V. Crepel et al., GLUTAMATE METABOTROPIC RECEPTORS INCREASE A CA2-ACTIVATED NONSPECIFICCATIONIC CURRENT IN CA1 HIPPOCAMPAL-NEURONS(), Journal of neurophysiology, 72(4), 1994, pp. 1561-1569
1. We studied the currents evoked in CA1 pyramidal neurons by the sele
ctive metabotropic glutamate receptor (mGluR) agonist 1S,3R-1-aminocyc
lopentane-1,3-dicarboxylate (1S,3R-ACPD; 100 mu M, 2.30-5 min) with th
e single-electrode voltage-clamp technique in the continuous presence
of tetrodotoxin (1 mu M), bicuculline(10 mu M), 6-cyano-7-nitroquinoxa
line-2,3-dione (15 mu M), and D-2-amino-5-phosphonovaleric acid (50 mu
M) to depress action potentials and synaptic activity. Microelectrode
s were filled with 3M CsCl or 2 M Cs2SO4. 2. With CsCl-filled microele
ctrodes, 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 potential
s: -99.6 +/- 3.4 and -17.5 a 3.0 mV (n = 12). 3. In contrast, in the p
resence 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 l
ess than or equal to 0.0117, n = 8), decreased linearly with depolariz
ation (from -130 to -60 mV), and reversed polarity at an estimated pot
ential of -20.7 +/- 3.6 mV(n = 8). We refer to this current recorded i
n 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 rea
dily activated in the presence of K+ channel blockers) and a low Ca2concentration(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 rec
ording electrode, the I-V relation of the 1S,3R-ACPD-induced current i
nvestigated using ramp voltage commands from -130 to +10 mV decreased
linearly with hyperpolarization (from -130 to -50 mV), was invariant b
etween -50 and +10 mV, and had a single reversal potential (-103.4 +/-
4.7 mV). Moreover, in the presence of K+ channel blockers and intrace
llular BAPTA, I-ACPD was totally suppressed at all the potentials test
ed. This suggests that I-ACPD is a Ca2+-sensitive current activated by
a rise of internal Ca2+ concentration ([Ca2+](i)). 6. I-ACPD was redu
ced by substitution of 100 mM external Na+ with equimolar amounts of c
holine (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 l
ess 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 thei
r 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 ext
rapolated 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 s
uggests that I-APCD uses Na+ and Li+ but not Cl- as charge carriers. 7
. I-APCD amplitude was not significantly reduced by lowering the tempe
rature of the chamber from 33 to 23 degrees C. The temperature coeffic
ient of I-APCD was 1.2, a value consistent with the mechanisms of ion
permeation through ionic channels. This experiment, as well as that wi
th external Li+, suggests that I-APCD is not due to the activation of
an electrogenic sodium-calcium exchanger. 8. We propose that activatio
n of mGluRs increases a Ca2+-activated nonspecific cationic (CAN) curr
ent in addition to reducing several K+ currents. Because in response t
o a rise in [Ca2+](i), CAN currents generate a long-lasting depolariza
tion, this dual action of mGluRs may facilitate the activation of high
-threshold voltage- and ligand-gated channels.