A new device for behavioral analysis on rats exposed to high pressure

Effects of repeated hyperbaric nitrogen–oxygen exposures on the striatal dopamine release and on motor disturbances in rats

Previous studies have demonstrated disruptions of motor activities and a decrease of extracellular dopamine level in the striatum of rats exposed to high pressure of nitrogen. Men exposed to nitrogen pressure develop also motor and cognitive disturbances related to inert gas narcosis. After repetitive exposures, adaptation to narcosis was subjectively reported. To study the effects of repetitive exposures to hyperbaric nitrogen-oxygen, male Sprague-Dawley rats were implanted in the striatum with multifiber carbon dopamine-sensitive electrodes. After recovery from surgery, free-moving rats were exposed for 2 h up to 3 MPa of nitrogen-oxygen mixture before and after one daily exposure to 1 MPa of nitrogen-oxygen, for 5 consecutive days. Dopamine release was measured by differential pulse voltammetry and motor activities were quantified using piezo-electric captor. At the first exposure to 3 MPa, the striatal dopamine level decreased during the compression (-15%) to reach -20% during the stay at 3 MPa. Motor activities were increased during compression (+15%) and the first 60 min at constant pressure (+10%). In contrast, at the second exposure to 3 MPa, an increase of dopamine of +15% was obtained during the whole exposure. However, total motor activities remained unchanged as compared to the first exposure. Our results confirm that nitrogen exposure at 3 MPa led to a decreased striatal dopamine release and increased motor disturbances in naïve rats. Repetitive exposures to 1 MPa of nitrogen induced a reversal effect on the dopamine release which suggests a neurochemical change at the level of the neurotransmitter regulation processes of the basal ganglia. In contrast, motor activity remained quantitatively unchanged, thus suggesting that dopamine is not involved alone in modulating these motor disturbances.

Striatal dopamine release and biphasic pattern of locomotor and motor activity under gas narcosis

Inert gas narcosis is a neurological syndrome appearing when humans or animals are exposed to hyperbaric inert gases (nitrogen, argon) composed by motor and cognitive impairments. Inert gas narcosis induces a decrease of the dopamine release at the striatum level, structure involved in the regulation of the extrapyramidal motricity. We have investigated, in freely moving rats exposed to different narcotic conditions, the relationship between the locomotor and motor activity and the striatal dopamine release, using respectively a computerized device that enables a quantitative analysis of this behavioural disturbance and voltammetry. The use of 3 MPa of nitrogen, 2 MPa of argon and 0.1 MPa of nitrous oxide, revealed after a transient phase of hyperactivity, a lower level of the locomotor and motor activity, in relation with the decrease of the striatal dopamine release. It is concluded that the striatal dopamine decrease could be related to the decrease of the locomotor and motor hyperactivity, but that other(s) neurotransmitter(s) could be primarily involved in the behavioural motor disturbances induced by narcotics. This biphasic effect could be of major importance for future pharmacological investigations, and motor categorization, on the basic mechanisms of inert gas at pressure.

Gamma-aminobutyric acid neuropharmacological investigations on narcosis produced by nitrogen, argon, or nitrous oxide

Inhaled anesthetics, including the gaseous anesthetics nitrous oxide and xenon, are thought to act by interacting directly with ion-channel receptors. In contrast, little is known about the mechanism of action of inert gases that show only narcotic potency at high pressures, such as nitrogen or argon. In the present study, we investigated the effects of selective gamma-aminobutyric acid (GABA) receptor antagonists on narcosis produced by nitrogen, argon, and nitrous oxide. Pretreatment with the competitive GABA(A) receptor antagonist gabazine (0.2 nmol) but not the GABA(B) receptor antagonist 2-hydroxysaclofen (10 nmol) increased the nitrogen and argon threshold pressure for loss-of-righting-reflex (P < 0.005) but had no effect on nitrous oxide narcosis. Pretreatment with the GABA(A) benzodiazepine receptor antagonist flumazenil (5 nmol) also increased the narcosis threshold pressure of argon (P < 0.025). Given that neither 2-hydroxysaclofen, gabazine, nor flumazenil at the doses used induced hyperexcitability, our results support a selective antagonism by gabazine and flumazenil of the narcotic action of nitrogen and argon. Some mechanisms of nitrogen and argon narcotic action might be similar to those of clinical inhaled anesthetics.

IMPLICATIONS

We studied the effects in the rat of gamma-aminobutyric acid (GABA) receptor antagonists on narcosis induced by nitrogen and argon that act only at high pressures. Our results show that the GABA (A) receptor may play a significant role, suggesting that some mechanisms might be similar to those of clinical inhaled anesthetics.

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.

High pressure enhanced NMDA activity in the striatum and the globus pallidus: relationships with myoclonia and locomotor and motor activity in rat

In mammals high pressure of helium-oxygen (He-O2) breathing mixture leads to the high pressure neurological syndrome (HPNS) which includes a set of behavioural disorders such as locomotor and motor hyperactivity (LMA) and myoclonia. In rats, i.c.v. administrations of competitive NMDA antagonists decrease some of these symptoms suggesting that He-O2 pressure could enhance NMDA neurotransmission within the central nervous system. More recently, we have shown using microdialysis that the extracellular glutamate level is increased in the striatum by He-O2 pressure. Neurochemical data have suggested that this structure is probably involved in the LMA development but not in the myoclonia expression. When considering myoclonia, recent neuropathological studies performed at normal pressure in humans suggest that the globus pallidus extern (equivalent to the globus pallidus in the rat) could be involved in this behavioural disorder. The aim of this study was to compare the role of striatal and pallidal NMDA activity on the LMA development and the myoclonia expression in the model of rat exposed to 8 MPa of He-O2 mixture. The intrastriatal administration of D(-)-2-amino-7-phosphonoheptanoic acid (2-APH) (10 nmol/slide) reduced the LMA development but only slightly reduced myoclonia. In contrast, the intrapallidal administration of 2-APH (10 nmol/slide) reduced both LMA and myoclonia. These results suggest that the LMA development requires NMDA activity at both striatal and pallidal level. In contrast, the myoclonia expression mainly requires NMDA activity at pallidal level. Consequently, NMDA neurotransmission at input and output levels of the striato-pallidal pathway play different roles in some of the behavioural disorders induced by He-O2 pressure.

Administration of the glutamate uptake inhibitor L-trans-PDC in the globus pallidus and the substantia nigra, but not in the striatum, attenuates the psychostimulant effect of high helium pressure on locomotor activity in the rat

High helium pressure of more than 2 MPa produces central neuroexcitatory motor behavior. In rodents, symptoms comprise locomotor and motor activity (LMA), myoclonia, and, at pressure greater than 9-10 MPa, convulsions and tonic-clonic seizures. We studied the behavioral effects of bilateral injection of the glutamate uptake inhibitor L-trans-pyrollidine-2,4-dicarboxylic acid (L-trans-PDC), in either the substantia nigra reticulata (SNr), the globus pallidus (GP), or the striatum on high helium pressure-induced LMA and myoclonia. Injection of L-trans-PDC in the GP and the SNr attenuated LMA, whereas injection in the striatum enhanced it. Alternatively, injection of L-trans-PDC in the SNr increased myoclonia, whereas injection in the GP or the striatum showed no effects on myoclonia. These results confirm that helium pressure-induced LMA and myoclonia have different neural origins. According to current thinking on basal ganglia function and previous data, it is suggested that high helium pressure would lead to a reduction of glutamate transmission in the SNr that could contribute to a reduction in activity of the nigrothalamic GABA pathway and then to the occurrence of LMA. It is further suggested that glutamate and DA transmissions in the striatum could have synergistic, rather than antagonistic, influences on motor activity.

Administration of either non-NMDA receptor agonists or NMDA receptor antagonists into the substantia nigra or the globus pallidus reduces the psychostimulant effect of high helium pressure on locomotor activity in rats

Helium pressure of >2 MPa is a well known factor underlying pressure-dependent central neuroexcitatory disorders that include locomotor and motor activity (LMA) and myoclonia. We investigated the effects of bilateral injection in either the substantia nigra (SN) or the globus pallidus (GP) of the AMPA receptor agonist (+/-)AMPA, the kainate receptor agonist kainic acid, the NMDA receptor agonist (+/-)-cis-piperidine-2,3-dicarboxylic acid (PDA), and the NMDA receptor antagonist (+/-)-2-amino-7-phosphono-heptanoic acid (AP-7) in the occurrence of helium pressure-induced LMA and myoclonia. Administration of AMPA, kainate, or AP-7 in either the SN or the GP significantly reduced high helium pressure-induced LMA, whereas the NMDA receptor agonist showed no significant effect. Injection in the SN of the non-NMDA receptor agonist AMPA and the NMDA receptor agonist PDA increased the development of high helium pressure-induced myoclonia, whereas injection of the NMDA receptor antagonist AP-7 into the SN or the GP decreased it. This confirms that NMDA transmission in the SN and the GP would play a major role in the development of helium pressure-induced LMA; manipulation of AMPA and kainate systems may have therapeutic potential. The opposite effects of AMPA on LMA and myoclonia also confirm the neural substrates involved in the motor disorder produced by helium pressure differ substantially between LMA and myoclonia.

Modulation by GABA transmission in the substantia nigra compacta and reticulata of locomotor activity in rats exposed to high pressure

Helium pressure of > 20 bar causes neuroexcitatory changes referred to as the high pressure neurological syndrome. In rodents, symptoms include locomotor and motor activity (LMA), myoclonia and, at greater pressure, convulsions. We studied the effects of the GABA reuptake inhibitor nipecotic acid, the GABA transaminase inhibitor gamma-vinyl-GABA (GVG), the GABAA receptor agonist muscimol, and the GABAB receptor agonist baclofen. Whatever the drug used, bilateral administration in the substantia nigra reticulata (SNR) or in the substantia nigra compacta (SNC) showed no significant effects on myoclonia. In contrast, administration in the SNR of nipecotic acid, GVG, and baclofen resulted in a significant decrease of LMA; administration of muscimol in the SNR increased LMA. No significant effect was seen when drugs were injected in the SNC. These results suggest that changes in GABA transmission in the SNR, but not in the SNC, play a crucial role in the control of motor activity and the regulation of movement.

Low-cost automatic activity data recording system

We describe a low-cost, high quality device capable of monitoring indirect activity by detecting touch-release events on a conducting surface, i.e., the animal's cage cover. In addition to the detecting sensor itself, the system includes an IBM PC interface for prompt data storage. The hardware/software design, while serving for other purposes, is used to record the circadian activity rhythm pattern of rats with time in an automated computerized fashion using minimal cost computer equipment (IBM PC XT). Once the sensor detects a touch-release action of the rat in the upper portion of the cage, the interface sends a command to the PC which records the time (hours-minutes-seconds) when the activity occurred. As a result, the computer builds up several files (one per detector/sensor) containing a time list of all recorded events. Data can be visualized in terms of actograms, indicating the number of detections per hour, and analyzed by mathematical tools such as Fast Fourier Transform (FFT) or cosinor. In order to demonstrate method validation, an experiment was conducted on 8 Wistar rats under 12/12-h light/dark cycle conditions (lights on at 7:00 a.m.). Results show a biological validation of the method since it detected the presence of circadian activity rhythm patterns in the behavior of the rats.

Role of 5-HT1b receptor in the pressure-induced behavioral and neurochemical disorders in rats

When human divers and experimental animals are exposed to increasing environmental pressure, they develop the high-pressure neurologic syndrome (HPNS) that has been recently demonstrated to include an increase in striatal dopamine (DA) release. This increase has been correlated with enhanced locomotor and motor activity (LMA). In the present study, we investigated the effect of the 5-HT1b receptor antagonist (+/-)cyanopindolol, which has been shown to block at normal pressure the increase in striatal DA release induced by the administration of the 5-HT1b receptor agonist CGS 12066B. Our data clearly showed that the administration of (+/-)cyanopindolol partially blocked both the pressure-induced increase in striatal DA release and the development of LMA. These results suggest the contribution of the 5-HT neurotransmission in the DA-related neurochemical and behavioral disorders that occur in rats exposed to high pressure.

Involvement of 5-HT3 receptor in the pressure-induced increase in striatal and accumbens dopamine release and the occurrence of behavioral disorders in free-moving rats

Rats exposed to high pressure developed locomotor and motor activity (LMA) that correlated with an increase of DA release in both the nucleus accumbens and the caudate-putamen. We investigated the effects of the 5-HT3 receptor antagonist MDL 72222 on these pressure-induced neurochemical and behavioral disorders. MDL 72222 totally blocked the pressure-induced increase in accumbens DA release and the development of LMA, whereas it only reduced the increase in striatal DA release. This suggest that both LMA and the increase of DA release in the nucleus accumbens, but not in the caudate-putamen, could specifically result from a 5-HT3 receptor activation in rats exposed to high pressure.

Pressure-induced disorders in neurotransmission and spontaneous behavior in rats: an animal model of psychosis

Disorders in neurotransmission and spontaneous behavior in rats exposed to a high pressure helium-oxygen mixture that shows interesting parallels with the dopaminergic hypothesis of schizophrenia at both the biochemical and the therapeutic responding levels are reviewed. Furthermore, as human subjects exposed to a very high pressure have shown psychotic episodes, we conclude that the pressure-induced disorders in neurotransmission and spontaneous behavior in rats could constitute a valid animal model of schizophreniform psychosis and a useful tool for both the investigation of the biological mechanisms underlying schizophrenia and the development of new antipsychotic drugs.

Relationship between emotional status and spontaneous behavior in rats exposed to high pressure

When humans and experimental animals are exposed to increased environmental pressure, they develop the high pressure neurological syndrome. In the present study, we have investigated the relationship between the emotional status and the development of pressure-induced behavioral disorders, such as locomotor and motor activity (LMA) and myoclonia, in two genetically selected lines of rats (Roman low-(RLA/verh) and high-(RLA/verh) avoidance), which differ drastically in their level of emotionality. RLA/verh rats presented a greater level of LMA than RHA/Verh rats; RLA/Verh rats also showed myoclonia while RHA/Verh rats did not. These results are discussed in the light of recent experiments in rats exposed to stressful situations or pressure environment. It is concluded that the use of both RLA/Verh and RHA/Verh rats would be of particular interest to better understand the neurochemical and neurological processes underlying the development of LMA and myoclonia in rats exposed to high pressure.

Effects of the administration of a-methyl-p-tyrosine on the striatal dopamine increase and the behavioral motor disturbances in rats exposed to high pressure

When human divers and experimental animals are exposed to increased environmental pressure, they develop the high pressure neurological syndrome (HPNS). Moreover, it has been recently demonstrated that pressure exposure induced an increase in striatal dopamine (DA) release. In this study, the effects of intracerebroventricular administration of a-methyl-p-tyrosine on the pressure-induced striatal DA increase, and the behavioral motor disturbances of HPNS, including hyperlocomotor activity (HLA), tremor, and myoclonia were monitored in free-moving rats. Striatal DA release was monitored by in vivo voltammetry, and behavioral symptoms using piezoelectrical sensors. Results suggested that the pressure-induced striatal DA increase could be the consequence of a release in both newly synthesized and vesicular DA. Elsewhere, data also confirmed that the pressure-induced DA disturbances would be involved in the development of HLA.

Role of dopamine receptors in the occurrence of the behavioral motor disturbances in rats exposed to high pressure

When human divers and experimental animals are exposed to an increasing environmental pressure, they develop the high pressure neurological syndrome (HPNS) characterized by electroencephalographic changes and sleep and behavioral disturbances. In rats, behavioral disturbances essentially include hyperlocomotor activity (HLA), tremor and myoclonia. Moreover, HLA has recently been demonstrated to be linked to a pressure-induced striatal increase of dopamine (DA). In these experiments, it was proposed to investigate in rats, at the behavioral level, the role of DA receptors in the occurrence of the pressure-induced DA disturbances. DA receptor agonists were found to induce no significant changes in the development of HLA, tremor, and myoclonia. Alternatively, HLA was found to be dramatically antagonized by the use of DA receptor antagonists (SCH 23390, sulpiride, and haloperidol), while tremor and myoclonia only decreased in SCH 23390 experiments.