In vivo and in vitro structure - activity studies with peptide and pseudopeptide neuroten - sin analogs suggest the existence of distinct central neurotensin receptor subtypes

The present study was designed to compare, with respect to structure-activity relationships, the receptors that subserve the hypothermic and analgesic effects of neurotensin (NT) to the receptor that mediates the effects of NT in mesencephalic dopamine (DA) neurons, and to compare these receptors to the cloned adult rat brain NT receptor and to newborn mouse and rat brain NT receptors. The results show that NT receptors in homogenates from newborn mouse and rat brain and from COS 7 cells transfected with the cloned high-affinity NT receptor from the adult rat brain displayed virtually identical structure-activity relationships toward a series of 12 peptide and pseudopeptide NT analogs, as assessed by the ability of the compounds to inhibit the binding of [I-125]NT binding in these systems. Furthermore, when eight of these analogs were tested for their ability to inhibit [I-125]NT binding and to potentiate K+-evoked DA release in primary cultures of rat mesencephalic neurons, it was found that they all behaved as agonists with binding and biological potencies quite similar to those observed in the other binding assays. Finally and strikingly, when seven of these analogs with checked metabolic stability were tested in vivo for their hypothermic and analgesic (tail-flick test) effects after i.c.v. injection in the mouse, they exhibited relative potencies that were completely different from those obtained in vitro. It is concluded that: I)the receptor which subserves the DA releasing effect of NT in rat embryo mesencephalic neurons is pharmacologically similar to the cloned high-affinity NT receptor from adult rat brain; 2) the cloned receptor in turn is similar to the NT receptors detected in whole brain homogenates from newborn rat and mouse; and 3) the hypothermic and analgesic actions of NT in the mouse brain appear to be mediated through a receptor whose pharmacological properties are distinct from those of the cloned NT receptor and of the receptors in mesencephalic neurons and brain homogenates.