Dissociable mesolimbic dopamine circuits control responding triggered by alcohol-predictive discrete cues and contexts.

Context can influence reactions to environmental cues and this elemental process has implications for substance use disorder. Using an animal model, we show that an alcohol-associated context elevates entry into a fluid port triggered by a conditioned stimulus (CS) that predicted alcohol (CS-triggered alcohol-seeking). This effect persists across multiple sessions and, after it diminishes in extinction, the alcohol context retains the capacity to augment reinstatement. Systemically administered eticlopride and chemogenetic inhibition of ventral tegmental area (VTA) dopamine neurons reduce CS-triggered alcohol-seeking. Chemogenetically silencing VTA dopamine terminals in the nucleus accumbens (NAc) core reduces CS-triggered alcohol-seeking, irrespective of context, whereas silencing VTA dopamine terminals in the NAc shell selectively reduces the elevation of CS-triggered alcohol-seeking in an alcohol context. This dissociation reveals new roles for divergent mesolimbic dopamine circuits in the control of responding to a discrete cue for alcohol and in the amplification of this behaviour in an alcohol context.

Dopamine induces release of calcium from internal stores in layer II lateral entorhinal cortex fan cells.

The entorhinal cortex plays an important role in temporal lobe processes including learning and memory, object recognition, and contextual information processing. The alteration of the strength of synaptic inputs to the lateral entorhinal cortex may therefore contribute substantially to sensory and mnemonic functions. The neuromodulatory transmitter dopamine exerts powerful effects on excitatory glutamatergic synaptic transmission in the entorhinal cortex. Interestingly, inputs from midbrain dopamine neurons appear to specifically target clusters of excitatory cells located in the superficial layers of the entorhinal cortex. We have previously demonstrated that dopamine facilitates synaptic transmission through the activation of D1-like receptors. This facilitation of synaptic transmission is dependent on both activation of classical D1-like-receptors, and upon activation of dopamine receptors linked to increases in phospholipase C, inositol triphosphate (IP3), and intracellular calcium. In the present study we combined electrophysiological recordings of evoked excitatory postsynaptic currents with imaging of intracellular calcium using the fluorescent indicator fluo-4 to monitor calcium transients evoked by dopamine in electrophysiologically identified putative fan and pyramidal cells of the lateral entorhinal cortex. Bath application of dopamine (1 µM), or the phosphatidylinositol (PI)-linked D1-like-receptor agonist SKF83959 (5 µM), induced reliable and reversible increases in fluo-4 fluorescence and excitatory postsynaptic currents in fan cells, but not in pyramidal cells. In contrast, application of the classical D1-like-receptor agonist SKF38393 (10 µM) did not result in significant increases in fluorescence. Blocking release of calcium from internal stores by loading cells with the IP3 receptor blocker heparin (1 mM) or the ryanodine receptor blocker dantrolene (20 µM) abolished both the calcium transients and the facilitation of evoked synaptic currents induced by dopamine. Dopamine also induced calcium transients in fan cells when calcium was excluded from the extracellular medium, further indicating that the calcium transients are linked to release from internal stores. These results indicate that following D1-like-receptor binding, dopamine selectively induces transient elevations in intracellular calcium via activation of IP3 and ryanodine receptors, and that these elevations are linked to the facilitation of synaptic responses in putative layer II entorhinal cortex fan cells.

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

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 D1 receptors 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.