AMPA/kainate receptor activation inhibits neuronal delayed rectifier K+ current via Na+ entry in rat cortical neurons

Neurophotonics Approaches for the Study of Pattern Separation

Successful memory involves not only remembering over time but also keeping memories distinct. Computational models suggest that pattern separation appears as a highly efficient process to discriminate between overlapping memories. Furthermore, lesion studies have shown that the dentate gyrus (DG) participates in pattern separation. However, these manipulations did not allow identifying the neuronal mechanism underlying pattern separation. The development of different neurophotonics techniques, together with other genetic tools, has been useful for the study of the microcircuit involved in this process. It has been shown that less-overlapped information would generate distinct neuronal representations within the granule cells (GCs). However, because glutamatergic or GABAergic cells in the DG are not functionally or structurally homogeneous, identifying the specific role of the different subpopulations remains elusive. Then, understanding pattern separation requires the ability to manipulate a temporal and spatially specific subset of cells in the DG and ideally to analyze DG cells activity in individuals performing a pattern separation dependent behavioral task. Thus, neurophotonics and calcium imaging techniques in conjunction with activity-dependent promoters and high-resolution microscopy appear as important tools for this endeavor. In this work, we review how different neurophotonics techniques have been implemented in the elucidation of a neuronal network that supports pattern separation alone or in combination with traditional techniques. We discuss the limitation of these techniques and how other neurophotonic techniques could be used to complement the advances presented up to this date.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Inhibition of delayed rectifier K+ conductance in cultured rat cerebellar granule neurons by activation of calcium-permeable AMPA receptors

Activation of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors in cerebellar granule cells during perforated-patch whole-cell recordings activated an inward current at negative voltages which was followed, after a delay, by the inhibition of an outward potassium current at voltages positive to -20 mV. The activated inward current was inwardly rectifying suggesting that the AMPA receptors were Ca2+-permeable. This was confirmed by direct measurements of intracellular calcium where Ca2+ rises were seen following AMPA receptor activation in Na+-free external solution. Ca2+ rises were equally large in the presence of 100 microM Cd2+ to block voltage-gated Ca2+ channels. Specific voltage-protocols, allowing selective activation of the delayed rectifier potassium current (KV) and the transient A current (KA), showed that kainate inhibited KV, but not to any great extent KA. The inhibition of KV was blocked by the AMPA receptor antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and was no longer observed when the KV current was abolished with high concentrations of Ba2+. The responses to kainate were not altered by pre-treating the cells with pertussis toxin, suggesting that the AMPA receptor stimulation of the G-protein Gi cannot account for the effects observed. Replacing extracellular Na+ with choline did not alter the inhibition of KV by kainate, however, removing extracellular Ca2+ reduced the kainate response. The inhibition of KV by kainate was unaffected by the presence of 100 microM Cd2+. The guanylyl cyclase inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), did not alter kainate inhibition of KV. It is concluded that ion influx (particularly Ca2+ ions) through AMPA receptor channels following receptor activation leads to an inhibition of KV currents in cerebellar granule neurons.