Maria Luisa Sotgiu

Cooperative N-methyl-d-aspartate (NMDA) receptor antagonism and μ-opioid receptor agonism mediate the methadone inhibition of the spinal neuron pain-related hyperactivity in a rat model of neuropathic pain

Methadone (Racemic methadone) exerts its antinociceptive effect by activation of mu-opioid receptors and/or blockade of NMDA receptors. The aim of this study is to determine whether the methadone analgesic effect on neuropathic pain is achieved only by the agonism of the mu-opioid receptors or cooperatively with the antagonism of the NMDA receptors. To this purpose, in rats with neuropathic pain model of chronic constriction of one sciatic nerve (CCI rats), we administered methadone before or after opioid receptor blockade with naloxone and checked its effects on the spinal Wide Dynamic Range (WDR) neuron dynamics in three experimental conditions: on the spontaneous and noxious evoked neuronal activities in control rats (sham operated and naïve); on iontophoretic NMDA induced neuronal hyperactivity in intact rats; on pain-related spontaneous and noxious evoked hyperactivities in CCI rats. The results, as from the spike-frequency analysis, show that: (i) in control rats, methadone inhibits the noxious evoked neuronal activity and naloxone prevents or reverses about 94% of methadone inhibitory effect; (ii) in intact rats, pretreated with naloxone, methadone reduces the NMDA induced neuronal hyperactivity; (iii) in CCI rats, methadone inhibits the neuronal spontaneous and noxious evoked hyperactivities, and naloxone prevents or reverses about 60% of methadone inhibitory effect. These findings allow to conclude that methadone inhibition of the noxious evoked activity in normal rats is achieved predominantly through the agonism of the mu-opioid receptors, while the inhibition of the pain-related hyperactivity in rats with signs of neuropathic pain (CCI rats), involves also the NMDA receptors antagonism.

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.

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.

Contribution by DRt descending facilitatory pathways to maintenance of spinal neuron sensitization in rats

We investigated in different experimental rat models the potential facilitatory contribution of the medullary dorsal reticular nucleus (DRt) descending pathway to the expressions of the sensory spinal neuron sensitization such as increased spontaneous and noxious evoked activities, responsivity to heterotopic afferences stimulation and long lasting afterdischarges (ADs). We carried out experiments by recording from ipsilateral lumbar Wide Dynamic Range (WDR) neurons and by simultaneously monitoring the DRt neuron activity in neuropathic pain rats with chronic constriction injury of one sciatic nerve (CCI), in sham-operated and in intact rats. In particular, we recorded the spinal neuron spontaneous activities and the activities evoked by noxious stimulations of ipsi- and contralateral sciatic supplied areas before and during DRt activity blockade. Additionally, in intact rats we modulated WDR activity by iontophoretic NMDA to mimic CCI WDR hyperactivity without peripheral damage. We found that during DRt activity blockade in CCI rat neurons and in intact rat NMDA-treated neurons, the spontaneous activity was significantly reduced, the responses to contralateral sciatic area stimulation were reduced or suppressed, the responses to ipsilateral sciatic area were poorly affected (slightly reduced or unaffected), except for the poststimulus afterdischarges that were mostly suppressed. In sham-operated rats, the neuronal activity was not affected by DRt blockade. The finding that during the DRt nucleus blockade some expressions of spinal neurons sensitization, seemingly associated to sensory disorders in neuropathic pain, fade or extinguish designates a likely facilitatory role of DRt in the maintenance of neuronal sensitization and thus a contribution to neuropathic pain state.

Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: effects of acute reversible blockade of the dorsal column nuclei.

Abstract The effects of reversible cooling of the dorsal column nuclei on the peripheral responsiveness of 55 specific somatosensory neurons (24 of which were identified as relay cells) belonging to the forepaw focus of the ventralis posterolateralis nucleus of the cat thalamus were studied. The effectiveness of cooling was checked for each unit by observing the disappearance of its responsiveness to dorsal column stimulation. The neurons were isolated from locally anesthetized preparations (17 units), as well as from animals anesthetized with chloralose (21 units) or Nembutal (17 units). Fifteen cells (six from locally anesthetized and nine from chloralose-treated cats) were seen to retain their peripheral responsiveness in spite of the dorsal column nucleus transmission blockade. The area and location of receptive fields showed no changes in 10 such units and exhibited small enlargements and/or displacements in the remaining 5 (3 from locally anesthetized and 2 from chloralose-treated cats). In locally anesthetized preparations, intravenous injections of chloralose administered after removal of the cold block produced the same effects as those seen during cooling on the three units that exhibited enlargements of their receptive fields. The evidence points to a similar mechanism explaining the “unmasking” effects of chloralose and of dorsal column nuclei transmission blockade. Disinhibition of a redundant specific pathway converging on a fraction of thalamocortical relay cells might be suggested. Such a redundant pathway is tentatively identified with the spinocervicothalamic tract.

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.

Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: Effects of chloralose treatment

Abstract The effects of the intravenous administration of α-chloralose on the static and dynamic properties of 20 highly specific somatosensory neurons isolated within the forepaw focus of the thalamic ventralis posterolateralis (VPL) nucleus of locally anesthetized cats were studied. Observations were made (extracellular recording) before, during, and after injection. Nine units of the sample were identified as thalamocortical relay cells. Spontaneous activity, activity during tonic afferent drive maintained by weak peripheral stimuli, responsiveness to stimulation of the dorsal column and dorsolateral funiculus, and location and extent of peripheral receptive fields were examined. In none of the sample units did the drug bring to light aspecific properties such as those previously described in sizable proportions of the cell populations sampled from the VPL nucleus of cats anesthetized with chloralose or subjected to surgical interventions which lower the amount of the ascending or corticofugal influx to the thalamus. After the treatment, eight units (six of which were identified as thalamocortical relay cells) exhibited continuous or (in two cases) discontinuous enlargements of their peripheral receptive fields, which, however, remained rather small compared to the aspecific fields. The effects are tentatively explained by hypothesizing the “unmasking” of a convergent specific input, directed only to a fraction of thalamocortical relay cells and normally blocked by afferent or corticofugal inhibition. It is suggested that the unmasked input could be mediated by the spinocervicothalamic pathway.

Interactions between spinocervicolemniscal and dorsal column pathways in thalamic ventrobasal neurons in the cat

Ventrobasal (VB) thalamic cells responding to electrical stimulation of the dorsal column (DC) and of the dorsolateral funiculus DLF in cats with DC intact or cut at C4 and isolated, were studied. Among the cells observed, about 30% showed convergence to the two pathways. Different patterns of interaction between the two inputs were found, but the most frequent effect observed was an inhibition exerted by dorsal column (conditioning) stimulation on the activity evoked by dorsolateral funiculi (test) stimulation. From these results a dominance of the DC over the spinocervico-thalamic (SCT) inputs was confirmed.

Effects of intravertebral injection of a barbiturate on unit activity in lower brain stem

Abstract In acute encephale isole cats whose basilar artery had previously been tied at midpontine level, the effect of intravertebral injection of thiopental sodium (0.4–0.8 mg) on spontaneous and evoked activity of caudal brain stem neurones (n. reeticularis gigantocellularis) was investigated. Out of 90 cells studied 63 were classified as responsive units since they showed a response to at least two of the stimuli used (electrical stimulation of infraorbital nerve and gingival tissue, acoustic stimulation, tactile stimulation of the face) and 27 were classified as unresponsive units. Out of the 63 responsive units 75% showed a marked reduction or disappearance of spontaneous and evoked activity following intravertebral injection of the barbiturate. The unit returned to the preinjection frequency discharge in a time varying from a few seconds to 30 min. 14% were slightly increased, whilst 11% were not affected by the same injection. Out of the 27 unresponsive units 40% showed a diminution or abolition of their spontaneous activity. The results indicate that thiopental sodium intravertebrally injected may have a deactivating action on caudal brain stem neurones.

Projections from lateral reticular nucleus neurons to trigeminal motor nucleus revealed by antidromic activation in rabbits.

In anesthetized rabbits, electrophysiological experiments were conducted to determine if neurons of the lateral reticular nucleus that modified their activity during masticatory movements project directly into the trigeminal motor nucleus. Of the 125 neurons tested, 50 responded to stimulation of the trigeminal motor nucleus; of these, 22 were antidromically excited. Among this last population, 9 were antidromically driven also by cerebellar stimulation. In these neurons the antidromic response evoked from one structure collided with the response antidromically evoked from the other, thus indicating that both potentials are from the same neuron projecting to the trigeminal motor nucleus and the cerebellum. The remaining 28 neurons responding to stimulation of the trigeminal motor nucleus were orthodromically activated with a wider range of latencies. The possible significance of these findings in the organization of the rhythmic masticatory movements is briefly discussed.