Characterization of kappa opioid receptors in the rabbit ear artery.

Quantitative characterization of the kappa opioid receptor in the rabbit ear artery was carried out using three kappa-selective agonist compounds, dynorphin-(1-13), U-69593 and ethylketocyclazocine. Kinetic analysis was performed using the antagonist, MR 2266. Two other in vitro preparations were studied for comparison: the mouse was deferens and rabbit was deferens. To avoid mu receptor action in the mouse was deferens the irreversible mu receptor antagonist, beta-funaltrexamine, was used. It was demonstrated that, using the highly selective kappa agonist compound U-69593, Ke values for MR 2266 obtained in the three assay systems were not significantly different. These results suggest that kappa receptors present in these three tissues share identical properties.

Characterization of rabbit ear artery opioid receptors using a delta-selective agonist and antagonist.

In order to characterize pre-junctional delta opioid receptors in the rabbit ear artery, the delta-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE), and delta-selective antagonist, ICI 174,864, were used. The selectivity of these compounds for the delta receptor was confirmed using a standard opioid sensitive preparation, the mouse was deferens. Relative potencies of a series of opioid agonists were similar in the rabbit ear artery and the mouse vas deferens. Furthermore, Ke values for IC 174,864 were not different in the two tissues. These findings demonstrate that delta opioid receptors in the rabbit ear artery and mouse vas deferens have similar properties.

Delta and kappa opioid receptors in the rabbit ear artery.

Delta and kappa opioid receptors have been characterized in the central ear artery of the rabbit. The delta selective D-Pen2,D-Pen5-enkephalin and the highly selective delta antagonist ICI 174864 have been used for the delta opioid receptors. Ke values were determined and compared to that of the electrically stimulated mouse vas deferens preparation. In case of kappa opioid receptors, Dynorphin (1-13) and Ethylketazocine were used as agonists and Mr 2266 as kappa antagonist. Because of the relatively low selectivity of the kappa antagonist (it is only twice as active at the kappa than at the mu receptor sites) the Ke values obtained in the rabbit ear artery were compared to both the normal and beta-funaltrexamine treated vasa deferentia. In the rabbit ear artery, the Ke values of Mr 2266 were also determined in the presence of the delta selective antagonist ICI 174864. The Ke values for the delta receptors were close in the two assay systems, suggesting that the delta receptors have similar properties in rabbit ear artery and mouse vas deferens. As for the kappa receptors, the Ke values in the rabbit ear artery were not significantly different in the presence or absence of the delta antagonist compound, nor were they different from the Ke values obtained on the normal mouse vas deferens. However, they were significantly different from that of the beta-funaltrexamine treated mouse vas deferens. Further experiments need to be done to interpret the meaning of these results.

The effect of GYKI-32 887 on dopamine (D2) and serotonin (5-HT1 and 5-HT2) receptors.

Investigations were carried out with a dopamine agonist compound GYKI-32 887 to compare its binding capacity towards D2, 5-HT1 and 5-HT2 receptors. Synaptosomal membranes were prepared from corpus striatum, hippocampus and frontal cortex of rats. The tritiated ligands used were: 3H-spiperone for D2 and 5-HT2 receptors and 3H-5-HT for 5-HT1 receptors. Comparing the results obtained, IC50 and Ki values, one can conclude that GYKI-32 887 has higher affinity towards D2 receptors than serotonin ones and shows better selectivity than bromocriptine, the reference substance.

The effect of neonatal treatment of mice with opioid and dopaminergic agents on the late responsiveness of vasa deferentia to opioids in vitro.

Neonatal treatment of mice with opioid and dopamine antagonists (naloxone, haloperidol and sulpiride) failed to alter the in vitro responsiveness of vasa deferentia to opioid agonists in the adulthood. Single neonatal administration of some opioid or dopaminergic agonists, viz. Met-enkephalin and piribedil, tended to enhance the sensitivity of in vitro preparations to opioid agonists, tested in adult animals. Behavioural differences and late mortality were also observed.

Opioid effects in developing rat vas deferens.

The inhibitory effects of various opiates on developing rat vas deferens were studied by determining the degree of depression of mechanical responses elicited by electrical field stimulation. All agonists showed decreased effects with maturation, but the decrease occurred at different times. With normorphine the loss of agonist activity was greatest at days 12-16, while with D-Met2,Pro5-enkephalinamide it was greatest at days 16-30. beta-Endorphin also was less effective in adult than 30-day preparations, but methionine enkephalin was ineffective at all ages. Morphine and normorphine were weak antagonists of opiate agonists in the adult preparations. These results indicate that the nature and pharmacologic sensitivity of opiate actions change with development.

Enkephalin-like character and analgesia.

The opioid activities of enkephalin analogues bearing D- or L-aminopentane-sulfonic/phosphonic acid at position 5 were studied in vitro, in electrically stimulated longitudinal muscle strip of guinea-pig ileum and mouse vas deferens preparations and in vivo in the rat tail-flick test. Using their in vitro effects Met-enkephalin-like, beta-endorphin-like, (nor)morphine-like and derivatives of intermediate character could be differentiated. Correlating the in vitro activities with the analgesic activity in vivo it is concluded that the enkephalin-like character in a pentapetide may hinder the expression of analgesic activity, when the compounds are given into the cerebroventricular system.