Effects of bicarbonate ion on serotonin binding to rat frontal cortex membranes

Several factors are known to regulate ligand binding to 5-hydroxytryptamine (5-HT) receptors. In the present experiments we have investigated the mechanism by which bicarbonate ion modify central 5-HT receptor sensitivity in rats. Mn²⁺ (10⁻⁶–10⁻³M) increased specific [³H]5-HT binding to 5-HT₁ receptor sites (+60–70%), this effect being further enhanced by the addition of HCO⁻₃ (+300–400%), while the binding of [³H]spiperone binding to 5-HT₂ receptor sites was not affected by Mn²⁺ and HCO⁻₃. The effect of other divalent cations, Mg²⁺, Cu²⁺, Ca²⁺ and Fe²⁺, however, were not enhanced by the addition of HCO⁻₃. Scatchard analysis indicated that the effect of bicarbonate ion was associated with increase in the number of high affinity binding sites and appearance of low affinity binding sites. This effect of bicarbonate ion was characterized by decreased dissociation rate of the specific binding, was temperature-dependent, reduced by N-ethylmaleimide and iodoacetamide, and was completely inhibited by ascorbate, dithiothreitol and 2-mercaptoethanol. The effect was not influenced by GTP or GppNHp but it was significantly inhibited by ATP. Pretreatment of membranes with Triton X-100 (0.1%) increased the effect of bicarbonate ion. From these results, it is suggested that bicarbonate ion specifically interacts with Mn²⁺ and selectively increases [³H]5-HT binding.