Specific effects of drugs at pressure: animal investigations

High-pressure neurological syndrome (HPNS)

High-pressure neurological syndrome (HPNS) is a condition encountered in diving beyond a depth of 100 m. Manifestations include headache, tremor, myoclonus, neuropsychiatric disturbances and EEG changes. Convulsions are seen only in experimental animals. Most of the changes are reversible on surfacing but some such as memory disturbances may linger on for long periods. Excessive atmospheric pressure is the most important factor in the pathogenesis of HPNS. Neurotransmitter changes occur of which serotonin appears to be a more likely mediator because of the resemblance of HPNS to serotonin syndrome. Anesthetics and anticonvulsants have been used in experimental animals but are unsuitable for use in human divers. Breathing gas mixtures such as heliox have enabled the extension of depth of diving without HPNS. Use of 5-HT1A receptor antagonists may provide an interesting approach to prevention of HPNS.

Effects of general anesthetics and pressure on mammalian excitatory receptors expressed in Xenopus oocytes

The effects of general anesthetics and pressure on receptors from the mammalian central nervous system have been investigated using oocyte expression techniques. Poly A+ mRNA extracted from rat whole brain was injected into mature Xenopus oocytes producing depolarizing responses to the fast excitatory neurotransmitters NMDA and kainate and the inhibitory neurotransmitters GABA and glycine. An apparatus was constructed to allow agonist dose-response curves to be determined at high pressures using voltage-clamped oocytes. This was used to investigate the excitatory transmitter kainate. It was found that anesthetics depress the current induced by kainate whereas pressure does not appear to affect the responses associated with this transmitter. Furthermore it was found that pressure does not reverse (or modify in any way) the changes in response brought about by application of anesthetics.

The high pressure neurological syndrome in genetically epilepsy prone rats: protective effect of 2-amino-7-phosphono heptanoate

Genetically epilepsy prone rats (GEPR) are hypersensitive to various epileptogenic treatments and undergo characteristic generalized seizures when exposed to potent acoustic stimulation. We have studied the sensitivity of GEPR to high atmospheric pressure. Threshold pressures for behavioral symptoms of the high pressure neurological syndrome (HPNS) were recorded in normal Sprague-Dawley (SD) and GEPR (which originate from the SD strain) of both sexes. The threshold pressure (TP) for tremor and for convulsion was significantly lower in GEPR than in SD rats. The protective action of the NMDA receptor antagonist D-2-amino-7-phosphono-heptanoate (D-APH) was tested on both strains of rats. D-APH, 90 mg/kg ip was more protective against tremor in SD than in GEPR. Female GEPR were not protected against tremor. Protection against clonic seizures was similar in both sexes of GEPR and female SD rats while SD males were not significantly protected. None of the animals treated with D-APH developed the tonic phase of seizures. Blockade of the NMDA receptor with D-APH brought the threshold for convulsions in GEPR to a similar pressure to that obtained in SD vehicle-injected controls. This findings suggests the involvement of the excitatory amino acid system in the hypersensitivity of GEPR to high atmospheric pressure.

Studies on the role of the NMDA receptor in the substantia nigra pars reticulata and entopeduncular nucleus in the development of the high pressure neurological syndrome in rats

The effect of the focal injection of N-methyl-D-aspartate (NMDA) and 2-amino-7-phosphonoheptanoate (APH) into the substantia nigra pars reticulata (SNR) and entopeduncular nucleus (EP) on behavioural signs of the high pressure neurological syndrome (HPNS) in rats was studied. Doses of 1, 5 and 10 nmoles of NMDA or APH were injected into the SNR or EP, 10-30 min prior to the exposure of animals to a high pressure. Injection of NMDA into either SNR or EP results in a lowering of the threshold pressure for tremor by about 30%. Injection of NMDA into the SNR has no significant effect on clonic seizures whereas its injection into the EP results in a decrease of threshold pressure for clonic seizures. NMDA also facilitates the occurrence of forelimb clonus when injected into the EP. Injection of the NMDA antagonist, APH, into the SNR or EP significantly increases the threshold pressure of tremor (32.8 and 48.2% respectively). Seizure threshold is also increased by the injection of APH into either area, but nigral injections (especially the higher doses) are more protective against seizures than the EP injections. Comparing the two sites blockade of NMDA receptors within the EP is more protective against tremor, whereas in the SNR NMDA blockade is more protective against seizures.

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.

Systemically administered glycine protects against strychnine convulsions, but not the behavioural effects of high pressure, in mice

1. The effects of intraperitoneal administration of glycine were studied on the behavioural effects of raised ambient pressure in mice, compared with the effects of such administration on the actions of chemical convulsants. 2. Glycine did not alter the onset pressures for the occurrence of tremor, myoclonic jerks or clonic convulsions, when the ambient pressure was raised using helium. 3. Glycine showed a protective action against the convulsant effects of strychnine. 4. No protective action of glycine was found against the convulsant actions of pentylenetetrazol or bicuculline. 5. It is suggested that the results provide evidence that the high pressure neurological syndrome and strychnine convulsions have different neurophysiological origins.

Sleep disturbances in man during different compression profiles up to 62 bars in helium-oxygen mixture

Studies of sleep organization of 8 subjects were performed during 4 dives between 500 and 610 msw (51 and 62 bar) in a helium-oxygen mixture. The results of the 156 sleep records showed that sleep was disrupted from 30 bar: awake periods and stages I and II increased, stages III and IV and REM periods decreased. These disturbances, which were more intense with fast compression and recovered during decompression, can be considered as further symptoms of the high pressure nervous syndrome (HPNS).

Neurotransmitter glutamate: its clinical importance

Excitatory amino acid glutamate has several important functions in the mammalian central nervous system (CNS). This review focuses on the transmitter role of glutamate and discusses anatomical and pharmacological data of clinical neurological relevance. Experimental and clinical conditions which have been associated with altered content, uptake, membrane binding or release of glutamate in the CNS are discussed. Such conditions include, epilepsy, disorders of the basal ganglia, cerebral ischemia, hypoxia, hypoglycemia, metabolic encephalopathies, olivopontocerebellar atrophy and cerebellar ataxias, amino acidopathies, mental and other neurological disorders. With the exception of a few fibre systems, it is very difficult to differentiate between glutamate and aspartate as CNS transmitters. The term glutamate is, thus, used in the sense glutamate and/or aspartate unless specifically stated.