GLUTAMATERGIC MODULATION OF CORTICAL ACETYLCHOLINE-RELEASE IN THE RAT- A COMBINED IN-VIVO MICRODIALYSIS, RETROGRADE TRACING AND IMMUNOHISTOCHEMICAL STUDY

The microdialysis technique with one or two probes was used to investi gate the modulation of cortically projecting cholinergic neurons by gl utamatergic input in the rat in vivo, Male albino Wistar rats (250-300 g) were used. Under chloral hydrate anaesthesia microdialysis membran es were positioned in the parietal cortex, nucleus basalis magnocellul aris (NBM) or medial septum, Acetylcholine was assayed using high-perf ormance liquid chromatography (HPLC) with electrochemical detection wh ile GABA was detected using HPLC with fluorimetric detection after der ivatization of the amino acid with o-phthalaldehyde. Septo-cortical ne urons were retrogradely labelled with fluoro-gold. Double labelling wi th choline acetyltransferase (ChAT) immunoreactivity was performed to identify these neurons. Our main findings were that: (i) i.c.v. admini stration of the NMDA antagonist -((R)-2-carboxypiperazin-4-yl)-propyl- 1-phosphonic acid (CPP, 1-5 nmol) increased cortical acetylcholine out flow; (ii) local administration of CPP (100 mu M) to the cortex had no effect on cortical acetylcholine outflow; (iii) local administration of CPP (100 mu M) to the NBM decreased cortical acetylcholine outflow; (iv) local administration of CPP (100-200 mu M) to the septum increas ed cortical GABA and acetylcholine outflow, (v) administration of musc imol to the septum prevented the effect of CPP on cortical acetylcholi ne outflow; (vi) retrograde tracing with fluoro-gold labelled cell bod ies in the medial septum; (vii) septal fluoro-gold-positive neurons we re not ChAT-immunoreactive. Our in vivo neurochemical results, in comb ination with retrograde tracing and immunohistochemistry, indicate tha t the cortically projecting cholinergic system is indirectly regulated by a glutamatergic input via a polysynaptic GABAergic circuitry locat ed in the septum.