Role of neurotensin and opioid receptors in the cardiorespiratory effects of [Ile9]PK20, a novel antinociceptive chimeric peptide

Ile9PK20 is a novel hybrid of opioid–neurotensin peptides synthesized from the C-terminal hexapeptide of neurotensin and endomorphin-2 pharmacophore. This chimeric compound shows potent central and peripheral antinociceptive activity in experimental animals, however nothing is known about its influence on the respiratory and cardiovascular parameters.The present study was designed to determine the cardiorespiratory effects exerted by an intravenous injection (i.v.) of [Ile9]PK20. Share of the vagal afferentation and the contribution of NTS1 neurotensin and opioid receptors were tested.Intravenous injection of the hybrid at a dose of 100 μg/kg in the intact, anaesthetized rats provoked an increase in tidal volume preceded by a prompt short-lived decrease. Immediately after the end of injection brief acceleration of the respiratory rhythm appeared, and was ensued by the slowing down of breathing. Changes in respiration were concomitant with a bi-phasic response of the blood pressure: an immediate increase was followed by a sustained hypotension. Midcervical vagotomy eliminated the increase in tidal volume and respiratory rate responses. Antagonist of opioid receptors – naloxone hydrochloride eliminated only [Ile9]PK20-evoked decline in tidal volume response. Blockade of NTS1 receptors with an intravenous dose of SR 142,948, lessened the remaining cardiorespiratory effects.This study depicts that [Ile9]PK20 acting through neurotensin NTS1 receptors augments the tidal component of the breathing pattern and activates respiratory timing response through the vagal pathway. Blood pressure effects occur outside vagal afferentation and might result from activation of the central and peripheral vascular NTS1 receptors. In summary the respiratory effects of the hybrid appeared not to be profound, but they were accompanied with unfavourable prolonged hypotension.

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