Is spinal cord dorsolateral funiculus involved in hypoalgesia induced by counter-irritation?

Developments in Understanding Diffuse Noxious Inhibitory Controls: Pharmacological Evidence from Pre-Clinical Research

Bulbospinal pathways regulate nociceptive processing, and inhibitory modulation of nociception can be achieved via the activity of diffuse noxious inhibitory controls (DNIC), a unique descending pathway activated upon application of a conditioning stimulus (CS). Numerous studies have investigated the effects of varied pharmacological systems on the expression status of a) DNIC (as measured in anaesthetised animals) and b) the descending control of nociception (DCN), a surrogate measure of DNIC-like effects in conscious animals. However, the complexity of the underlying circuitry that governs initiation of a top-down inhibitory response in reaction to a CS, coupled with the methodological limitations associated with using pharmacological tools for its study, has often obscured the exact role(s) of a given drug. In this literature review, we discuss the pharmacological manipulation interrogation strategies that have hitherto been used to examine the functionality of DNIC and DCN. Discreet administration of a substance in the spinal cord or brain is considered in the context of action on one of four hypothetical systems that underlie the functionality of DNIC/DCN, where interpreting the outcome is often complicated by overlapping qualities. Systemic pharmacological modulation of DNIC/DCN is also discussed despite the fact that the precise location of drug action(s) cannot be pinpointed. Chiefly, modulation of the noradrenergic, serotonergic and opioidergic transmission systems impacts DNIC/DCN in a manner that relates to drug class, route of administration and health/disease state implicated. The advent of increasingly sophisticated interrogation tools will expedite our full understanding of the circuitries that modulate naturally occurring pain-inhibiting pathways.

The onset of diffuse noxious inhibitory controls in postnatal rat pups: a C-Fos study

The development of diffuse noxious inhibitory controls (DNIC) was studied in postnatal rats aged 12, 21 and 42-days-old, using immunoreactive localization of the c-fos protein products induced in the dorsal horn by noxious stimulation. The presence of DNIC was revealed by the reduction in the levels of Fos-like immunoreactivity that are normally induced by a standardized primary pinch stimulus when this stimulus was accompanied by a concurrent noxious stimulus (formalin) to a heterotopic body part. Significant reductions were seen at postnatal day 42 (P42; 15% reduction) and P21 (17% reduction), but concurrent stimulation had no significant effect at P12. These results suggest that the system subserving DNIC is functionally mature by P21, but not effective at P12. This delayed maturation of an inhibitory system may underlie the extreme sensitivity to somatosensory stimulation seen in neonatal pups and premature infants.

Inhibition of a cutaneous nociceptive reflex by a noxious visceral stimulus is mediated by spinal cholinergic and descending serotonergic systems in the rat

The present study examined the spinal pathway and receptors that mediate nocigenic inhibition of the tail-flick (TF) reflex produced by conditioning colorectal distension (CRD). Conditioning CRD (80 mmHg; 30 s) inhibited the TF reflex in all rats studied (n = 29). In 19 rats where intensity-dependent effects of CRD were studied, conditioning CRD in 7 rats facilitated the TF reflex at lesser, non-noxious intensities (mean 7.9 +/- 2.1 mmHg) and inhibited the TF reflex at greater, noxious intensities (40-100 mmHg); conditioning CRD at all intensities tested only inhibited the TF reflex in the other 12 rats. Inhibition of the TF reflex produced by 30 s CRD was short-lasting, repeatable and graded with the intensity of CRD. The mean threshold of CRD for inhibition of the TF reflex to cut off (10 s) was 61.4 +/- 3.3 mmHg (n = 29). Intrathecal pretreatment with atropine or methysergide significantly attenuated the inhibitory effect of CRD on the TF reflex; the effects were time- and dose-related. Intrathecal pretreatment with mecamylamine, phentolamine or naloxone was without effect. Intrathecal administration of physostigmine, an acetylcholinesterase inhibitor, significantly reduced the threshold intensity of conditioning CRD necessary to inhibit the TF reflex to cut off (mean 36.0 +/- 4.0 mmHg; n = 5). Bilateral transections of the spinal dorsolateral funiculi (DLF) did not affect the inhibitory effect of CRD in 4/7 rats and attenuated the inhibitory effect of CRD in the other 3 rats. The antagonistic effect of methysergide on CRD-produced inhibition of the TF reflex was abolished following the DLF transections, while scopolamine retained its efficacy in rats with bilateral DLF transections. These findings provide evidence for involvement of spinal cholinergic interneurons as well as a descending serotoninergic pathway traveling in the DLF in CRD-produced inhibition of the TF reflex.