N. Harata et al., 2 COMPONENTS OF METABOTROPIC GLUTAMATE RESPONSES IN ACUTELY DISSOCIATED CA3 PYRAMIDAL NEURONS OF THE RAT, Brain research, 711(1-2), 1996, pp. 223-233
The excitatory and inhibitory actions of metabotropic glutamate recept
or (mGluR) agonists were investigated in acutely dissociated rat hippo
campal CA3 pyramidal neurons, using the conventional whole-cell and ny
statin-perforated patch recording configurations under the voltage-cla
mp condition. With the conventional whole-cell recording, glutamate (G
lu) and quisqualic acid (QA) induced only ionotropic inward currents a
ccompanied by increased membrane conductance at a holding potential (V
-H) of -45 mV. The response was reversibly blocked in the presence of
D-2-amino-5-phosphonopentanoic acid (D-AP5) and 6-cyano-7-nitroquinoxa
line-2,3-dione (CNQX), the antagonists of N-methyl-D-aspartate (NMDA)
receptor and non-NMDA receptor, respectively. With nystatin-perforated
patch recording, mGlu responses insensitive to both D-AP5 and CNQX we
re observed. Fifty-five % of the cells responded by a slow inward curr
ent accompanied by conductance decrease (I-mGlui) at a V-H of -44 mV.
One % of the neurons showed an outward current with conductance increa
se (I-mGluo), and 34% of the neurons showed I-mGluo followed by I-mGlu
i. The onset of I-mGluo occurred approximately 900 ms after the respon
se to 30 mM K+. The time to peak of I-mGluo were 32- to 79-times longe
r than those of ionotropic responses. I-mGlui appeared at lower concen
trations than ionotropic Glu responses, whereas I-mGluo appeared at si
milar concentrations as ionotropic responses. The rank order of affini
ty was QA > Glu >(+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid
(tACPD) for both I-mGlui and I-mGluo. Half-maximal effective concentr
ations (EC(50)) and the threshold concentrations for the three agonist
s were four- to tenfold lower for I-mGlui than for I-mGluo. The curren
t-voltage relationship showed that the reversal potentials of I-mGlui
and I-mGluo shifted 55 and 59 mV, respectively, for a tenfold change i
n extracellular K+ concentration, indicating that K+ is the charge car
rier of both mGlu responses. During I-mGlui, both the leakage current
and muscarine-sensitive voltage-dependent K+ current (M current) were
suppressed. I-mGluo induced by 10(-4) M tACPD was abolished by 3 . 10(
-7) M charybdotoxin and 10(-6) M ryanodine. These results show that th
ere are two components of mGlu responses in CA3 pyramidal neurons and
that I-mGlui and I-mGluo show different pharmacological properties.