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The specific binding of 3H-serotonin to calf caudate homogenate was studied. The dissociation constant was 2nM and the number of specific sites was 14fmoles/mg protein. Of many drugs tested, inhibition of specific 3H-serotonin binding occurred almost exclusively with serotonin agonists and antagonists. The concentrations for 50% inhibition of 3H-serotonin binding by serotonergic agonists follow: bufotenin, 6nM; 5-methoxytryptamine, 12 nM; psilocin, 35nM; dimethyltryptamine, 220 nM; and tryptamine, 270 nM. The concentrations for the antagonists were: LSD 9.5 nM; methysergide 16nM and metergoline 25nM.

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Brain astroglial cells, whether from a bulk isolated preparation or in culture, have been shown to take up serotonin actively. [3H]imipramine has been proposed as a specific label for serotonin uptake sites in brain. We therefore studied the binding of [3H]imipramine to C6 astroglial cells in culture to determine if some of the binding of this radioligand in brain homogenates is actually to serotonin transporting sites on glia. [3H]Imipramine binds saturably (Bmax = 202 fmol/mg protein) and with high affinity (KD = 1.72 nM) to C6 cells. This binding is competitively inhibited by other tricyclic antidepressants. The C6 cells actively transport [3H]serotonin with a Km of 2 microM and a Vmax of 1080 fmol/10(6) cells/min. However, the pharmacological profile for inhibition of serotonin uptake does not correlate with the pharmacological profile for inhibition of [3H]imipramine binding. These results suggest that the binding of [3H]imipramine to astroglial cells is not related to their capacity for active uptake of serotonin. Further, in brain homogenates, some of the binding of [3H]imipramine may not be to neuronal uptake sites but rather may be to sites on astroglial cells.

Since it was known that d-lysergic acid diethylamide (LSD) affected catecholaminergic as well as serotoninergic neurons, the objective in this study was d-Lysergic acid diethylamide (LSD) has been extensively studied since its discovery (1). The structural resemblance of serotonin (2) to LSD was subsequently noted, and evidence accumulated suggesting that LSD exerts its effects through the serotoninergic neurotransmitter system (3-11). Convincing evidence also indicates dopaminergic involvement in LSD activity (12)(13)(14)(15)(16)(17)(18)(19) clearance); this piston, rotating at 500 rpm, was passed up and down 10 times. The crude homogenate was incubated at 370C for 60 min and then stored in 5-ml aliquots at -20°C for future use. Before use, the samples were thawed, resuspended with an additional 5 ml of buffer, and centrifuged at 39,000 X g for 15 min at 4°C. The supernatant was discarded and the pellet was resuspended in 15 ml of buffer by using five up-and-down strokes of a ground-glass homogenizer. This suspension was then homogenized by a Polytron homogenizer (Brinkmann, Westbury, NY) at a setting of 7 (full range = 10) for 10 sec, using a PT-10 homogenizer probe and a 50-ml polycarbonate tube to contain the suspension. This resulted in a final protein concentration of 0.2 mg per tube. Because the membrane homogenate was made up immediately prior to use, it was not necessary to store it on ice.Serotonin-Specific [3HJLSD Binding Assays. The assays were done by using 12 X 75 mm glass test tubes in which the following aliquots were placed (Eppendorf Brinkmann pipettes with polypropylene tips) for control binding: 0.1 ml of nonradioactive LSD (final concentration, 200 nM) or 0.1 ml of buffer; 0.1 ml of buffer containing apomorphine, phentolamine, and spiperone (final concentration, 50 nM for each, referred to as the APS system); 0.2 ml of [3H]LSD (11.5 Ci/mmol; New England Nuclear; final concentration, 2.0 nM); and 0.2 ml of brain homogenate. For tubes with varying concentrations of drugs competing against [3H]LSD, the final contents of the tubes were the same as for control except that the 0.1 ml of buffer or nonradioactive LSD was replaced with 0.1 ml of drug solution.Abbreviations: LSD, d-lysergic acid diethylamide; IC50, concentration that inhibits [3HJLSD binding by 50%; APS system, mixture containing 50 nM each of apomorphine, phentolamine, and spiperone.

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