Recent developments in the bioassay of opioids

Effect of mu-, kappa-, and delta-selective opioid agonists on thermoregulation in the rat

The effect of selective mu-, kappa-, and delta-agonists on brain surface temperature (Tb), oxygen consumption (Vo2), and heat exchange (Q) was studied in unrestrained, male Sprague-Dawley rats using whole-body calorimetry. Hyperthermia, produced by PL-017 (1.86 nM) given ICV, resulted from increased Vo2 and reduced Q during the first 15-45 min postinjection. Tb returned to control levels due to a combination of increased Q and reduced Vo2. PL-017-induced hyperthermia was abolished by the mu-selective antagonist CTAP (0.75 nM). Dynorphin A1-17 (4.65 nM), a kappa-selective agonist, reduced both Vo2 and Q, resulting in hypothermia that was blocked by the kappa-selective antagonist nor-binaltorphimine (25 nM). The delta-selective agonist DPDPE (4.64 nM) caused no significant changes in Tb, Vo2, or Q. The data indicate that central stimulation of the mu- and kappa-opioid receptors affects both oxidative metabolism and heat exchange, which result in a change in Tb. These alterations can be prevented with selective opioid antagonist pretreatment.

The inhibitory effect of somatostatin peptides on the rat anococcygeus muscle in vitro

1. Electrically evoked contractions of the rat anococcygeus muscle were inhibited in a concentration-dependent manner by somatostatin-14 (SS14), -28 (SS28) and two synthetic hexapeptide analogues: L-363,301 (Pro-Phe-D-Trp-Lys-Thr-Phe) and L-363,586 (N-Me-Ala-Tyr-D-Trp-Lys-Val-Phe), with pIC50 values of 7.41, 7.38, 7.07 and 8.34, respectively. 2. The inhibitory effects of SS14 were dependent on stimulation frequency and external calcium ion concentration. Calcium behaved as a non-competitive antagonist of SS14, it reduced the maximal inhibitory effect of the peptide and at a concentration of 5.08 mM it significantly affected the pIC50 value. 3. SS14 (3 x 10(-7) M) did not affect the tonic actions of bath-applied noradrenaline in the absence of field stimulation. 4. The effects of SS14 persisted in naloxone (10(-5) M) and were, therefore, not due to an action at opiate receptors. Furthermore, experiments involving the lyophilization of bath contents, showed no evidence to support an indirect mechanism involving the release of an endogenous inhibitory substance. 5. High concentrations (10(-5) M) of SS14 or L-363,301 inhibited the relaxation response evoked by electrical stimulation of guanethidine (3 x 10(-4) M)-treated preparations. 6. These results are consistent with similar actions of SS14 on other smooth muscle preparations and are presumed to reflect a presynaptic inhibition of transmitter release by a direct action on somatostatin receptors. The antagonistic effect of calcium on this response is discussed with reference to a possible role in receptor desensitization.

Opioid receptor binding profile of selected dermorphin-like peptides

The receptor binding profile of a selected group of dermorphin-like peptides was determined and correlated with the results of the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays and with the currently used antinociception tests in the rat. For the peptides with the characteristic dermorphin D-Ala2-Phe3-Gly4 sequence, a linear negative correlation was found between the reciprocal of sodium shift and relative affinity for the mu-type opioid receptor. For the same peptides, a positive correlation was evidenced between relative potency on GPI and MVD and relative affinity for mu- and delta-type receptors, respectively.