The full expression of locomotor and motor hyperactivities induced by pressure requires both striatal dopaminergic and N-methyl-D-aspartate receptor activities in the rat

The efficacy of physiological and pharmacological N-methyl-D-aspartate receptor block is greatly reduced under hyperbaric conditions

Human divers exposed to hyperbaric pressure may suffer from cognitive and motor impairments thought to be related to high pressure effects per ce. These effects, termed high pressure neurological syndrome (HPNS), appear at pressure above 1.1 MPa. HPNS involves CNS hyperexcitability that is partially attributed to augmented responses of the glutamatergic N-methyl-d-aspartate receptor (NMDAR). NMDAR is blocked by Mg(2+) (physiologically) and by dl-2-Amino-5-phosphonopentanoic acid (AP5, pharmacologically). We have recently reported that hyperbaric pressure augments rat hippocampus NMDAR synaptic response and generates hyperexcitability. We now test pressure effects on the blockade efficacy of Mg(2+)and AP5. Under high pressure conditions more than double [Mg(2+)](o) and [AP5](o) were needed to achieve similar effects on NMDAR synaptic response's amplitude, decay time, and time integral comparable to control conditions. [Mg(2+)](o) and [AP5](o) concentration-response curves and the concentration for 50% responses' inhibition (IC(50)s) showed similar normalized pattern at control and pressure for each parameter. We conclude that hyperbaric pressure reduces the efficacy of these NMDAR blockers that may be associated with the receptor conformational change(s). This provides additional mechanism for pressure over activation of NMDAR. Taken together with our previous reports, high pressure modification of NMDAR activity significantly contributes to CNS hyperexcitability and possibly for long term vulnerability.

Integrated digital image and accelerometer measurements of rat locomotor and vibratory behaviour

This study developed a combined IC-type accelerometer and video camera system to simultaneously measure vibration and locomotion activities in rats. A personal computer, adopted as an image frame grabber, was combined with a digital image processing algorithm to measure the precise location of an animal in an experimental cage. An accelerometer-based vibration subsystem, based on an 89C51 single-chip microprocessor, was designed. The acceleration sensor module was attached directly to the shaved back of the rat's body to directly measure the animal's vibration. This module can detect a wide range of vibrations from movements of the entire body to micro-tremors. Along with hardware, this study also proposes novel software for video enhancement and data analysis to calculate the behaviour parameters from recorded movements. In normal mode, three vibration activities (locomotor activity, tremor and twitch) are auto-analyzed every 10 min. The results are saved, and various display, statistical and data organization options are available. The primary merits of this system are the ability to simultaneously record locomotion and vibrational data, the rapid set-up and operation, the low cost, the reduced illumination requirements, the reduction of environmental noise and the high precision. The proposed method will be of interest to researchers in various behavioural, biological and medical fields.

High pressure modulation of NMDA receptor dependent excitability

Pressure above 1.1 MPa induces in mammals and humans the high pressure neurological syndrome (HPNS). HPNS is characterized by cognitive and motor decrements associated with sleep disorders, EEG changes, tremor, and convulsions that ultimately may lead to death. Previous theories proposed that augmented response of the glutamatergic N-methyl-D-aspartate receptor (NMDAR) or reduced GABAergic inhibition may be involved. Recently, we have reported that isolated NMDAR response was augmented at high pressure. We now test whether this augmentation induces neuronal hyperexcitability. We studied high pressure effects on pharmacologically isolated NMDAR field excitatory postsynaptic potentials (fEPSPs) and on their efficacy in generating population spikes (PSs). Sprague-Dawley male rats were used. Hippocampal coronal brain slices were prepared, constantly superfused with physiological solutions, gas-saturated at normobaric pressure, and compressed up to 10.1 MPa with helium. fEPSPs and PSs were recorded from the dendritic and the somatic layers of CA1 pyramidal neurons in response to Schaefer collaterals stimulation with trains of five stimuli at 25 Hz. Pressure caused PSs to appear earlier in the train. However, PS delay, rise time and decay time were increased and PS amplitude, frequency, and number were decreased in the last responses in the train. The decrease in late fEPSPs was associated with a reduction of the total number of PSs in the train, apparently without a change in the synaptic efficacy. These results may partially explain the neuronal hyperexcitability observed at pressure. Therefore, it is postulated that significant hyperexcitability is attained at pressure only when the normal fast fEPSP is intact.

Modulation of isolated N-methyl-d-aspartate receptor response under hyperbaric conditions

In humans, hyperbaric pressure induces the high-pressure neurological syndrome (HPNS). HPNS is characterized by tremor, sleep disorders, electroencephalographic changes, and impairment of cognitive and motor performances. In animals, higher pressures result in convulsions and death. An increased N-methyl-d-aspartate receptor (NMDAR) response has been implicated with HPNS. We studied high-pressure effects on pharmacologically isolated NMDAR field excitatory postsynaptic potentials (fEPSPs). Hippocampal coronal brain slices from male Sprague-Dawley rats were prepared, constantly superfused with physiological solutions, gas-saturated at normobaric pressure and compressed up to 10.1 MPa with helium. fEPSPs were recorded from the dendritic layer of CA1 pyramidal neurones. High pressure significantly increased the single fEPSP delay, maximal initial slope, amplitude, decay time and time integral (elevated Na(+) and Ca(2+) influx) despite the known general decrease in glutamatergic synaptic release. The estimated negative and positive activation volumes (DeltaV*) for various kinetic segments of the fEPSP suggest a complex response of the receptor to pressure. The NMDAR frequency response was tested by a train of five stimuli. At 50-100 Hz, high pressure did not increase the fEPSPs' frequency-dependent depression and the train's time integral remained unchanged. At 25 Hz, pressure induced a larger frequency-dependent depression and significantly increased the time integral. Our results provide, for the first time, direct information on the isolated brain NMDAR response under hyperbaric conditions. These observations may explain some increase in the excitability of single normal glutametergic fEPSPs and their frequency responses.

Microdialysis study of striatal dopaminergic dysfunctions induced by 3 MPa of nitrogen– and helium–oxygen breathing mixtures in freely moving rats

Previous studies have demonstrated opposite effects of high-pressure helium and nitrogen on extracellular dopamine (DA) levels, which may reflect disturbances on the synthesis, release or metabolic mechanisms. Intrastriatal microdialysis was used to measure the precursor (tyrosine), DA and its metabolites (DOPAC, HVA) levels under nitrogen- or helium- at pressure up to 3 MPa. Under 3 MPa of helium-oxygen breathing mixtures, the extracellular concentration of tyrosine is decreased while the extracellular concentration of DA is increased. On the contrary, nitrogen-oxygen breathing mixture at the same pressure increased extracellular tyrosine concentration and decreased DA release. Under both conditions, an increment of the DOPAC and HVA levels could be noted. Our results suggest that changes in DA release and metabolism during high-pressure helium exposure reflect the effect of the pressure per se, whereas the intrinsic effects of narcotic gases, although sensitive to pressure, would be revealed by hyperbaric nitrogen exposure.

GABAergic modulation in the substantia nigra of the striatal dopamine release and of the locomotor activity in rats exposed to helium pressure

Helium-oxygen pressure induces in rodents an increase of both locomotor and motor activity (LMA) and of the striatal dopamine release, which could result from a decrease of GABA transmission in the substantia nigra. The effects of the GABA(A) receptor agonist muscimol and of the GABA(B) receptor agonist baclofen on the striatal dopamine release were measured using differential pulse voltammetry. Behavioural studies were performed in freely moving rats using actimetry. Whatever the drug used under helium pressure, bilateral administration in the substantia nigra pars reticulata (SNr) or in the substantia nigra pars compacta (SNc) counteracted the evoked dopamine release. However, only the baclofen reduced the LMA, while the muscimol administration in the SNr, but not in the SNc, increased it. These results indicate that different subtypes of GABA receptors would be involved in the control of the DA release and in the occurrence of LMA under helium pressure.

Pallidal administrations of gabazine and 5-AVA affect pressure-induced behavioral disorders in rats

The aim of this work was to study the role of pallidal GABAa and GABAb neurotransmission in the behavioral disorders induced by pressure. The effects of GABAb antagonist 5-aminovalleric acid (5-AVA) or GABAa antagonist gabazine administrations in the globus pallidus (GP) on locomotor and motor hyperactivity (LMA) and myoclonia expressions in the model of the rat submitted to 8 MPa of helium-oxygen breathing mixture were analyzed. The administration of GABAa antagonist gabazine enhances the occurrence of the epileptic seizures, slightly increases LMA but decreases myoclonia. In contrast, the administration of GABAb antagonist 5-AVA decreases both LMA and myoclonia during the compression and the beginning of the holding time at 8 MPa. These data indicate that some behavioral disorders induced by pressure are in relation with GABAergic neurotransmission and establish clearly that GABAa and GABAb receptor mediations have distinct functions in the GP of the rat.

Helium-oxygen pressure induces striatal glutamate increase: a microdialysis study in freely-moving rats

In rat, helium pressures induce locomotor and motor activity which requires dopaminergic and N-methyl-D-aspartate (NMDA) receptor activities at striatal level. However, biochemical studies have suggested that pressure exposure may increase striatal glutamate level. We used microdialysis technique to study the effects of pressure on glutamate level in the striatum and the effects of local administration of D1 (SCH23390) or D2 (sulpiride) on these changes. Pressures increase both glutamate and glutamine levels in striatal microdialysates. Administration of sulpiride (1 microM) or SCH23390 (1 microM) by reverse microdialysis did not affect significantly pressure induced glutamate increase. So, protective effects of D1 and D2 antagonists against locomotor and motor hyperactivity (LMA) are probably independent of the processes involved in the striatal glutamate increase evoked by pressure.