NON-STEROIDAL ANTI-INFLAMMATORY DRUGS'S ANTINOCICEPTION MEDIATED BY THE OPIOID MECHANISM IN THE NUCLEUS RAPHE MAGNUS.

It has been established that the midbrain periaqueductal gray matter (PAG) and rostral ventro-medial medulla (RVM) are involved in the descending pain control system. The latter involves the midline nucleus raphe magnus (NRM) and adjacent reticular formation. These brain structures are is one of important parts of CNS circuit that controls nociceptive transmission at the level of spinal cord. Here we report that microinjection of commonly used non-steroidal anti-inflammatory drugs (NSAIDs), diclofenac, ketorolac, metamizol, and xefocam into the NRM produces strong antinociception which is mediated by the opioid mechanism. The experiments were carried out on experimental and control (saline) white albino male rats. Animals were implanted with a guide cannula in the NRM and tested for antinociception following microinjection of NSAIDs into the NRM in the tail flick (TF) and hot plate (HP) tests. The analysis of variance (ANOVA) with post-hoc Tukey-Kramer multiple comparison tests were used for statistical evaluation. The obtained data show that microinjection of these NSAIDs into the NRM produced antinociception as revealed by a latency increase in the tail-flick (TF) and hot plate (HP) latencies compared to the saline control microinjected into the same nucleus. Furthermore, we definitely showed that pre-treatment with opioid antagonist naloxone in the NRM diminishes NSAID-induced antinociception expressing in significant decrease in TF and HP latencies (P<0.001). The present findings support the concept that antinociceptive effects of NSAIDs are mediated via an endogenous opioid system possibly involving the descending pain modulatory circuit.

Low- and high-frequency subcortical SEP amplitude reduction during pure passive movement.

OBJECTIVES: To investigate the effect of pure passive movement on both cortical and subcortical somatosensory evoked potentials (SEPs). METHODS: Median nerve SEPs were recorded in 8 patients suffering from Parkinson's disease (PD) and two patients with essential tremor. PD patients underwent electrode implantation in the subthalamic (STN) nucleus (3 patients) and pedunculopontine (PPTg) nucleus (5 patients), while 2 patients with essential tremor were implanted in the ventral intermediate nucleus (VIM) of the thalamus. In anesthetized patients, SEPs were recorded at rest and during a passive movement of the thumb of the stimulated wrist from the intracranial electrode contacts and from the scalp. Also the high-frequency oscillations (HFOs) were analyzed. RESULTS: Amplitudes of both deep and scalp components were decreased during passive movement, but the reduction was higher at cortical than subcortical level. Also the HFOs were reduced by movement. CONCLUSION: The different amount of the movement-related decrease suggests that the cortical SEP gating is not only the result of a subcortical somatosensory volley attenuation, but a further mechanism acting at cortical level should be considered. SIGNIFICANCE: Our results are important for understanding the physiological mechanism of the sensory-motor interaction during passive movement.

Role of the red nucleus in suppressing the jaw-opening reflex following stimulation of the raphe magnus nucleus.

In a previous study, we found that electrical and chemical stimulation of the red nucleus (RN) suppressed the high-threshold afferent-evoked jaw-opening reflex (JOR). It has been reported that the RN receives bilaterally projection fibers from the raphe magnus nucleus (RMg), and that stimulation of the RMg inhibits the tooth pulp-evoked nociceptive JOR. These facts imply that RMg-induced inhibition of the JOR could be mediated via the RN. The present study first examines whether stimulation of the RMg suppresses the high-threshold afferent-evoked JOR. The JOR was evoked by electrical stimulation of the inferior alveolar nerve (IAN), and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was 4.0 (high-threshold) times the threshold. Conditioning electrical stimulation of the RMg significantly suppressed the JOR. A further study then examined whether electrically induced lesions of the RN or microinjection of muscimol into the RN affects RMg-induced suppression of the JOR. Electrically induced lesions of the bilateral RN and microinjection of muscimol into the bilateral RN both reduced the RMg-induced suppression of the JOR. These results suggest that RMg-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the RN.