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In this study we have examined the effects of N-methyl-D-aspartate (NMDA) receptor activation on the release of cholecystokinin and somatostatin from rat neocortical nerve endings. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused synaptosomes, but not the spontaneous release, was increased by NMDA in a concentration-dependent manner. The effects of NMDA could be prevented by antagonists selective for the glutamate recognition site, the receptor channel and the glycine site of the NMDA receptor. In the absence of NMDA, glycine increased on its own and in a concentration-dependent manner the depolarization-evoked release of both CCK-LI and SRIF-LI. This effect of glycine was strychnine-insensitive and could be mimicked by D-serine, a stereoselective agonist at the NMDA receptor glycine site. Antagonists selective for the glycine site or for the NMDA receptor channel prevented the effects of glycine/D-serine; these effects were, however, insensitive to blockade of the glutamate recognition site of the NMDA receptor, suggesting that glutamate released from synaptosomes or present as contaminant was not involved. The neuropeptide release elicited by D-serine was strongly inhibited by ifenprodil (0.3 microM) and by Zn2+ ions (50 nM), selective ligands at the NR2B and NR2A subunits of NMDA receptors, respectively. It is concluded that nerve terminals of CCK- and SRIF-releasing neurons possess non-conventional NMDA receptors whose channels can be operated by glycine or D-serine without apparent activation of the glutamatergic coagonist site. These receptors may display the triple subunit combination NR1/NR2A/NR2B.

1 The effects of 5-hydroxytryptamine (5-HT) on the release of cholecystokinin-like immunoreactivity (CCK-LI) were examined in synaptosomes prepared from rat cerebral cortex and nucleus accumbens and depolarized by superfusion with 15 mM KCl. 2 In both areas 5-HT, tested between 0.1 and 100 nm, increased the calcium-dependent, depolarizationevoked CCK-LI release in a concentration-related manner. The concentration-response curves did not differ significantly between the two brain areas (EC50: 0.4 + 0.045 nm and 0.48 + 0.053 nm, respectively, in cortical and n. accumbens synaptosomes; maximal effect: about 60% at 10 nm 5-HT).3 The 5-HT,/5-HT2 receptor antagonist methiothepin (300 nM) did not affect the CCK-LI release elicited by 10nm 5-HT. However, the effects of 10nm 5-HT were antagonized in a concentration-dependent manner by the 5-HT3 receptor antagonists (3a-tropanyl)-lH-indole-3-carboxylic acid ester 0.1-100 nM; IC50: 3.56 + 0.42 nm in the cortex and 3.90 + 0.50 nm in the n. accumbens) and ondasetron (IC50: 8.15 + 0.73nm in the cerebral cortex). 5-HT (10nM) was also strongly antagonized by l00nM laH, 3a5aH-tropan-3-yl-3,5-dichlorobenzoate (MDL 72222) another blocker of the 5-HT3 receptor. Moreover, the 5-HT3 receptor agonist 1-phenylbiguanide (tested in the cerebral cortex between 0.1 and 100nM) enhanced CCK-LI release in a manner almost identical to that of 5-HT (EC5o = 0.64 + 0.071 nM).4 It is concluded that 5-HT can act as a potent releaser of CCK-LI in rat cerebrocortex and nucleus accumbens through the activation of receptors of the 5-HT3 type situated on the CCK-releasing terminals. This interaction may provide a rationale for the clinical development of both 5-HT3 and CCK receptor antagonists as novel anxiolytic drugs.

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