Adán Hernández

Dynamics of the Parkinsonian Striatal Microcircuit: Entrainment into a Dominant Network State

Neuronal synchronization in basal ganglia circuits plays a key role in the encoding of movement, procedural memory storage and habit formation. Striatal dopamine (DA) depletion during Parkinsonism causes abnormal synchronization in corticobasal ganglia loops resulting in motor dysfunction. However, the dynamics of the striatal microcircuit underlying abnormal synchronization in Parkinsonism is poorly understood. Here we used targeted whole-cell recordings, calcium imaging allowing the recording from dozens of cells simultaneously and analytical approaches, to describe the striking alterations in network dynamics that the striatal microcircuit undergoes following DA depletion in a rat model of Parkinson disease (PD): In addition to a significant enhancement of basal neuronal activity frequent periods of spontaneous synchronization were observed. Multidimensional reduction techniques of vectorized network dynamics revealed that increased synchronization resulted from a dominant network state that absorbed most spontaneously active cells. Abnormal synchronous activity can be virtually abolished by glutamatergic antagonists, while blockade of GABAergic transmission facilitates the engagement of striatal cell assemblies in the dominant state. Finally, a dopaminergic receptor agonist was capable of uncoupling neurons from the dominant state. Abnormal synchronization and “locking” into a dominant state may represent the basic neuronal mechanism that underlies movement disorders at the microcircuit level.

Muscarinic Enhancement of Persistent Sodium Current Synchronizes Striatal Medium Spiny Neurons

Network dynamics denoted by synchronous firing of neuronal pools rely on synaptic interactions and intrinsic properties. In striatal medium spiny neurons, N-methyl-d-aspartate (NMDA) receptor activation endows neurons with nonlinear capabilities by inducing a negative-slope conductance region (NSCR) in the current-voltage relationship. Nonlinearities underlie associative learning, procedural memory, and the sequential organization of behavior in basal ganglia nuclei. The cholinergic system modulates the function of medium spiny projection neurons through the activation of muscarinic receptors, increasing the NMDA-induced NSCR. This enhancement is reflected as a change in the NMDA-induced network dynamics, making it more synchronous. Nevertheless, little is known about the contribution of intrinsic properties that promote this activity. To investigate the mechanisms underlying the cholinergic modulation of bistable behavior in the striatum, we used whole cell and calcium-imaging techniques. A persistent sodium current modulated by muscarinic receptor activation participated in the enhancement of the NSCR and the increased network synchrony. These experiments provide evidence that persistent sodium current generates bistable behavior in striatal neurons and contributes to the regulation of synchronous network activity. The neuromodulation of bistable properties could represent a cellular and network mechanism for cholinergic actions in the striatum.

DOPAMINE INHIBITS GABA TRANSMISSION FROM THE GLOBUS PALLIDUS TO THE THALAMIC RETICULAR NUCLEUS VIA PRESYNAPTIC D4 RECEPTORS

The globus pallidus sends a significant GABAergic projection to the thalamic reticular nucleus. Because pallidal neurons express D4-dopamine receptors, we have explored their presence on pallidoreticular terminals by studying the effect of dopamine and D4-receptor agonists on the GABAergic transmission in the thalamic reticular nucleus. We made whole-cell recordings of inhibitory postsynaptic currents (IPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) in the thalamic reticular neurons. Dopamine consistently reduced the IPSCs. The effect of dopamine was associated with paired-pulse facilitation, indicating a presynaptic location of the receptors. The effect of dopamine was also measured on the mIPSCs, reducing their frequency but not affecting their amplitude, which also suggests a presynaptic site of action. The selective D4-receptor agonist PD 168,077 also reduced the IPSCs, which was also associated with paired-pulse facilitation. In addition, this agonist reduced the frequency of the mIPSCs with no effect on their amplitude. The D4-receptor antagonist L-745,870 totally blocked the effect of the D4-receptor agonist, indicating the specificity of its effect. To verify the location of the receptors on the pallidal terminals, these were eliminated by injecting kainic acid into the globus pallidus. Kainic acid produced a drastic (80%) fall in the globus pallidus neuronal population. In this condition, the effect of the activation of D4 receptors both on the IPSCs and mIPSCs was prevented, thus indicating that the location of the receptors was on the pallidal terminals. Our results demonstrate that dopamine controls the activity of the thalamic reticular neurons by regulating the inhibitory input from the globus pallidus.

Comparison of Ca2+ Currents of Chromaffin Cells from Normotensive Wistar Kyoto and Spontaneously Hypertensive Rats

Spontaneously hypertensive rats (SHR) are widely used as model to investigate the pathophysiological mechanisms of essential hypertension. Catecholamine plasma levels are elevated in SHR, suggesting alterations of the sympathoadrenal axis. The residual hypertension in sympathectomized SHR is reduced after demedullation, suggesting a dysfunction of the adrenal medulla. Intact adrenal glands exposed to acetylcholine or high K+ release more catecholamine in SHR than in normotensive Wistar Kyoto (WKY) rats, and adrenal chromaffin cells (CCs) from SHR secrete more catecholamines than CCs from WKY rats. Since Ca2+ entry through voltage-gated Ca2+ channels (VGCC) triggers exocytosis, alterations in the functional properties of these channels might underlie the enhanced catecholamine release in SHR. This study compares the electrophysiological properties of VGCC from CCs in acute adrenal slices from WKY rats and SHR at an early stage of hypertension. No significant differences were found in the macroscopic Ca2+ currents (current density, I–V curve, voltage dependence of activation and inactivation, kinetics) between CCs of SHR and WKY rats, suggesting that Ca2+ entry through VGCC is not significantly different between these strains, at least at early stages of hypertension. Ca2+ buffering, sequestration and extrusion mechanisms, as well as Ca2+ release from intracellular stores, must now be evaluated to determine if alterations in their function can explain the enhanced catecholamine secretion reported in CCs from SHR.

Down-Modulation of Ca2+ Channels by Endogenously Released ATP and Opioids: from the Isolated Chromaffin Cell to the Slice of Adrenal Medullae

Modifications in Ca2+ influx may lead to profound changes in the cell activity associated with Ca2+-dependent processes, from muscle contraction and neurotransmitter release to calcium-mediated cell death. Therefore, calcium entry into the cell requires fine regulation. In this context, understanding of the modulation of voltage-dependent Ca2+ channels seems to be critical. The modulatory process results in the enhancement or decrement of calcium influx that may regulate the local and global cytosolic Ca2+ concentrations. Here, we summarize the well-established data on this matter described in isolated chromaffin cells by our laboratory and others, and the new results we have obtained in a more physiological preparation: freshly isolated slices of mouse adrenal medullae.