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Pharmacological Demarcation of the Growth Hormone, Gut Motility and Feeding Effects of Ghrelin Using a Novel Ghrelin Receptor Agonist

Departments of Pediatrics and Neurology and Neurosurgery (G.S.T.), McGill University, and the Neuropeptide Physiology Laboratory, McGill University Health Centre-Montréal Children’s Hospital Research Institute, Montréal, Québec, Canada H3H 1P3; and Tranzyme Pharma Inc. (G.L.F., H.R.H.) Sherbrooke, Québec, Canada J1H 5N4

Address all correspondence and requests for reprints to: Dr. Gloria S. Tannenbaum, Neuropeptide Physiology Laboratory, McGill University-Montréal Children’s Hospital Research Institute, 2300 Tupper Street, Montréal, Québec, Canada H3H 1P3. E-mail: gloria.tannenbaum{at}mcgill.ca.

The peptide hormone ghrelin exerts a wide spectrum of activities including the stimulation of GH release, feeding, and gastrointestinal motility, purportedly via the activation of a common receptor, GH secretagogue receptor (since renamed the GRLN-R) The aim of the present study was to determine whether these effects can be separated pharmacologically. Tranzyme Pharma (TZP)-101 is a small-molecule agonist with potent binding affinity (inhibitory constant = 16 nM) and full agonist activity (EC₅₀ = 29 nM, maximum response = 111%) at the human recombinant GRLN-R. Pharmacokinetic profiling of TZP-101 in rat determined a plasma elimination half-life of 99 min and low blood-brain barrier permeability (0.09%). The pharmacological response to TZP-101, administered centrally [intracerebroventricular (icv)] or peripherally (iv), was evaluated in comparison with that of acylated ghrelin. Thus, TZP-101 (iv) accelerated gastric emptying of a liquid meal (2% methylcellulose) similarly to ghrelin (iv). IAlso, TZP-101 (icv) stimulated spontaneous, cumulative food intake in a similar manner to ghrelin (icv). However, unlike ghrelin, TZP-101 did not elicit significant GH release on either central or peripheral administration. Moreover, TZP-101 did not alter ghrelin-induced GH release. n total, these data demonstrate that the GH response can be pharmacologically demarcated from the orexigenic and gastrointestinal responses to ghrelin in rats. The observation that the centrally mediated orexigenic response and the peripherally mediated gastric motility response are pharmacologically associated is consistent with the classification of ghrelin as a brain-gut peptide, whereas the additional action of ghrelin to stimulate GH release (possibly via a distinct signaling pathway) may be considered a complementary mechanism to harmonize somatic growth and body composition with the regulation of energy homeostasis.

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