Gastrin-releasing peptide

Gastrin releasing peptide (GRP) is released by the post-ganglionic fibres of the vagus nerve which innervate the G cells of the stomach and stimulate them to release gastrin.

Role in circadian rhythm
GRP is also involved in the maintenance of the circadian system, playing a role in the signalling of light to the master circadian oscillator in the suprachiasmatic nuclei of the hypothalamus. Furthermore GRP seems to mediate certain aspects of stress.

This is the reason for the observed fact that atropine does not block the vagal effect on gastrin release.