Glutamate has been known as an excitatory neurotransmitter
in both vertebrate and invertebrate
neural systems. We found that glutamate depolarized
the membrane potential of the ventral nerve cord in
the earthworm,
Eisenia foetida (Oka et al., 1994).
We extensively characterized the role of glutamate
from Ca
2+ and membrane potential responses using
the optical recording method on the ventral nerve
cord of the earthworm. Glutamate and four types of
glutamate receptor subtype agonists, N-methyl-D-aspartate
(NMDA), kainate, quisqualate, and trans-1-aminocyclopentane-1,3-dicarboxylic acid (transACPD),
increased intracellular calcium concentration
([Ca
2+]
i in the MGF. Based on the temporal
pattern of Ca
2+ responses, the Ca
2+ elevations induced
by NMDA, kainate, and trans-ACPD were
transient, and [Ca
2+]
i was restored to basal levels
during the stimulation, while a Ca
2+ increase induced
by quisqualate was sustained and weak compared
with the other tested agonists. From the
spatial pattern of Ca
2+ responses, NMDA and kainate-induced responses were regionally different in
contrast to the uniform Ca
2+ increase by trans-ACPD
stimulation. Ionotropic glutamate receptor
agonists (NMDA, quisqualate, and kainate) depolarized
the membrane potential of neurons in the
ventral nerve cord, but a metabotropic glutamate
receptor agonist, trans-ACPD, did not affect the
membrane potential. Our results indicate that there
are at least three types of glutamate receptors,
NMDA, non-NMDA, and metabotropic ones, on the
earthworm ventral nerve cord.
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