Action of 5‐hydroxytryptamine in facilitating N‐methyl‐D‐aspartate depolarization of cortical neurones mimicked by calcimycin, cyclopiazonic acid and thapsigargin

1 The ability of calcimycin, cyclopiazonic acid and thapsigargin to facilitate the N‐methyl‐D‐aspartate (NMDA)‐mediated depolarization of cortical projection neurones was investigated by use of grease‐gap recording and the results compared with the facilitation that results from activation of 5‐hydroxytryptamine2A receptors. 2 Calcimycin (0.25 to 3 μm), cyclopiazonic acid (5 to 30 μm), and thapsigargin (10 to 300 nM) reversibly facilitated the NMDA (50 μm)‐induced depolarization in the presence of tetrodotoxin. The concentration‐response relationships were bell‐shaped with a mean enhancement of 550% for calcimycin (1 μm) and approximately 400% for cyclopiazonic acid (20 μm) and thapsigargin (100 nM). At the highest concentration of each agent tested, no facilitation was observed. 3 Chlorpromazine (1 μm) partially restored a facilitation at 3 μm calcimycin and 300 nM thapsigargin. Myo‐inositol (10 mM) and 100 nM staurosporine were both ineffective in this regard. 4 The depolarization elicited by 10 μm quisqualate or 5 μm kainate was not facilitated by 10 μm cyclopiazonic acid. 5 Calcimycin (0.5 μm), cyclopiazonic acid (20 μm), and thapsigargin (100 nM) elicited a significant facilitation in the presence of an antagonist cocktail consisting of D, L‐2‐amino‐3‐phosphonopropionic acid, prazosin, ritanserin, and scopolamine, although the magnitude of the facilitation was reduced. 6 Facilitation of the NMDA depolarization elicited by both 30 μm 5‐hydroxytryptamine and 10 μm phenylephrine was eliminated in nominally Mg2+‐free medium. In contrast, the facilitation induced by 0.5 μm calcimycin remained intact. 7 Bis‐(o‐aminophenoxy)‐ethane‐N,N,N,N, tetraacetic acid aminoethoxy (50 μm) or perfusion with nominally Ca2+‐free medium eliminated facilitation of the NMDA depolarization induced by 30 μm 5‐hydroxytryptamine and 100 nM thapsigargin. 8 The facilitation induced by both 30 μm 5‐hydroxytryptamine and 1 μm calcimycin was reduced in a concentration‐dependent manner by nifedipine (1 to 10 μm). 9 Calcimycin, cyclopiazonic acid and thapsigargin facilitate the NMDA depolarization in a manner which closely mimics the facilitation induced by 5‐hydroxytryptamine. It is concluded that enhancement of the NMDA depolarization at cortical projection neurones results from an elevation of Ca2+ in the cytosol and that several sources of Ca2+ contribute to the facilitation.