Ascending inhibition of nociceptive neurons in the nucleus ventralis posterolateralis following conditioning stimulation of the nucleus raphe magnus

Changes in electrophysiological properties and sodium channel Nav1.3 expression in thalamic neurons after spinal cord injury

Spinal cord contusion injury (SCI) is known to induce pain-related behaviour, as well as hyperresponsiveness in lumbar dorsal horn nociceptive neurons associated with the aberrant expression of Na(v)1.3, a rapidly repriming voltage-gated sodium channel. Many of these second-order dorsal horn neurons project to third-order neurons in the ventrobasal complex of the thalamus. In this study we hypothesized that, following SCI, neurons in the thalamus undergo electrophysiological changes linked to aberrant expression of Na(v)1.3. Adult male Sprague-Dawley rats underwent contusion SCI at the T9 thoracic level. Four weeks post-SCI, Na(v)1.3 protein was upregulated within thalamic neurons in ventroposterior lateral (VPL) and ventroposterior medial nuclei, where extracellular unit recordings revealed increased spontaneous discharge, afterdischarge, hyperresponsiveness to innocuous and noxious peripheral stimuli, and expansion of peripheral receptive fields. Altered electrophysiological properties of VPL neurons persisted after interruption of ascending spinal barrage by spinal cord transection above the level of the injury. Lumbar intrathecal administration of specific antisense oligodeoxynucleotides generated against Na(v)1.3 caused a significant reduction in Na(v)1.3 expression in thalamic neurons and reversed electrophysiological alterations. These results show, for the first time, a change in sodium channel expression within neurons in the thalamus after injury to the spinal cord, and suggest that these changes contribute to altered processing of somatosensory information after SCI.

The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a noxious visceral stimulus (colorectal balloon distension; CRD) and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity. Sixty-four (59%) responded to CRD, of which 52 (81 %) had excitatory and 12 (19%) had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralateral or bilateral. Ninety-four percent of these neurons responded to both noxious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/ kg, i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are consistent with a role of VPL in sensory-discriminative aspects of nociception.

Comparison of antinociception induced by supraspinally administered L-arginine and kyotorphin

1. Intracerebroventricular (i.c.v.) or intracisternal (i.cist.) administration of kyotorphin (KTP), an endogenous Met-enkephalin releaser, at 5 micrograms per mouse, and L-arginine (L-Arg), a possible KTP precursor, at 30 micrograms per mouse, elicited antinociception in mice to a similar extent, as assessed by the tail-flick test. 2. Intracisternal preadministration of anti-KTP serum abolished the effect of i.cist. KTP and i.c.v. or i.cist. L-Arg, but not of i.c.v. KTP. 3. The antinociceptive effects of i.cist. KTP and of i.c.v. or i.cist. L-Arg disappeared in reserpinized mice, whereas the effect of i.c.v. KTP was unaffected by treatment of mice with reserpine. 4. Intrathecal (i.t.) phentolamine markedly reduced the antinociception induced by i.cist. KTP and by i.c.v. or i.cist. L-Arg, but not by i.c.v. KTP. 5. Intrathecal methysergide attenuated the antinociceptive effects of i.cist. KTP, but not of i.c.v. KTP and i.c.v. or i.cist. L-Arg. 6. These results suggest that the antinociception produced by i.cist. KTP, but not by i.c.v. KTP, is mediated by the brainstem-spinal noradrenergic and 5-hydroxytryptaminergic systems, and that L-Arg given i.c.v. or i.cist. increases KTP formation in the lower brain, possibly the brainstem, resulting in antinociception mediated by the descending noradrenergic system. Therefore, the regional distribution of KTP receptors and KTP synthetase in the brain does not appear to be common.

The dorsal raphe: an important nucleus in pain modulation

The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. It has abundant 5-HT neurons and many other neurotransmitter and/or neuromodulator containing neurons. Its vast fiber connections to other parts of the central nervous system provide a morphological basis for its pain modulating function. Its descending projections, via the nucleus raphe magnus or directly, modulate the responses caused by noxious stimulation of the spinal dorsal horn neurons. In ascending projections, it directly modulates the responses of pain sensitive neurons in the thalamus. It can also be involved in analgesia effects induced by the arcuate nucleus of the hypothalamus. Neurophysiologic and neuropharmacologic results suggest that 5-HT neurons and ENKergic neurons in the DRN are pain inhibitory, and GABA neurons are the opposite. The studies of the intrinsic synapses between ENKergic neurons, GABAergic neurons, and 5-HT neurons within the DRN throw light on their relations in pain modulation functions, and further explain their functions in pain mediation.