Protein kinase associated with gating and closing transmission mechanisms in temporoammonic pathway

The entorhinal cortex (EC) is a major source of afferent input to the hippocampus via the perforant and temporoammonic pathways; however, the detailed transmission mechanism in the temporoammonic pathway remains to be clarified. Thus, we determined interaction among GABA(A), AMPA/glutamate receptors and protein kinases (PKA and PKC) in the exocytosis of GABA and glutamate using multiprobe microdialysis, as well as propagation of neuronal excitability using optical recording in the EC-Hippocampal formation. Multiprobe microdialysis demonstrated that EC-evoked GABA release in ventral CA1 was predominantly regulated by the PKC-related rather than PKA-related exocytosis mechanism and was augmented by the activation of glutamatergic transmission. Contrary to GABA release, EC-evoked glutamate release was predominantly regulated by PKA-related rather than PKC-related mechanisms and was suppressed by activation of GABAergic transmission. Optical recording demonstrated that there are two sub-pathways in the temporoammonic pathway; direct projects from EC layers (II-IV) to dendrites on pyramidal cells and GABAergic interneurons in ventral hippocampal CA1. PKC activation enhanced trisynaptic transmission, whether the GABA(A) receptor was functional or blocked, whereas PKC activation enhanced and inhibited temporoammonic transmission when the GABA(A) receptor was functional and blocked, respectively. Thus, GABAergic inhibition, which is regulated by PKC activity, in the temporoammonic pathway is more significant than that in the trisynaptic pathway.