Spiking Patterns and Their Functional Implications in the Antennal Lobe of the Tobacco Hornworm Manduca sexta

Bursting as well as tonic firing patterns have been described in various sensory systems. In the olfactory system, spontaneous bursts have been observed in neurons distributed across several synaptic levels, from the periphery, to the olfactory bulb (OB) and to the olfactory cortex. Several in vitro studies indicate that spontaneous firing patterns may be viewed as “fingerprints” of different types of neurons that exhibit distinct functions in the OB. It is still not known, however, if and how neuronal burstiness is correlated with the coding of natural olfactory stimuli. We thus conducted an in vivo study to probe this question in the OB equivalent structure of insects, the antennal lobe (AL) of the tobacco hornworm Manduca sexta. We found that in the moth's AL, both projection (output) neurons (PNs) and local interneurons (LNs) are spontaneously active, but PNs tend to produce spike bursts while LNs fire more regularly. In addition, we found that the burstiness of PNs is correlated with the strength of their responses to odor stimulation – the more bursting the stronger their responses to odors. Moreover, the burstiness of PNs was also positively correlated with the spontaneous firing rate of these neurons, and pharmacological reduction of bursting resulted in a decrease of the neurons' responsiveness. These results suggest that neuronal burstiness reflects a physiological state of these neurons that is directly linked to their response characteristics

Spatiotemporal Properties of the Action Potential Propagation in the Mouse Visual Cortical Slice Analyzed by Calcium Imaging

The calcium ion (Ca2+) is an important messenger for signal transduction, and the intracellular Ca2+ concentration ([Ca2+]i) changes in response to an excitation of the cell. To reveal the spatiotemporal properties of the propagation of an excitatory signal with action potentials in the primary visual cortical circuit, we conducted a Ca2+ imaging study on slices of the mouse visual cortex. Electrical stimulation of layer 4 evoked [Ca2+]i transients around the stimulus electrode. Subsequently, the high [Ca2+]i region mainly propagated perpendicular to the cortical layer (vertical propagation), with horizontal propagation being restricted. When the excitatory synaptic transmission was blocked, only weak and concentric [Ca2+]i transients were observed. When the action potential was blocked, the [Ca2+]i transients disappeared almost completely. These results suggested that the action potential contributed to the induction of the [Ca2+]i transients, and that excitatory synaptic connections were involved in the propagation of the high [Ca2+]i region in the primary visual cortical circuit. To elucidate the involvement of inhibitory synaptic connections in signal propagation in the primary visual cortex, the GABAA receptor inhibitor bicuculline was applied. In this case, the evoked signal propagated from layer 4 to the entire field of view, and the prolonged [Ca2+]i transients were observed compared with the control condition. Our results suggest that excitatory neurons are widely connected to each other over the entire primary visual cortex with recurrent synapses, and inhibitory neurons play a fundamental role in the organization of functional sub-networks by restricting the propagation of excitation signals

Tamapin, a venom peptide from the Indian red scorpion (mesobuthus tamulus) that targets small conductance Ca2+-activated K+ channels and afterhyperpolarization currents in central neurons

The biophysical properties of small conductance Ca2+-activated K+ (SK) channels are well suited to underlie afterhyperpolarizations (AHPs) shaping the firing patterns of a conspicuous number of central and peripheral neurons. We have identified a new scorpion toxin (tamapin) that binds to SK channels with high affinity and inhibits SK channel-mediated currents in pyramidal neurons of the hippocampus as well as in cell lines expressing distinct SK channel subunits. This toxin distinguished between the SK channels underlying the apamin-sensitive IAHP and the Ca2+-activated K+ channels mediating the slow IAHP (sIAHP) in hippocampal neurons. Compared with related scorpion toxins, tamapin displayed a unique, remarkable selectivity for SK2 versus SK1 (~1750-fold) and SK3 (~70-fold) channels and is the most potent SK2 channel blocker characterized so far (IC50 for SK2 channels = 24 pM). Tamapin will facilitate the characterization of the subunit composition of native SK channels and help determine their involvement in electrical and biochemical signaling.</p

Distribution of Bacillus thuringiensis subsp. israelensis in Soil of a Swiss Wetland Reserve after 22 Years of Mosquito Control▿†

Recurrent treatments with Bacillus thuringiensis subsp. israelensis are required to control the floodwater mosquito Aedes vexans that breeds in large numbers in the wetlands of the Bolle di Magadino Reserve in Canton Ticino, Switzerland. Interventions have been carried out since 1988. In the present study, the spatial distribution of resting B. thuringiensis subsp. israelensis spores in the soil was measured. The B. thuringiensis subsp. israelensis concentration was determined in soil samples collected along six transects covering different elevations within the periodically flooded zones. A total of 258 samples were processed and analyzed by quantitative PCR that targeted an identical fragment of 159 bp for the B. thuringiensis subsp. israelensis cry4Aa and cry4Ba genes. B. thuringiensis subsp. israelensis spores were found to persist in soils of the wetland reserve at concentrations of up to 6.8 log per gram of soil. Continuous accumulation due to regular treatments could be excluded, as the decrease in spores amounted to 95.8% (95% confidence interval, 93.9 to 97.7%). The distribution of spores was correlated to the number of B. thuringiensis subsp. israelensis treatments, the elevation of the sampling point, and the duration of the flooding periods. The number of B. thuringiensis subsp. israelensis treatments was the major factor influencing the distribution of spores in the different topographic zones (P < 0.0001). These findings indicated that B. thuringiensis subsp. israelensis spores are rather immobile after their introduction into the environment

Neuronal Responses to Lemniscal Stimulation in Laminar Brain Slices of the Inferior Colliculus

The central nucleus of the inferior colliculus (ICC) receives inputs from all parts of the auditory brainstem and transmits the information to the forebrain. Fibrodendritic laminae of the ICC provide a structural basis for a tonotopic organization, and the interaction of inputs within a single layer is important for ICC processing. Transverse slice planes of the ICC sever the layers and many of the ascending axons that enter through the lateral lemniscus. Consequently, the activity initiated within a lamina by a pure lemniscal stimulus is not well characterized. Here, we use a slice plane that maintains the integrity of the laminae in ICC and allows the axons in the lateral lemniscus to be stimulated at a distance from the ICC. We examined both the postsynaptic currents and potentials of the same neurons to lemniscal stimuli in this laminar brain slice. Our main finding is that lemniscal stimulation evokes prolonged synaptic potentials in ICC neurons. Synaptic potential amplitudes and durations increase with lemniscal shock strength. In ∼50% of ICC neurons, the postsynaptic potential is equal in duration to the postsynaptic current, whereas in the remaining neurons it is three to four times longer. Synaptic responses to single shocks or shock trains exhibit plateau potentials that enable sustained firing in ICC neurons. Plateau potentials are evoked by N-methyl-d-aspartate (NMDA) receptor activation, and their amplitudes and durations are regulated by both NMDA-R and gamma-aminobutyric acid A (GABA(A))-R activation. These data suggest that in the intact laminae of the ICC, lemniscal inputs initiate sustained firing through monosynaptic and polysynaptic NMDA-mediated synapses regulated by GABA(A) synapses