In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.