The Ontogeny of Delta and Kappa Opioid Receptors in The Rat.

Saturation binding studies with the κ1-selective agonists [3H]U-69,593 and [3H]CI-977 have been used to quantify κ1-receptor development in the neonatal rat brain. Full development of κ1-receptors has been demonstrated at post-natal day 1 and characterised as κ1 by competition studies using the selective κ1-agonist U-69,593, the selective μ-agonist DAGO, the selective δ-agonist DPDPE and the opioid antagonist naloxone v. [3H]CI-977. Functional κ1-mediated antinociception against thermal and mechanical noxious stimuli and naloxone reversal has been demonstrated using the κ1-selective agonists PD117,302, U-69,593 and CI-977 in 5 and 10 day-old rat pups. The density of the δ-opioid receptor in rat brain homogenates was quantified from adulthood down to as early an age as possible by means of saturation binding studies using the δ-receptor-selective antagonist [3H]naltrindole and the δ-receptor-selective agonist [3H]Ile5,6Deltorphin II and by homologous displacement studies using the d-receptor-selective agonist Deltorphin I. δ-binding was measurable at post-natal day 3 and steadily increased to a level not significantly different from adult levels by day 25. Comparison of δ-receptor densities in the brain and spinal cord of the 25-day-old weaned and non-weaned rat showed no difference when measured with [3H]naltrindole or [3H]Ile5,6Deltorphin II. Using [3H]Deltorphin I, the δ-receptor density in the weaned animals was approximately 25% higher than in the non-weaned animals in both tissues. Saturation binding studies in the presence of 100mM Na+, 50μM GMPPNP or both, conditions designed to investigate the possible influence of the G-protein coupled state of the receptors on the ligand binding, were carried out with all three ligands. The only significant difference observed was a 32% decrease in the binding of [3H]Deltorphin I in the weaned animals in the presence of 50μM GMPPNP which did not occur in the non-weaned animals, suggesting that the weaned/non-weaned difference in δ-receptor density seen by [3H]Deltorphin I may be a consequence of different or incomplete G-protein-coupling prior to weaning