Hippocampal cholinergic neurostimulating peptide (HCNP), which derives from phosphatidylethanolamine-binding protein (also named Raf kinase inhibitor protein), enhances acetylcholine synthesis in the hippocampal medial septal nuclei. It is present in the chromaffin secretory granules of the adrenal cells and under stress is cosecreted with peptide hormones and catecholamines. Using the isolated rat heart perfused according to Langendorff to reveal the cardiotropic action of HCNP on the mammalian heart, we showed that rat HCNP exerts, at concentrations of 5x10(-13) to 10(-6) M, a negative inotropism under basal conditions (left ventricular pressure variations ranging from -8.34+/-0.94% to -21+/-3.5%) and enhances the cholinergic-mediated negative inotropy through direct interaction with G-protein-coupled muscarinic receptor pathway. Under adrenergic stimulation (isoproterenol), the peptide exerts an antiadrenergic action. The analysis of the percentage of rate pressure product variations in terms of EC50 values of isoproterenol alone (-8.5+/-0.3; r2=0.90) and in the presence of rat HCNP at 0.01 nM (-6.9+/-0.36; r2=0.88) revealed a competitive type of antagonism of the peptide. HCNP does not affect either heart rate or coronary pressure. The evidence that HCNP in mammals may play a novel role as an inhibitory cardiac modulator throughout an involvement of the myocardial G-protein-coupled receptor pathway provides new insights regarding the neurohumoral control of heart function under normal and physiopathological conditions.