Brain Research 887 2000 359–377 www.elsevier.com locate bres
Research report
Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn
c a
b c ,
´ ´
Jie Zhong , Gabor Gerber , Ljubomir Kojic , Mirjana Randic
a
¨ ´
Department of Anatomy , Histology and Embryology, Semmelweis University of Medicine, Tuzolto utca 58., 1094 Budapest, Hungary
b
Department of Ophthalmology , Faculty of Medicine, University of British Columbia, 2550 Willow Street, Vancouver, BC, Canada V5Z 3N9
c
Department of Biomedical Sciences , Iowa State University, Ames, IA 50011, USA
Accepted 3 October 2000
Abstract
The effects of group I metabotropic glutamate mGlu receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist
1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid 1S,3R-ACPD, the group I mGlu receptor selective agonist S -3,5-dihydrox- yphenylglycine DHPG, and the selective mGlu subtype 5 agonist RS -2-chloro-5-hydroxyphenylglycine CHPG, all induce long-
lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential EPSP, and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was
partially or fully reversed by S -4-carboxyphenylglycine S-4CPG, the mGlu subtype 1 preferring antagonist. 2-Methyl-6- phenylethynyl-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic
EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-
D
-aspartate NMDA receptor antagonist
D
2-2-amino-5-phosphonopentanoic acid AP5. A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-
aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn
neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and S -4-CPG, the mGlu
receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression LTD and long-term potentiation
LTP of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity LTP or antinociception LTD.
2000 Elsevier Science B.V. All rights reserved.
Theme : Excitable membranes and synaptic transmission
Topic : Long-term potentiation: physiology
Keywords : Excitatory postsynaptic potential; Group I metabotropic glutamate receptors; Spinal cord dorsal horn; Slice-intracellular recording technique
1. Introduction Glutamate acts through two broad classes of receptors,
ion channel-linked ionotropic receptors, which include It is now well established that primary afferent fibers use
a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate as a principal fast excitatory transmitter in the
AMPA, kainate, and N-methyl-
D
-aspartate NMDA re- dorsal horn DH of the spinal cord, the first modulatory
ceptors, and metabotropic receptors mGluRs which cou- site in the relay of sensory information to the brain [21,98].
ple via G-proteins to the intracellular second messenger cascades and ion channels. Eight mGlu receptor subtypes
have been cloned to date and are classified into three
Corresponding author. Tel.: 11-515-294-7793; fax: 11-515-294-
groups based on structural homology, pharmacology and
2315. ´
E-mail address : mrandiciastate.edu M. Randic.
signal transduction mechanisms: group I mGlu receptors 1
0006-8993 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 3 0 6 6 - 3
360 J
and 5 are coupled to phospholipase C and stimulate mm thick were cut with attached dorsal roots in an
21
phosphoinositide hydrolysis and intracellular Ca signal
oxygenated 95 O , 5 CO Krebs-bicarbonate solu-
2 2
transduction, whereas group II mGlu receptors 2 and 3 tion 48C on a vibratome and placed in a holding chamber
and group III mGlu receptors 4 and 6–8 are negatively 36618C to recover for at least 1 h. A single slice was
coupled to adenyl cyclase [1,16,79]. These receptors then transferred into an interface-type recording chamber
modulate synaptic transmission and neuronal excitability. where it was submerged beneath an oxygenated superfus-
Class I mGlu receptors have been postulated to play a role ing medium flow-rate of about 3 ml min, 34–358C
in synaptic plasticity such as long-term potentiation LTP containing in mM: NaCl, 124; KCl, 1.9; KH PO , 1.2;
2 4
[3,9,106] and long-term depression LTD of synaptic CaCl , 2.4; MgSO , 1.3; NaHCO , 26; glucose, 10; pH
2 4
3
transmission [53,55,71] in the brain, as well as in the 7.4, 310–320 mOsm, and was equilibrated with 95 O ,
2
spinal cord [32,82]. 5 CO .
2
Although the presence of multiple mGlu receptors subtypes 1–5 and 7 in the spinal cord DH has been
2.2. Dorsal root stimulation and intracellular recording shown [10,46,54,70,89,92–94], their roles in physiology
and pathophysiology of synaptic transmission are not clear Intracellular recordings with sharp microelectrodes were
and are complicated by the diversity of pre- and postsynap- made from DH neurons laminae I–V, including sub-
tic receptors. There is behavioral and electrophysiological stantia gelatinosa SG, lamina II cells. When viewed
evidence that the activation of mGlu receptors, in par- under a dissecting microscope at a magnification of 10–
ticular, mGlu subtypes 1 5, increases the excitability of the 403 with transmitted illumination, the SG was distinguish-
rat spinal DH neurons [51,64,65] and facilitates responses able as a translucent bend in the superficial DH, although it
to NMDA
and AMPA
receptor activation
was difficult to discern with certainty the border between [5,8,12,24,25,48,61,69,102–105]. Studies of the actions of
laminae I and II. Under visual control, a single fiberglass mGlu receptor agonists and antagonists on responses of
[6010; O.D. and I.D., 1.0 and 0.58 mm, respectively; AM deep DH neurons [68,69,103–105] to noxious and non-
Systems microelectrode filled with 4 M potassium acetate noxious stimuli indicate that mGlu receptors are involved
pH 7.2 DC resistance: 140–220 MV was placed in the in mediating nociceptive inputs. In particular, group I
SG or deep DH DDH, and neurons were impaled by mGlu 1 5 receptors have been implicated in mediating
oscillating the capacity compensation circuit of a high- nociception following a sustained noxious input [24–
input impedance bridge amplifier Axoclamp 2A, Axon 26,28,29,104,105]. However, the synaptic and cellular
Instruments. A DC pen-recorder was used to record mechanisms underlying functional plasticity following
membrane potentials continuously and a Digidata 1200 tissue or nerve injury are not known. Moreover, the data
system with
PCLAMP
version 6 software Axon Instru- described above are largely based on observations obtained
ments was used for data acquisition and analysis. Most from wide-dynamic-range neurons in deep DH laminae of
recordings were obtained from cells with a stable resting rats and primates in vivo.
membrane potential more negative than 255 mV and In the present study we examined the effects of group I
with overshooting action potentials. The protocol for mGlu receptor activation by the group I and II mGlu
assessing the effects of mGlu receptor agonists and antago- agonist
1S,3R-1-aminocyclopentane-1,3-dicarboxylate nists on EPSPs was as follows. Monosynaptic and poly-
1S,3R-ACPD, S -3,5-dihydroxyphenylglycine DHPG, synaptic EPSPs in DH neurons were evoked by ortho-
a selective agonist for the group I mGlu receptors [88], and dromic electrical stimulation of primary afferent fibers in
RS -2-chloro-5-hydroxyphenylglycine CHPG, a selec- the lumbar dorsal root L4 and or L5 using a bipolar
tive mGlu subtype 5 agonist [20] on the synaptic responses platinum wire electrode or glass suction electrode with the
mostly of rat superficial dorsal horn neurons to primary cathode internal. Single shocks 0.01–0.5 ms pulses, 2–35
afferent stimulation in vitro. Some of the results have been V, repeated at 2-min intervals, were given for at least 10
reported previously in abstract form [51,84,108]. min before, during 10 min, and for a 30–60 min period
after bath administrations of chemicals. This frequency of stimulation was chosen for sampling data because it did
2. Materials and methods not result in response facilitation or depression. A stimulus
