Electrophysiological characterization of laminar synaptic inputs to the olfactory tubercle of the rat studied in vitro: modulation of glutamatergic transmission by cholinergic agents is pathway-specific

We have exploited the complementary arrangement of afferents in a coronal slice (300–400 µm) of the rat olfactory tubercle (OT) maintained in vitro to investigate transmission in two separate synaptic pathways. We recorded extracellular responses within the OT dense cell layer in slices and stimulated either the outermost layer to activate primary olfactory fibres or deeper to activate secondary input. Superficial stimulation produced a synaptic potential with superimposed population spike. This interpretation was based on blockade by calcium removal from the bathing medium and the use of the glutamate antagonist DNQX (10 µm); the spike was found to be selectively suppressed by tetrodotoxin applied near the cells. The spike, but not the synaptic wave, was depressed by 12 mm Ca2+ and enhanced by 1 mm Ba2+ in the bathing medium. Deep stimulation to activate association and intrinsic fibres elicited a nerve volley followed by a later response, also blocked by Ca2+ removal or 10 µm DNQX. It was unaffected by high Ca2+ or Ba2+, hence resulting from synaptic and not action current flow. Removal of Mg2+ from the bathing medium revealed an NMDA component of synaptic transmission at both loci that was selectively blocked by D‐AP‐5. The deep synaptic response, only, was depressed by carbachol IC50 7 µm or muscarine IC50 13 µm. This depression was also induced by AChE inhibitors eserine or tacrine and was antagonized by 1 µm atropine or 5–10 µm clozapine. These results characterize transmission in the OT and demonstrate a role for muscarinic modulation of deeper synapses in the OT that is influenced by psychotherapeutic drugs.