Effects of lateral reticular nucleus stimulation on trigeminal sensory and motor neuron activity related to the jaw opening reflex.

The effects of lateral reticular nucleus (LRN) stimulation on the responses to tooth pulp (TP) stimulation of neurons located in the trigeminal (V) sensory (39 units) and motor (33 units) nuclei were assessed in anesthetized rats. Only neurons which responded to TP stimulation with bursts of spikes that were in a constant temporal relationship with the digastric EMG signal were studied. The LRN-stimulating electrodes were positioned at optimal sites to suppress the TP-evoked jaw-opening reflex (JOR) recorded simultaneously with the neuronal activity related to it. It was found that: (1) the neurons in the V nucleus oralis responded to TP stimulation with 3-8 msec latency excitatory responses that were suppressed during LRN conditioning stimulation with a time course comparable to that of the JOR suppression, and (2) the neurons in the V nucleus motor responded to TP stimulation with 5-15 msec latency excitatory responses. This activity was suppressed during LRN-conditioning stimulation with a time course that parallels the inhibition of the activity of nucleus oralis neurons and of the JOR. However, assuming that the excitatory interneurons for the V motoneurons are located in the nucleus oralis, the suppression of this input by LRN may account for the lack of response in V motor neurons. Thus, we suggest that LRN inhibits the TP-evoked JOR by acting on the sensitive afferent limb of the reflex.

Novel effect of nefopam preventing cGMP increase, oxygen radical formation and neuronal death induced by veratridine.

Nefopam hydrochloride is a potent analgesic compound that possesses a profile distinct from that of opiods or anti-inflammatory drugs. Previous evidence suggested a central action of nefopam but the detailed mechanisms remain unclear. Here we have used cultured cerebellar neurons to test the hypothesis that nefopam may modulate voltage sensitive sodium channel (VSSC) activity. Nefopam (100 microM) effectively prevented NMDA receptor-mediated early appearance (30 min) of toxicity signs induced by the VSSC activator veratridine. Delayed neurotoxicity by veratridine occurring independently from NMDA receptor activation, was also prevented by nefopam. In contrast, excitotoxicity following direct exposure of neurons to glutamate was not affected. Neuroprotection by nefopam was dose-dependent. 50% protection was obtained at 57 microM while full neuroprotection was achieved at 75 microM nefopam. Veratridine-induced sodium influx was completely abolished in nefopam-treated neurons. Intracellular cGMP and oxygen radical formation following VSSC stimulation by veratridine were also effectively prevented by nefopam. Our data are consistent with an inhibitory action of nefopam on VSSC and suggest that nefopam may modulate the release of endogenous glutamate following activation of these channels. This novel action of nefopam may be of great interest for the treatment of neurodegenerative disorders involving excessive glutamate release and neurotransmission.

Repeated treatment with the synthetic cannabinoid WIN 55,212-2 reduces both hyperalgesia and production of pronociceptive mediators in a rat model of neuropathic pain.

The antinociceptive properties of cannabinoids in persistent pain are not fully elucidated. We investigated the effect of repeated treatment with the synthetic cannabinoid receptor agonist WIN 55,212-2 on the neuropathic pain induced in rats by chronic constriction of the sciatic nerve. WIN 55,212-2 administered daily throughout the development of neuropathy reversed the hyperalgesia, at a dose (0.1 mg x kg(-1), s.c.) that had no effect on the nociceptive responses of either paw contralateral to the sciatic ligation or of animals subjected to sham surgery. At 14 days after injury, the levels of mediators known to be involved in neuropathic pain, such as prostaglandin E2, NO and the neuronal NOS, were increased. Repeated treatment with WIN 55,212-2 abolished these increases. In the light of the current clinical need for neuropathic pain treatments, these findings indicate that cannabinoid agonists, at doses devoid of psychoactive effects, could constitute important compounds for the development of new analgesics.

Topical axonal transport blocker vincristine prevents nerve injury-induced spinal neuron sensitization in rats.

The effect of vincristine (Vin, a fast axonal transport blocker) to prevent any alteration in the excitability of dorsal horn neurons, following peripheral nerve injury, was investigated on 31 rats: 20 with chronic constriction injury (CCI) of the sciatic nerve and 11 sham preparations. In 15 of the 20 CCI rats, a small piece of gelfoam soaked with Vin was applied to the sciatic nerve before ligation (Vin+); in the remaining 5 rats the nerve was ligated without Vin (Vin-). The 11 sham rats were 7 Vin+ and 4 Vin-. The dorsal horn neuronal activity was recorded after 2-3 postoperative (PO) weeks. In the CCI Vin- rats, the neurons showed increased spontaneous activity and hyperresponsiveness to noxious stimulus with prolonged afterdischarges, events considered to signal central neuron sensitization. In the CCI Vin+ rats, the neuronal spontaneous and stimulated activity values were significantly lower (p < 0.001) than in the CCI Vin- rats being comparable to normal values. In sham Vin+ and Vin- rats, the neuronal activities had normal values. Given the crucial role attributed to central neuron sensitization for the development of neuropathic pain, the possibility that vincristine, by blocking the axonal transport, exerts a preventive action on this syndrome is discussed.

Bulbar influences on the periaqueductal gray. Potential role in nociception.

Unit activity was recorded from 96 neurons in the periaqueductal gray (PAG) in response to focal electrical stimulation of the lateral reticular nucleus (LRN) and the nucleus gigantocellularis (NGC), in anesthetized, curarized rats. Consistent with anatomical studies showing direct projections from these nuclei to PAG, short latency excitatory responses (less than 5 ms) were found in 63 neurons and inhibition of the activity in 28 neurons. Peripheral noxious heat stimulation was effective in altering the activity in 36 of these neurons (38%). The LRN and NGC stimuli inhibited the noxious evoked responses in 26 of these neurons. The inhibition could also be induced when the dorsolateral funiculi (DLFs) were cut. These results indicate that LRN and NGC can have a direct influence on PAG neurons, which are probably involved in the processing of noxious information.

Spinal expansion of saphenous afferents after sciatic nerve constriction in rats.

In rats with chronic constriction of one sciatic nerve, neurone pair responses to saphenous electrical stimulation were simultaneously recorded in sciatic (L5-6) and in saphenous (L2) spinal areas. In 16 rats with thermal hyperalgesia, 43 pairs of neurons were recorded, 4 and 14 days after constriction, on both sides of the spinal cord. On the side ipsilateral to nerve constriction, stimulation of the saphenous evoked excitatory responses in 90% of neurones recorded in the L5-L6 sciatic area, regardless of the post-constriction time. No responses were evoked by saphenous stimulation of the L5-L6 contralateral spinal cord neurones. The possibility that pre-existing connections are unmasked after nerve injury and impinge on sensitized neurones, contributing to abnormal pain sensations such as, for instance, extraterritorial pain, is discussed.

A study of early ongoing activity in dorsal horn units following sciatic nerve constriction.

An analysis was made of the background activity of spinal neurones involved in nociceptive processes during, and in the first hour after, sciatic nerve constriction in normal rats and in rats in which sciatic nerve conduction had been previously blocked with local anaesthetic. In both pure noxious and WDR neurones the results showed high frequency discharges at the time of ligation, followed by a slight but persistent increase in activity that reached, after 1 h, values 6-8 times those preligature. The increased post-injury discharge could be reduced by secondarily applying lidocaine on the peripheral site of constriction. There were no neuronal activity changes after the ligatures in rats with sciatic nerve pretreated with local anaesthetic. The likelihood that the early persistent increase in activity may have a role in the excitability change of spinal neurones observed in neuropathic pain is discussed.

Selective inhibition by systemic lidocaine of noxious evoked activity in rat dorsal horn neurons.

The effect of systemically injected lidocaine (3-4 mg kg-1) on the responses to noxious and non-noxious stimuli on 28 wide dynamic range (WDR) neurons in the dorsal horn was studied in anesthetized and curarized rats. It was consistently found that lidocaine reduced or suppressed the responses to noxious stimuli whereas it did not act on the responses to non-noxious stimulation and on the spontaneous activity. Furthermore the noxious stimuli were completely ineffective from 10-15 min following the lidocaine injection while the non-noxious stimuli maintained their efficacy. The control responses, in all the cases, returned within 20 min. The results suggest that lidocaine exerts a selective inhibitory effect on nociceptive transmission at the spinal level.

Poststimulus afterdischarges of spinal WDR and NS units in rats with chronic nerve constriction.

Forty pairs of wide dynamic range (WDR) and nociceptive specific (NS) neurones were simultaneously recorded in the dorsal horn of rats with chronic constriction injury (CCI) of the sciatic nerve. The responses to noxious mechanical stimulation were analysed and the afterdischarges were compared in the two neuronal populations. The number of neurones presenting an afterdischarge was higher in WDR than in NS population. Furthermore afterdischarges had significantly longer duration and greater magnitude with slower time course decays in WDR than in NS neurones. Given the role that afterdischarge may have in the prolonged transmission of nociceptive information, the possibility that WDR neurones could give a major contribute to the abnormal processing of noxious signals in CCI rats is proposed.

Injured nerve block alters adjacent nerves spinal interaction in neuropathic rats.

Baseline activity and responses to simultaneous saphenous stimulation of pairs of neurones recorded from sciatic (L5-6) and saphenous (L2) spinal cord segments, in rats with thermal hyperalgesia following sciatic constriction, were analysed before, during and after a sciatic nerve block with a local anaesthetic. In sciatic neurones, during the block, reductions of baseline activity (p < 0.001), increases in threshold of saphenous electrical stimulation (p < 0.01) and reductions of responses to electrical and to natural noxious saphenous stimuli (p < 0.001) were consistently found. The neuronal baseline and evoked activities remained unmodified in saphenous neurones. The contribution of input from injured periphery to central neurone circuitry mechanisms underlying the unmasking of improper afferents is discussed.

Different time-courses of i.v. lidocaine effect on ganglionic and spinal units in neuropathic rats.

The effect of intravenous lidocaine (4 mg kg-1) on ganglionic and spinal neuronal hyperactivity following sciatic chronic constriction injury (CCI) was studied in anaesthetized and curarized rats. A significant difference in the time course and magnitude of the lidocaine effect on the two neuronal populations was found. Longer lasting and more potent inhibitory effects on the dorsal horn neurones in comparison with ganglionic neurones were observed. By contrast the magnitude and time course of the inhibitory effects were highly comparable in dorsal horn neurones before and after acute rhizotomy. The results indicate that peripheral and central effects of lidocaine are not sequentially related. The likelihood that lidocaine inhibition at central sites may have a role in its analgesic effect, at least in the neuropathic model, is discussed.

Rough annealing by two-step clustering, with application to neuronal signals.

To accomplish analyses on the properties of neuronal populations it is mandatory that each unit activity is identified within the overall noise background and the other unit signals merged in the same trace. The problem, addressed as a clustering one, is particularly difficult as no assumption can be made on the prior data distribution. We propose an algorithm that achieves this goal by a two-phase agglomerative hierarchical clustering. First, an inflated estimation (overly) of the number of clusters is cast down and, by a maximum entropy principle (MEP) approach, is made to collapse towards an arrangement near natural ones. In the second step consecutive partitions are created by merging, two at time previously aggregated partitions, according to similarity criteria, in order to reveal a cluster solution. The procedure makes no assumptions about data distributions and guarantees high robustness with respect to noise. An application on real data out of multiple unit recordings from spinal cord neurons of mixed gas-anaesthetized rats is presented.

The effects of periaqueductal gray and nucleus raphe magnus stimulation on the spontaneous and noxious-evoked activity of lateral reticular nucleus neurons in rabbits.

In urethane-anesthetized rabbits the effects of periaqueductal gray (PAG) and nucleus raphe magnus (NRM) stimulation on the spontaneous and noxious-evoked activity of the lateral reticular nucleus (LRN) neurons were studied. The PAG and the NRM stimulating electrodes were located in the optimal sites for suppressing the jaw-opening reflex (JOR) evoked by the tooth pulp stimulation. It was found that the 12% of neurons tested were affected by one or both stimuli. A total of 80 responsive neurons (52% antidromically activated by the cerebellum) were analyzed. Out of these neurons, 31 showed a convergence to both stimuli, 43 responded only to PAG and 6 only to NRM. Noxious heat stimulation of the contralateral foot was effective in altering the activity of 60% of these neurons. The PAG and NRM stimuli modified the noxious-evoked responses in most of these units. While the excitation was the predominant effect on the spontaneous activity (52 cells), the inhibition was predominant on the noxious-evoked activity (29 cells). These results indicate the presence of connections from PAG and NRM to LRN, probably devoted to the processing of the nociceptive information.

Effects of noxious stimuli on neurons of the lateral reticular nucleus region in rabbits.

The responses of neurons in the lateral reticular nucleus region (LRN) to both noxious spinal (radiant heat) and trigeminal inputs (tooth pulp stimulation), were analyzed in 12 anesthetized and paralyzed rabbits. It was found that 20% of neurons tested were affected by one or both noxious stimuli and 58% of these cells could be antidromically activated by stimulation of the cerebellum. In the majority of cell (85% of the heat-responding, and 72% of the tooth pulp-responding) the effect of noxious stimuli was excitatory and in the remaining cells was inhibitory. Furthermore 53% of responding neurons showed a convergence to spinal and trigeminal input. The type of response was the same (excitation) in 63% of these units, but was different in the others. These data support the view of an involvement of the LRN region in the mechanisms of the nociceptive information and suggest the possibility that the cerebellum may also contribute to some aspects of these mechanisms.

Central effects of systemic lidocaine mediated by glycine spinal receptors: an iontophoretic study in the rat spinal cord.

The objective of this investigation was to demonstrate the possible interactions of systemic lidocaine (lido) with inhibitory receptors in the spinal cord. In the lumbar dorsal horn of anesthetized and curarized rats, 60 physiologically identified, wide dynamic range (WDR) neurons, were recorded extracellularly. Glutamate, glycine and its selective antagonist, strychnine, were iontophoretically applied onto the neurons either singularly or concurrently. The effects of systemic lido on the drug-induced frequency changes and the interaction with the glycine receptors, using strychnine as a probe, were studied. It was consistently found that: (i) lido (3-4 mg/kg) inhibited the excitatory responses to iontophoretic glutamate, (ii) this inhibition was significantly antagonized by concurrent iontophoretic strychnine, (iii) iontophoretic glycine induced comparable glutamate inhibition that was reversed by strychnine. In contrast, no effect on glutamate-induced excitations was observed when lido was applied by micropressure or a different local anesthetic was systemically administered. The results suggest that central inhibitory effects of lido could be mediated by spinal strychnine-sensitive glycine receptors, activated by lido itself or possibly by its glycine residue-bearing metabolites.

Electrophysiological identification of pontomedullary reticular neurons directly projecting into dorsal column nuclei.

In cats 'encephale isole', cerebellectomized and curarized, neurons projecting from the bulbopontine reticular formation to the cuneate nucleus were identified by stimulating their axons in this nucleus by means of Eide micropipettes and by recording both field potential and antidromic responses from single somata in the nucleus gigantocellularis. Of 130 neurons responsive to cuneate stimulation, 17 (12%) were found that were antidromically activated. The small proportion of antidromic responses might reflect the small size of the projection pathway. However, there are reasons to believe that a certain number of neurons might have been missed because of biasing factors introduced by the microstimulation technique. Neurons that were activated orthodromically were also observed, responding after latencies spread over rather wide ranges. The possible pathways for this activation are discussed.

Single unit activity in lateral reticular nucleus during cortically evoked masticatory movements in rabbits.

The activity pattern of the lateral reticular nucleus (LRN) neurones was analyzed during cortically evoked masticatory movements in anesthetized rabbits. Antidromic activation from the cerebellum and histological reconstruction of recorded neurones location from electrode tracks and microdrive readings were the criteria for neuronal identification. Tonic changes as well as rhythmic modulation of neuronal activity were found in a subpopulation of LRN cells during mastication. The same results were observed also in curarized rabbits during fictive mastication. These data support the view that LRN neurones are involved in a central mechanism controlling the masticatory movements.

Effect of different concentrations of iontophoretic nociceptin on distinct classes of nociceptive neurons in rat spinal cord.

Iontophoretically applied nociceptin (NC) was tested at different concentrations on the activity of spinal nociceptive specific (NS) and wide dynamic range (WDR) neurons. Low NC dosages inhibited the noxious response of NS neurons, higher dosages inhibited the noxious responses of the WDR neurons but had little effect on the non-noxious response. Naloxone did not antagonize the NC effect. Thus, appropriate dosages of NC may be selective, both for neuronal classes and for sensory modalities.

Facilitatory role of calcitonin gene-related peptide (CGRP) on excitation induced by substance P (SP) and noxious stimuli in rat spinal dorsal horn neurons. An iontophoretic study in vivo.

The effects of iontophoretically applied calcitonin gene-related peptide (CGRP) and/or substance P (SP) onto physiologically identified neurons in the rat lumbar dorsal horn were studied. The consistent excitatory effects of SP found on the spontaneous and the noxious evoked activity were significantly potentiated by CGRP application. This peptide facilitated the noxious evoked activity and did not affect the spontaneous activity of the neurons. The responses to non-noxious stimuli were unaffected in all cases.

Central effect of ketorolac involving NMDA receptors activity.

Possible central mechanisms underlying the analgesic action of Ketorolac, a non-steroidal antiinflammatory drug (NSAID) have been investigated using an iontophoretic approach. We found that the excitation induced by N-methyl-d-aspartate (NMDA) on spinal wide dynamic range (WDR) neurons was prevented, or reduced, by Ketorolac applied before or after the start of the NMDA ejection. The data suggest that Ketorolac can achieve its central analgesic effect by interfering with the NMDA receptor activity on the spinal neurons.