IN-VIVO AND IN-VITRO STRUCTURE-ACTIVITY STUDIES WITH PEPTIDE AND PSEUDOPEPTIDE NEUROTENSIN ANALOGS SUGGEST THE EXISTENCE OF DISTINCT CENTRAL NEUROTENSIN RECEPTOR SUBTYPES

The present study was designed to compare, with respect to structure-a ctivity relationships, the receptors that subserve the hypothermic and analgesic effects of neurotensin (NT) to the receptor that mediates t he effects of NT in mesencephalic dopamine (DA) neurons, and to compar e these receptors to the cloned adult rat brain NT receptor and to new born mouse and rat brain NT receptors. The results show that NT recept ors in homogenates from newborn mouse and rat brain and from COS 7 cel ls transfected with the cloned high-affinity NT receptor from the adul t rat brain displayed virtually identical structure-activity relations hips toward a series of 12 peptide and pseudopeptide NT analogs, as as sessed by the ability of the compounds to inhibit the binding of [I-12 5]NT binding in these systems. Furthermore, when eight of these analog s were tested for their ability to inhibit [I-125]NT binding and to po tentiate 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 wi th checked metabolic stability were tested in vivo for their hypotherm ic and analgesic (tail-flick test) effects after i.c.v. injection in t he mouse, they exhibited relative potencies that were completely diffe rent from those obtained in vitro. It is concluded that: I)the recepto r which subserves the DA releasing effect of NT in rat embryo mesencep halic neurons is pharmacologically similar to the cloned high-affinity NT receptor from adult rat brain; 2) the cloned receptor in turn is s imilar to the NT receptors detected in whole brain homogenates from ne wborn rat and mouse; and 3) the hypothermic and analgesic actions of N T in the mouse brain appear to be mediated through a receptor whose ph armacological properties are distinct from those of the cloned NT rece ptor and of the receptors in mesencephalic neurons and brain homogenat es.