Fundamental pharmacological research linked to the pharmacotherapy of limbic epilepsy and the involved neurotransmitter systems.

Project Details

Description

This postdoctoral research project aims at finding alternative strategies for the drug therapy of limbic epilepsy. We will evaluate the effects of some new, interesting receptor subtype selective ligands in the well characterised in vivo pilocarpine model. Using this in vivo technology, we will also be able to study some relevant interactions of the neurotransmitters involved. The brain is indeed a complex neuronal network, in which the subtle balance between the different neurotransmitter systems is essential for normal functioning. On the other hand, we are also intrigued by the diversity of intracellular signal transduction mechanisms, after interaction of the in vivo therapeutically active molecules with their receptor subtypes. We would like to study some of these aspects in this research project as well with in vitro techniques in hippocampal slices.
Objectives
- to investigate if subtype selective metabotropic glutamate receptor ligands, such as group I mGluR antagonists and group II/III mGluR agonists, possess in vivo neuroprotective effects in limbic epilepsy, using intracerebral microdialysis-electrocorticography.
- to concentrate with the group I mGluR antagonists on some intracellular mechanisms, using in vitro intracellular Ca2+ determinations and inositol phosphate (IP) accumulation.
- to test a possible involvement of serotonin and selective 5-HT receptor ligands in limbic epilepsy.
- to investigate the in vivo effects of the reversal of the day/night cycle of the experimental animals on hippocampal glutamate-serotonin interactions.
1. In vivo and in vitro studies with subtype selective metabotropic glutamate receptor (mGluR) ligands
An important hypothesis for the future drug therapy of limbic epilepsy is the search for strategies to suppress the excessive glutamate mediated processes. In this regard, we may refer to our own recent work, which contributed to some major breakthroughs in this field, since we demonstrated that iGluR5 kainate receptors play a critical role in synaptic plasticity and in epilepsy. The hippocampus contains a rich glutamatergic in- and output innervation. We would like to continue this research with subtype selective ligands for mGluRs. mGluR ligands have a modest influence on fast excitatory synaptic transmission but possess important modulatory properties, which is an advantage for chronic drug therapies. These compounds would also possess few side effects in the periphery. Group I (mGluR1, mGluR5) activation increases neuronal excitability, so that inhibitory and anticonvulsant effects are to be expected for group I mGluR antagonists. We would like to compare the effects of a selective mGluR5 antagonist, MPEP, with the results obtained with mGluR1 antagonists. Group II (mGluR2, mGluR3) and III (mGluR4, mGluR6, mGluR7, mGluR8) stimulation is presumed to inhibit synaptic signal transmission, so it is of interest to plan in vivo experiments with subtype selective group II and III mGluR agonists. With group I mGluR ligands, we would like to start in vitro research for intracellular (IC) signal transduction mechanisms. mGluR1/mGluR5 receptors are coupled via G-proteins to phosphoinositide hydrolysis, a metabolic process leading to mobilisation of IC Ca2+ and activation of protein kinase C (PKC), as a result of formation of the second messengers inositol-1,4,5-triphosphate (IP3) and diacylglycerol, respectively. Knowing that increases in free Ca2+ and PKC activation are part of the IC cascades that can lead to cell death, one can predict that endogenous stimulation of mGluR1/mGluR5 receptors is potentially neurotoxic. This hypothesis fits our finding that the mGluR1a antagonist is anticonvulsant. Results with agonists however seem to be controversial, both amplification and attenuation of excitotoxicity were demonstrated. Several hypotheses have been put forward and point to complex interactions between mGluRs, IC Ca2+ and Ca2+ sensitive ion channels in the cell membrane. We are especially interested - in the frame of drug therapy of epilepsy - in the till now unknown effects of subtype selective antagonists on [3H]IP accumulation. In simultaneous experiments, we can determine IC Ca2+. Dependent on obtained results, we can strategically continue research with ryanodine receptor antagonists, IP3 antagonists, tyrosine kinase inhibitors...
2. Study with subtype selective serotonin (5-hydroxytryptophan or 5-HT)-ligands
Seven 5-HT receptors with different pharmacological profiles are known, some of them subdivided in many subtypes. In the central nervous system, 5-HT modulates endocrine secretion, sexual behaviour, pain, mood, sleep, ... 5-HT possesses also anticonvulsant properties in a genetic animal model for generalised tonic-clonic convulsions. 5-HT1A agonists seem neuroprotective in in vitro models and 5-HT2C receptors, also coupled to phosphoinositide hydrolysis, modulate neuronal excitability. 5-HT might be an interesting target involved in limbic epilepsy. We would like to screen in this research project a few 5-HT ligands in the pilocarpine model. Hippocampus receives rich serotonergic innervation from the raphe nucleus. We can study if indeed selective 5-HT1A and 5-HT2C agonists can suppress seizures in vivo. There is no literature available on eventual roles of other 5-HT receptors in limbic epilepsy. It might be of interest to evaluate effects of e.g. a selective 5-HT4 antagonist, such as RS39604.
3. Transmitter interactions in hippocampus: effect of reversal of day/night rhythm of the experimental animals?
Finally, we would like to spend some time of our postdoctoral research to glutamate-5-HT interactions in the hippocampus under normal physiological conditions (i.e. without initiation of epilepsy). More specifically, we want to investigate if the interactions will be influenced by the level of activation of the hippocampus. This idea grew as result of reading about clinically very relevant data obtained for cortical acetylcholine and its role in information processing: modulation of cortical acetylcholine release by benzodiazepines is clearly dependent on activity of the neuronal pathway. Rats are nocturnal animals. The hippocampus, also phylogenetically of cortical origin, is presumed to be strongly activated during the nocturnal dark phase. Therefore, reversal of their day/night rhythm might be able to make an important difference for characterisation of baseline glutamate in dialysates. We also would like to investigate possible effects of the reversal of the day/night cycle of the rats on modulation of hippocampal 5-HT release by group I mGluR liganden.
AcronymFWOTM180
StatusFinished
Effective start/end date1/10/9713/03/08

Flemish discipline codes

  • Basic sciences
  • Pharmaceutical sciences

Keywords

  • serotonin
  • epilepsy
  • pharmacology
  • glutamate