Pyramidal cells in piriform cortex receive a convergence of inputs from monoamine activated GABAergic interneurons

Previously, serotonin (5-HT) was shown to increase inhibitory post-synaptic potentials (IPSPs) in layer II pyramidal cells, and excite a subpopulation of interneurons located on the layer II/III border of piriform cortex in rat in vitro brain slices. In the present study, the effects of norepinephrine (NE) and dopamine (DA) on these two populations of neurons were examined in brain slices using intracellular and extracellular recordings. All three monoamines increased GABAergic IPSPs in many pyramidal cells; overall, 5-HT was most effective in eliciting IPSPs (58% of cells), followed by NE (45%), then DA (24%). Commonly, pyramidal cells responded with an increase in IPSPs to more than one of the monoamines. The increase in IPSPs was found to include an increase in the frequency of IPSPs present at baseline, as well as recruitment of additional IPSPs of different amplitudes. In interneurons the effects of the monoamines paralleled that which was found for the pyramidal cells. Thus, all three monoamines increased the firing rate of many interneurons; again 5-HT was most effective (56%), followed by NE (51%), then DA (42%). In about 10% of the interneurons the monoamines inhibited cell firing. Interneurons frequently had responses to more than one of the monoamines. The excitatory amino acid (EAA) antagonist, kynurenic acid (200-400 microM), spared most 5-HT and NE responses on interneurons, suggesting that these effects were directly mediated. We conclude that IPSPs elicited by monoamines in pyramidal cells result from a convergence of inputs from populations of layer II/III interneurons that are activated by one, two or all three of the monoamines.