IN-VITRO CHARACTERIZATION OF COCAINE BINDING-SITES IN HUMAN HAIR

In vitro studies were performed to characterize [H-3]cocaine binding t o dark and light ethnic hair types. In vitro binding to hair was selec tive, was reversible and increased linearly with increasing hair conce ntration. Scatchard analyses revealed high-affinity (6-112 nM) and low -affinity (906-4433 nM) binding in hair. Competition studies demonstra ted that the potencies of 3 beta-(4-bromophenyl)tropane-2 beta-carboxy lic acid methyl ester, and 5-(4-chlorophenyl)-2,5-dihydro-3H-imidazo l [2,1-alpha]isoindole-5-ol and 2 beta-carbomethoxy-3 beta-(4-fluorophen yl)tropane were similar to or less than that of (-)-cocaine. The poten cy of (-)-cocaine was 10-fold greater than that of (+)-cocaine at inhi biting radioligand specific binding to hair. Multivariate analysis ind icated that significantly greater nonspecific and specific radioligand binding occurred in dark hair than in light hair. Multivariate analys is also demonstrated a significant ethnicity x sex effect on specific and nonspecific binding to hair. Greater radioligand binding occurred in male Africoid hair than in female Africoid hair and in all Caucasoi d hair types. Melanin was considered the most likely binding site for cocaine in hair. Typically, the concentration of melanin is much great er in dark than in light hair. Scatchard analysis indicated that dark hair had a 5- to 43-fold greater binding capacity than light hair. Dif ferences in radioligand binding between hair types appeared to be due to differences in the density of binding sites formed by melanin in ha ir.