Ling-Li Zhang

Regulation of KCC2 and NKCC during development: membrane insertion and differences between cell types.

The developmental switch of GABAs action from excitation to inhibition is likely due to a change in intracellular chloride concentration from high to low. Here we determined if the GABA switch correlates with the developmental expression patterns of KCC2, the chloride extruder K+‐Cl− cotransporter, and NKCC, the chloride accumulator Na+‐K+‐Cl− cotransporter. Immunoblots of ferret retina showed that KCC2 upregulated in an exponential manner similar to synaptophysin (a synaptic marker). In contrast, NKCC, which was initially expressed at a constant level, upregulated quickly between P14 and P28, and finally downregulated to an adult level that was greater than the initial phase. At the cellular level, immunocytochemistry showed that in the inner plexiform layer KCC2s density increased gradually and its localization within ganglion cells shifted from being primarily in the cytosol (between P1–13) to being in the plasma membrane (after P21). In the outer plexiform layer, KCC2 was detected as soon as this layer started to form and increased gradually. Interestingly, however, KCC2 was initially restricted to photoreceptor terminals, while in the adult it was restricted to bipolar dendrites. Thus, the overall KCC2 expression level in ferret retina increases with age, but the time course differs between cell types. In ganglion cells the upregulation of KCC2 by itself cannot explain the relatively fast switch in GABAs action; additional events, possibly KCC2s integration into the plasma membrane and downregulation of NKCC, might also contribute. In photoreceptors the transient expression of KCC2 suggests a role for this transporter in development. J. Comp. Neurol. 499:132–143, 2006.

The Slow Afterhyperpolarization: A Target of β1-Adrenergic Signaling in Hippocampus-Dependent Memory Retrieval

In rodents, adrenergic signaling by norepinephrine (NE) in the hippocampus is required for the retrieval of intermediate-term memory. NE promotes retrieval via the stimulation of β1-adrenergic receptors, the production of cAMP, and the activation of both protein kinase A (PKA) and the exchange protein activated by cAMP. However, a final effector for this signaling pathway has not been identified. Among the many targets of adrenergic signaling in the hippocampus, the slow afterhyperpolarization (sAHP) is an appealing candidate because its reduction by β1 signaling enhances excitatory neurotransmission. Here we report that reducing the sAHP is critical for the facilitation of retrieval by NE. Direct blockers of the sAHP, as well as blockers of the L-type voltage-dependent calcium influx that activates the sAHP, rescue retrieval in mutant mice lacking either NE or the β1 receptor. Complementary to this, a facilitator of L-type calcium influx impairs retrieval in wild-type mice. In addition, we examined the role of NE in the learning-related reduction of the sAHP observed ex vivo in hippocampal slices. We find that this reduction in the sAHP depends on the induction of persistent PKA activity specifically in conditioned slices. Interestingly, this persistent PKA activity is induced by NE/β1 signaling during slice preparation rather than during learning. These observations suggest that the reduction in the sAHP may not be present autonomously in vivo, but is likely induced by neuromodulatory input, which is consistent with the idea that NE is required in vivo for reduction of the sAHP during memory retrieval.

mGluR6 Transcripts in Non-neuronal Tissues

To study mGluR6 expression, the authors investigated two transgenic mouse lines that express enhanced green fluorescent protein (GFP) under control of mGluR6 promoter. In retina, GFP was expressed exclusively in all ON bipolar cell types, either uniformly across all cells of this class (line 5) or in a mosaic (patchy) fashion (line 1). In brain, GFP was found in certain cortical areas, superior colliculus, axons of the corpus callosum, accessory olfactory bulb, and cells of the subcommissural organ. Outside the nervous system, GFP was seen in the corneal endothelium, testis, the kidney’s medulla, collecting ducts and parietal layer that surround the glomeruli, and B lymphocytes. Furthermore, RT-PCR showed that most tissues that expressed GFP in the transgenic mouse also transcribed two splice variants of mGluR6 in the wild-type mouse. The alternate variant was lacking exon 8, predicting a protein product of 545 amino acids that lacks the 7-transmembrane domains of the receptor. In cornea, immunostaining for mGluR6 gave strong staining in the endothelium, and this was stronger in wild-type than in mGluR6-null mice. Furthermore, calcium imaging with Fura-2 showed that application of L-AP4, an agonist for group III metabotropic glutamate receptors including mGluR6, elevated calcium in endothelial cells.

GABA, Glycine and Cation-Chloride Cotransporters in Retinal Function and Development

This chapter introduces the GABA, glycine and chloride cotransporters and their functions. The diversity of GABA and glycines actions in mature nervous system are also discussed. It focuses mainly on the retina because the wealth of information regarding specific cell types and their functions in this tissue greatly facilitates the understanding of computations and performance of neuronal circuits. It also discusses the function of inhibitory neurotransmitters and cation-coupled Clˉ cotransporters in retinal development. The retina is a thin sheet of brain tissue that grows out into the eye to provide neural processing for image processing. The retina includes photoreceptors and two stages of neural processing. Its output cells project centrally and the information they convey is analyzed by about half of the cerebral cortex. Neuronal processes in the cerebral cortex can span millimeters to centimeters while in the retina the two synaptic layers span only 60μm, and most lateral processes span only several hundred μm. The general design of the retina encompasses several parallel feedforward pathways and many intricate feedback circuits. The feedforward pathways comprise three orders of hierarchical neurons that transmit information in two stages, using glutamate as their neurotransmitter.