The overall goal of research conducted in this lab is to examine the role of metabotropic glutamate receptors (mGluRs) in epilepsy. Eight mGluR subtypes have been identified thus far, and they are subgrouped based on their sequence similarities, signal transduction mechanisms, and agonist selectivities. Group I mGluRs are coupled to phosphoinositide (PI) hydrolysis and intracellular calcium mobilization, while groups II and III are negatively coupled to cyclic AMP formation. Progress in this lab to date reveals that the separate mGluR subgroups serve distinct functions in the production of epileptiform activity in the hippocampus. Activation of group II mGluRs reversibly modulates interictal burst frequency (Merlin et al., 1995), while group I mGluR activation prolongs epileptiform burst duration, converting interictal discharges into ictal events (Merlin and Wong, 1997; see figure).

Current projects involve studies on the effects of group I agonists and synaptic activation of the group I metabotropic glutamate receptors (mGluRs). The main results are: (1) Sufficient activation of group I mGluRs by synaptically-released glutamate can elicit seizure discharges in guinea pig hippocampal slices. (2) Application of group I mGluR agonists will enhance the responsiveness of group I mGluRs to synaptically-released glutamate and result in the production of seizure discharges. (3) Transient exposure of the pyramidal cells to the agonist induces long-lasting seizure activity which persists for hours following removal of the agonist. (4) The long-lasting seizure activity is sustained by the synaptic activation of group I mGluRs now occurring in the absence of any exogenous agonists. And (5), while both mGluR1 and mGluR5 participate in the induction and maintenance of group mGluR-mediated long-lasting seizure activity, mGluR1 plays a greater role in the maintenance of the effect whereas mGluR5 may be more critical for the induction process.

Evidence for the above conclusions was obtained via studies using group I mGluR agonists and antagonists (see below for full names and descriptions of actions). Experiments using hippocampal slices revealed that application of any group I mGluR agonist in the presence of ongoing epileptiform activity dramatically prolongs the epileptiform burst duration, converting the synchronized discharges from brief (300-500 ms) bursts to seizure-like discharges (1-8 sec duration). The modification was a slowly-progressive one, requiring 1.5-2 hrs to peak. When selective group I agonist was used, the burst prolongation was persistent for hours following removal of the agonist. Studies revealed that the persistence could not be explained by inadequate removal of the drug from the system. The results therefore suggest a long-term modification has occurred, probably mediated by the PI hydrolysis and intracellular calcium mobilization known to result from group I mGluR activation.

Antagonist application during the appearance of the persistent prolonged discharges reversibly reduced the burst length, indicating that the group I mGluR response had been up-regulated by the transient exposure to agonist, allowing synaptically-released glutamate to now activate the group I mGluRs and elicit these seizure-like events.

Activation of group I mGluRs has been shown to produce long-lasting potention of postsynaptic ionotropic glutamate responses mediated by AMPA and NMDA receptors. My results show for the first time that the efficacy of the mGluR itself is autoregulated, and that the process may be a mechanism for epileptogenesis and the interictal-to-ictal transition.

Figure 1. A & B: Control - Intracellular recording from a CA3 pyramidal cell demonstrating rhythmic spontaneous synchronized discharges ("interictal bu") in the presence of picrotoxin, an antagonist of GABA_A receptor-mediated inhibition. The broad-spectrum mGluR antagonist MCPG (1 mM) slowed the burst frequency without affecting interictal burst duration. ACPD in low concentrations (5-10 mM), a group II mGluR agonist, had the opposite effect on burst frequency. C: 1. Control burst elicited with picrotoxin. 2. Transient exposure to the selective group I mGluR agonist DHPG elicited persistent prolongation of burst duration. 3 & 4. The group I mGluR antagonist 4CPG reversibly suppressed the persistent prolonged bursts, demonstrating that the group I mGluR response can be autopotentiated by transient group I activation.

Chemical agents

ACPD = (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, an agonist at group I (high concentrations only) and group II mGluRs
DHPG = (R,S)-3,5-dihydroxyphenylglycine, another specific agonist at group I mGluRs
4CPG = (S)-4-carboxyphenylglycine, a group II mGluR agonist and group I mGluR antagonist
MCPG = (+)-a-methyl-4-carboxyphenylglycine, a competitive antagonist at group I and group II mGluRs

Selected Publications

Merlin, L. R., Bergold, P. J., and Wong, R. K. S. (1998). Requirement of protein synthesis for group I mGluR-mediated induction of epileptiform discharges. J. Neurophysiol. 80, 989-993.

Merlin, L. R. (1998). Metabotropic glutamate receptors in the plasticity of excitatory responses in the hippocampus: Clinical Impact. Adv. Exp. Med. Biol. 446, 131-144.

Merlin, L. R. (1999). Group I mGluR-mediated silent induction of long-lasting epileptiform discharges. J. Neurophysiol. 82, 1078-1081.

Galoyan, S. M., and Merlin, L. R. (2000). Long-lasting potentiation of epileptiform bursts by group I mGluRs is NMDA receptor independent. J. Neurophysiol. 83, 2463-2467.

Merlin, L. R. (2002). Differential roles for mGluR1 and mGluR5 in the persistent prolongation of epileptiform bursts. J. Neurophysiol. 87, 621-625.

Rico, M. J., and Merlin, L. R. (2004). Evidence that phospholipase D activation prevents group I mGluR-induced persistent prolongation of epileptiform bursts. J. Neurophysiol. 91, 2385-2388.

Huszár, P., and Merlin, L. R. (2004). Contribution of GABAB receptor-mediated inhibition to the expression and termination of group I mGluR-induced ictaform bursts. Epilepsy Res. 61, 161-165.

List of Publications (Pub Med)

Personnel

Leonardo Faria, Ph.D., Postdoctoral Fellow
Michaelangelo Fuortes, B.S., Graduate Student

Service Functions

Member, Research and Training Committee, American Epilepsy Society
Member, A.B. Baker Section on Neurological Education, American Academy of Neurology
Member, Clinical Neuroplasticity and Neurotransmitters (CNNT) Study Section, National Institutes of Health
Member, Committee for Standard Setting of the NBME Clinical Neurology Subject Examination, National Board of Medical Examiners
Reviewer for various scientific journals