Reduced Inhibition of Dentate Granule Cells in a Model of Temporal Lobe Epilepsy

• Published in: The Journal of neuroscience Vol. 23; no. 6; pp. 2440 - 2452
• Main Authors: Kobayashi, Masayuki, Buckmaster, Paul S
• Format: Journal Article
• Language: English
• Published: United States Soc Neuroscience 15.03.2003; Society for Neuroscience
• Subjects: Action Potentials; Animals; Atropine Derivatives; Cell Count; Dentate Gyrus - drug effects; Dentate Gyrus - pathology; Dentate Gyrus - physiopathology; Disease Models, Animal; Electric Stimulation; Epilepsy, Temporal Lobe - chemically induced; Epilepsy, Temporal Lobe - pathology; Epilepsy, Temporal Lobe - physiopathology; Evoked Potentials; In Vitro Techniques; Interneurons - pathology; Male; Membrane Potentials; Neural Inhibition - drug effects; Neurons - drug effects; Neurons - pathology; Neurons - physiology; Patch-Clamp Techniques; Pilocarpine; Rats; Rats, Sprague-Dawley; Receptors, GABA-A - metabolism; Sensory Thresholds; Status Epilepticus - chemically induced; Status Epilepticus - physiopathology
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/jneurosci.23-06-02440.2003

Patients and models of temporal lobe epilepsy have fewer inhibitory interneurons in the dentate gyrus than controls, but it is unclear whether granule cell inhibition is reduced. We report the loss of GABAergic inhibition of granule cells in the temporal dentate gyrus of pilocarpine-induced epileptic rats. In situ hybridization for GAD65 mRNA and immunocytochemistry for parvalbumin and somatostatin confirmed the loss of inhibitory interneurons. In epileptic rats, granule cells had prolonged EPSPs, and they discharged more action potentials than controls. Although the conductances of evoked IPSPs recorded in normal ACSF were not significantly reduced and paired-pulse responses showed enhanced inhibition of granule cells from epileptic rats, more direct measures of granule cell inhibition revealed significant deficiencies. In granule cells from epileptic rats, evoked monosynaptic IPSP conductances were <40% of controls, and the frequency of GABA(A) receptor-mediated spontaneous and miniature IPSCs (mIPSCs) was <50% of controls. Within 3-7 d after pilocarpine-induced status epilepticus, miniature IPSC frequency had decreased, and it remained low, without functional evidence of compensatory synaptogenesis by GABAergic axons in chronically epileptic rats. Both parvalbumin- and somatostatin-immunoreactive interneuron numbers and the frequency of both fast- and slow-rising GABA(A) receptor-mediated mIPSCs were reduced, suggesting that loss of inhibitory synaptic input to granule cells occurred at both proximal/somatic and distal/dendritic sites. Reduced granule cell inhibition in the temporal dentate gyrus preceded the onset of spontaneous recurrent seizures by days to weeks, so it may contribute, but is insufficient, to cause epilepsy.