activation of the pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system decreases the postural activity. This effect can be achieved either by local injection into the dorsal pontine tegmentum of cholinergic agonists which excite cholinoceptive pRF neurons, or by injection of noradrenergic agents which block the inhibitory influence exerted by the locus coeruleus (LC) neurons on the pRF. The main aim on the present study was to analyze the effects of tonic activation of these pRF neurons on the postural adjustments accompanying limb movements induced by motor cortex stimulation. In particular, electrodes were implanted chronically in the motor cortex of cats and stainless steel guide tubes of small size, later used for drug injection, were set bilaterally into sites just above the responsive regions. 2. Limb flexion elicited by stimulation of the motor cortex was accompanied by a diagonal pattern of postural adjustment, characterized by a decreased force exerted by the limb diagonally opposite to the moving one and an increased force exerted by the other two. 3. Microinjection into the pRF of both sides of 0.25 microliter of the muscarinic agonist bethanechol at the concentration of 8 or 16 micrograms/microliters in buffered artificial cerebrospinal fluid produced a short-lasting episode of postural atonia followed by a period of reduced postural activity, during which the cats were still able to stand on the measurement platform. Under this condition no changes in threshold, latency and amplitude of the flexion response were observed in the performing limb; however, the postural responses were considerably affected. In particular, when the performing limb was a forelimb, the other anterior limb showed a dissociation of the postural response in two distinct components. The first anticipatory component, which had a short latency (12-15 msec) and was considered to be centrally triggered, decreased in amplitude after injection of bethanechol and sometimes disappeared; on the other hand the second component, which had a long latency (50-60 msec) and was thus considered to be of reflex origin, increased in amplitude, due to the instability resulting from the depression of the early postural response. Similar results also affected to a lesser extent the hindlimbs. Moreover, body oscillations were observed and monitored from the force platforms following the late component of the postural responses.(ABSTRACT TRUNCATED AT 400 WORDS)