Optical study of interactions among propagation waves of neural excitation in the rat somatosensory cortex evoked by forelimb and hindlimb stimuli.

Multisite optical recording has revealed that the neural excitation wave induced by a sensory stimulation begins at a focus and propagates in the cortex. This wave is considered to be important for computation in the sensory cortex, particularly the integration of sensory information; however, the nature of this wave remains largely unknown. In the present study, we examined the interaction between two waves in the rat sensory cortex induced by hindlimb and forelimb stimuli with different interstimulus intervals. We classified the resultant patterns as follows: 1) the collision of two waves, 2) the hindlimb response being evoked while the forelimb-induced wave is passing the hindlimb focus, and 3) the hindlimb response being evoked after the forelimb-induced wave has passed the hindlimb focus. In pattern 1, the two waves fused into a single wave, but the propagation pattern differed from that predicted by the superimposition of two singly induced propagation courses. In pattern 2, the state of the interaction between the two waves varied depending on the phase of optical signals constituting the forelimb-induced wave around the hindlimb focus. Although no hindlimb-induced wave was observed in the rising phase, the propagating velocity of the forelimb-induced wave increased. At the peak, neither the hindlimb-induced response nor a modulatory effect on the forelimb-induced wave was detected. In pattern 3, the hindlimb-induced wave showed a reduced amplitude and spatial extent. These results indicate that the state of the interaction between waves was strongly influenced by the relative timing of sensory inputs. NEW & NOTEWORTHY Sensory stimulation-induced cortical excitation propagates as a wave and spreads over a wide area of the sensory cortex. To elucidate the characteristics of this relatively unknown phenomenon, we examined the interaction between two individually induced waves in the somatosensory cortex. Either the waves collided or the preceding wave affected the emergence of the following one. Our results indicate that the state of the interaction was strongly influenced by the relative timing of sensory inputs.

Optical Analysis of Acute Changes after Peripheral Nerve Injury in Spatio-Temporal Pattern of Neural Response to Forelimb Stimulation in Rat Somatosensory Cortex.

Peripheral nerve injury induces functional reorganization of the central nervous system. The mechanisms underlying this reorganization have been widely studied. Our previous study involving multiple-site optical recording reported that a neural excitatory wave induced by somatic stimulation begins in a small area and propagates in the cortex. In the present study, to examine the possible role of this propagation wave in cortical reorganization, we analyzed the early changes in the spatio-temporal pattern of the sensory-evoked wave immediately, and 30 min, after nerve injury. The response to hypothenar stimulation, innervated by the ulnar nerve and adjoining the median nerve area, persisted after injury to either the ulnar or median nerve. Initially, we assessed changes in the response pattern at the focus. The latency increased after ulnar nerve injury, whereas no change was observed after median nerve injury. Similarly, no change was noted in the duration of the response signal with either nerve injury. Second, changes in the propagation wave pattern were analyzed. Ulnar nerve injury decreased the propagation velocity in the medial direction but the median nerve injury induced no changes. These results indicated that the propagation wave pattern is readily altered, even immediately after nerve injury, and suggest that this immediate change in the spatio-temporal pattern is one of the factors contributing to the cortical reorganization.

Concave-shaped transparent electrode to simultaneously monitor electrical activity from multiple sites within the optical sampling area of the intact rat cerebral cortex.

We have developed a concave-shaped transparent electrode unit that enables the placement of several electrodes within the optical sampling area on the spherical surface of the rat brain. This concave-shaped transparent electrode unit consists of an insulator base (a plano-concave lens) and a gallium-doped zinc oxide film that is a transparent conductor coating the base. Most of the unit is wrapped in an insulator film made of silicon dioxide, and the few areas left unwrapped act as electrodes. In the study reported here this newly developed transparent electrode unit worked well within the optical detection area without affecting optical recording. We applied this unit to our multiple-site optical recording system for membrane potential in order to eliminate pulsation artifacts and succeeded in optically recording spontaneous neural activity, including small changes in membrane potential, in the cerebral cortex in a single-sweep recording.

An optical telemetry system for underwater recording of electromyogram and neuronal activity from non-tethered crayfish.

We have developed an optical telemetry system for recording electrical signals associated with muscle and neuronal activities from freely walking crayfish under water. The device was made from conventional electronic parts which are commercially available, utilizing infrared light (880 nm) for signal transmission. Two or four channels of biological signals were multiplexed, the voltage of each data point modulated to the duration of subcarrier pulses and further to the interval of narrower carrier pulses that directly drove the infrared light emission diode (IRLED) under water. The light-pulse modulated signals were received by photodiodes and demodulated to restore the original two or four channel signals. Electrical recordings using wired electrodes and conventional amplifiers revealed that the optically transmitted signals were consistent with the wire-transmitted ones. In order to test the performance of this system, we recorded electromyograms (EMGs) from the second and third walking legs on each side of crayfish together with the neuronal activity in the ventral nerve cord. The results confirmed our previous observation in tethered crayfish that the background tonus of leg muscles showed an increase preceding their rhythmic activation.

Improvement of the optical imaging technique for intact rat brain using a plano-concave lens.

Use of a plano-concave lens improved the quality of optical signals from the rat cerebral cortex by improving the focus. When detecting neural activity from a curved surface of an in vivo brain by optical techniques, it is not possible to adjust the focus equally over the entire detecting area in the two-dimensional plane, since the active window of the optical detector is usually flat, while the intact brain surface is spherical. It has been known that the size of the optical signal is reduced as the distance of the real image to the active window of the detector increases; therefore, the level of the signal-to-noise ratio obtained from the unfocused area often becomes insufficient for quantitative physiological analyses. By placing a plano-concave lens on the cerebral cortex, we succeeded in obtaining a two-dimensional image that has no unfocused area over an entire image recorded by the detector.

Memory trace in feeding neural circuitry underlying conditioned taste aversion in Lymnaea.

BACKGROUND: The pond snail Lymnaea stagnalis can maintain a conditioned taste aversion (CTA) as a long-term memory. Previous studies have shown that the inhibitory postsynaptic potential (IPSP) evoked in the neuron 1 medial (N1M) cell by activation of the cerebral giant cell (CGC) in taste aversion-trained snails was larger and lasted longer than that in control snails. The N1M cell is one of the interneurons in the feeding central pattern generator (CPG), and the CGC is a key regulatory neuron for the feeding CPG. METHODOLOGY/PRINCIPLE FINDINGS: Previous studies have suggested that the neural circuit between the CGC and the N1M cell consists of two synaptic connections: (1) the excitatory connection from the CGC to the neuron 3 tonic (N3t) cell and (2) the inhibitory connection from the N3t cell to the N1M cell. However, because the N3t cell is too small to access consistently by electrophysiological methods, in the present study the synaptic inputs from the CGC to the N3t cell and those from the N3t cell to the N1M cell were monitored as the monosynaptic excitatory postsynaptic potential (EPSP) recorded in the large B1 and B3 motor neurons, respectively. The evoked monosynaptic EPSPs of the B1 motor neurons in the brains isolated from the taste aversion-trained snails were identical to those in the control snails, whereas the spontaneous monosynaptic EPSPs of the B3 motor neurons were significantly enlarged. CONCLUSION/SIGNIFICANCE: These results suggest that, after taste aversion training, the monosynaptic inputs from the N3t cell to the following neurons including the N1M cell are specifically facilitated. That is, one of the memory traces for taste aversion remains as an increase in neurotransmitter released from the N3t cell. We thus conclude that the N3t cell suppresses the N1M cell in the feeding CPG, in response to the conditioned stimulus in Lymnaea CTA.

An improved multiple-site optical membrane potential-recording system to obtain high-quality single sweep signals in intact rat cerebral cortex.

We improved our optical recording system to record epifluorescence optical signals from the intact cerebral cortex. Using custom-made fiber optic illumination equipment and tandem lens optics excluding a dichroic mirror, we successfully obtained high-quality single sweep optical signals in vivo in rat sensorimotor cortex. After reduction of pulsation artifacts with software originally designed for prolonged continuous recording under spontaneous breathing, the signal-to-noise ratio of the optical signal was sufficient to analyse evoked responses quantitatively. We were able to make isochrones for the onset of responses based on single sweep optical recordings. We further examined the effect of baclofen using this isochrone map, and succeeded in quantitatively demonstrating the inhibitory effect on the spatial pattern of neural activity in the rat sensorimotor cortex by calculating the area of response region on the isochrone map. Thus, our improved system provides sustained single sweep records suitable for quantitative analysis using epifluorescence optics.

Static electric field detection and behavioural avoidance in cockroaches.

Electric fields are pervasively present in the environment and occur both as a result of man-made activities and through natural occurrence. We have analysed the behaviour of cockroaches to static electric fields and determined the physiological mechanisms that underlie their behavioural responses. The behaviour of animals in response to electric fields was tested using a Y-choice chamber with an electric field generated in one arm of the chamber. Locomotory behaviour and avoidance were affected by the magnitude of the electric fields with up to 85% of individuals avoiding the charged arm when the static electric field at the entrance to the arm was above 8-10 kV m(-1). Electric fields were found to cause a deflection of the antennae but when the antennae were surgically ablated, the ability of cockroaches to avoid electric fields was abolished. Fixation of various joints of the antennae indicated that hair plate sensory receptors at the base of the scape were primarily responsible for the detection of electric fields, and when antennal movements about the head-scape joint were prevented cockroaches failed to avoid electric fields. To overcome the technical problem of not being able to carry out electrophysiological analysis in the presence of electric fields, we developed a procedure using magnetic fields combined with the application of iron particles to the antennae to deflect the antennae and analyse the role of thoracic interneurones in signalling this deflection. The avoidance of electric fields in the context of high voltage power lines is discussed.

Peripheral targets of centrally located putative accessory neurons of MRO in the isopod Ligia exotica.

The three centrally located putative accessory neurons of the muscle receptor organ (MRO) of the isopod Ligia exotica were identified to the third segmental nerve (N3) of the thoracic ganglion by backfilling with Lucifer Yellow. These neurons were then studied intracellularly and extracellularly to determine whether they suppressed the stretch-activated responses of thoracic stretch receptors. Intracellular injection of depolarizing currents into these three putative accessory neurons revealed that only neuron #3 had an inhibitory effect, suggesting that it is an inhibitory accessory neuron related to thoracic stretch receptors. We searched for the peripheral targets of neurons #1 and #2 by intracellular filling with Lucifer Yellow or by recording of junctional potentials in extensor muscles, and show that they are motor neurons that innervate the deep extensor and superficial extensor muscles, respectively.