Julius S. Heyman

Opioid δ-receptor involvement in supraspinal and spinal antinociception in mice

The possibility that the opioid r morphine) and b ([D-Pen2,D-PenS]enkephalin, DPDPE)-agonists in the absence, and in the presence of the/~ non-surmountable antagonist, fl-funaltrexamine (fl-FNA) or the b-antagonist ICI 174,864 (N,N-diallyl-Tyr-Aib-Aib-Phe-LeuOH, where Aib is a-amino-isobutyric acid), Agonists and 1CI 174,864 were given alone or in the same intracerebroventricular (i.c.v.) or intrathecal (i.th.) injection to mice 20 min prior to testing in the warm-water (55 °C) tail-withdrawal test (+10 min for i.th. DPDPE); fl-FNA was given as a single i.c.v, or i.th. pretreatment dose (20 and 0.01 nM, respectively) 4 h prior to testing. I.c.v. pretreatment with fl-FNA resulted in a rightward displacement of the DAGO and morphine antinociceptive dose-response lines, but failed to displace the i.c.v. DPDPE curve. Similarly, i.th. pretreatment with/3-FNA displaced the i.th. morphine dose-response curve to the right without affecting the i.th. DPDPE antinociceptive dose-response line. ICI 174,864 (1 and 3 #g) produced a dose-related antagonism of i.c.v, or i.th. DPDPE, but did not alter the antinociceptive effects of DAGO or morphine given by the same routes. Co-administration of ICI 174,864 (3/~g) with i.c.v, morphine in fl-FNA pretreated (but not control) mice resulted in a further rightward displacement of the morphine dose-response line. The effective antagonism of DPDPE but not morphine or DAGO antinociception by ICI 174,864, together with the effectiveness of fl-FNA against morphine and DAGO but not DPDPE antinociception, provide strong and direct evidence for the involvement of cerebral and spinal b-receptors in the mediation of antinociception in tests where heat is employed as the noxious stimulus. Additionally, the effectiveness of ICI 174,864 against morphine in/3-FNA pretreated (but not control) mice demonstrates a b-effect of morphine, in vivo.

Modulation of μ-mediated antinociception by δ agonists in the mouse: selective potentiation of morphine and normorphine by [D-Pen2, D-Pen5]enkephalin

The effect of the delta-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) on the antinociception produced by intracerebroventricular (i.c.v.) administration of the mu agonists morphine, [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO), [NMePhe3,D-Pro4]morphiceptin (PLO17), beta-endorphin, phenazocine, etorphine and sufentanil was studied in mice. Only the antinociceptive effects of morphine and normorphine were modulated by i.c.v. coadministration of a dose of DPDPE which did not produce any significant antinociception alone. Both the morphine and normorphine dose-response lines were displaced to the left in the presence of DPDPE. The delta-selective antagonist ICI174,864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH) (where Aib is alpha-aminoisobutyric acid) blocked the modulation of morphine antinociception by DPDPE. ICI 174,864 alone failed to produce either a significant increase or decrease of morphine, phenazocine, etorphine or beta-endorphin antinociception. The results of the present study provide support for the hypothesis that the enkephalins may function to modulate antinociception produced at the mu receptor; such modulation may come about via the existence of an opioid mu-delta receptor complex. The mu receptors existing in such a complex may be selectively activated by morphine and normorphine, but not the other mu agonists studied here. Thus, the enkephalins may function both to directly initiate, as well as to modulate, some forms of supraspinal mu receptor-mediated antinociception.

Can supraspinal δ-opioid receptors mediate antinociception?

Abstract The antinociception mediated by supraspinal opioid receptors has traditionally been thought to arise solely from activation of the μ-opioid receptor subtype. There is a growing body of evidence, however, which suggests that supraspinal δ-opioid receptors may also be able to directly initiate antinociception. Frank Porreca and colleagues review the evidence favoring exclusive involvement of μ-opioid receptors (as well as studies implicating δ-opioid receptors) in supraspinal antinociceptors. They conclude that δ-opioid receptors can be pharmacologically activated to initiate antinociception, suggesting the existence of an alternative mechanism for pain relief which may be of significant clinical importance.

Estimation of the affinity of naloxone at supraspinal and spinal opioid receptors in vivo: Studies with receptor selective agonists

The apparent affinity of naloxone at cerebral and spinal sites was estimated using selective mu [D-Ala2, Gly-o15]-enkephalin (DAGO) and delta [D-Pen2, D-Pen5]enkephalin] (DPDPE) opioid agonists in the mouse warm water tail-withdrawal test in vivo; the mu agonist morphine was employed as a reference compound. The approach was to determine the naloxone pA2 using a time-dependent method with both agonist and antagonist given intracerebroventricularly (i.c.v.) or intrathecally (i.th.); naloxone was always given 5 min before the agonist. Complete time-response curves were determined for each agonist at each site in the absence, and in the presence, of a single, fixed i.c.v. or i.th. dose of naloxone. From these i.c.v. or i.th. pairs of time-response curves, pairs of dose-response lines were constructed at various times; these lines showed decreasing displacement with time, indicative of the disappearance of naloxone. The graph of log (dose ratio-1) vs. time was linear with negative slope, in agreement with the time-dependent form of the equation for competitive antagonism. From this plot, the apparent pA2 and naloxone half-life was calculated at each site and against each agonist. The affinity of naloxone was not significantly different when compared between agonists after i.c.v. administration. A small difference was seen between the affinity of i.th. naloxone against DPDPE and DAGO; the i.th. naloxone pA2 against morphine, however, was not different than that for DPDPE and DAGO. The naloxone half-life varied between 6.6 and 16.9 min, values close to those previously reported for this compound. These results suggest that the agonists studied may produce their i.c.v. analgesic effects at the same receptor type or that alternatively, the naloxone pA2 may be fortuitously similar for mu and delta receptors in vivo. Additionally, while the affinity of naloxone appears different for the receptors activated by i.th. DAGO and DPDPE, further work may be necessary before firm conclusions regarding the nature of the spinal analgesic receptor(s) can be drawn.

Intrathecal FMRFamide (Phe-Met-Arg-Phe-NH2) induces excessive grooming behavior in mice

The molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) was administered intrathecally (i.t.) to mice and their behavior was monitored for 30 min. FMRFamide induced a dramatic and dose-related (5-12 micrograms) increase in grooming-related activities compared to saline-treated controls. The grooming behavior produced by 8 micrograms FMRFamide was not blocked by simultaneous i.t. administration of 10 micrograms of the following antagonists: atropine, phentolamine, methysergide, naloxone or spantide; peripheral administration of naloxone (3.5 mg/kg, s.c.) also failed to antagonize FMRFamide grooming. These data constitute the first report that FMRFamide produces behavioral changes in mammals.