A functional glutamatergic neurone network in the medial septum and diagonal band area

Abstract

The medial septum and diagonal band complex (MS/DB) is important for learning and memory and is known to contain cholinergic and GABAergic neurones. Glutamatergic neurones have also been recently described in this area but their function remains unknown. Here we show that local glutamatergic neurones can be activated using 4-aminopyridine (4-AP) and the GABA(A) receptor antagonist bicuculline in regular MS/DB slices, or mini-MS/DB slices. The spontaneous glutamatergic responses were mediated by AMPA receptors and, to a lesser extend, NMDA receptors, and were characterized by large, sometimes repetitive activity that elicited bursts of action potentials postsynaptically. Similar repetitive AMPA receptor-mediated bursts were generated by glutamatergic neurone activation within the MS/DB in disinhibited organotypic MS/DB slices, suggesting that the glutamatergic responses did not originate from extrinsic glutamatergic synapses. It is interesting that glutamatergic neurones were part of a synchronously active network as large repetitive AMPA receptor-mediated bursts were generated concomitantly with extracellular field potentials in intact half-septum preparations in vitro. Glutamatergic neurones appeared important to MS/DB activation as strong glutamatergic responses were present in electrophysiologically identified putative cholinergic, GABAergic and glutamatergic neurones. In agreement with this, we found immunohistochemical evidence that vesicular glutamate-2 (VGLUT2)-positive puncta were in proximity to choline acetyltransferase (ChAT)-, glutamic acid decarboxylase 67 (GAD67)- and VGLUT2-positive neurones. Finally, MS/DB glutamatergic neurones could be activated under more physiological conditions as a cholinergic agonist was found to elicit rhythmic AMPA receptor-mediated EPSPs at a theta relevant frequency of 6-10 Hz. We propose that glutamatergic neurones within the MS/DB can excite cholinergic and GABAergic neurones, and that they are part of a connected excitatory network, which upon appropriate activation, may contribute to rhythm generation.