Identification and distribution of 5-HT3 recognition sites within the human brainstem

The present studies demonstrate the presence of specific [3H]GR65630 binding sites within the human brainstem using the techniques of in vitro receptor autoradiography and ligand binding to homogenates. Autoradiography revealed the greatest accumulation of specific binding in the area postrema and subpostrema (AP/ASP). A lower level of specific binding was identified in the nucleus tractus solitarius (excluding area subpostrema). No specific binding was evident in the remainder of the hindbrain at this level. Discrete dissection followed by ligand binding to homogenates revealed that the specific binding of [3H]GR65630 (defined by the presence of 30 microM metoclopramide) was differentially distributed with highest levels in the AP/ASP (112.1 fmol/mg protein) and lower levels in the dorsal vagal complex (nucleus tractus solitarius--excluding the area subpostrema--dorsal motor nucleus of the vagus and hypoglossal nucleus) (DVC) and olivary nucleus (ON) (22.9 and 3.9 fmol/mg, respectively). No specific binding was detectable in the reticular formation (RF) located ventral to the dorsal vagal complex. The specific [3H]GR65630 binding site was pharmacologically similar to the 5-HT3 receptor since the potent and selective 5-HT3 receptor antagonists ICS 205-930 and zacopride (100 nM) and the agonist 5-HT (10 microM) inhibited binding to the same extent as metoclopramide in each of the individual areas (90, 60 and 20% in the AP/ASP, DVC and ON, respectively). The 5-HT1-like and 5-HT2 receptor antagonist methysergide (10 microM) failed to compete for the binding site. 5-HT3 receptor recognition sites within the AP/ASP and the DVC may be functionally involved in the ability of 5-HT3 receptor antagonists to control emesis.