| Since NT was hypothesized to act as an endogenous neuroleptic, studies have shown that all clinically effective neuroleptics illicit pronounced effects on the NT system. Hence, the development of NT agonist and antagonist to probe this phenomenon is of significant clinical interest. This work describes the development of NT[8-13] analogues that incorporate non-natural homologues of arginine (Arg) and lysine (Lys) in the Arg(8) position. These analogues were designed to exhibit enhanced binding affinity through desolvation of the ionic interaction between the novel, cationically charged side-chains and neurotensin receptors (NTRs). In addition, these analogues were designed to resist peptidase degradation and facilitate transport across the blood-brain barrier (BBB). Based upon the data accumulated in this dissertation, we have identified three distinct classes of NT[8-13] analogues. Class 1 analogues possess alkyl-arginines substituted for Arg in the Arg(8) position [KK16-19]. In general, these peptides bind with greater affinity to NTRs. However, these analogues are only slightly more stable to peptidase degradation in rat serum (t1/2 14--32 mins) compared to NT[8-13] (t1/2 = 5.78 min). Therefore, these analogues are useful leads to investigate the topography of NTRs. However, they lack the stability for further in vivo analysis. Class 2 analogues possess azido-acid homologues of Arg, homolysine (Hlys), Lys and ornithine (Orn) in the Arg(8) position [KK1-12 and KK15]. These peptides bind with varying affinity NTRs compared to NT. Incorporation of the azido group at the N-terminus dramatically increased the stability of these peptides (t1/2 103--216 mins). However, none of these peptides exhibited in vivo effects when administered intraperitoneally (I.P.). In addition to azido-acid homologues of Hlys and Arg substituted for Arg(8) position, Class 3 analogues possess tert-Leu substituted in the Ile(12) position [ KK13, KK14, and KK20]. These analogues bind with limited affinity (0.3--34.5%) to NTRs compared to NT. However, these analogues are completely (>24 hrs) stable to peptidase degradation in rat serum. Therefore, these analogues are able to cross the BBB and elicit specific CNS mediated events such as induction of hypothermia. Behavioral assays indicate that KK13 is likely to mediate activities implicated in the pathophysiology of schizophrenia as indicated by its ability to inhibit d-amphetamine induced hyperlocomotion without superfluous side effects such as antinociception and induced catalepsy. This compound is considered an important lead for the development of an antipsychotic drug that would work via a novel mechanism, direct agonism of the NT system. |
