TRANS-ACPD, A METABOTROPIC RECEPTOR AGONIST, PRODUCES CALCIUM MOBILIZATION AND AN INWARD CURRENT IN CULTURED CEREBELLAR PURKINJE NEURONS

1. 1-aminocyclopentane-trans-1,3-dicarboxyl acid (t-ACPD), a racemic m ixture of 1-aminocyclopentane-1S,3R-dicarboxylic acid and 1-aminocyclo pentane-1R,3S-dicarboxylic acid, a selective agonist of the metabotrop ic glutamate receptor, was applied to mouse Purkinje neurons (PNs) in culture. Measurements of free intracellular Ca2+ were made using fura- 2 microfluorimetric imaging and of membrane current using perforated-p atch voltage-clamp recording in separate experiments. 2. Brief pulses of t-ACPD (less than or equal to 100 mu M, 1-5 s) consistently produce d a large (200-600 nM) increase in dendritic Ca2+ that was sometimes f ollowed by a somatic increase. The dendrites typically returned to bas al Ca2+ levels within 10-30 s. 3. Ca2+ increases produced by t-ACPD we re measured in Ca2+-free external saline [0.5 mM ethylene glycol-bis(b eta-aminoethyl ether)-N,N,N:N'-tetraacetic acid(EGTA)], suggesting tha t they result from intracellular mobilization rather than influx. In a ddition, Ca2+ increases were not attenuated by a mixture of DL-AP5 and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) [antagonists of N-methyl- D-aspartate (NMDA) and AMPA/kainate receptors, respectively], but were almost entirely eliminated by L-AP3 (100 mu M), a putative metabotrop ic receptor antagonist or by preincubation of the cultures in pertussi s toxin. 4. Brief pulses of t-ACPD (10 mu M) produced a small inward c urrent that was associated with an increase in membrane conductance. T his current was reversibly blocked by L-AP3 but not by treatments that attenuate some voltage-gated K+ currents. Thus this current is unlike ly to underlie the depolarization that is produced by metabotropic ago nists in hippocampal pyramidal cells by K+-channel closure. 5. The t-A CPD induced inward current was attenuated by substitution of external Na+ with Li+ or choline, or by application of the membrane-permeable C a2+ chelator, bis-(2-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)/ AM. One mechanism that could mediate this current is electrogenic Na-o /Ca-i exchange, triggered by Ca2+ mobilization.