Ba. Mccool et al., RAT GROUP-I METABOTROPIC GLUTAMATE RECEPTORS INHIBIT NEURONAL CA2-TRANSDUCTION PATHWAYS IN HEK-293 CELLS( CHANNELS VIA MULTIPLE SIGNAL), Journal of neurophysiology, 79(1), 1998, pp. 379-391
Rat group I metabotropic glutamate receptors inhibit neuronal Ca2+ cha
nnels via multiple signal transduction pathways in HEK 293 cells. J. N
europhysiol. 79: 379-391, 1998. We have shown previously that metabotr
opic glutamate receptors with group I-like pharmacology couple to N-ty
pe and P/Q-type calcium channels in acutely isolated cortical neurons
using G proteins most likely belonging to the G(i)/G(o) subclass. To b
etter understand the potential mechanisms forming the basis for group
I mGluR modulation of voltage-gated calcium channels in the CNS, we ha
ve examined the ability of specific mGluRs to couple to neuronal N-typ
e (alpha(1B-1)/alpha(2) delta/beta(1b)) and P/Q-type (alpha 1(A-2)/alp
ha(2) delta/beta(1b)) voltage-gated calcium channels in an HEK 293 het
erologous expression system. Using the whole cell patch-clamp techniqu
e where intracellular calcium is buffered to low levels, we have shown
that group I receptors inhibit both N-type and P/Q-type calcium chann
els in a voltage-dependent fashion. Similar to our observations in cor
tical neurons, this voltage-dependent inhibition is mediated almost en
tirely by N-ethylmaleimide (NEM)-sensitive heterotrimeric G proteins,
strongly suggesting that these receptors can use G(i)/G(o)-like G prot
eins to couple to N-type and P/Q-type calcium channels. However, incon
sistent with the apparent NEM sensitivity of group I modulation of cal
cium channels, modulation of N-type channels in group I mGluR-expressi
ng cells was only partially sensitive to pertussis toxin (PTX), indica
ting the potential involvement of both PTX-sensitive and -resistant G
proteins. The PTX-resistant modulation was voltage dependent and entir
ely resistant to NEM and cholera toxin. A time course of treatment wit
h PTX revealed that this toxin caused group I receptors to slowly shif
t from using a primarily NEM-sensitive G protein to using a NEM-resist
ant form. The PTX-induced switch from NEM-sensitive to -resistant modu
lation was also dependent on protein synthesis, indicating some relian
ce on active cellular processes. In addition to these voltage-dependen
t pathways, perforated patch recordings on group I mGluR-expressing ce
lls indicate that another slowly developing, calcium-dependent form of
modulation for N-type channels may be seen when intracellular calcium
is not highly buffered. We conclude that group I mGluRs can modulate
neuronal Ca2+ channels using a variety of signal transduction pathways
and propose that the relative contributions of different pathways may
exemplify the diversity of responses mediated by these receptors in t
he CNS.