Pressure induces striatal serotonin and dopamine increases: a simultaneous analysis in free-moving microdialysed rats

Neurochemistry of Pressure-Induced Nitrogen and Metabolically Inert Gas Narcosis in the Central Nervous System

Gases that are not metabolized by the organism are thus chemically inactive under normal conditions. Such gases include the "noble gases" of the Periodic Table as well as hydrogen and nitrogen. At increasing pressure, nitrogen induces narcosis at 4 absolute atmospheres (ATAs) and more in humans and at 11 ATA and more in rats. Electrophysiological and neuropharmacological studies suggest that the striatum is a target of nitrogen narcosis. Glutamate and dopamine release from the striatum in rats are decreased by exposure to nitrogen at a pressure of 31 ATA (75% of the anesthetic threshold). Striatal dopamine levels decrease during exposure to compressed argon, an inert gas more narcotic than nitrogen, or to nitrous oxide, an anesthetic gas. Inversely, striatal dopamine levels increase during exposure to compressed helium, an inert gas with a very low narcotic potency. Exposure to nitrogen at high pressure does not change N-methyl-d-aspartate (NMDA) glutamate receptor activities in Substantia Nigra compacta and striatum but enhances gama amino butyric acidA (GABAA) receptor activities in Substantia Nigra compacta. The decrease in striatal dopamine levels in response to hyperbaric nitrogen exposure is suppressed by recurrent exposure to nitrogen narcosis, and dopamine levels increase after four or five exposures. This change, the lack of improvement of motor disturbances, the desensitization of GABAA receptors on dopamine cells during recurrent exposures and the long-lasting decrease of glutamate coupled with the higher sensitivity of NMDA receptors, suggest a nitrogen toxicity induced by repetitive exposures to narcosis. These differential changes in different neurotransmitter receptors would support the binding protein theory. © 2016 American Physiological Society. Compr Physiol 6:1579-1590, 2016.

Effects of cyclic hydrostatic pressure on the brain biogenic amines concentrations in the flounder, Platichthys flesus

The present study evaluated the effects of cyclic variations of hydrostatic pressure (HP) on neurotransmitters in the whole brain of flounder. The concentrations of the biogenic amines L-3,4-dihydroxyphenylalanine (L-DOPA), dopamine (DA), norepinephrine (NE), epinephrine (E), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA) were measured. Fish were subjected to HP cyclic variations which mimic naturally occurring conditions for a period of 14 days. DA, NE and 5-HT concentrations were significantly smaller by 21, 24 and 36%, respectively, compared to control fish. The concentrations of monoamine metabolites HVA, 3-MT and 5-HIAA were also smaller than those in control fish. These results suggest that central monoaminergic systems were influenced during long exposure to cyclic HP. The decreases of central neurotransmitters content might be involved in the physiological and behavioral responses to intermittent HP in fish.

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.

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.

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.

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

High pressure induced locomotor and motor hyperactivities (LMA), tremor and myoclonia in rat. The LMA has been reported to be reduced by intracerebroventricular (i.c.v.) administration of dopaminergic receptor antagonists. Moreover, the LMA but not myoclonia correlate with pressure induced striatal dopamine increase. Nevertheless the role of dopaminergic and NMDA receptor activities at striatal level in the development of LMA remained unclear. In this study, the microdialysis technique associated to a behavioural device was used to test the effects of intra-striatal administration of D1 antagonist SCH23390 (1 microM), D2 antagonist sulpiride (1 microM) and NMDA antagonist AP-5 (10 microM) on LMA, tremor and myoclonia expression. Data clearly showed that LMA was drastically reduced by each treatment. In contrast, tremor and myoclonia were poorly affected. These data suggest that both dopaminergic and NMDA receptor activities at striatal level are needed for the full expression of the pressure-induced LMA and confirm that striatal neurotransmission changes are principally involved in this behavioural disorders. At the light of recent studies on dopaminergic neurotransmission and glutamate evoked-NMDA activity, we suggest that blockage of D1 or D2 receptors should reduced the LMA by reducing glutamate-evoked activity.