Faculty Bibliography

Characterizing the responses of different mouse strains to experimentally-induced seizures can provide clues to the genes that are responsible for seizure susceptibility, and factors that contribute to epilepsy. This approach is optimal when sequenced mouse strains are available. Therefore, we compared two sequenced strains, DBA/2J (DBA) and A/J. These strains were compared using the chemoconvulsant pilocarpine, because pilocarpine induces status epilepticus, a state of severe, prolonged seizures. In addition, pilocarpine-induced status is followed by changes in the brain that are associated with the pathophysiology of temporal lobe epilepsy (TLE). Therefore, pilocarpine can be used to address susceptibility to severe seizures, as well as genes that could be relevant to TLE. A/J mice had a higher incidence of status, but a longer latency to status than DBA mice. DBA mice exhibited more hippocampal pyramidal cell damage. DBA mice developed more ectopic granule cells in the hilus, a result of aberrant migration of granule cells born after status. DBA mice experienced sudden death in the weeks following status, while A/J mice exhibited the most sudden death in the initial hour after pilocarpine administration. The results support previous studies of strain differences based on responses to convulsants. They suggest caution in studies of seizure susceptibility that are based only on incidence or latency. In addition, the results provide new insight into the strain-specific characteristics of DBA and A/J mice. A/J mice provide a potential resource to examine the progression to status. The DBA mouse may be valuable to clarify genes regulating other seizure-associated phenomena, such as seizure-induced neurogenesis and sudden death

BACKGROUND: Obtaining a complete phenotypic characterization of a freely moving organism is a difficult task, yet such a description is desired in many neuroethological studies. Many metrics currently used in the literature to describe locomotor and exploratory behavior are typically based on average quantities or subjectively chosen spatial and temporal thresholds. All of these measures are relatively coarse-grained in the time domain. It is advantageous, however, to employ metrics based on the entire trajectory that an organism takes while exploring its environment. METHODOLOGY/PRINCIPAL FINDINGS: To characterize the locomotor behavior of Drosophila melanogaster, we used a video tracking system to record the trajectory of a single fly walking in a circular open field arena. The fly was tracked for two hours. Here, we present techniques with which to analyze the motion of the fly in this paradigm, and we discuss the methods of calculation. The measures we introduce are based on spatial and temporal probability distributions and utilize the entire time-series trajectory of the fly, thus emphasizing the dynamic nature of locomotor behavior. Marginal and joint probability distributions of speed, position, segment duration, path curvature, and reorientation angle are examined and related to the observed behavior. CONCLUSIONS/SIGNIFICANCE: The measures discussed in this paper provide a detailed profile of the behavior of a single fly and highlight the interaction of the fly with the environment. Such measures may serve as useful tools in any behavioral study in which the movement of a fly is an important variable and can be incorporated easily into many setups, facilitating high-throughput phenotypic characterization

Neurotrophins are potent survival factors for developing and injured neurons. However, they are not being used to treat neurodegenerative diseases because of difficulties in administration and numerous side effects that have been encountered in previous clinical trials. Their biological activities use Trk (tropomyosin-related kinase) transmembrane tyrosine kinases. Therefore, one alternative approach is to use transactivation pathways such as adenosine 2A receptor agonists, which can activate Trk receptor signaling independent of neurotrophin binding. However, the relevance in vivo and applicability of these transactivation events during neurodegenerative and injury conditions have never been extensively studied. Here we demonstrate that motoneuron survival after facial nerve lesioning is significantly enhanced by transactivation of Trk receptor tyrosine kinases by adenosine agonists. Moreover, survival of motoneurons directly required the activation of the BDNF receptor TrkB and an increase in Akt (AKT8 virus oncogene cellular homolog) activity. The ability of small molecules to activate a trophic response by using Trk signaling provides a unique mechanism to promote survival signals in motoneurons and suggests new strategies for using transactivation in neurodegenerative diseases

Although cerebellar Purkinje cell complex spikes occur at low frequency (approximately 1/s), each complex spike is often associated with a high-frequency burst (approximately 500/s) of climbing fiber spikes. We examined the possibility that signals are present within the climbing fiber bursts. By intracellularly recording from depolarized, nonspiking Purkinje cells in anesthetized pigmented rabbits, climbing fiber burst patterns were investigated by determining the number of components in the induced compound EPSPs during spontaneous activity and during visual stimulation. For our sample of 43 cells, >70% of all EPSPs were of the compound type composed of two or three EPSPs. During spontaneous activity, the number of components in each compound EPSP was not related to the latency to the succeeding compound EPSP. Conversely, the number of components in each compound EPSP was related to its latency after the preceding compound EPSP. This latency increased from 0.62 s for one-component EPSPs to 1.69 s for compound EPSPs with four or more components. The effect of visual stimulation on the climbing fiber activity was studied in 19 floccular Purkinje cells whose low-frequency interburst climbing fiber response was modulated by movement about the vertical axis. During sinusoidal oscillation (0.1 Hz, +/-10 degrees), compound EPSPs with a larger number of components tended to be more prevalent during movement in the excitatory direction than in the inhibitory direction. Thus, climbing fibers can, in addition to modulation of their low interburst frequency, transmit signals in the form of the number of spikes within each high-frequency burst

Sensory signals of widely differing dynamic range and intensity are transformed into a common firing rate code by thalamocortical neurons. While a great deal is known about the ionic currents, far less is known about the specific channel subtypes regulating thalamic firing rates. We hypothesized that different K(+) and Ca(2+) channel subtypes control different stimulus-response curve properties. To define the channels, we measured firing rate while pharmacologically or genetically modulating specific channel subtypes. Inhibiting Kv3.2 K(+) channels strongly suppressed maximum firing rate by impairing membrane potential repolarization, while playing no role in the firing response to threshold stimuli. By contrast, inhibiting Kv1 channels with alpha-dendrotoxin or maurotoxin strongly increased firing rates to threshold stimuli by reducing the membrane potential where action potentials fire (V(th)). Inhibiting SK Ca(2+)-activated K(+) channels with apamin robustly increased gain (slope of the stimulus-response curve) and maximum firing rate, with minimum effects on threshold responses. Inhibiting N-type Ca(2+) channels with omega-conotoxin GVIA or omega-conotoxin MVIIC partially mimicked apamin, while inhibiting L-type and P/Q-type Ca(2+) channels had small or no effects. EPSC-like current injections closely mimicked the results from tonic currents. Our results show that Kv3.2, Kv1, SK potassium and N-type calcium channels strongly regulate thalamic relay neuron sensory transmission and that each channel subtype controls a different stimulus-response curve property. Differential regulation of threshold, gain and maximum firing rate may help vary the stimulus-response properties across and within thalamic nuclei, normalize responses to diverse sensory inputs, and underlie sensory perception disorders

Cajal-Retzius (CR) cells, the earliest-born neurons in the neocortex, arise from discrete sources within the telencephalon, including the dorsal midline and the pallial-subpallial boundary (PSB). In particular, the cortical hem, a region of high bone morphogenetic proteins (BMPs) and Wnt (wingless-type MMTV integration site family) expression but lacking in Foxg1 (forkhead box G1) is a major source of CR neurons. Whether CR cells from distinct origins arise from disparate developmental processes or share a common mechanism is unclear. To elucidate the molecular basis of CR cell development, we assessed the role of both Foxg1 and dorsal midline signaling in the production of cortical hem- and PSB-derived CR cells. We demonstrate that the loss of Foxg1 results in the overproduction of both of these CR populations. However, removal of Foxg1 at embryonic day 13, although expanding the number of CR cells with a PSB phenotype, does not result in an expansion of BMPs or Wnts in the dorsomedial signaling center. Conversely, loss of the dorsal midline ligands as observed in Gli3 (glioma-associated oncogene homolog 3) mutants results in the loss of the cortical hem-derived CR character but does not affect the specification of PSB-derived CR cells. Hence, our findings demonstrate that, although the specification of cortical hem-derived CR cells is dependent on signaling from the dorsal midline, Foxg1 functions to repress the generation of both cortical hem- and PSB-derived CR cells

The visual system, with its ability to perceive motion, is crucial for most animals to walk or fly steadily. Theoretical models of motion detection exist, but the underlying cellular mechanisms are still poorly understood. In this issue of Neuron, Rister and colleagues dissect the function of neuronal subtypes in the optic lobe of Drosophila to reveal their role in motion detection.

Phosphodiesterase type 5 (PDE5) acts specifically on cyclic guanosine monophosphate (cGMP) and terminates cGMP-mediated signalling. PDE5 has a well established role in vascular smooth muscle, where specific inhibitors of PDE5 such as sildenafil correct erectile dysfunction by augmenting cGMP-mediated vascular relaxation. However, the role of PDE5 outside of the vasculature has received little attention. The present study tested PDE5 inhibitors on the cGMP-mediated modulation of K(+) channels in the neurohypophysis (posterior pituitary). Photolysis of caged-cGMP enhanced current through Ca(2+)-activated K(+) channels, and this enhancement recovered in about 2 min. Sildenafil essentially eliminated this recovery, suggesting that the reversal of K(+) current enhancement depends on cGMP breakdown. Activation of nitric oxide synthase during trains of activity in pituitary nerve terminals enhances excitability. When trains of stimulation were applied at regular intervals, sildenafil enhanced the excitability of neurohypophysial nerve terminals and increased the action potential firing probability. T-1032, a compound with high specificity for PDE5 over PDE6, had a similar action. Voltage imaging in intact neurohypophysis with a voltage sensitive absorbance dye showed that T-1032 reduced the failure of propagating action potentials during trains of activity. This indicates that PDE5 activity limits action potential propagation in neurohypophysial axons. Immunoassay of oxytocin, a neuropeptide hormone secreted by the posterior pituitary, demonstrated that sildenafil increased electrically evoked release. Thus, PDE5 plays an important role in the regulation of neurohypophysial function, and blockade of this enzyme can enhance the use-dependent facilitation of neurohypophysial secretion

We report that temporal spike sequences from hippocampal place neurons of rats on an elevated track recurred in reverse order at the end of a run, but in forward order in anticipation of the run, coinciding with sharp waves. Vector distances between the place fields were reflected in the temporal structure of these sequences. This bidirectional re-enactment of temporal sequences may contribute to the establishment of higher-order associations in episodic memory