GLUTAMATE METABOTROPIC RECEPTORS INCREASE A CA2+-ACTIVATED NONSPECIFIC CATIONIC CURRENT IN CA1 HIPPOCAMPAL-NEURONS

1. We studied the currents evoked in CA1 pyramidal neurons by the selective metabotropic glutamate receptor (mGluR) agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD; 100 mu M, 2.30-5 min) with the single-electrode voltage-clamp technique in the continuous presence of tetrodotoxin (1 mu M), bicuculline(10 mu M), 6-cyano-7-nitroquinoxaline-2,3-dione (15 mu M), and D-2-amino-5-phosphonovaleric acid (50 mu M) to depress action potentials and synaptic activity. Microelectrodes were filled with 3M CsCl or 2 M Cs2SO4. 2. With CsCl-filled microelectrodes, bath application of 1S,3R-ACPD induced an inward current of -308 +/- 50 (SE) pA amplitude [holding potential (V-H -60 mV, n = 12)] associated with a conductance decrease (26.5 +/- 5.6%, P less than or equal to 0.0022, n = 12). The current-voltage (I-V) relation of the 1S,3R-ACPD-induced (difference) current investigated using ramp voltage commands from -130 to +10 mV had a V shape with two reversal potentials: -99.6 +/- 3.4 and -17.5 a 3.0 mV (n = 12). 3. In contrast, in the presence of external K+ channel blockers (2 mM Ba2+ and 6 mM Cs+ or 25 mM tetraethylammonium, 6 mM Cs+, and 3 mM 4-aminopyridine), 1S,3R-ACPD also generated an inward current, albeit of smaller amplitude (-114.2 +/- 27.5 pA, P less than or equal to 0.003, V-H -60 mV, n = 8). This current was associated with a conductance increase (20.7 +/- 3.1%, P less than or equal to 0.0117, n = 8), decreased linearly with depolarization (from -130 to -60 mV), and reversed polarity at an estimated potential of -20.7 +/- 3.6 mV(n = 8). We refer to this current recorded in the presence of K+ channel blockers as I-ACPD. 4. In the presence of Cd2+ (200 mu M, to block voltage-dependent Ca2+ channels that are readily activated in the presence of K+ channel blockers) and a low Ca2+ concentration(100 mu M), I-ACPD decreased linearly from -130 to +10 mV and reversed polarity at -15.8 +/- 8.5 mV (n = 5). This validates the linear extrapolation of the reversal potential of I-ACPD and suggests that voltage-dependent Ca2+ currents do not trigger I-ACPD. 5. In the presence of the Ca2+ chelator agent bis-(-o-amino-phenoxy)-ethane-N,N,N,N-tetraacetic acid (BAPTA) intracellularly injected through the recording electrode, the I-V relation of the 1S,3R-ACPD-induced current investigated using ramp voltage commands from -130 to +10 mV decreased linearly with hyperpolarization (from -130 to -50 mV), was invariant between -50 and +10 mV, and had a single reversal potential (-103.4 +/- 4.7 mV). Moreover, in the presence of K+ channel blockers and intracellular BAPTA, I-ACPD was totally suppressed at all the potentials tested. This suggests that I-ACPD is a Ca2+-sensitive current activated by a rise of internal Ca2+ concentration ([Ca2+](i)). 6. I-ACPD was reduced by substitution of 100 mM external Na+ with equimolar amounts of choline (67.2 +/- 7.6%, P less than or equal to 0.04, n = 5). In these conditions the extrapolated value of the reversal potential of I-ACPD was significantly shifted (from -17.0 +/- 3.8 to -43.4 +/- 1.8 mV, P less than or equal to 0.04, n = 5). In contrast, when 50 mM external Na+ concentration was substituted with equimolar amounts of Li+, I-ACPD amplitude (-207.8 +/- 46 pA, V-H -60 mV) and its extrapolated reversal potential(-4.6 +/- 6.2 mV) were not significantly different from their values recorded in the absence of Li+ (P less than or equal to 0.07 and P less than or equal to 0.071, respectively). Furthermore, the extrapolated reversal potential of I-APCD was not significantly different (-30.8 +/- 7.0, n = 5, P less than or equal to 0.18) when recordings were performed with CsCl- or Cs2(S)O(4)-filled microelectrodes. This suggests that I-APCD uses Na+ and Li+ but not Cl- as charge carriers. 7. I-APCD amplitude was not significantly reduced by lowering the temperature of the chamber from 33 to 23 degrees C. The temperature coefficient of I-APCD was 1.2, a value consistent with the mechanisms of ion permeation through ionic channels. This experiment, as well as that with external Li+, suggests that I-APCD is not due to the activation of an electrogenic sodium-calcium exchanger. 8. We propose that activation of mGluRs increases a Ca2+-activated nonspecific cationic (CAN) current in addition to reducing several K+ currents. Because in response to a rise in [Ca2+](i), CAN currents generate a long-lasting depolarization, this dual action of mGluRs may facilitate the activation of high-threshold voltage- and ligand-gated channels.