The binding of [³H]opioid ligands to homogenates prepared from the
spinal cords of rat and other species has been studied. Similar numbers
of sites were seen in all areas of the cord when measured in a rostrocaudal
direction. There was found to be approximately 2 x higher density
of sites in the dorsal half of the cord compared with the ventral half.
Binding studies suggested a similar relative distribution of mu, delta
and kappa sites in all areas of the cord. The results are discussed in
relation to the reported distribution of opioid peptides.
In the above study the kappa binding site was defined as the binding of
[³H] unselective opioids in the presence of cold ligands to suppress
binding to mu- and delta-sites. Competitive binding assays, however,
suggested this site did not have the properties of a single homogeneous
group. Approximately 50% of the apparent kappa binding was consistent
with a classical kappa site. Saturated binding assays afforded Bmax values
which suggested lower 'true' kappa site numbers than previously supposed,
values which were confirmed using the kappa peptide' [³H]Dynorphin A-(1-9),
and the kappa selective [³H]U-69593. Heterogeneity was also seen in other
central nervous system tissues.
The heterogeneous nature of the kappa site may be due to different sites,
due to interactions at a non-opioid site or may represent different
conformations of the same site. The second possibility was discounted since
observed binding followed the cellular distribution of the plasma marker
Na+/K+-ATPase was stereoselective for levorphanol over dextrorphan, and
fully displaceable by naloxone. The third possibility was investigated by
studying the role of Na+ and MG2+ ions, which are reported to affect
receptor conformation in binding assays employing brain tissues. None of
the results obtained suggested that conformational changes were responsible
for the observed effects, although the experiments were not exhaustive.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University. Appendices have been removed for copyright reasons.