Spinal cord seizures elicited by high pressures of helium

Glycine activation of human homomeric alpha 1 glycine receptors is sensitive to pressure in the range of the high pressure nervous syndrome

The effect of hyperbaric pressure on the inhibitory glycine receptor has been investigated in voltage-clamped Xenopus oocytes microinjected with cRNA encoding the human alpha-1 glycine receptor subunit. Heterologous expression of the human alpha-1 subunit generated functional glycine-gated channels with properties typical of native receptors. Glycine elicited a concentration-dependent inward current which reversed polarity at -25 mV and was antagonised by nanomolar concentrations of strychnine. Concentration-response curves established for the homomeric alpha-1 glycine receptor at 5, 10 and 15 MPa were progressively shifted to the right with respect to the concentration response curve established at atmospheric pressure (0.1 MPa). Pressure had no effect on the maximal response. The EC(50) values at 0.1, 5, 10 and 15 MPa were 190 mu M, 222 mu M, 338 mu M and 482 mu M, respectively. The results demonstrate that a receptor comprised solely of the human alpha-subunit is sensitive to pressure in the range that affects divers and at which the native rat spinal cord receptor is affected. This finding is discussed in the context of the postulated binding sites for glycine and the implications for the design of drugs to protect divers from the effects of pressure.

Electroencephalographic and behavioural correlates of seizure development in rats in response to hyperbaric exposure

Electroencephalographic (EEG) activity was continuously monitored from the hippocampus, amygdala, reticular formation and frontal cortex in freely moving Sprague-Dawley rats exposed to 91 atmospheres absolute pressure (ATA) using compression rates of 1 or 3 ATA/min. Videotape recordings were made for subsequent behavioural analysis. Tremor, myoclonic jerks and tonic extensions of the tail were observed in all animals but did not appear to correlate with epileptiform activity. Convulsions occurred between 66.5 and 91 ATA in all subjects compressed at 3 ATA/min, but in only 1 rat (at 91 ATA) in the 1 ATA/min group. Tonic-clonic motor seizures developed explosively and involved the entire body. EEG records showed continuous spiking at all sites during the generalized convulsive state. There was no evidence of differential susceptibility of the various brain regions examined to the epileptogenic effects of high pressure. The behavioural and EEG data indicate that hyperbarically induced seizures differ from the classical limbic type.

Hyperbaric pressure depresses potentiation of polysynaptic medullospinal reflexes in newborn rats

Hyperbaric pressure induces hyperexcitability and convulsions in intact animals by mechanisms that are not understood. In the present experiments we examined the effects of pressure on medullospinal reflexes and synaptic interactions in the vitro brainstem-spinal cord of newborn rats. Reflex activity was recorded extracellularly from the cut ventral root of the 1st cervical nerve; the Vth and Xth cranial nerves were stimulated. Exposure to pressure of 10.1 MPa increased the amplitude and duration of individual reflex responses. Hyperbaric pressure inhibited post-tetanic potentiation, reduced recovery time and decreased the marked heterosynaptic potentiation caused by Vth nerve stimulation on the Xth nerve reflex. Xth nerve stimulation caused weak heterosynaptic potentiation of the Vth nerve reflex and was not affected by pressure. In contrast to crustacean neuromuscular junction, hyperbaric pressure in the mammalian central nervous system enhanced single polysynaptic responses but depressed frequency-dependent potentiation of medullospinal reflexes.

Activity of hypothalamic neurons in conscious rats decreased by hyperbaric environment

Single-unit activity of neurons in rat hypothalamus, prefrontal cortex (PFC) and central medial thalamus (CM) was recorded through chronically implanted electrodes. Upon being subjected to 7 atmospheres absolute (ATA) hyperbaric air (709 kPa) environment, about two-thirds of the hypothalamic neurons tested reversibly decreased their firing rate to 71 +/- 12% (mean +/- SD, n = 25) compared to the precompression control activity, and the remaining one-third did not change their firing rate (100 +/- 6%, n = 13). It is suggested that the decrease in firing rate was attributable to increased partial pressure of both oxygen and nitrogen. In contrast to the hypothalamus, the firing rates of most neurons in the PFC and CM did not decrease (99 +/- 9% for PFC, 107 +/- 12% for CM). In separate acute experiments, 3 out of 5 hypothalamic neurons tested in the isolated forebrain also decreased their firing rates in hyperbaric environment. The results suggest that the hyperbaric air acted directly on the forebrain and depressed more than half of the hypothalamic neurons.

Hyperbaric effects on sensorimotor reactivity studied with acoustic startle in the rat

In three experiments, startle responses to brief intense tone-bursts (30 msec, 110 dB, 6000 Hz) and single pulse (0.1 msec) stimulation of the cochlear nucleus and the nucleus reticularis pontis caudalis are studied under high pressures of heliox (from 0 bar to 50 bars) in the rat. For each rat (N = 12), mean amplitude and latency changes in startle responses (nuchal electromyography and whole-body accelerometry) are compared at normobaric pressure and during compression, at a speed of 100 bar/H. The results indicate that high pressures decrease (50% of control size) tone evoked startle by acting on the peripheral auditory organ, probably through middle and/or inner ear barotrauma. The large increases in electrically-elicited startle (250% of control size) from the cochlear and reticular nuclei, under hyperbaric conditions, suggest that high pressures affect sensorimotor reactivity by excitatory action on synaptic transmission in the relays of the acoustic startle reflex arc at the lower brainstem and spinal levels. Startle latencies remain unaffected by the heliox high pressures studied here.

Effects of high-pressure helium on gamma-[3H]aminobutyric acid release from the isolated frog spinal cord

The application of high pressure in vivo causes a hyperexcitability syndrome involving tremors and convulsions. Drugs that potentiate GABA transmission protect animals against this syndrome. It is possible that changes in gamma-aminobutyric acid (GABA) transmission may underlie the hyperexcitability. We have therefore investigated the effects of pressure on the components of GABA transmission in vitro. After incubation with [3H]GABA, hemisected frog spinal cords were superfused inside a pressure chamber and perfusate fractions were collected every 10 min. Helium, at 50 or 100 atm, did not alter the spontaneous release of GABA, but if electrical stimulation had been applied previously, then pressure caused a prolonged increase in GABA release. Helium at 50 atm did not alter the evoked release during electrical stimulation, but at 100 atm this was increased. This increase was smaller in the absence of calcium. No corresponding changes in [14C]urea efflux were seen, suggesting that the effects were not due to nonselective membrane permeability changes. The results are consistent with the known effects of pressure on neuronal activity, such as repetitive firing, but they do not suggest a selective action on the GABA release process.

The possible mechanisms of the high pressure-induced motor disturbances in the cat

High pressures elicit a high-frequency tremor (8-12 c/sec) in mammals, the mechanisms of which are still unknown. The present study shows that: (1) in spite of many similarities observed between the EMG characteristics of harmaline-induced tremor and pressure-induced tremor, cerebellar lesions which suppress the harmaline-induced tremor, do not modify the characteristics of the pressure-induced tremor; (2) at depth, the caudal part of the spinal cat (section at T9-T10 spinal level) displays irregular spontaneous EMG activities which can be clonic or rhythmic (4-8 c/sec), and a neuromuscular stretch hyperreflexivity. These data suggest that the origin of the pressure-induced tremor is spinal and neuromuscular rather than cerebellar.

Enhancement of cortical evoked potentials by high atmospheric pressures of helium

Adult guinea pigs with electrodes chronically implanted in the optic chiasm (OC(, lateral geniculate nucleus (LGN), and visual cortex (CX) were compressed at 1 bar/min to 120 bars pressure in helium-oxygen. Body temperature was controlled to within +/- 1 degree C, CO2 was removed by a soda-lime absorbent. Electrical stimuli (50 microA, 0.05 msec) were delivered to the OC at 9.6/sec at 10 bar intervals, with pressure held constant. Pressure caused only negligible changes inthe amplitudes of pre- and postsynaptic components of potentials in the LGN. Amplitudes of evoked potentials in the CX increased approximately linearly with pressure, reaching values of up to 300% above normobaric. Latencies did not change independently of temperature. Excitability of nerve fibers changed very little, but intracortical synaptic transmission was substantially enhanced. Differences in the effects of elevated helium pressure at the LGN and CX may be attributed to differences in organization of local circuits.