Identification and Characterisation of 5‐Hydroxytryptamine3 Recognition Sites in Human Brain Tissue

Abstract

[³H]Zacopride displayed regional saturable specific binding to homogenates of human brain tissues, as defined by the inclusion of BRL43694 [endo-N-(9-methyl-9-azabicyclo[3.3.1]non-3-yl)-l-methylindazole-3-carboxamide] in the incubation media. Scatchard analysis of the saturation data obtained from amygdaloid and hippocampal tissues identified the binding as being of high affinity and to a homogeneous population of binding sites (K_D= 2.64 ± 0.75 and 2.93 ± 0.41 nmol/L and B_max= 55 ± 7 and 44 ± 9 fmol/ mg of protein in the amygdala and hippocampus, respectively). 5-Hydroxytryptamine₃ (5-HT₃) receptor agonists and antagonists competed for the [³H]zacopride binding site, competing with up to 40% of total binding with a similar rank order of affinity in both tissues; agents acting on various other neurotransmitter receptors failed to inhibit binding. Kinetic data revealed a fast association that was fully reversible (k₊₁= 6.61 × 10⁵ and 7.65 × 10⁵/mol/L/s and k_-1= 3.68 × 10⁻³ and 3.45 × 10⁻³/s in the amygdala and hippocampus, respectively). It is concluded that [³H]zacopride selectively labels with high affinity 5-HT₃ recognition sites in human amygdala and hippocampus and, if these binding domains represent 5-HT₃ receptors, may provide the opportunity for 5-HT₃ receptor antagonists to modify 5-HT function in the human brain