Periaqueductal gray stimulation produces a spinally mediated, opioid antinociception for the inflamed hindpaw of the rat

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.

Autotomy in rats following peripheral nerve transection is attenuated by preceding formalin injections into the same limb

The influence of an evolving painful inflammatory lesion on the development of autotomy, a behavioural model of denervation pain, was studied in rats suffering sciatic and saphenous nerves transection 30 or 60 min, and 1, 3, 7 or 14 days after being injected with formalin (50 microl, 5%, s.c). Hindpaws pressure and heat nociceptive thresholds and volume of the injected paw were assessed, in non-operated rats, at the above time-points. The main effects on autotomy were: (1) a significant attenuation when formalin injection preceded the neurectomies by 1 day or more, a period characterized by hypalgesia of the injected paw to both mechanical (during the first week) and thermal (spanning up to the third day after formalin) stimuli and inflammation (lasting for 14 days); (2) a significantly earlier onset when formalin was injected 30 min before neurectomies. Possible mechanisms linking nociceptive responsiveness and inflammation to the development of autotomy are discussed.

Endogenous opioid peptides acting at mu-opioid receptors in the dorsal horn contribute to midbrain modulation of spinal nociceptive neurons

Activation of neurons in the midbrain periaqueductal gray (PAG) inhibits spinal dorsal horn neurons and produces behavioral antinociception in animals and analgesia in humans. Although dorsal horn regions modulated by PAG activation contain all three opioid receptor classes (mu, delta, and kappa), as well as enkephalinergic interneurons and terminal fields, descending opioid-mediated inhibition of dorsal horn neurons has not been demonstrated. We examined the contribution of dorsal horn mu-opioid receptors to the PAG-elicited descending modulation of nociceptive transmission. Single-unit extracellular recordings were made from rat sacral dorsal horn neurons activated by noxious heating of the tail. Microinjections of bicuculline (BIC) in the ventrolateral PAG led to a 60-80% decrease in the neuronal responses to heat. At the same time, the responses of the same neurons to iontophoretically applied NMDA or kainic acid were not consistently inhibited. The inhibition of heat-evoked responses by PAG BIC was reversed by iontophoretic application of the selective mu-opioid receptor antagonists, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). A similar effect was produced by naloxone; however, naloxone had an excitatory influence on dorsal horn neurons in the absence of PAG-evoked descending inhibition. This is the first demonstration that endogenous opioids acting via spinal mu-opioid receptors contribute to brain stem control of nociceptive spinal dorsal horn neurons. The inhibition appears to result in part from presynaptic inhibition of afferents to dorsal horn neurons.

Inflammation-induced Fos protein expression in the rat spinal cord is enhanced following dorsolateral or ventrolateral funiculus lesions

Previous studies have shown an enhanced expression of Fos protein-like immunoreactivity in the lumbar spinal cord of rats with complete spinal transection following persistent hindpaw inflammation. To further locate the spinal pathways responsible for these effects, we compared the inflammation-evoked Fos expression in rats with bilateral lesions of the dorsolateral (DLFX) or ventrolateral (VLFX) funiculus, and with rats with a sham operation. The results indicate that the number of Fos-labeled neurons was significantly increased in all laminae of the dorsal horn ipsilateral to the inflamed hindpaw and in contralateral deep dorsal horn in both DLFX and VLFX rats compared to sham-operated rats. Moreover, when comparing DLFX and VLFX rats, in the ipsilateral spinal cord, DLFX resulted in more Fos expression in the deep dorsal horn; in contrast, a larger number of Fos-labeled cells in superficial laminae was observed in VLFX rats. These results suggest that modulatory systems, which descend in both DLF and VLF pathways, mediate the enhanced net descending nociceptive inhibition after persistent inflammation, although the supraspinal sites of origin of each pathway are likely functionally diverse.

Immunoelectron microscopy of enkephalinergic innervation of GABAergic neurons in the periaqueductal gray

The pre-embedding double immunoreaction method was used to study synaptic relations of enkephalinergic and GABAergic neuronal elements in the ventrolateral part of the periaqueductal gray of the Wistar albino rat. The enkephalin-like neuronal elements were immunoreacted by the silver-gold intensified peroxidase-antiperoxidase method and the GABA-like immunoreactive neurons were immunoreacted by the unintensified peroxidase-antiperoxidase method. GABA-like immunoreactive neuronal somata were post-synaptic to both the enkephalin-like immunoreactive and the non-immunoreactive axon terminals. Enkephalin-like immunoreactive axon terminals were found to make synapses with GABA-like immunoreactive and non-immunoreactive dendrites. The synapses between the two kinds of chemically characterized neurons appeared to be both asymmetrical and symmetrical. Possible functional activity related to pain modulation, and synaptic relations between the enkephalinergic and GABAergic neurons in the periaqueductal gray and the dorsal raphe nucleus, are discussed.