Identification and Characterization of a New Serotonergic Recognition Site with High Affinity for 5‐Carboxamidotryptamine in Mammalian Brain

Abstract

We analyzed the existence of an additional serotonin (5‐HT) receptor subtype, sensitive to 5‐carboxamidotryptamine, in the mammalian brain. Radioligand binding studies with [3H]5‐HT were carried out in rat, guinea pig, and human brain membranes, in the presence of unlabeled drugs to mask the binding to all known 5‐HT receptors, with the exception of 5‐HT1E sites. Under these conditions, unlabeled 5‐carboxamidotryptamine still showed a biphasic competition curve with a nanomolar affinity component. Saturation studies with 5‐[3H]carboxamidotryptamine were carried out in the presence of (±)‐8‐hydroxy‐2‐(di‐n‐propylamino)tetralin, mesulergine, and ergotamine, to mask the binding to all receptors known to be labeled by 5‐carboxamidotryptamine. These studies showed the existence in cortex and hippocampus from guinea pig and human brain of a remaining binding site with high affinity (pKD = 7.8–8.1) and a unique pharmacological profile. 5‐HT and 5‐carboxamidotryptamine showed nanomolar affinity, whereas 5‐methoxytryptamine recognized this binding site with intermediate affinity. Other drugs exhibited low or very low potency in inhibiting this binding. The addition of 5′‐guanylylimidodiphosphate significantly reduced the number of binding sites labeled by 5‐[3H]carboxamidotryptamine, in the presence of the masking drugs described above, indicating the interaction with a GTP‐binding protein. Preliminary autoradiographic studies in human brain appear to indicate that this 5‐HT binding site is present in areas such as the globus pallidus, neocortex, and hippocampus, among others.

Reconsideration of 5-Hydroxytryptamine (5-HT)7Receptor Distribution Using [3H]5-Carboxamidotryptamine and [3H]8-Hydroxy-2-(di-n-propylamino)tetraline: Analysis in Brain of 5-HT1AKnockout and 5-HT1A/1BDouble-Knockout Mice

“…Agonist radioligands, which only recognize the high-affinity state of the receptor are also more sensitive to differing salt conditions than antagonists. We repeated the different protocols described in the literature (different incubation buff- (Waeber and Moskowitz, 1995;Castro et al, 1997). These sites, described by Castro and colleagues as a new serotoninergic recognition site, most likely correspond to the "atypical" 5-HT 1A binding sites described by Waeber and Moskowitz (1995).…”

“…3 H]5-CT binding sites observed in 5-HT 1A receptor-containing regions (Waeber and Moskowitz, 1995;Castro et al, 1997).…”

Pharmacological characterisation of the decrease in 5-HT synthesis in the mouse brain evoked by the selective serotonin re-uptake inhibitor citalopram

“…However, some of these compounds themselves decreased the synthesis of 5-HT, which complicated the analysis. Recently a new putative G-protein-coupled 5-HT receptor in mammalian brain was described to which 5-carboxamidotryptamine (5-CT) and 5-HT had high affinity but all 5-HT receptor antagonists tested had low affinity, methiothepin being the most potent with a pK i -value of 6.5 (K i =316 nM; Castro et al 1997). It is therefore possible that this or another unknown 5-HT receptor might be involved in the citalopram induction of the decrease in 5-HT synthesis.…”

Autoradiographic distribution of 5‐HT7 receptors in the human brain using [3H]mesulergine: comparison to other mammalian species

“…In this regard, the authors concluded that masking drugs added in these autoradiographic studies occluded full visualization of the receptor. In addition, a further recognition site with high affinity for 5‐CT has been identified and characterized in mammalian brain ( Castro et al , 1997 ). In line with these results, a recent report indicates that, despite its high affinity, [ 3 H]5‐CT is not a good tracer for measuring 5‐HT 7 ‐binding sites autoradiographically ( Bonaventure et al , 2002 ).…”