Dendritic Ca2+ transient increase evoked by wind stimulus in the cricket giant interneuron

ENCODING OF SMALL-SCALE AIR MOTION DYNAMICS IN THE CRICKET ACHETA DOMESTICUS

The cercal sensory system of the cricket mediates the detection, localization and identification of air current signals generated by predators, mates and competitors. This mechanosensory system has been used extensively for experimental and theoretical studies of sensory coding at the cellular and system levels. It is currently thought that sensory interneurons in the terminal abdominal ganglion extract information about the direction, velocity, and acceleration of the air currents in the animal's immediate environment, and project a coarse-coded representation of those parameters to higher centers. All feature detection is thought to be carried out in higher ganglia by more complex, specialized circuits. We present results that force a substantial revision of current hypotheses. Using multiple extracellular recordings and a special sensory stimulation device, we demonstrate that four well-studied interneurons in this system respond with high sensitivity and selectivity to complex dynamic multi-directional features of air currents which have a spatial scale smaller than the physical dimensions of the cerci. The INs showed much greater sensitivity for these features than for unidirectional bulk-flow stimuli used in previous studies. Thus, in addition to participating in the ensemble encoding of bulk air flow stimulus characteristics, these interneurons are capable of operating as feature detectors for naturalistic stimuli. In this sense, these interneurons are encoding and transmitting information about different aspects of their stimulus environment: they are multiplexing information. Major aspects of the stimulus-response specificity of these interneurons can be understood from the dendritic anatomy and connectivity with the sensory afferent map.

Ca2+ imaging of cricket protocerebrum responses to air current stimulation

Crickets (Gryllus bimaculatus) use the cercal sensory system at the rear of the abdomen to detect air currents and direct predator avoidance behavior. Sensory information regarding the direction and dynamic properties of air currents is processed within the terminal abdominal ganglion, and conveyed by ascending giant interneurons (GIs) to higher centers including the brain. However, the brain region responsible for decoding cercal sensory information has not yet been identified, nor the response properties within the brain characterized. In this study, we performed in vivo Ca(2+) imaging to investigate wind-evoked neural activities within the cricket protocerebrum. Ca(2+) responses to air current stimuli were observed at peripheral regions of the ventrolateral neuropile (VLNP) where projection of GIs' axon terminals has been observed in larvae. The wind-evoked Ca(2+) response had temporal dynamics and directional sensitivity that varied with different recorded regions displaying transient or sustained Ca(2+) increases. Individual cells showed Ca(2+) elevation in response to air currents from a specific angle, while stimuli from a different angle evoked decreased signals. Removing the antennae reduced the air-current-evoked responses in VLNP, suggesting contribution of sensory inputs from antennae in addition to the cercal inputs. The VLNP is presumably an integrative center for mechanosensory processing from antennae and cerci where directional information is primarily decoded by protocerebral neurons.

The morphology and fine structure of the giant interneurons of the wood cricket Nemobius sylvestris

The structural and ultrastructural characteristics of giant interneurons in the terminal abdominal ganglion of the cricket Nemobius sylvestris were investigated by means of cobalt and fluorescent dye backfilling and transmission electron microscopy. The projections of the 8 eight pairs of the biggest ascending interneurons (giant interneurons) are described in detail. The somata of all interneurons analyzed are located contralateral to their axons, which project to the posterior region of the terminal ganglion and arborise in the cercal glomerulus. Neuron 7-1a is an exception, because its arborisation is restricted to the anterior region of the ganglion. The fine structure of giant interneurons shows typical features of highly active cells. We observed striking indentations in the perineural layer, enabling the somata of the giant interneurons to be very close to the haemolymph. The cercal glomerulus exhibits a high diversity of synaptic contacts (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic), as well as areas of tight junctions. Electrical synapses seem to be present, as well as mixed synapses. The anatomical organization of the giant interneurons is finally discussed in terms of functional implications and on a comparative basis.

The terminal abdominal ganglion of the wood cricket Nemobius sylvestris

The abdominal cerci of the wood cricket, Nemobius sylvestris, are covered by a variety of hair-like sensilla that differ in length, thickness, and articulation. Fillings from the cercal nerves with cobalt chloride and fluorescent dyes revealed the projection of sensory axons into the terminal abdominal ganglion of the ventral nerve chain. Two projection areas on each side of the terminal abdominal ganglion midline could be identified: a posterior cercal glomerulus and an anterior bristle neuropil. Axons from some cercal sensilla ascend through the connectives to reach the metathoracic ganglionic mass. As their axons pass through each segmental abdominal ganglion, they project medial arborization. Cross-sections of the terminal abdominal ganglion and retrograde fills with cobalt chloride and fluorescent dyes from connectives revealed several small cells and seven pairs of giant ascending interneurons organized symmetrically. Giant somata are located contralateral to their axons (diameters between 20 and 45 mum). The cercal projections overlap extensively with the dendritic fields of the giant interneurons. In the terminal abdominal ganglion, we identified nine longitudinal tracts, two major tracts, and seven smaller ones. The functional implications of the neuranatomical organization of the system are discussed on a comparative basis.

Dendritic design implements algorithm for synaptic extraction of sensory information

While sensory information is encoded by firing patterns of individual sensory neurons, it is also represented by spatiotemporal patterns of activity in populations of the neurons. Postsynaptic interneurons decode the population response and extract specific sensory information. This extraction of information represented by presynaptic activities is a process critical to defining the input-output function of postsynaptic neuron. To understand the "algorithm" for the extraction, we examined directional sensitivities of presynaptic and postsynaptic Ca(2+) responses in dendrites of two types of wind-sensitive interneurons (INs) with different dendritic geometries in the cricket cercal sensory system. In IN 10-3, whose dendrites arborize with various electrotonic distances to the spike-initiating zone (SIZ), the directional sensitivity of dendritic Ca(2+) responses corresponded to those indicated by Ca(2+) signals in presynaptic afferents arborizing on that dendrite. The directional tuning properties of individual dendrites varied from each other, and the directional sensitivity of the nearest dendrite to the SIZ dominates the tuning properties of the spiking response. In IN 10-2 with dendrites isometric to the SIZ, directional tuning properties of different dendrites were similar to each other, and each response property could be explained by the directional profile of the spatial overlap between that dendrite and Ca(2+)-elevated presynaptic terminals. For IN 10-2, the directional sensitivities extracted by the different dendritic-branches would contribute equally to the overall tuning. It is possible that the differences in the distribution of synaptic weights because of the dendritic geometry are related to the algorithm for extraction of sensory information in the postsynaptic interneurons.

Visualization of ensemble activity patterns of mechanosensory afferents in the cricket cercal sensory system with calcium imaging

The cercal sensory system of the cricket mediates the detection and analysis of low velocity air currents in the animal's immediate environment, and is implemented around an internal representation of air current direction that demonstrates the essential features of a continuous neural map. Previous neurophysiological and anatomical studies have yielded predictions of the global spatio-temporal patterns of activity that should be evoked in the sensory afferent map by air current stimuli of different directions. We tested those predictions by direct visualization of ensemble afferent activity patterns using Ca2+ -sensitive indicators. The AM ester of the fluorescent Ca2+ indicator (Oregon Green 488 BAPTA-1 AM) was injected under the sheath of a cercal sensory nerve containing all of the mechanosensory afferent axons from one cercus. Optical signals were recorded with a digital intensified CCD camera. Control experiments using direct electrical stimulation of stained and unstained nerves demonstrated that the observed Ca2+ signals within the terminal abdominal ganglion (TAG) were due to activation of the dye-loaded sensory afferent neurons. To visualize the spatial patterns of air-current-evoked ensemble activity, unidirectional air currents were applied repeatedly from eight different directions, and the optically recorded responses from each direction were averaged. The dispersion of the optical signals by the ganglion limited the spatial resolution with which these ensemble afferent activity patterns could be observed. However, resolution was adequate to demonstrate that different directional stimuli induced different spatial patterns of Ca2+ elevation in the terminal arbors of afferents within the TAG. These coarsely- resolved, optically-recorded patterns were consistent with the anatomy-based predictions.

Directional sensitivity of dendritic calcium responses to wind stimuli in the cricket giant interneuron

We examined directional sensitivities in the dendritic activity of the identified giant interneurons (GIs) in the cricket, using in vivo Ca(2+) imaging during different directional air-current stimuli. Air current stimulus evoked action potential burst and quick Ca(2+) increase in GI. The stimulus direction of the maximal Ca(2+) responses corresponded to that of the maximal voltage response. However, the shapes of the directional tuning curves based on the Ca(2+) responses for each dendritic branch were different from the overall tuning curve based on spike counts for the cell. Moreover, different dendritic branches displayed distinct directional sensitivity profiles to the air-current stimuli. We propose that postsynaptic activities will influence the local Ca(2+) signals in the distal dendrites, and produce the difference in directional sensitivity of the dendritic Ca(2+) response.

Natural patterns of neural activity: how physiological mechanisms are orchestrated to cope with real life

Physiological mechanisms of neuronal information processing have been shaped during evolution by a continual interplay between organisms and their sensory surroundings. Thus, when asking for the functional significance of such mechanisms, the natural conditions under which they operate must be considered. This has been done successfully in several studies that employ sensory stimulation under in vivo conditions. These studies address the question of how physiological mechanisms within neurons are properly adjusted to the characteristics of natural stimuli and to the demands imposed on the system being studied. Results from diverse animal models show how neurons exploit natural stimulus statistics efficiently by utilizing specific filtering capacities. Mechanisms that allow neurons to adapt to the currently relevant range from an often immense stimulus spectrum are outlined, and examples are provided that suggest that information transfer between neurons is shaped by the system-specific computational tasks in the behavioral context.

Direction of action potential propagation influences calcium increases in distal dendrites of the cricket giant interneurons

To understand the relationship between the propagation direction of action potentials and dendritic Ca(2+) elevation, simultaneous measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)) and intradendritic membrane potential were performed in the wind-sensitive giant interneurons of the cricket. The dendritic Ca(2+) transients induced by synaptically-evoked action potentials had larger amplitudes than those induced by backpropagating spikes evoked by antidromic stimulation. The amplitude of the [Ca(2+)](i) changes induced by antidromic stimuli combined with subthreshold synaptic stimulation was not different from that of the Ca(2+) increases evoked by the backpropagating spikes alone. This result means that the synaptically activated Ca(2+) release from intracellular stores does not contribute to enhancement of Ca(2+) elevation induced by backpropagating spikes. On the other hand, the synaptically evoked action potentials were also increased at distal dendrites in which the Ca(2+) elevation was enhanced. When the dendritic and axonal spikes were simultaneously recorded, the delay between dendritic spike and ascending axonal spike depended upon which side of the cercal nerves was stimulated. Further, dual intracellular recording at different dendritic branches illustrated that the dendritic spike at the branch arborizing on the stimulated side preceded the spike recorded at the other side of the dendrite. These results suggest that the spike-initiation site shifts depending on the location of the activated postsynaptic site. It is proposed that the difference of spike propagation manner could change the action potential waveform at the distal dendrite, and could produce synaptic activity-dependent Ca(2+) dynamics in the giant interneurons.

Ca2+ imaging in the mammalian brain in vivo

Changes in intracellular free calcium ion concentration ([Ca(2+)](i)) have been visualized over more than two decades using fluorescent dyes and optical microscopy. So far, however, most imaging studies have been performed on isolated cells or brain tissue. Here, we review approaches to measure cellular [Ca(2+)](i) changes in vivo, i.e. within the intact brain of a living animal. In particular we describe the application of two-photon microscopy to the mammalian central nervous system, which has recently enabled studies of Ca(2+) dynamics in individual dendrites in anaesthetized rats. New developments in microscopy and labeling techniques are creating further opportunities to study Ca(2+) dynamics in vivo and are likely to make measurements of spatio-temporal [Ca(2+)](i) distributions feasible even in awake, behaving mammals.

Spike-triggered dendritic calcium transients depend on synaptic activity in the cricket giant interneurons

The relationship between electrical activity and spike-induced Ca2+ increases in dendrites was investigated in the identified wind-sensitive giant interneurons in the cricket. We applied a high-speed Ca2+ imaging technique to the giant interneurons, and succeeded in recording the transient Ca2+ increases (Ca2+ transients) induced by a single action potential, which was evoked by presynaptic stimulus to the sensory neurons. The dendritic Ca2+ transients evoked by a pair of action potentials accumulated when spike intervals were shorter than 100 ms. The amplitude of the Ca2+ transients induced by a train of spikes depended on the number of action potentials. When stimulation pulses evoking the same numbers of action potentials were separately applied to the ipsi- or contra-lateral cercal sensory nerves, the dendritic Ca2+ transients induced by these presynaptic stimuli were different in their amplitude. Furthermore, the side of presynaptic stimulation that evoked larger Ca2+ transients depended on the location of the recorded dendritic regions. This result means that the spike-triggered Ca2+ transients in dendrites depend on postsynaptic activity. It is proposed that Ca2+ entry through voltage-dependent Ca2+ channels activated by the action potentials will be enhanced by excitatory synaptic inputs at the dendrites in the cricket giant interneurons.

Neuron-independent Ca(2+) signaling in glial cells of snail's brain

To directly monitor the glial activity in the CNS of the pond snail, Lymnaea stagnalis, we optically measured the electrical responses in the cerebral ganglion and median lip nerve to electrical stimulation of the distal end of the median lip nerve. Using a voltage-sensitive dye, RH155, we detected a composite depolarizing response in the cerebral ganglion, which consisted of a fast transient depolarizing response corresponding to a compound action potential and a slow depolarizing response. The slow depolarizing response was observed more clearly in an isolated median lip nerve and also detected by extracellular recording. In the median lip nerve preparation, the slow depolarizing response was suppressed by an L-type Ca(2+) channel blocker, nifedipine, and was resistant to tetrodotoxin and Na(+)-free conditions. Together with the fact that a delay from the compound action potential to the slow depolarizing response was not constant, these results suggested that the slow depolarizing response was not a postsynaptic response. Because the signals of the action potentials appeared on the saturated slow depolarizing responses during repetitive stimulation, the slow depolarizing response was suggested to originate from glial cells. The contribution of the L-type Ca(2+) current to the slow depolarizing response was confirmed by optical recording in the presence of Ba(2+) and also supported by intracellular Ca(2+) measurement. Our results suggested that electrical stimulation directly triggers glial Ca(2+) entry through L-type Ca(2+) channels, providing evidence for the generation of glial depolarization independent of neuronal activity in invertebrates.

Spike-dependent calcium influx in dendrites of the cricket giant interneuron

Identified wind-sensitive giant interneurons in the cricket's cercal sensory system integrate cercal afferent signals and release an avoidance behavior. A calcium-imaging technique was applied to the giant interneurons to examine the presence of the voltage-dependent Ca(2+) channels (VDCCs) in their dendrites. We found that presynaptic stimuli to the cercal sensory nerve cords elevated the cytosolic Ca(2+) concentration ([Ca(2+)](i)) in the dendrites of the giant interneurons. The dendritic Ca(2+) rise coincided with the spike burst of the giant interneurons, and the rate of Ca(2+) rise depended on the frequency of the action potentials. These results suggest that the action potentials directly caused [Ca(2+)](i) increase. Observation of the [Ca(2+)](i) elevation induced by depolarizing current injection demonstrates the presence of the VDCCs in the dendrites. Although hyperpolarizing current injection into the giant interneuron suppressed action potential generation, EPSPs could induce no [Ca(2+)](i) increase. This result means that ligand-gated channels do not contribute to the synaptically stimulated Ca(2+) elevation. On the other hand, antidromically stimulated spikes also increased [Ca(2+)](i) in all cellular regions including the dendrites. And bath application of a mixture of Ni(2+), Co(2+), and Cd(2+) or tetrodotoxin inhibited the [Ca(2+)](i) elevation induced by the antidromic stimulation. From these findings, we suppose that the axonal spikes antidromically propagate and induce the Ca(2+) influx via VDCCs in the dendrites. The spike-dependent Ca(2+) elevation may regulate the sensory signals processing via second-messenger cascades in the giant interneurons.