Relationship of yeast-injection induced changes in brain met-enkephalin and analgesia

Subplantar yeast injection induces a non-naloxone reversible antinociception in spontaneously hypertensive rats

Spontaneously hypertensive (SH), Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats were tested for their responsiveness to noxious mechanical pressure before and after a subplantar yeast injection to the right rear paw. Prior to the yeast injection, hypertensive animals were less responsive to pain relative to normotensive animals, as seen in the significantly greater pre-yeast latencies of SH compared to WKY or SD rats. Subplantar yeast injection produced hyperreactivity in the inflamed paws of WKY or SD rats, with no effect on the contralateral non-injected paw. However, identical subplantar yeast injections to hypertensive animals produced a robust, long-lasting antinociceptive effect in both rear paws of SH rats. This effect was not reversed by naloxone (opiate antagonist), labetalol (beta-blocker/vasodilator antihypertensive), or hydralazine (peripheral vasodilator antihypertensive); the antinociception was not potentiated by thiorphan (enkephalinase inhibitor). However, the alpha 2-receptor antagonist yohimbine (0.1-5 mg/kg s.c.), produced a dose-related reversal of the yeast-induced antinociception in SH rats. These results suggest that the subplantar yeast injection is triggering descending noradrenergic pain inhibitory pathways in SH rats.

Local inhibition of inflammatory pain by naloxone and its N-methyl quaternary analogue

Inflammation was induced in rats by intraplantar administration of carrageenan (500 micrograms in 0.1 ml). Nociceptive thresholds were measured on both inflamed and contralateral hindpaws with the pressure test of Randal and Sellito. Low doses (0.03-10 micrograms/kg) of naloxone hydrochloride (Nx) or of methylnaloxone methylsulfonate (MeNx) were injected 4 h after carrageenan in the inflamed paw: Nx (3 micrograms) and MeNx (1-10 micrograms) diminished inflammatory pain; a slight, nonspecific hyperalgesia was observed in the contralateral paw after Nx, MeNx or NaCl. Neither drug was effective when injected s.c. at the same doses and time; however activities were shown to be influenced by the experimental schedule. Low s.c. doses of Nx antagonized the analgesia produced by s.c. morphine whereas MeNx did not. This work demonstrates the local site of the analgesic action of the doses of Nx and MeNx used here, bringing new evidence in favour of the existence of cutaneous opioid receptors. Opiate antagonists might inhibit inflammatory pain by interacting with a particular population of cutaneous receptors and (or) by being dealkylated locally into agonists.

Determination of analgesic drug efficacies by modification of the Randall and Selitto rat yeast paw test

This report describes a modified Randall and Selitto (1957) rat yeast paw test that can evaluate differences in efficacy of different analgesics. The modifications consist of a decrease in the rate of acceleration of the noxious stimulus (mechanical pressure) on the inflamed paw from 20 to 12.5 mmHg/sec and an extension of the cut-off time from 15 to 60 sec. All the narcoticlike drugs tested (morphine, codeine, and pentazocine) increased the response latencies of the inflamed paws to the cut-off time. The nonsteroidal antiinflammatory-like drugs tested (acetylsalicylic acid, acetaminophen, indomethacin, phenylbutazone, and proquazone) showed plateaus in their analgesic effects (i.e., increasing the dose failed to produce significantly greater increases in the response latencies compared to the next lower dose). Zomepirac (80-240 mg/kg p.o.) did not show this plateau effect, but was unable to increase response latencies to greater than 30 sec because of the toxicity of higher doses (320 mg/kg p.o.). Flunixin NMG (the meglumine salt of flunixin), a nonnarcotic analgesic, did not display a plateau effect and increased response latencies to maximum values. The methodology was therefore able to discriminate analgesics active against mild to severe clinical pain (narcoticlike) from those only useful against mild to moderate pain (nonnarcotic-like).