Dopaminergic Enhancement of Excitatory Synaptic Transmission in Layer II of the Lateral Entorhinal Cortex

Previous research demonstrated that dopamine produces concentration-dependent changes in synaptic transmission in the entorhinal cortex, wherein high concentrations of dopamine (50 µM) suppress evoked excitatory postsynaptic potentials (EPSPs) and lower concentrations of dopamine (1 to 10 µM) facilitate them. Whole-cell current clamp recordings were used to investigate the dopaminergic facilitation of synaptic responses in layer II neurons of the lateral entorhinal cortex. Surprisingly, the pattern of changes in EPSPs was dependent on cell type. During bath applications of 1 µM dopamine, fan cells showed a facilitation of the amplitude of EPSPs evoked by layer I stimulation. In contrast, pyramidal cells showed mixed modulation of EPSPs in response to dopamine, with different cells showing either facilitation or suppression effects. Voltage clamp recordings of excitatory postsynaptic currents suggest that dopamine facilitates AMPA glutamate receptor-mediated EPSCs. To determine the dopaminegic receptor subtype involved, either the D1 receptor blocker SCH23390 (50 µM) or the D2 receptor blocker sulpiride (50 µM) was bath applied to the slices prior to dopamine. Application of SCH 23390 blocked the facilitation of EPSCs, whereas application of sulpiride had no significant effect. Therefore, the dopaminergic enhancement of EPSCs is likely to be mediated primarily through D1-like receptors. D1 receptors can act through a variety of intracellular signaling pathways to modulate synaptic strength. The role of signaling via protein kinase A was tested by including the PKA inhibitor H-89 in the recording pipette solution. Cells filled with H-89 did not show a facilitation of EPSCs in response to dopamine application. Thus, the dopamine-induced facilitation of AMPA receptor-mediated synaptic responses in the lateral entorhinal cortex appears to be mediated via a D1 receptor-dependent increase in PKA activity

Dopaminergic Facilitation of Synaptic Transmission in Layer II of the Lateral Entorhinal Cortex

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

Dataset: 'Activation of phosphatidylinositol-linked dopamine receptors induces a facilitation of glutamate-mediated synaptic transmission in the lateral entorhinal cortex.'

Measures of the amplitudes of synaptic responses

Activation of Phosphatidylinositol-Linked Dopamine Receptors Induces a Facilitation of Glutamate-Mediated Synaptic Transmission in the Lateral Entorhinal Cortex

<div><p>The lateral entorhinal cortex receives strong inputs from midbrain dopamine neurons that can modulate its sensory and mnemonic function. We have previously demonstrated that 1 µM dopamine facilitates synaptic transmission in layer II entorhinal cortex cells via activation of D₁-like receptors, increased cAMP-PKA activity, and a resulting enhancement of AMPA-receptor mediated currents. The present study assessed the contribution of phosphatidylinositol (PI)-linked D₁receptors to the dopaminergic facilitation of transmission in layer II of the rat entorhinal cortex, and the involvement of phospholipase C activity and release of calcium from internal stores. Whole-cell patch-clamp recordings of glutamate-mediated evoked excitatory postsynaptic currents were obtained from pyramidal and fan cells. Activation of D₁-like receptors using SKF38393, SKF83959, or 1 µM dopamine induced a reversible facilitation of EPSCs which was abolished by loading cells with either the phospholipase C inhibitor U-73122 or the Ca²⁺ chelator BAPTA. Neither the L-type voltage-gated Ca²⁺ channel blocker nifedipine, nor the L/N-type channel blocker cilnidipine, blocked the facilitation of synaptic currents. However, the facilitation was blocked by blocking Ca²⁺ release from internal stores via inositol 1,4,5-trisphosphate (InsP₃) receptors or ryanodine receptors. Follow-up studies demonstrated that inhibiting CaMKII activity with KN-93 failed to block the facilitation, but that application of the protein kinase C inhibitor PKC(19-36) completely blocked the dopamine-induced facilitation. Overall, in addition to our previous report indicating a role for the cAMP-PKA pathway in dopamine-induced facilitation of synaptic transmission, we demonstrate here that the dopaminergic facilitation of synaptic responses in layer II entorhinal neurons also relies on a signaling cascade dependent on PI-linked D₁ receptors, PLC, release of Ca²⁺ from internal stores, and PKC activation which is likely dependent upon both DAG and enhanced intracellular Ca²⁺. These signaling pathways may collaborate to enhance sensory and mnemonic function in the entorhinal cortex during tonic release of dopamine.</p></div

Dopaminergic facilitation of EPSCs is dependent on intracellular calcium, but not L- or N-type VGCCs.

<p><b>A.</b> Bath application of the dopamine D₁-receptor agonist SKF38393 (10 <b>μ</b>M) induces a reversible facilitation of the amplitudes of glutamate-mediated excitatory postsynaptic currents in layer II lateral entorhinal cortex neurons (A₁). Traces show averaged EPSCs for a neuron before, during, and after 5-min application of SKF38893. The histogram at right shows mean EPSC amplitudes for the group of cells. Bars indicate ± one SEM and the asterisk indicates p < 0.05. In addition, the facilitation induced by SKF38393 is blocked in the presence of the D₁ receptor blocker SCH23990 (A₂). <b>B.</b> Amplitudes of EPSCs recorded from cells filled with the Ca²⁺ chelator BAPTA (10 <b>μ</b>M) remained stable during dopamine (DA) application. <b>C,D.</b> Bath-application of the L-type voltage-gated calcium channel (VGCC) blocker nifedipine (10 <b>μ</b>M; C) or of the L/N-type VGCC channel blocker cilnidipine (10<b>μ</b>M; D) failed to block the dopaminergic facilitation of EPSCs (*, p < 0.001). Traces for the baseline and VGCC blockers are superimposed.</p

The DA-induced facilitation is dependent upon activation of both InsP₃ and ryanodine receptors.

<p><b>A.</b> Including the InsP₃-receptor blocker heparin (1 mM) in the intracellular recording solution blocked the dopaminergic facilitation of EPSCs. <b>B.</b> Including the ryanodine receptor blocker dantrolene (20 <b>μ</b>M) in the recording solution also blocked the facilitation of EPSCs by dopamine.</p

Electrophysiologic Management and Considerations for D-transposition of the Great Arteries After Atrial Baffle Repair.

This unique case highlights the electrophysiologic management and risk assessment of sudden cardiac death in a 35-year-old woman with a history of D-transposition of the great arteries status post-Mustard atrial switch repair