Rewarding stimulation of the lateral hypothalamus induces a dopamine-dependent suppression of synaptic responses in the entorhinal cortex

Activation of Dopamine D2 Receptors Alleviates Neuronal Hyperexcitability in the Lateral Entorhinal Cortex via Inhibition of HCN Current in a Rat Model of Chronic Inflammatory Pain

Functional changes in synaptic transmission from the lateral entorhinal cortex to the dentate gyrus (LEC-DG) are considered responsible for the chronification of pain. However, the underlying alterations in fan cells, which are the predominant neurons in the LEC that project to the DG, remain elusive. Here, we investigated possible mechanisms using a rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. We found a substantial increase in hyperpolarization-activated/cyclic nucleotide-gated currents (Ih), which led to the hyperexcitability of LEC fan cells of CFA slices. This phenomenon was attenuated in CFA slices by activating dopamine D2, but not D1, receptors. Chemogenetic activation of the ventral tegmental area -LEC projection had a D2 receptor-dependent analgesic effect. Intra-LEC microinjection of a D2 receptor agonist also suppressed CFA-induced behavioral hypersensitivity, and this effect was attenuated by pre-activation of the Ih. Our findings suggest that down-regulating the excitability of LEC fan cells through activation of the dopamine D2 receptor may be a strategy for treating chronic inflammatory pain.

Distinct Transcriptomic Cell Types and Neural Circuits of the Subiculum and Prosubiculum along the Dorsal-Ventral Axis

Subicular regions play important roles in spatial processing and many cognitive functions, and these are mainly attributed to the subiculum (Sub) rather than the prosubiculum (PS). Using single-cell RNA sequencing, we identify 27 transcriptomic cell types residing in sub-domains of the Sub and PS. Based on in situ expression of reliable transcriptomic markers, the precise boundaries of the Sub and PS are consistently defined along the dorsoventral axis. Using these borders to evaluate Cre-line specificity and tracer injections, we find bona fide Sub projections topographically to structures important for spatial processing and navigation. In contrast, the PS sends its outputs to widespread brain regions crucial for motivation, emotion, reward, stress, anxiety, and fear. The Sub and PS, respectively, dominate dorsal and ventral subicular regions and receive different afferents. These results reveal two molecularly and anatomically distinct circuits centered in the Sub and PS, respectively, providing a consistent explanation for historical data and a clearer foundation for future studies.

Dopamine suppresses persistent firing in layer III lateral entorhinal cortex neurons

Persistent firing in layer III entorhinal cortex neurons that can be evoked during muscarinic receptor activation may contribute to mechanisms of working memory. The entorhinal cortex receives strong dopaminergic inputs which may modulate working memory for motivationally significant information. We used whole cell recordings in in vitro rat brain slices to assess the effects of dopamine on persistent firing in layer III neurons initiated by depolarizing current injection. Persistent firing during pharmacological block of ionotropic excitatory and inhibitory synaptic transmission, and in the presence of the cholinergic agonist carbachol (10 μM), was observed in 39% of layer III pyramidal cells. Addition of 1 μM dopamine suppressed the incidence of persistent firing and similarly reduced the mean probability of induction of persistent firing at each current step, without significantly affecting the latency, duration, plateau potential, or frequency of persistent firing that was induced. These results indicate that dopamine can result in a suppression of the induction of persistent firing in layer III entorhinal neurons, while still being permissive of persistent firing once it is initiated.

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.

Dopaminergic enhancement of excitatory synaptic transmission in layer II entorhinal neurons is dependent on D₁-like receptor-mediated signaling

The modulatory neurotransmitter dopamine induces concentration-dependent changes in synaptic transmission in the entorhinal cortex, in which high concentrations of dopamine suppress evoked excitatory postsynaptic potentials (EPSPs) and lower concentrations induce an acute synaptic facilitation. Whole-cell current-clamp recordings were used to investigate the dopaminergic facilitation of synaptic responses in layer II neurons of the rat lateral entorhinal cortex. A constant bath application of 1 μM dopamine resulted in a consistent facilitation of EPSPs evoked in layer II fan cells by layer I stimulation; the size of the facilitation was more variable in pyramidal neurons, and synaptic responses in a small group of multiform neurons were not modulated by dopamine. Isolated inhibitory synaptic responses were not affected by dopamine, and the facilitation of EPSPs was not associated with a change in paired-pulse facilitation ratio. Voltage-clamp recordings of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptor-mediated excitatory postsynaptic currents (EPSCs) were facilitated by dopamine, but N-methyl-D-aspartate receptor-mediated currents were not. Bath application of the dopamine D₁-like receptor blocker SCH23390 (50 μM), but not the D₂-like receptor blocker sulpiride (50 μM), prevented the facilitation, indicating that it is dependent upon D₁-like receptor activation. Dopamine D₁ receptors lead to activation of protein kinase A (PKA), and including the PKA inhibitor H-89 or KT 5720 in the recording pipette solution prevented the facilitation of EPSCs. PKA-dependent phosphorylation of inhibitor 1 or the dopamine- and cAMP-regulated protein phosphatase (DARPP-32) can lead to a facilitation of AMPA receptor responses by inhibiting the activity of protein phosphatase 1 (PP1) that reduces dephosphorylation of AMPA receptors, and we found here that inhibition of PP1 occluded the facilitatory effect of dopamine. The dopamine-induced facilitation of AMPA receptor-mediated synaptic responses in layer II neurons of the lateral entorhinal cortex is therefore likely mediated via a D₁ receptor-dependent increase in PKA activity and a resulting inhibition in PP1-dependent dephosphorylation of AMPA receptors.

Exposure to cues associated with palatable food reward results in a dopamine D₂ receptor-dependent suppression of evoked synaptic responses in the entorhinal cortex

BACKGROUND

The lateral entorhinal cortex receives inputs from ventral tegmental area dopamine neurons that are activated by exposure to food-related cues, and exogenously applied dopamine is known to modulate excitatory synaptic responses within the entorhinal cortex.

METHODS

The present study used in vivo synaptic field potential recording techniques to determine how exposure to cues associated with food reward modulates synaptic responses in the entorhinal cortex of the awake rat. Chronically implanted electrodes were used to monitor synaptic potentials in the entorhinal cortex evoked by stimulation of the piriform (olfactory) cortex, and to determine how synaptic responses are modulated by food-related cues.

RESULTS

The amplitudes of evoked synaptic responses were reduced during exposure to cues associated with delivery of chocolate, and during delivery of chocolate for consumption at unpredictable intervals. Reductions in synaptic responses were not well predicted by changes in behavioural mobility, and were not fully blocked by systemic injection of either the D₁-like receptor antagonist SCH23390, or the muscarinic receptor antagonist scopolamine. However, the reduction in synaptic responses was blocked by injection of the D₂-like receptor antagonist eticlopride.

CONCLUSIONS

Exposure to cues associated with palatable food results in a suppression of synaptic responses in olfactory inputs to the entorhinal cortex that is mediated in part by activation of dopamine D₂ receptors.