Cholecystokinin-induced ventral root depolarization of neonate rat hemicord in vitro.

1. Sulphated cholecystokinin octapeptide (CCK8S, 0.03-1.00 microM), pentapeptide (CCK5) and tetrapeptide (CCK4) elicited concentration dependent depolarizations of neonate rat ventral roots in vitro. 2. CCK5 was equipotent with CCK8S although CCK4 was weaker (equipotent molar ratio 17.5). 3. CCK8S-induced depolarizations were depressed by tetrodotoxin (0.1 microM), Mg2+ ions (0.75 mM) and the NMDA receptor antagonist 2-amino-5-phosphonopentanoate (AP5, 10 microM). These results suggest that CCK8S-induced depolarizations were predominantly mediated through the release of an excitatory amino acid from interneuronal sites. 4. The selective CCKA and CCKB receptor antagonists, L-364,718 and L-365,260 both depressed CCK8S-induced depolarizations. CCK8S dose ratios in the presence of 1 microM L-364,718 or L-365,260 were 4.5 and 11.2 respectively, suggesting the response was mediated predominantly through stimulation of CCKB receptors. 5. These results suggest that the neonate rat hemicord preparation is a suitable tissue for functional CCK receptor assays.