Selective destruction of medial septal cholinergic neurons attenuates pyramidal cell suppression, but not excitation in dorsal hippocampus field CA1 induced by subcutaneous injection of formalin

Using extracellular recording techniques in urethane- (1 g/kg, i.p.) anaest hetized rats, we investigated the influence exercised by medial septal chol inergic neurons on dorsal hippocampus field CA1 neural responses to a kind paw injection of formalin (5%, 0.05 ml, s.c.). Cholinergic neurons of the m edial septal region were destroyed by local microinjection of the immunotox in 192 IgG-saporin. Compared to control vehicle microinjected animals, immu notoxin-treatment attenuated the amplitude, but not frequency, of CA1 theta induced by intraseptal injection of carbachol. This suggested a selective destruction of medial septal cholinergic neurons by the immunotoxin. Such d estruction also abolished; (i) intraseptal carbachol-induced suppression of CA1 population spike, and (ii) stimulation-intensity dependent increase in amplitude, but not frequency, of theta evoked on electrical stimulation in the region of oral part of pontine reticular nucleus. Further, in comparis on to vehicle-treated animals, selective cholinergic destruction attenuated formalin-induced; (i) theta activation, (ii) suppression of CA1 pyramidal cell population spike and dendritic field excitatory post synaptic potentia l, (iii) inhibition of complex spike cell extracellular activity, and (iv) excitation and theta-rhythmicity of local putative GABAergic interneurons. However, pretreatment with the immunotoxin did not alter the strength and p roportion of complex spike cells excited following injection of formalin From these findings we suggest that medial septal cholinergic neurons media te, at least partly, the amplitude of theta and pyramidal cell suppression via an inhibitory network involving CA1 interneurons. The data also indicat es that during formalin theta, the cholinergic-mediated inhibitory processi ng does not modulate the strength and selectivity of complex spike cell exc itation. This points to formalin-induced, non-overlapping inhibitory and ex citatory processes that might have different functional relevance. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.