Trans-ACPD, a metabotropic receptor agonist, produces calcium mobilization and an inward current in cultured cerebellar Purkinje neurons

1. 1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), a racemic mixture of 1-aminocyclopentane-1S,3R-dicarboxylic acid and 1- aminocyclopentane-1R,3S-dicarboxylic acid, a selective agonist of the metabotropic 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-patch voltage-clamp recording in separate experiments. 2. Brief pulses of t-ACPD (≤100 μM, 1-5 s) consistently produced a large (200-600 nM) increase in dendritic Ca²⁺ that was sometimes followed by a somatic increase. The dendrites typically returned to basal Ca²⁺ levels within 10-30 s. 3. Ca²⁺ increases produced by t-ACPD were measured in Ca²⁺-free external saline [0.5 mM ethylene glycol-bis(β-amino-ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)], suggesting that they result from intracellular mobilization rather than influx. In addition, 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 μM), a putative metabotropic receptor antagonist or by preincubation of the cultures in pertussis toxin. 4. Brief pulses of t-ACPD (10 μM) produced a small inward current that was associated with an increase in membrane conductance. This current was reversibly blocked by L-AP3 but not by treatments that attenuate some voltage-gated K⁺ currents. Thus this current is unlikely to underlie the depolarization that is produced by metabotropic agonists in hippocampal pyramidal cells by K⁺-channel closure. 5. The t-ACPD induced inward current was attenuated by substitution of external Na⁺ with Li⁺ or choline, or by application of the membrane-permeable Ca²⁺ 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.