Magnetic field dependence of spin-lattice relaxation enhancement using piperidinyl nitroxyl spin-labels.

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.

Effects of trimethyltin chloride on differential-reinforcement-of-low-rate responding.

The effects of a range of doses of trimethyltin chloride (1.8-7.0 mg/kg, TMT) on responding maintained under a differential-reinforcement-of-low-rate 15-sec schedule of water presentation were examined in rats. TMT produced a large decrease in the percent of reinforced responses and consequently a decrease in the rate of reinforcement. These effects were due primarily to prolonged bursts of nonreinforced responses. In contrast to the gradual dose-related decrease in body weight produced by TMT, responding was disrupted only at the highest dose tested, i.e., 7.0 mg/kg. The results were discussed in terms of the neurotoxic effects of TMT on the limbic system.

Ultrasonic equipment for the measurement of backfat on unshorn live sheep.

Because of the ease of application to the animal's exposed skin, the measurement by A-scan ultrasonics of backfat on pigs is an established technique; but difficulties are experienced with unshorn sheep because the fleece presents an obstacle, as a parting of the wool offers only a limited aperture for insonification of the subcutaneous tissues. Also, movement of the typical nervous sheep usually provides somewhat intermittent return echo signals, rendering difficult an otherwise simple measurement. The present instrument has overcome these problems by an accumulator-averaging technique, implemented by a microprocessor, allowing estimation of live backfat thickness to the nearest 0.5 mm. This paper describes the instrument function, and presents results of a series of experiments which examined the correlation with the carcass backfat thickness.

Variations in the time course of the synchronization of intercostal motoneurones in the cat.

1. Synchronization of intercostal motoneurones was studied by the construction of cross-correlation histograms which related the firing times of paired groups of efferent inspiratory or expiratory discharges recorded from filaments of the external or internal nerves of anaesthetized or decerebrate cats.2. The principal feature of the histograms was always a central peak but the time course of the central peak showed considerable variation. Three forms of synchronization were defined on the basis of the time course of the central peak: (i) short-term synchronization (Sears & Stagg, 1976), where the peak was narrow, extending over about +/-3 ms but sometimes with weak shoulders to about +/-5 ms; (ii) broad-peak synchronization where the peak was wider than this (often +/-20 ms or more) but where there were no strong periodicities; (iii) high-frequency oscillation (h.f.o) synchronization, which was named from the related phenomena in medullary and phrenic recordings (Cohen, 1979), where there were periodic peaks on either side of the central peak with a frequency in the range 60-120 Hz. Combinations of these forms of synchronization were seen in some histograms.3. When different animals were compared, broad peak synchronization was seen in association with light anaesthesia and with polysynaptic excitation of the motoneurones from muscle spindle afferents.4. In individual animals, additional anaesthesia depressed both broad peak and h.f.o. synchronization.5. Raising P(A, CO2), which increased the respiratory drive to the motoneurones, favoured short-term or h.f.o. synchronization at the expense of broad-peak synchronization.6. In three decerebrate animals only short-term or h.f.o. synchronization was seen.7. Spinal cord lesions above or below the segments of interest promoted broad-peak synchronization, even with high P(A, CO2) or deep anaesthesia.8. We conclude: (i) that short-term synchronization, due mainly to the branching of presynaptic axons, is generated mainly by those axons which transmit the respiratory drive, that drive providing most of the excitation of the motoneurones in moderately deep anaesthesia; (ii) that h.f.o. synchronization arises from the periodic synchronization of the discharges in these same presynaptic axons; (iii) that broad-peak synchronization is generated by the activity of other presynaptic neurones whose discharges are also synchronized, but aperiodically, these neurones most likely including spinal cord interneurones which are active in light anaesthesia or when released by spinal cord lesions.9. These conclusions are supported by comparisons between intracellular recordings from inspiratory motoneurones in animals showing different forms of motoneurone synchronization, the comparison including the measurements of ;average common excitation' (a.c.e.) potentials (Kirkwood & Sears, 1978).