Abstract

The lateral entorhinal cortex (LEC) plays an important role in the sensory and mnemonic functions of the medial temporal lobe, most notably in regard to object-related information and olfaction processing. The modulation of synaptic inputs in the LEC may have important implications for learning and memory, in part by affecting the synaptic output of the superficial layers of the LEC to the hippocampus. The superficial layers of the lateral entorhinal cortex are strongly innervated by midbrain dopaminergic neurons. Dopamine can modulate synaptic strength in a dose-dependent manner; high concentrations of dopamine suppress excitatory synaptic transmission, whereas lower concentrations of dopamine (1-10 µM) facilitate it. Therefore, low dopamine levels are likely to promote synaptic transmission in the LEC, and thus provide a mechanism for promoting mnemonic processes. However, the underlying intracellular signalling cascade linking dopamine receptor activation to glutamatergic transmission in layer II LEC neurons has remained hitherto unknown.

The work presented here used single-cell, patch-clamp recordings to characterize the signalling pathway linking dopamine-receptor activation to increases in synaptic transmission in layer II LEC cells. The first set of experiments in this thesis used bath application of dopamine and current-clamp recordings to investigate the dopaminergic facilitation of excitatory postsynaptic potentials (EPSPs) in layer II neurons of the rat lateral entorhinal cortex in vitro. Results indicated that activation of dopamine D1-like receptors lead to increases in AMPA receptor-mediated responses in a manner that was dependent upon the cAMP-protein kinase A (PKA) pathway and protein phosphatase 1 (PP-1). The second series of experiments assessed the contribution of phosphatidylinositol (PI)-linked D1-like receptors to the dopaminergic facilitation of synaptic transmission. Experiments demonstrated that, in addition to activation of the cAMP-PKA pathway, dopamine can lead to the facilitation of synaptic transmission that is reliant on a signaling cascade dependent on PI-linked D1-like receptors, phospholipase C, release of calcium from internal stores, and protein kinase C. In a third series of experiments, fluorescence calcium imaging was used to monitor changes in intracellular calcium induced by dopamine. Bath application of dopamine and the PI-linked dopamine agonist induced a reliable and reversible increase in fluorescence in fan, but not pyramidal, entorhinal cells. This increased fluorescence was correlated with a reversible increase in the amplitude of evoked synaptic currents. Together, the results demonstrate that both the cAMP-PKA and the PLC-DAG-IP3 signalling pathways may contribute to transient increases in synaptic strength that could mediate enhanced sensory and mnemonic function in the entorhinal cortex during release of dopamine.