Discrepancies between cortical and behavioural long-term readouts of hyperalgesia in awake freely moving rats.

BACKGROUND: It is still unclear to what extent the most common animal models of pain and analgesia, based on indirect measures such as nocifensive behaviours, provide valid measures of pain perception. METHODS: To address this issue, we developed a novel animal model comprising a more direct readout via chronically (>1 month) implanted multichannel electrodes (MCE) in rat primary somatosensory cortex (S1; known to be involved in pain perception in humans) and compared this readout to commonly used behavioural pain-related measures during development of hyperalgesia. A translational method to induce hyperalgesia, UVB irradiation of the skin, was used. Localized CO2 laser stimulation was made of twenty skin sites (20 stimulations/site/observation day) on the plantar hind paw, before and during the time period when enhanced pain perception is reported in humans after UVB irradiation. RESULTS: We demonstrate a 2-10 fold significant enhancement of cortical activity evoked from both irradiated and adjacent skin and a time course that corresponds to previously reported enhancement of pain magnitude during development of primary and secondary hyperalgesia in humans. In contrast, withdrawal reflexes were only significantly potentiated from the irradiated skin area and this potentiation was significantly delayed as compared to activity in S1. CONCLUSIONS: The present findings provide direct evidence that chronic recordings in S1 in awake animals can offer a powerful, and much sought for, translational model of the perception of pain magnitude during hyperalgesia. WHAT DOES THIS STUDY ADD?: In a novel animal model, chronic recordings of nociceptive activity in primary somatosensory cortex (S1) in awake freely moving rats are compared to behavioural readouts during UVB-induced hyperalgesia. Evoked activity in rat S1 replicates altered pain perception in humans during development of hyperalgesia, but withdrawal reflexes do not.

Spike-feature based estimation of electrode position in extracellular neural recordings.

Detecting and sorting spikes in extracellular neural recordings are common procedures in assessing the activity of individual neurons. In chronic recordings, passive electrode movements introduce changes in the shape of detected spike waveforms, and may thus lead to problems with identification and tracking of spikes recorded at separate instances in time, which is an important step in long-term monitoring of individual neurons. Information about electrode movements after implantation is crucial to the evaluation of mechanical stability of different electrode designs. In this paper, we present a preliminary study of the relationship between electrode movements and the resulting movements of spike-features in feature space. We show that there is a characteristic relationship between the two movements and that this relationship can be modeled as a linear transformation between two coordinate systems. Finally, we show how the relationship can be used for estimating electrode positions based on measured spike waveforms without any prior knowledge about the type of neuron by introducing a learning procedure during electrode insertion.

Statistical modelling of spike libraries for simulation of extracellular recordings in the cerebellum.

Brain machine interfaces with chronically implanted microelectrode arrays for signal acquisition require algorithms for successful detection and classification of neural spikes. During the design of such algorithms, signals with a priori known characteristics need to be present. A common way to establish such signals is to model the recording environment, simulate the recordings and store ground truth about spiking activity for later comparison. In this paper, we present a statistical method to expand the spike libraries that are used in a previously presented simulation tool for the purpose described above. The method has been implemented and shown to successfully provide quick access to a large assembly of synthetic extracellular spikes with realistic characteristics. Simulations of extracellular recordings using synthesized spikes have shown to possess characteristics similar to those of in-vivo recordings in the cat cerebellum.

Flexible multi electrode brain-machine interface for recording in the cerebellum.

A new type of chip based microelectrode for acute electrophysiological recordings in the CNS has been developed. It's designed to be adaptable to a multitude of specific neuronal environments, in this study the cerebellar cortex of rat and cat. Photolithographically patternened SU-8 is used to yield flexible and biocompatible penetrating shanks with gold leads. Electrodes with an impedance of about 300 kOmega at 1kHz have excellent signal to noise ratio in acute recordings in cat cerebellum.

Spike library based simulator for extracellular single unit neuronal signals.

A well defined set of design criteria is of great importance in the process of designing brain machine interfaces (BMI) based on extracellular recordings with chronically implanted micro-electrode arrays in the central nervous system (CNS). In order to compare algorithms and evaluate their performance under various circumstances, ground truth about their input needs to be present. Obtaining ground truth from real data would require optimal algorithms to be used, given that those exist. This is not possible since it relies on the very algorithms that are to be evaluated. Using realistic models of the recording situation facilitates the simulation of extracellular recordings. The simulation gives access to a priori known signal characteristics such as spike times and identities. In this paper, we describe a simulator based on a library of spikes obtained from recordings in the cat cerebellum and observed statistics of neuronal behavior during spontaneous activity. The simulator has proved to be useful in the task of generating extracellular recordings with realistic background noise and known ground truth to use in the evaluation of algorithms for spike detection and sorting.

Implementation of a telemetry system for neurophysiological signals.

With an ever increasing need for assessment of neurophysiological activity in connection with injury and basic research, the demand for an efficient and reliable data acquisition system rises. Brain-machine interfaces is one class of such systems that targets the central nervous system. A necessary step in the development of a brain-machine interface is to design and implement a reliable and efficient measurement system for neurophysiological signals. The use of telemetric devices increases the flexibility of the devices in terms of subject mobility and unobtrusiveness of the equipment. In this paper, we present a complete system architecture for a wearable telemetry system for the acquisition of neurophysiological data. The system has been miniaturized and implemented using surface-mount technology. System performance has been successfully verified and bottlenecks in the architecture have been identified.

Classification of motor commands using a modified self-organising feature map.

In this paper, a control system for an advanced prosthesis is proposed and has been investigated in two different biological systems: (1) the spinal withdrawal reflex system of a rat and (2) voluntary movements in two human males: one normal subject and one subject with a traumatic hand amputation. The small-animal system was used as a model system to test different processing methods for the prosthetic control system. The best methods were then validated in the human set-up. The recorded EMGs were classified using different ANN algorithms, and it was found that a modified self-organising feature map (SOFM) composed of a combination of a Kohonen network and the conscience mechanism algorithm (KNC) was superior in performance to the reference networks (e.g. multi-layer perceptrons) as regards training time, low memory consumption, and simplicity in finding optimal training parameters and architecture. The KNC network classified both experimental set-ups with high accuracy, including five movements for the animal set-up and seven for the human set-up.

How best to fight that nasty itch - from new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus to novel therapeutic approaches.

While the enormous clinical and psychosocial importance of pruritus in many areas of medicine and the detrimental effects of chronic 'itch' on the quality of life of an affected individual are widely appreciated, the complexity of this sensation is still often grossly underestimated. The current Controversies feature highlights this complexity by portraying pruritus as a truly interdisciplinary problem at the crossroads of neurophysiology, neuroimmunology, neuropharmacology, protease research, internal medicine, and dermatology, which is combated most successfully if one keeps the multilayered nature of 'itch' in mind and adopts a holistic treatment approach - beyond the customary, frequently frustrane monotherapy with histamine receptor antagonists. In view of the often unsatisfactory, unidimensional, and altogether rather crude standard instruments for pruritus management that we still tend to use in clinical practice today, an interdisciplinary team of pruritus experts here critically examines recent progress in pruritus research that future itch management must take into consideration. Focusing on new insights into the neuroimmunological, neuroendocrine, and neurophysiological bases of pruritus, and discussing available neuropharmacological tools, specific research avenues are highlighted, whose pursuit promises to lead to novel, and hopefully more effective, forms of pruritus management.

Profound inhibition of chronic itch induced by stimulation of thin cutaneous nerve fibres.

BACKGROUND: Despite the fact that severe itch is common in many dermatological diseases, the therapeutic arsenal against itching is limited. From neurophysiological experiments, using a new technique termed cutaneous field stimulation, it is known that acute itch can be effectively relieved by stimulation of cutaneous nociceptors. METHODS: We tested the effects of cutaneous field stimulation (25 min, 16 electrodes, 4 Hz per electrode, up to 0.8 mA) on chronic itch due to atopic dermatitis. Transcutaneous electrical nerve stimulation (100 Hz, up to 26 mA) was used for comparison. In 27 patients, itch was measured just prior to, during and at regular intervals up to 12 h after either type of treatment. RESULTS: Both treatments augmented the itch sensation during ongoing stimulation, presumably reflecting an altered sensory processing in the somatosensory pathways of chronic itch patients. However, after cessation of cutaneous field stimulation, but not transcutaneous electrical nerve stimulation, the itch sensation was significantly depressed for up to 7 h. The peak inhibitory effect (about 25% of control) was reached between 1 and 5 h poststimulation. Neither treatment had any significant effect on alloknesis, as measured before and 10 min after stimulation. CONCLUSION: It is concluded that cutaneous field stimulation strongly depresses chronic itch, and is a potentially useful symptomatic treatment of itch.

An imaging system for monitoring receptive field dynamics.

The paper describes a computerized method, termed receptive field imaging (RFI), for the rapid mapping of multiple receptive fields and their respective sensitivity distributions. RFI uses random stimulation of multiple sites, in combination with an averaging procedure, to extract the relative contribution from each of the stimulated sites. Automated multi-electrode stimulation and recording, with spike detection and counting, are performed on-line by the RFI programme. Direct user interpretation of receptive field changes is made possible by a user-friendly graphic interface. A series of imaging experiments was carried out to evaluate the functional capacity of the system. RFI was tested on the receptive fields in the nociceptive withdrawal reflex (NWR) system in the rat. RFI replicates the results obtained with conventional methods and allows the display of receptive field dynamics induced by topical spinal cord application of morphine and naloxone on a minute-to-minute time scale. Data variance was estimated, and proved to be small enough to yield a stable representation of the receptive field, thereby achieving a high sensitivity in dynamic imaging experiments. The large number of stimulation and registration sites that can be monitored in parallel permits detailed network analysis of synaptic sets, corresponding to 'connection weights' between individual neurones.

Functional connections are established in the deafferented rat spinal cord by peripherally transplanted human embryonic sensory neurons.

Functionally useful repair of the mature spinal cord following injury requires axon growth and the re-establishment of specific synaptic connections. We have shown previously that axons from peripherally grafted human embryonic dorsal root ganglion cells grow for long distances in adult host rat dorsal roots, traverse the interface between the peripheral and central nervous system, and enter the spinal cord to arborize in the dorsal horn. Here we show that these transplants mediate synaptic activity in the host spinal cord. Dorsal root ganglia from human embryonic donors were transplanted in place of native adult rat ganglia. Two to three months after transplantation the recipient rats were examined anatomically and physiologically. Human fibres labelled with a human-specific axon marker were distributed in superficial as well as deep laminae of the recipient rat spinal cord. About 36% of the grafted neurons were double labelled following injections of the fluorescent tracers MiniRuby into the sciatic and Fluoro-Gold into the lower lumbar spinal cord, indicating that some of the grafted neurons had grown processes into the spinal cord as well as towards the denervated peripheral targets. Electrophysiological recordings demonstrated that the transplanted human dorsal roots conducted impulses that evoked postsynaptic activity in dorsal horn neurons and polysynaptic reflexes in ipsilateral ventral roots. The time course of the synaptic activation indicated that the human fibres were non-myelinated or thinly myelinated. Our findings show that growing human sensory nerve fibres which enter the adult deafferentated rat spinal cord become anatomically and physiologically integrated into functional spinal circuits.

Sensorimotor transformation in cat nociceptive withdrawal reflex system.

The withdrawal reflex system of higher vertebrates has been extensively used as a model for spinal sensorimotor integration, nociceptive processing and plasticity. In the rat, the nociceptive withdrawal reflex system appears to have a modular organization. Each reflex module controls a single muscle or a few synergistic muscles, and its cutaneous receptive field corresponds to the skin area withdrawn upon contraction of the effector muscle(s) when the limb is in the standing position. This organization principle is at odds with the 'flexion reflex' concept postulated from cat studies. To assess the generality of the modular organization principle we have therefore re-examined the cutaneous input to the withdrawal reflex system of the cat. The cutaneous receptive fields of hindlimb and forelimb muscles were mapped using calibrated noxious pinch stimulation and electromyographic recording technique in barbiturate anaesthetized animals. The investigated muscles had specific cutaneous receptive fields that appeared to correspond to the area of the skin withdrawn upon contraction of the muscle when the limb is in the standing position. The spatial organization of receptive fields in the cat was similar to that in the rat. However, differences in gain properties of reflexes to some anatomically equivalent muscles in the two species were observed, possibly reflecting adaptations to the biomechanics characteristic of the digitigrade and plantigrade stance in cats and rats, respectively. Implications of the findings for the generality of the modular organization of the withdrawal reflex system and for its adaptive properties are discussed.

Developmental tuning in a spinal nociceptive system: effects of neonatal spinalization.

Recent studies indicate a modular organization of the nociceptive withdrawal reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and adequate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord transection on the postnatal tuning of withdrawal reflexes was studied. The nociceptive receptive fields of single hindlimb muscles and compound withdrawal reflexes were examined in decerebrate unanesthetized and awake rats, respectively. Noxious thermal CO(2) laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal distribution of sensitivity not matching the withdrawal action of the effector muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for supraspinal systems in the postnatal tuning of spinal nociceptive systems.

Perforated silicon nerve chips with doped registration electrodes: in vitro performance and in vivo operation.

An in vitro model was developed for the study of signal transduction between a Cu-wire, miming a neural signal source, and recording electrodes on perforated silicon chips. Phosphorous doped electrodes were used to achieve an all silicon device. The model was used to study signal amplitude as a function of the spatial position, and distance to the signal source. Recordings of the signal crosstalk to neighboring electrodes on the chips were made. It was found that the amplitude decreased by a factor of two at a distance of 50 microns between the electrode surface and the signal source. The chip electrode signal crosstalk was found to be 6 dB using an external reference electrode. Improvements were accomplished with an on chip reference electrode giving a crosstalk suppression of 20 dB. Impedance analysis showed that doped silicon electrodes displayed similar characteristics as Cu-electrodes at frequencies above 3 kHz. Sieve electrodes were implanted in the rat sciatic nerve and following a 10-week nerve regeneration period the dorsal and ventral (L5) roots in the spinal cord were stimulated. Compound action potentials were recorded via the chip. Stimulating the regenerated sciatic nerve via the sieve electrode also induced lower leg muscle contraction activity.

Differential inhibitory effect on human nociceptive skin senses induced by local stimulation of thin cutaneous fibers.

It is known that stimulation of thin cutaneous nerve fibers can induce long lasting analgesia through both supraspinal and segmental mechanisms, the latter often exhibiting restricted receptive fields. On this basis, we recently developed a new method, termed cutaneous field stimulation (CFS), for localized stimulation of A delta and C fibers in the superficial part of the skin. In the present study, we have evaluated the effects of CFS on non-nociceptive and nociceptive skin senses. We compared the effects of CFS with those of conventional transcutaneous electrical nerve stimulation (TENS), known to preferentially activate coarse myelinated fibers. A battery of sensory tests were made on the right volar forearm of 20 healthy subjects. CFS (16 electrodes, 4 Hz per electrode, 1 ms, up to 0.8 mA) and TENS (100 Hz, 0.2 ms, up to 26 mA) applied either on the right volar forearm (homotopically), or on the lower right leg (heterotopically) were used as conditioning stimulation for 25 min. The tactile threshold was not affected by either homo- or heterotopical CFS or TENS. The mean thresholds for detecting warming or cooling of the skin were increased by 0.4-0.9 degrees C after homo- but not heterotopical CFS and TENS. Regarding nociceptive skin senses, homo- but not heterotopical CFS, markedly reduced CO2-laser evoked A delta- and C fiber mediated heat pain to 75 and 48% of control, respectively, and mechanically evoked pain to 73% of control. Fabric evoked prickle, was not affected by CFS. Neither homo- nor heterotopical TENS induced any marked analgesic effects. It is concluded that different qualities of nociception can be differentially controlled by CFS.

GABAA receptor blockade inhibits A beta fibre evoked wind-up in the arthritic rat.

To clarify the mechanisms of allodynia we have examined whether 'wind-up' of nociceptive withdrawal reflexes (NWR), a phenomenon characteristic of nociceptive C fiber spinal processing, can be mimicked by stimulation of tactile A beta fibers in monoarthritic decerebrate spinal rats. Knee joint monoarthritis was induced by carrageenan/kaolin under halothane anaesthesia 5 h before recordings. In arthritic, but not in control rats, wind-up of NWR of the semitendinosus muscle could be evoked by repeated stimulation of A beta fibres. By contrast, peroneus longus reflexes did not exhibit marked wind-up. Bicuculline (0.03-0.3 mg/kg, i.v.) dose-dependently inhibited this wind-up. Hence, reflex wind-up can be elicited by tactile A beta fibers in arthritis rats through a GABAA dependent mechanism.

On the cutaneous receptors contributing to withdrawal reflex pathways in the decerebrate spinal rat.

Previous studies indicate that the withdrawal reflex system in the rat has a "modular" organization, each reflex pathway performing a specific sensorimotor transformation. Here, we wished to clarify which cutaneous receptors contribute to this system and to determine whether there are differences in this respect between reflex pathways of different muscles. Withdrawal reflexes of the peroneus longus, extensor digitorum longus, and semitendinosus muscles were recorded with EMG techniques during high reflex excitability in decerebrate spinal rats (n=26). While maintained innocuous pressure on glabrous skin could elicit a sustained reflex activity in all muscles studied, vibration of glabrous skin (10-300 Hz) always failed to evoke a reflex response, suggesting that slowly adapting, but not rapidly adapting, low-threshold mechanoreceptive fibers from this type of skin contribute to withdrawal reflex pathways. Thermal stimulation in the innocuous range, i.e., cooling from 32 to 17 degrees C, or warming the skin from 32 to 41 degrees C, always failed to produce reflex responses, indicating that neither cold nor warm receptors contribute to withdrawal reflex pathways. When either cooling or warming the skin to the noxious temperatures of 1 degrees C or above 45 degrees C, respectively, a reflex discharge was often evoked in the muscles studied. Intradermal administration of histamine, a potent pruritogenic substance, produced very weak, or no, reflex response. In contrast, mustard oil produced vigorous reflex responses in all muscles studied. These findings suggest that some chemonociceptors contribute only weakly, or not at all, to withdrawal reflex pathways. The present data suggest that a selective set of cutaneous receptors contribute to withdrawal reflex pathways and that different withdrawal reflex pathways receive input from essentially the same cutaneous receptor types.

Cutaneous field stimulation (CFS): a new powerful method to combat itch.

Scratching the skin, while instantly relieving itch, often aggravates itch over time due to skin injury. To relieve itch, without damaging the skin, a new technique termed cutaneous field stimulation (CFS) was developed and tested on 21 subjects. CFS uses a flexible plate with needle-like electrodes (n = 16) to electrically stimulate nerve fibres in the superficial skin. The electrodes were stimulated consecutively (4 Hz per electrode, pulse duration 1 ms, intensity 0.4-0.8 mA, 25 min). CFS resulted in a pricking and burning sensation that usually faded rather quickly. The burning sensation was still present during a selective block of impulse conduction in myelinated fibres indicating that nociceptive C-fibres are activated by CFS. Furthermore, a flare reaction developed around the CFS electrodes indicating activation of axon reflexes in nociceptive C-fibres. Itch, elicited by transdermal iontophoresis of histamine, was abolished within the skin area pre-treated with CFS, and was reduced to 14% of control 10 cm distally. Contralateral effects were small or non-existent. After 4 h, itch was reduced ipsilaterally to 32% of control. In comparison, 2 h after transcutaneous electrical nerve stimulation (TENS; 10-20 mA, 100 Hz, 25 min) ipsilateral itch was reduced to 56% of control. In conclusion, CFS offers a powerful new method for combating itch. It is suggested that CFS acts through endogenous central inhibitory mechanisms that are normally activated by scratching the skin.

Developmental adaptation of rat nociceptive withdrawal reflexes after neonatal tendon transfer.

Nociceptive withdrawal reflexes (NWRs) were studied in adult rats in which the movement patterns produced by single muscles had been altered by neonatal tendon transfer. NWRs evoked by cutaneous noxious mechanical and thermal (CO2-laser) stimulation were recorded using electromyography in a decerebrate spinal preparation. The sensitivity distribution within the receptive fields of the NWRs of the extensor digitorum longus and the peronei muscles exhibited changes corresponding to the altered movement patterns. No detectable change of NWRs was found in normal muscles whose receptive fields overlapped that of the modified muscle. Furthermore, NWRs of muscles that regained an essentially normal function after neonatal tendon transfer did not differ from normal. It is proposed that a developmental experience-dependent mechanism, which takes into account the hindlimb movement pattern caused by contraction of single muscles, underlies the functionally adapted organization of adult NWRs.

Developmental adaptation of withdrawal reflexes to early alteration of peripheral innervation in the rat.

1. In adult decerebrate spinal rats whose plantar nerves (PLN) had been transected at either postnatal day 1 (P1) or P21 the nociceptive withdrawal reflexes (NWR) of musculi extensor digitorum longus (EDL), peroneus longus (PER) and semitendinosus (ST) were characterized with respect to receptive field (RF) organization, magnitude and time course, using electromyography. Thermal (short CO2 laser pulses) and mechanical (calibrated pinch) stimulation were used. The innervation patterns in normal and lesioned adult rats were assessed by acute nerve lesions. 2. The spatial organization of the mean mechano- and thermonociceptive RFs of all the muscles studied was similar to normal in both P1- and P21-lesioned rats, although in some P21-lesioned rats atypical EDL RFs were encountered. 3. In P1-lesioned rats thermo-NWR of PER and EDL had normal magnitudes, while mechano-NWR were reduced. In P21-lesioned rats both thermo- and mechano-NWR of these muscles had reduced magnitudes. Except for thermo-NWR of ST in P1-lesioned rats, which were increased, NWR of ST had normal magnitudes in both P1- and P21-lesioned rats. The time course of thermonociceptive NWR of the muscles studied were near normal in both P1- and P21-lesioned rats. 4. Acute nerve lesions in adult P1-lesioned rats revealed an essentially abolished contribution to NWR from the PLN. Instead, the contribution to NWR from other hindpaw nerves, such as the superficial and deep peroneal nerves, was dramatically increased. By contrast, in P21-lesioned rats, the regenerated PLN contributed significantly to the NWR. 5. It is concluded that despite profound alterations of plantar hindpaw innervation induced by early PLN transection the cutaneous nociceptive input to NWR attained an essentially normal spatial organization. An experience-dependent mechanism is suggested to be instrumental in adapting the reflex connectivity to the peripheral innervation.