When the dimensionality of a neural circuit is substantially larger than the dimensionality of the variable it encodes, many different degenerate network states can produce the same output. In this review I will discuss three different neural systems that are linked by this theme. The pyloric network of the lobster, the song control system of the zebra finch, and the odor encoding system of the locust, while different in design, all contain degeneracies between their internal parameters and the outputs they encode. Indeed, although the dynamics of song generation and odor identification are quite different, computationally, odor recognition can be thought of as running the song generation circuitry backwards. In both of these systems, degeneracy plays a vital role in mapping a sparse neural representation devoid of correlations onto external stimuli (odors or song structure) that are strongly correlated. I argue that degeneracy between input and output states is an inherent feature of many neural systems, which can be exploited as a fault-tolerant method of reliably learning, generating, and discriminating closely related patterns.

The timely onset of metamorphosis in holometabolous insects depends on their reaching the appropriate size known as critical weight. Once critical weight is reached, juvenile hormone (JH) titers decline, resulting in the release of prothoracicotropic hormone (PTTH) at the next photoperiod gate and thereby inducing metamorphosis. How individuals determine when they have reached critical weight is unknown. We present evidence that in Drosophila, a component of the ring gland, the prothoracic gland (PG), assesses growth to determine when critical weight has been achieved.

The established strand-displacement model for mammalian mitochondrial DNA (mtDNA) replication has recently been questioned in light of new data using two-dimensional (2D) agarose gel electrophoresis. It has been proposed that a synchronous, strand-coupled mode of replication occurs in tissues, thereby casting doubt on the general validity of the "orthodox," or strand-displacement model. We have examined mtDNA replicative intermediates from mouse liver using atomic force microscopy and 2D agarose gel electrophoresis in order to resolve this issue. The data provide evidence for only the orthodox, strand-displacement mode of replication and reveal the presence of additional, alternative origins of lagging light-strand mtDNA synthesis. The conditions used for 2D agarose gel analysis are favorable for branch migration of asymmetrically replicating nascent strands. These data reconcile the original displacement mode of replication with the data obtained from 2D gel analyses.

The insulin signaling pathway, which is conserved in evolution from flies to humans, evolved to allow a fast response to changes in nutrient availability while keeping glucose concentration constant in serum. Here we show that, both in Drosophila and mammals, insulin receptor (InR) represses its own synthesis by a feedback mechanism directed by the transcription factor dFOXO/FOXO1. In Drosophila, dFOXO is responsible for activating transcription of dInR, and nutritional conditions can modulate this effect. Starvation up-regulates mRNA of dInR in wild-type but not dFOXO-deficient flies. Importantly, FOXO1 acts in mammalian cells like its Drosophila counterpart, up-regulating the InR mRNA level upon fasting. Mammalian cells up-regulate the InR mRNA in the absence of serum, conditions that induce the dephosphorylation and activation of FOXO1. Interestingly, insulin is able to reverse this effect. Therefore, dFOXO/FOXO1 acts as an insulin sensor to activate insulin signaling, allowing a fast response to the hormone after each meal. Our results reveal a key feedback control mechanism for dFOXO/FOXO1 in regulating metabolism and insulin signaling.

The blood-brain barrier of Drosophila is established by surface glia, which ensheath the nerve cord and insulate it against the potassium-rich hemolymph by forming intercellular septate junctions. The mechanisms underlying the formation of this barrier remain obscure. Here, we show that the G protein-coupled receptor (GPCR) Moody, the G protein subunits G alpha i and G alpha o, and the regulator of G protein signaling Loco are required in the surface glia to achieve effective insulation. Our data suggest that the four proteins act in a complex common pathway. At the cellular level, the components function by regulating the cortical actin and thereby stabilizing the extended morphology of the surface glia, which in turn is necessary for the formation of septate junctions of sufficient length to achieve proper sealing of the nerve cord. Our study demonstrates the importance of morphogenetic regulation in blood-brain barrier development and places GPCR signaling at its core.

We identified moody in a genetic screen for Drosophila mutants with altered cocaine sensitivity. Hypomorphic mutations in moody cause an increased sensitivity to cocaine and nicotine exposure. In contrast, sensitivity to the acute intoxicating effects of ethanol is reduced. The moody locus encodes two novel GPCRs, Moody-alpha and Moody-beta. While identical in their membrane-spanning domains, the two Moody proteins differ in their long carboxy-terminal domains, which are generated by use of alternative reading frames. Both Moody forms are required for normal cocaine sensitivity, suggesting that they carry out distinct but complementary functions. Moody-alpha and Moody-beta are coexpressed in surface glia that surround the nervous system, where they are actively required to maintain the integrity of the blood-brain barrier in the adult fly. We propose that a Moody-mediated signaling pathway functions in glia to regulate nervous system insulation and drug-related behaviors.

We performed simultaneous patch-electrode recordings from the soma and apical dendrite of CA1 pyramidal neurons in hippocampal slices, in order to determine the degree of voltage attenuation along CA1 dendrites. Fifty per cent attenuation of steady-state somatic voltage changes occurred at a distance of 238 microm from the soma in control and 409 microm after blocking the hyperpolarization-activated (H) conductance. The morphology of three neurons was reconstructed and used to generate computer models, which were adjusted to fit the somatic and dendritic voltage responses. These models identify several factors contributing to the voltage attenuation along CA1 dendrites, including high axial cytoplasmic resistivity, low membrane resistivity, and large H conductance. In most cells the resting membrane conductances, including the H conductances, were larger in the dendrites than the soma. Simulations suggest that synaptic potentials attenuate enormously as they propagate from the dendrite to the soma, with greater than 100-fold attenuation for synapses on many small, distal dendrites. A prediction of this powerful EPSP attenuation is that distal synaptic inputs are likely only to be effective in the presence of conductance scaling, dendritic excitability, or both.

In the hypothalamic arcuate nucleus (ARC), pro-opiomelanocortin (POMC) neurons inhibit feeding and neuropeptide-Y (NPY) neurons stimulate feeding. We tested whether neurons in the ventromedial hypothalamic nucleus (VMH), a known satiety center, activate anorexigenic neuronal pathways in the ARC by projecting either excitatory synaptic inputs to POMC neurons and/or inhibitory inputs to NPY neurons. Using laser scanning photostimulation in brain slices from transgenic mice, we found that POMC and NPY neurons, which are interspersed in the ARC, are nevertheless regulated by anatomically distinct synaptic inputs. POMC neurons received strong excitatory input from the medial VMH (mVMH), whereas NPY neurons did not and, instead, received weak inhibitory input only from within the ARC. The strength of the excitatory input from the mVMH to POMC neurons was diminished by fasting. These data identify a new molecularly defined circuit that is dynamically regulated by nutritional state in a manner consistent with the known role of the VMH as a satiety center.

Ligand-gated ion channels involved in the modulation of synaptic strength are the AMPA, kainate, and NMDA glutamate receptors. Small molecules that potentiate AMPA receptor currents relieve cognitive deficits caused by neurodegenerative diseases such as Alzheimer’s disease and show promise in the treatment of depression. Previously, there has been limited understanding of the molecular mechanism of action for AMPA receptor potentiators. Here we present cocrystal structures of the glutamate receptor GluR2 S1S2 ligand-binding domain in complex with aniracetam [1-(4-methoxybenzoyl)-2-pyrrolidinone] or CX614 (pyrrolidino-1,3-oxazino benzo-1,4-dioxan-10-one), two AMPA receptor potentiators that preferentially slow AMPA receptor deactivation. Both potentiators bind within the dimer interface of the nondesensitized receptor at a common site located on the twofold axis of molecular symmetry. Importantly, the potentiator binding site is adjacent to the "hinge" in the ligand-binding core "clamshell" that undergoes conformational rearrangement after glutamate binding. Using rapid solution exchange, patch-clamp electrophysiology experiments, we show that point mutations of residues that interact with potentiators in the cocrystal disrupt potentiator function. We suggest that the potentiators slow deactivation by stabilizing the clamshell in its closed-cleft, glutamate-bound conformation.

A theoretical formulation for optically active sum frequency generation (OA-SFG) from isotropic chiral solutions was proposed for molecules with a chiral side chain and an intrinsically achiral chromophore. Adapting an electron correlation model first proposed by Höhn and Weigang for linear optical activity, we presented a dynamic coupling model for OA-SFG near the electronic resonance of the achiral chromophore. As a demonstration, we used this model to explain the observed OA-SFG spectra of a series of amino acids near the electronic resonance of the intrinsically achiral carboxyl group. Our model shows that the nonlinear chiroptical response comes about by the through-space correlative electronic interactions between the chiral side chain and the achiral chromophore, and its magnitude is determined by the position and orientation of the bonds that make up the chiral side chain. Using the bond polarizability values in the literature and the conformations of amino acids obtained from calculation, we were able to reproduce the relative OA-SFG strength from a series of amino acids.