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AMPA-type ([alpha]-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) glutamate receptors (AMPARs) mediate post-synaptic depolarization and fast excitatory transmission in the central nervous system. AMPARs are tetrameric ion channels that assemble in the endoplasmic reticulum (ER) in a poorly understood process. The subunit composition determines channel conductance properties and gating kinetics, and also regulates vesicular traffic to and from synaptic sites, and is thus critical for synaptic function and plasticity. The distribution of functionally different AMPARs varies within and between neuronal circuits, and even within individual neurons. In addition, synapses employ channels with specific subunit stoichiometries, depending on the type of input and the frequency of stimulation. Taken together, it appears that assembly is not simply a stochastic process. Recently, progress has been made in understanding the molecular mechanisms underlying subunit assembly and receptor biogenesis in the ER. These processes ultimately determine the size and shape of the postsynaptic response, and are the subject of this review.

Our understanding of the role of GABA signaling in circuit development is rapidly expanding. Here, we review three recent refinements in our understanding of the diverse roles that GABA plays at different stages of neural circuit formation. First, we discuss recent evidence that depolarizing GABA plays at least a permissive role in promoting both excitatory and inhibitory synaptogenesis in developing neurons (including newly generated neurons in the adult). Next, we discuss recent evidence that GABAergic circuits sculpt the temporal and spatial aspects of synaptic integration. Consequently, early developmental events affecting the establishment of GABAergic circuits will control subsequent activity-dependent refinements of information processing and circuit function. In the third section, we review recent evidence of molecular mechanisms by which GABAergic signaling plays a role in the regulation of the balance between GABAergic and glutamatergic transmission in developing circuits. Throughout the review, we concentrate on the effects of the signaling by GABAA receptors, as told from the point of view of the GABA-responsive cells, and do not discuss mechanisms that govern GABA release or activity of GABAergic neurons per se.

During the past few decades, optical methods for imaging activity in networks composed of thousands of neurons have been developed. These techniques rely mainly on organic-chemistry-based dyes as indicators of [Ga.sup.2+] and membrane potential. However, recently a new generation of probes, genetically encoded fluorescent protein sensors, has emerged for use by physiologists studying the operation of neuronal circuits. We critically review the development of these new probes, and analyze objectives and experimental conditions in which classical probes are likely to prevail and where the fluorescent protein sensors will open paths to previously unexplored territories of functional neuroimaging.

Neural circuits are composed of a meshwork of numerous neuron types, each with their own distinctive morphological and intrinsic physiological properties, connectivity and biochemistry. How do distinct neural subcircuits, composed of different classes of neuron, contribute to brain function? Approaching this question requires methods that can target specific neurons types. This can be achieved by harnessing the same machinery that builds sophistication into the brain and using it to make novel tools for investigating and manipulating the brain: molecular and genetic technology. These tools can be used to target gene expression to specific neuron types within complicated neuronal circuits, and the transgenes that are expressed can be used to elucidate and manipulate these circuits with unprecedented precision and control. These methods are likely to become the archetype for future studies linking perception, cognition and behavior to specific components of the brain.

The biologically grounded approach for sensing and actuation of neuronal function is discussed. The analysis of connectivity, dynamics and plasticity of neuronal circuits is possible with the help of a combination of finely resolved optical field sensing and finely resolved optical field actuation.

The input, internal circuitry and output of primary visual cortex (V1) and secondary visual cortex (V2) are discussed. Two main streams, each composed of a mixture of magno, parvo, and konio geniculate signals, originate from cytochrome oxidase patches and interpatches and are projected towards V2.

The structure and function of the middle temporal visual area (MT or V5) is discussed. MT is useful for the study of neural circuits that underlie computations of motion and depth and for examining the relationship between neural activity and perception.