Intrathecal administration of thiorphan, bestatin, desipramine and fluoxetine differentially potentiate the antinociceptive effects induced by beta-endorphin and morphine, administered intracerebroventricularly

The effects of the intrathecal injection of thiorphan (an inhibitor of enkephalinase inhibitor), bestatin (an inhibitor of aminopeptidase), desipramine (an inhibitor of the uptake of noradrenaline) and fluoxetine (an inhibitor of the uptake of serotonin) on the antinociception induced by beta-endorphin and morphine, administered intracerebroventricularly, were studied in male ICR mice. Antinociceptive effects were assessed by the tail-flick and hot-plate tests. Thiorphan (16 micrograms) and bestatin (16 micrograms), injected intrathecally, potentiated inhibition of the tail-flick response, induced by beta-endorphin but not by morphine administered intracerebroventricularly, whereas desipramine (6 micrograms) and fluoxetine (6 micrograms), injected intrathecally potentiated inhibition of the tail-flick response induced by morphine, but not by beta-endorphin, administered intracerebroventricularly. Thiorphan, bestatin, desipramine or fluoxetine, given intrathecally, did not antagonize inhibition of the hot-plate response, induced by beta-endorphin or morphine administered intracerebroventricularly. The results indicate that inhibition of the tail-flick response, induced by beta-endorphin administered intracerebroventricularly, is mediated by the opioid system, but not by noradrenergic and serotonergic systems in the spinal cord. On the other hand, the inhibition of the tail-flick response, induced by morphine given intracerebroventricularly, is mediated by noradrenergic and serotonergic systems, but not by the opioid system in the spinal cord. The lack of effect of enzyme inhibitors and inhibitors of the uptake of biogenic amines intrathecally on beta-endorphin- and morphine-induced inhibition of the hot-plate response, is due to the supraspinal nature of the nociceptive hot-plate response. The present results further support the hypothesis, proposed previously, that intracerebroventricularly injected beta-endorphin and morphine elicit antinociception by activating different descending inhibitory systems.

Lack of antinociceptive cross-tolerance between intracerebroventricularly administered beta-endorphin and morphine or DPDPE in mice

Antinociceptive tolerance and cross-tolerance to intracerebroventricular (i.c.v.) beta-endorphin, morphine, and DPDPE (D-Pen2-D-Pen5-enkephalin) induced by a prior i.c.v. administration of beta-endorphin, morphine and DPDPE, respectively, were studied in mice. Acute tolerance was induced by i.c.v. pretreatment with beta-endorphin (0.58 nmol), morphine (6 nmol) and DPDPE (31 nmol) for 120, 180 and 75 min, respectively. Various doses of beta-endorphin, morphine or DPDPE were then injected. The tail-flick and hot-plate tests were used as antinociceptive tests. Pretreatment of mice with beta-endorphin i.c.v. reduced inhibition of the tail-flick and hot-plate responses to i.c.v. administered beta-endorphin, but not morphine and DPDPE. Pretreatment of mice with morphine i.c.v. reduced inhibition of the tail-flick and hot-plate responses to morphine but not beta-endorphin. Pretreatment of mice with DPDPE reduced inhibition of the tail-flick and hot-plate responses to DPDPE but not beta-endorphin. The results indicate that one injection of beta-endorphin, morphine or DPDPE induces acute antinociceptive tolerance to its own distinctive opioid receptor and does not induce cross-tolerance to other opioid agonists with different opioid receptor specificities. The data support the hypothesis that beta-endorphin, morphine and DPDPE produce antinociception by stimulating specific epsilon, mu- and delta-opioid receptors, respectively.

Intrathecal [Met5]enkephalin antibody blocks analgesia induced by intracerebroventricular beta-endorphin but not morphine in mice

Intrathecal (i.t.) injection of antibody directed to [Met5]enkephalin antagonized intracerebroventricularly (i.c.v.) administered beta-endorphin-induced inhibition of the tail-flick response but not the hot-plate response. [Met5]enkephalin antibody injected i.t., however, did not affect inhibition of either the tail-flick and hot-plate response induced by morphine given i.c.v. The antibodies to [Leu5]enkephalin, dynorphin A-(1-13) and beta-endorphin even at a high dose injected i.t. did not affect the inhibition induced by i.c.v. administered beta-endorphin or morphine either in the tail-flick and hot-plate test. Our results with antibodies support the results of previous studies that beta-endorphin but not morphine produces its analgesic action by selectively releasing [Met5]enkephalin.

Differential mechanisms mediating beta-endorphin- and morphine-induced analgesia in mice

Effects of yohimbine, methysergide and naloxone given intrathecally (i.t.) and naloxone given intracerebroventricularly (i.c.v.) on inhibition of the tail-flick and hot-plate response induced by beta-endorphin and morphine given i.c.v. were studied in male ICR mice. Yohimbine (1.5 and 15 micrograms) and methysergide (1.5 and 15 micrograms) injected i.t. antagonized inhibition of the tail-flick response induced by morphine but not beta-endorphin administered i.c.v. On the other hand, naloxone (20 ng) injected i.t. antagonized inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin but not morphine. Yohimbine and methysergide given i.t. did not antagonize inhibition of the hot-plate response induced by morphine nor did naloxone given i.t. antagonized i.c.v. beta-endorphin-induced inhibition of the hot-plate response. Naloxone given i.c.v. was more effective in antagonizing morphine-induced inhibition of the tail-flick and hot-plate response than inhibition induced by beta-endorphin given i.c.v. Naloxone at doses (0.1 and 1 microgram) which effectively reversed inhibition of the tail-flick response to i.c.v. morphine was not effective in reversing the i.c.v. beta-endorphin-induced inhibition of the tail-flick response. Our results indicate that beta-endorphin and morphine produce analgesia by stimulating separate types of opioid receptors, epsilon- for for beta-endorphin and mu- for morphine, and activate separate descending pain modulatory control systems. The supraspinal epsilon system stimulated by beta-endorphin is mediated by activation of spinal opioid receptors whereas the supraspinal mu system stimulated by morphine is mediated by activation of spinal alpha 2-adrenoceptors and serotonin receptors for the production of analgesia.

Intrathecal beta-funaltrexamine antagonizes intracerebroventricular beta-endorphin- but not morphine-induced analgesia in mice

We have reported previously that beta-endorphin and morphine administered supraspinally produce analgesia by activating different descending pain inhibitory systems in rats. The descending system activated by beta-endorphin involves a spinal endorphinergic system whereas the descending system activated by morphine does not. To determine if this differential action of intraventricular beta-endorphin and morphine also occurs in mice, the effects of pretreatment with intrathecal (i.t.) and i.c.v. beta-funaltrexamine (beta-FNA) on analgesic response induced by i.c.v. and i.t. beta-endorphin and morphine were studied in mice. beta-FNA (2.5 micrograms) was injected i.t. or i.c.v. 24 hr before beta-endorphin or morphine administration and hot-plate and tail-flick responses were measured. Intrathecal beta-FNA attenuated i.c.v. beta-endorphin- but not i.c.v. morphine-induced analgesia. On the other hand, i.t. beta-FNA blocked both i.t. beta-endorphin- and morphine-induced analgesia, but was more effective in blocking the effects of i.t. morphine than beta-endorphin. At the supraspinal sites, beta-FNA administered i.c.v. was found to antagonize i.c.v. morphine-induced analgesia but not i.c.v. beta-endorphin-induced analgesia. The present results in mice are consistent with previous studies in rats and indicate that beta-endorphin and morphine activate different supraspinal opioid receptors. Also, analgesia produced by these two opioids resulted from activation of different descending pain inhibitory systems. The spinal endorphinergic system was involved in the production of i.c.v. beta-endorphin-, but not morphine-induced analgesia.

Beta-endorphin-(1-27) antagonizes beta-endorphin-induced hypothermia in mice

The effects of beta-endorphin and beta-endorphin-(1-27) on the body temperature of mice was studied at an ambient temperature of 10 degrees C. Intracerebroventricular injection of beta-endorphin (0.25-4 micrograms) and beta-endorphin-(1-27) (0.61 to 10 micrograms) caused a dose-related hypothermia. The duration of hypothermia induced by beta-endorphin-(1-27) was shorter than that induced by beta-endorphin. The hypothermia induced by 2 micrograms of beta-endorphin was attenuated by 5 micrograms of beta-endorphin-(1-27). Our results indicated that beta-endorphin-(1-27) is a partial agonist which produces a small degree of hypothermia and an antagonist which blocks the beta-endorphin-induced hypothermia.