Specificity and sensitivity of commercially available assays for glucagon-like peptide-1 (GLP-1): implications for GLP-1 measurements in clinical studies.

AIMS: To evaluate the performances of commercially available glucagon-like peptide-1 (GLP-1) assays and the implications for clinical studies. METHODS: Known concentrations (5-300 pmol/l) of synthetic GLP-1 isoforms (GLP-1 1-36NH2, 7-36NH2, 9-36NH2, 1-37, 7-37 and 9-37) were added to the matrix (assay buffer) supplied with 10 different kits and to human plasma, and recoveries were determined. Assays yielding meaningful results were analysed for precision and sensitivity by repeated analysis and ability to discriminate low concentrations. Endogenous GLP-1 levels in clinical samples were assessed using three commercial kits. RESULTS: The USCN LIFE assay detected none of the GLP-1 isoforms. The active GLP-1 enzyme-linked immunosorbent assays (ELISAs) from Millipore and DRG appeared identical and were specific for intact GLP-1 in buffer and plasma. The Meso Scale Discovery (MSD) total GLP-1 kit detected all six GLP-1 isoforms, although recovery of non-active forms was incomplete, especially in plasma. Millipore total GLP-1 ELISA kit detected all isoforms in buffer, but mainly amidated forms in plasma. The Alpco, Phoenix and Bio-Rad kits detected only amidated GLP-1, but the Alpco kit had a limited measurement range (30 pmol/l), the Phoenix kit had incomplete recovery in plasma and the Bio-Rad kit was insensitive (detection limit in plasma 40 pmol/l). The pattern of postprandial GLP-1 responses in clinical samples was similar between the kits tested, but the absolute concentrations measured varied. CONCLUSIONS: The specificity and sensitivity of commercially available kits for the analysis of GLP-1 levels vary considerably. This should be taken into account when selecting which assay to use and when comparing data from different studies.

Feasibility of a visual prosthesis for the blind based on intracortical microstimulation of the visual cortex.

The feasibility of producing a visual prosthesis for the blind using intracortical microstimulation (ICMS) of the visual cortex was studied in a 42-year-old woman who had been totally blind for 22 years secondary to glaucoma. Thirty-eight microelectrodes were implanted in the right visual cortex, near the occipital pole, for a period of 4 months. Percepts reported as small spots of light, called phosphenes, were produced with 34 of the 38 implanted microelectrodes. Threshold currents for phosphene generation with trains of biphasic pulses were as low as 1.9 microA, and most of the microelectrodes had thresholds below 25 microA. Phosphene brightness could be modified with stimulus amplitude, frequency and pulse duration. Repeated stimulation over a period of minutes produced a gradual decrease in phosphene brightness. Phosphenes did not flicker. The apparent size of phosphenes ranged from a "pin-point' to a "nickel' (20 mm diameter coin) held at arm's length. Phosphene size usually decreased as stimulation current was increased but increased slightly as the train length (TL) was increased. At levels of stimulation near threshold, the phosphenes were often reported to have colours. As the stimulation level was increased, the phosphenes generally became white, greyish or yellowish. Individual phosphenes appeared at different distances from the subject. When two phosphenes were simultaneously generated, the apparent distances of the individual phosphenes sometimes changed to make them appear to be at about the same distance. When three or more phosphenes were simultaneously generated, they became coplanar. Except for rare occasions, phosphenes extinguished rapidly at the termination of the stimulation train. When stimulation TLs were increased beyond 1 s, phosphenes usually disappeared before the end of the train. The duration of phosphene perception could be increased by interrupting a long stimulation train with brief pauses in stimulation. Intracortical microelectrodes spaced 500 microns apart generated separate phosphenes, but microelectrodes spaced 250 microns typically did not. This two-point resolution was about five times closer than has typically been achieved with surface stimulation. With some individual microelectrodes, a second closely spaced phosphene was sometimes produced by increasing the stimulation current. Phosphenes moved with eye movements. When up to six phosphenes were simultaneously elicited, they all moved with the same relative orientation during eye movements. All phosphenes were located in the left hemi-field with the majority above the horizontal meridian. There was a clustering of most of the phosphenes within a relatively small area of visual space. The potentially greater microelectrode density and lower power requirements of ICMS compared with surface stimulation appears encouraging for a visual prosthesis. However, further studies with blind subjects are required to optimize stimulation parameters and test complex image recognition before the feasibility of a visual prosthesis based on ICMS can be established.

Laser exposure of Parylene-C insulated microelectrodes.

The polymer, Parylene-C, has proven to be a biocompatible insulation for microelectrodes. However, due to its inert nature, the removal of the insulation from the tips of microelectrodes is difficult. This paper describes the use of an ultraviolet laser system to micromachine Parylene-C insulation with photoablation to precisely expose an arbitrary shape recording or stimulating surface.

The responses of cat motor cortical units to electrical cutaneous stimulation during locomotion and during lifting, falling and landing.

Experiments were performed to examine the influence of cutaneous information on motor cortical cells during movement in intact, awake cats. The movements investigated were locomotion and a sequence in which the animal was repeatedly lifted and dropped. Electrical stimuli to distal skin areas were delivered periodically during the movements and responses of motor cortical cells were examined. The animals used in these experiments were chronically implanted with cortical microelectrodes, a pyramidal tract stimulating electrode, cutaneous stimulating electrodes in the forepaw, and a recording cuff electrode around the median nerve. EMG electrodes were implanted in several forelimb muscles and a length gauge was implanted across the elbow joint. Results included in this report were obtained from three cats. The twenty-two cortical units analysed in this study (seven were PT units) were selected from a larger sample by the following criteria: cutaneous receptive fields which included the distal part of the limb, consistent short latency responses to electrical cutaneous stimulation and spontaneous activity modulated in consistent patterns during the movement investigated. Sixteen units were recorded during locomotion, 12 during the lifting and dropping cycle and 6 of these during both conditions. Most of the cells were influenced by the cutaneous input during locomotion. Three units had no response to peripheral stimulation during locomotion though they were responsive to this stimulus when the animal was sitting quietly. All the cells were responsive to the cutaneous input during the lifting and dropping cycle. The responses to cutaneous stimuli were found to be modulated in relation to phases of the step cycle and the lifting and dropping cycle. In 13 units this modulation did not parallel the modulation of the unit's spontaneous firing during these activities. For these units a common finding during locomotion was that the response to cutaneous stimuli increased throughout stance, reached maximum during the flexion part of the swing, and then declined to a minimum during the beginning of the next stance. During the lifting and dropping cycle, the responses were greatest when the animal was held in the air and when starting to fall, and minimal just prior to and after landing. In both movements, cutaneous responses were reduced when the limb was used to support the animal's weight. There is apparently a movement phase-related modulation of cutaneous input to some motor cortical cells. This modulation of cutaneous input resembled the modulation of cutaneous reflexes during locomotion.(ABSTRACT TRUNCATED AT 400 WORDS)

A pulsed integrator for EMG analysis.

A novel device is described which converts EMG signals to an output which is proportional to the area under the rectified curve in discrete time intervals. This is achieved by resetting a long time constant integrator at the desired rate and saving each integral in a sample/hold circuit for output during the subsequent integration period. This output is particularly useful for quantifying fast reflex events since the signal is integrated without leakage in each period and leaves no residual signal in subsequent periods.

Long-term unit recording from somatosensory neurons in the spinal ganglia of the freely walking cat.

A new technique has been developed for stable, long-term recording from groups of individual primary afferent neurons in the freely walking cat. A number of fine, flexible wires are inserted into dorsal root ganglia via a small laminotomy in the lumbar spine. The cut end of each wire can record stable and separable action potentials from one to three dorsal root ganglion neurons; each unit has typically held for 1 to 4 days. A broad range of myelinated somatosensory afferent (conduction velocities of 30 to 120 meters per second) have been studies during locomotion. Most cutaneous and proprioceptive afferent studied have been sensitive monitors of complex combinations of step-cycle components, and their firing patterns would often have been difficult to predict from existing information.

An instrument for stable single cell recording from pulsating human cerebral cortex.

An instrument has been developed for positioning microelectrodes near neurons in the exposed pulsating cerebral cortex of man or animals. The electrode moves with the cortex and maintains a fixed relationship with the desired neuron. Stable extracellular single cell recordings have been obtained for 17.5 min from an exposed human brain that pulsated 1.5 mm at the surface.