Characterization of a [ 3 H]-5-hydroxytryptamine Binding Site in Rabbit Brain Binding Site in Rabbit Brain Dwne S

In the present study non-5-HT1A/non-5-HT1Q binding sites in the rabbit caudate nucleus (CN) were examined to determine if they might be identical to the recently discoverd 5-HT1D sites in the bovine CN. The characterizations were carried out measuring high-affinity [3H]5-HT binding under conditions where 5-HTja and 5-HTjq sites were pharmacologically masked in both tissues. Comparison of the pharmacologic profiles of the bovine 5-HTjq and rabbit non-5-HT1A/non5-HTlc sites revealed similarities, but showed distinct differences. For example, spiperone gave a Kj value of 2.8 uM in the rabbit CN, while concentrations of up to 100 uM did not produce even 50% inhibition of [3H]5-HT binding in the bovine CN. [3H]5-HT binding in the bovine CN was significant­ ly more sensitive to inhibition by GTP than was [3H]5-HT binding in the rabbit CN, and this effect was differentially sensitive to calcium and other divalent cations (i.e., Mg2+, Mn^+) in the two tissues. [3H]5-HT binding in the bovine CN was significantly more sensitive to inhibition by NEM than it was in the rabbit CN. Thus, It may be concluded that the non-5-HTjA/non5-HTjC [3H]5-HT binding sites in rabbit CN are distinct from those in the bovine CN, and we propose that they be tentatively identified as 5-HTj^ to distinguish them from the 5-HTjp site.