Implication of acidic lipids in 5-hydroxytryptamine receptor mechanisms.

To establish the possible involvement of acidic lipids in 5-HT receptor mechanisms, we subjected whole rat brain synaptic plasma membranes to treatment with several kinds of lipid-modifying reagents and examined the [3H]5-HT and [3H]spiperone binding properties of the membranes. [3H]5-HT binding was decreased by treatment with Azure A, while [3H]spiperone binding was not altered. Similarly, prior treatment with arylsulphatase reduced the former binding, but had no effect on the latter binding. On the other hand, neither [3H]ligand binding was sensitive to phospholipases C and D. In contrast, prior treatment with phospholipase A2 (unheated) drastically decreased the [3H]5-HT binding and also affected the [3H]spiperone binding to some extent. Chelation of Ca2+ by EGTA (5 mM) prior to incubation of membranes with the unheated phospholipase A2 did not completely prevent the inhibitory effect of this enzyme on [3H]5-HT binding, while in the heated enzyme (at 100 degrees C for 10 min) EGTA exhibited this preventive effect perfectly. Furthermore, it was an interesting find that at least a low concentration of the heated phospholipase A2 (0.01 U) had no effect on the [3H]spiperone binding, as contrasted with the case of [3H]5-HT binding. In addition, the reduction of [3H]5-HT binding capacity in membranes treated with phospholipase A2 (heated and unheated) was restored only slightly by treatment with BSA (1%). Scatchard analysis of the [3H]5-HT binding showed that Azure A and phospholipase A2 (heated) decreased the Bmax values with no significant alteration in the KD values, whereas arylsulphatase increased only the KD value. All these observations infer that certain acidic lipids may play a role as the recognition site(s) or modulator(s) of 5-HT1 receptor molecules.