OPIOID RECEPTOR CONSTANTS IN VITRO--TEMPERATURE

The intimate sequence of events occurring during the formation
of the drug-receptor complex are characterized by fundamental
constants which include the dissociation constant (KA, KB,
reciprocals of affinity for agonists and antagonists), and the
thermodynamic parameters associated with the reaction, i.e.,
changes in Gibbs free energy reflected as changes in enthalpy and
entropy. Agonists and antagonists may bind to receptors
differently, with only agonists able to possibly induce a
conformational change in the receptor; this change is assumed
responsible for the information transfer which initiates events
leading to a measureable effect. This possible agonist-induced
change in conformation is reflected by the reaction
thermodynamics, with agonist and antagonists having different
characteristics. It is now accepted that opioids interact with at
least three types of opioid receptor, the mu, delta and kappa.
While the fundamental parameters of drug-receptor interaction
have been studied for many classes of pharmacological agents
(e.g. alpha & beta adrenergics, nicotinics, muscarinics,
benzodiazepines) surprisingly few studies have been attempted
with opioids. Selective opioid agonists and antagonists, together
with a non-surmountable antagonist, with affinity for all
receptors, have recently been developed; these novel tools now
allow the determination of opioid constants. Additionally, opioid
specific bioassays have been defined with only one type of
receptor (i.e. rabbit vas deferens for the kappa receptor) or
created through alkylation of non-relevant receptor leaving only
one functional opioid receptor. This proposal suggests the
determination of individual opioid receptor constants, and the
thermodynamic parameters of opioid agonists and antagonists in
multiple bioassays (guinea-pig ileum, mouse vas deferens, rabbit
vas deferens, hamster vas deferens). The approach focuses on
receptor constants at different temperatures. Such information
allows calculation of thermodynamic parameters of agonist and
antagonist binding. The specific questions which will be addressed
include: (a) do opioid agonists and antagonists bind to the same
type of opioid receptor in different tissues (i.e. are mu receptors
in the GPI the same as mu receptors in the MVD); (b) are the
three opioid receptors affected differently by temperature; (c)
are there differences in opioid agonist and antagonist binding, and
are peptide agonists and antagonists different from non-peptides;
(d) how are the binding parameters affected by tolerance, or
supersensitivity. Knowledge of these fundamental opioid
parameters should be instrumental in the process of rational drug
design, particularly for antagonists.