The effects of high hydrostatic pressure on transmission at the crustacean neuromuscular junction

Metabolic costs imposed by hydrostatic pressure constrain bathymetric range in the lithodid crab Lithodes maja

The changing climate is shifting the distributions of marine species, yet the potential for shifts in depth distributions is virtually unexplored. Hydrostatic pressure is proposed to contribute to a physiological bottleneck constraining depth range extension in shallow-water taxa. However, bathymetric limitation by hydrostatic pressure remains undemonstrated, and the mechanism limiting hyperbaric tolerance remains hypothetical. Here, we assess the effects of hydrostatic pressure in the lithodid crab Lithodes maja (bathymetric range 4-790 m depth, approximately equivalent to 0.1 to 7.9 MPa hydrostatic pressure). Heart rate decreased with increasing hydrostatic pressure, and was significantly lower at ≥10.0 MPa than at 0.1 MPa. Oxygen consumption increased with increasing hydrostatic pressure to 12.5 MPa, before decreasing as hydrostatic pressure increased to 20.0 MPa; oxygen consumption was significantly higher at 7.5-17.5 MPa than at 0.1 MPa. Increases in expression of genes associated with neurotransmission, metabolism and stress were observed between 7.5 and 12.5 MPa. We suggest that hyperbaric tolerance in Lmaja may be oxygen-limited by hyperbaric effects on heart rate and metabolic rate, but that Lmaja's bathymetric range is limited by metabolic costs imposed by the effects of high hydrostatic pressure. These results advocate including hydrostatic pressure in a complex model of environmental tolerance, where energy limitation constrains biogeographic range, and facilitate the incorporation of hydrostatic pressure into the broader metabolic framework for ecology and evolution. Such an approach is crucial for accurately projecting biogeographic responses to changing climate, and for understanding the ecology and evolution of life at depth.

Selective modulation of cellular voltage-dependent calcium channels by hyperbaric pressure-a suggested HPNS partial mechanism

Professional deep sea divers experience motor and cognitive impairment, known as High Pressure Neurological Syndrome (HPNS), when exposed to pressures of 100 msw (1.1 MPa) and above, considered to be the result of synaptic transmission alteration. Previous studies have indicated modulation of presynaptic Ca²⁺ currents at high pressure. We directly measured for the first time pressure effects on the currents of voltage dependent Ca²⁺ channels (VDCCs) expressed in Xenopus oocytes. Pressure selectivity augmented the current in Ca_V1.2 and depressed it in Ca_V3.2 channels. Pressure application also affected the channels' kinetics, such as Ʈ_Rise, Ʈ_Decay. Pressure modulation of VDCCs seems to play an important role in generation of HPNS signs and symptoms.

Modulation of presynaptic Ca(2+) currents in frog motor nerve terminals by high pressure

Presynaptic Ca(2+) -dependent mechanisms have already been implicated in depression of evoked synaptic transmission by high pressure (HP). Therefore, pressure effects on terminal Ca(2+) currents were studied in Rana pipiens peripheral motor nerves. The terminal currents, evoked by nerve or direct stimulation, were recorded under the nerve perineurial sheath with a loose macropatch clamp technique. The combined use of Na(+) and K(+) channel blockers, [Ca(2+) ]o changes, voltage-dependent Ca(2+) channel (VDCC) blocker treatments and HP perturbations revealed two components of presynaptic Ca(2+) currents: an early fast Ca(2+) current (ICaF ), possibly carried by N-type (CaV 2.2) Ca(2+) channels, and a late slow Ca(2+) current (ICaS ), possibly mediated by L-type (CaV 1) Ca(2+) channels. HP reduced the amplitude and decreased the maximum (saturation level) of the Ca(2+) currents, ICaF being more sensitive to pressure, and may have slightly shifted the voltage dependence. HP also moderately diminished the Na(+) action current, which contributed to the depression of VDCC currents. Computer-based modeling was used to verify the interpretation of the currents and investigate the influence of HP on the presynaptic currents. The direct HP reduction of the VDCC currents and the indirect effect of the action potential decrease are probably the major cause of pressure depression of synaptic release.

Enhanced Excitability Compensates for High-Pressure-Induced Depression of Cortical Inputs to the Hippocampus

High pressure (>1.0 MPa) induces the high-pressure neurological syndrome (HPNS) characterized by increased excitability of the CNS and cognitive impairments involving memory disorders. The perforant-path transfer of cortical information to the hippocampal formation is important for memory acquisition. High pressure may alter information transfer in this connection. We used rat corticohippocampal slices for studying the effect of pressure on the transfer function between synaptic inputs from the medial perforant path (MPP) and spike generation by granule cells (GC) of the dentate gyrus. High pressure (10.1 MPa) reduced single MPP field excitatory postsynaptic potential (fEPSP) amplitude and slope by nearly 50%. Field antidromic action potentials (AAPs) elicited by stimulation of GC axons, and population spike (PS) generation by the pressure-depressed MPP fEPSP were not significantly altered at hyperbaric conditions. Nevertheless the relationship PS/fEPSP increased at high pressure, indicating dendritic hyperexcitability in the GC. PSs elicited by paired-pulse MPP fEPSPs at 10- to 200-ms interstimulus intervals and PS generated by trains of five fEPSPs at 25 Hz were also not affected in spite of severe pressure-induced synaptic depression. Similarly, trains of AAPs at 25–50 Hz were not significantly changed. Trains of fEPSPs at higher frequency (50 Hz), however, induced additional spikes at high pressure, indicating pressure disruption of the regular low-pass filter properties of the DG. Such effect was closely mimicked by partial blockade of GABAA inhibition. High pressure depresses synaptic activity while increases excitability in the neuronal dendrites but not in the axons. This mechanism, allowing neuronal communication at low input signals, may partially cope with pressure effects at the low frequency range (<25 Hz) but losses reliability at higher frequencies (>50 Hz).

Modulation of rat corticohippocampal synaptic activity by high pressure and extracellular calcium: single and frequency responses

High pressure (>1.5 MPa) induces a series of disturbances of the nervous system that are generically termed high-pressure nervous syndrome (HPNS). HPNS is characterized by motor and cognitive impairments. The neocortex and the hippocampus are presumably involved in this last disorder. The medial perforant path (MPP) synapse onto the granule cells of the dentate gyrus is the main connection between these structures. We have studied high-pressure (HP) effects on single and frequency response of this synapse. Since effects of HP on various synapses were mimicked by reducing [Ca2+]o, results under these conditions were compared. Medial perforant path-evoked field excitatory postsynaptic potentials (fEPSPs) were recorded from granule cells in rat brain slices. Slices were exposed to high pressure of helium (0.1-10.1 MPa) at 30 degrees C. HP depressed single fEPSPs by 35 and 55% at 5.1 and 10.1 MPa, respectively, and increased paired-pulse facilitation (PPF) at 10- to 40-ms inter-stimulus intervals. Frequency-dependent depression (FDD) was enhanced by HP during trains of stimuli at 50 but not at 25 Hz. Depression of single fEPSPs by reduction of [Ca2+]o from 2 mM control to 1 mM at normal pressure was equivalent to the effect of 10.1 MPa at control [Ca2+]o. However, this low [Ca2+]o induced greater enhancement of PPF, and in contrast, turned FDD at 25-50 Hz into frequency-dependent potentiation. These results suggest that HP depresses single synaptic release by reducing Ca2+ entry, whereas slowing of synaptic frequency response is independent of Ca2+. These findings increase our understanding of HPNS experienced by deep divers.

Interaction of Ca-channel blockers and high pressure at the crustacean neuromuscular junction

Exposure to high pressure causes a significant depression of synaptic transmission. We examined the effects of various Ca-channel blockers and their interaction with high pressure on excitatory neuromuscular junction currents (EJCs) of lobster abdominal muscles. Reduced [Ca2+]o to half of normal concentration or exposure to 40-60 microM CdCl2, 10-20 microM NiCl2 and 1 microM omega-conotoxin decreased EJCs by 50%. Nifedipine, Nitrendipine and Bay K-8644 were ineffective. Either Ca-blockers or reduced [Ca2+]o, enhanced EJC suppression exerted by high pressure. The data suggest that high pressure primarily affects Ca2+ inflow at the presynaptic terminals through N-type voltage-gated Ca-channel.

Evidence for reduced presynaptic Ca2+ entry in a lobster neuromuscular junction at high pressure

1. Previous studies have shown that hyperbaric pressure depresses synaptic transmission and have suggested that the effect is primarily on transmitter release. The present study analysed the effects of pressure at a crustacean neuromuscular junction. Changes in pressure were compared to changes in extracellular calcium concentration [Ca2+]o with respect to effects on excitatory junction potential (EJP) amplitude, time course, facilitation and potentiation. 2. The effects of 10.1 MPa pressure on EJP amplitude, facilitation and potentiation, but not time course, were mimicked by reducing [Ca2+]o to approximately one-half the normal level. 3. The effects of pressure and the interaction between compression and calcium concentration were analysed in terms of a model of transmitter release. The model assumes that release is dependent on internal calcium concentration, as modulated by both influx and removal processes; that calcium influx is a saturating function of [Ca2+]o; and that release and removal are saturating functions of [Ca2+]i. 4. The results were consistent with the hypothesis that increased pressure acts primarily to reduce calcium influx into the nerve terminal.

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.

Effects of ketamine and of high pressure on the responses to gamma-aminobutyric acid of the rat superior cervical ganglion in vitro

1 The method of Brown & Marsh (1974) for recording of surface potentials from the rat superior cervical ganglion has been adapted for use in a high pressure chamber in order to study the effects of high pressure of helium and the possible interactions with the effects of general anaesthetics. 2 Helium pressure of 130 atm did not alter the amplitude of the responses recorded from the ganglion in response to gamma-aminobutyric acid (GABA) application (9.7 and 19.4 microM) but the amplitude of responses to a nicotinic agonist were depressed. 3 Ketamine, at concentration between 18 and 180 microM, considerably potentiated the responses of the ganglion to GABA. 4 Helium pressure (130 atm) did not reverse the potentiation of GABA by ketamine. 5 The results are discussed in connection with the ability of ketamine to oppose the behavioural effects of high pressure.

The effects of hydrostatic pressure on the spontaneous release of transmitter at the frog neuromuscular junction

1. The effects of hydrostatic pressure (0.1-15.55 MPa) on the spontaneous release of transmitter at the frog neuromuscular junction were investigated. 2. The major effect of high pressure is on the release mechanism, pressure (0.1-10.40 MPa) producing an exponential decrease in frequency of the miniature end-plate currents in normal Ringer solution. The frequency decreases to 0.52 and 0.24 of the control value at 5.25 and 10.40 MPa respectively. This effect is reversible on decompression. 3. The sensitivity of the release process to high pressure is unaltered in 10 mM-K+, 6 mM- and 10 mM-Ca2+ and hypertonic (165 mM-NaCl) Ringer solution, although the high Ca2+ media shift the threshold for the pressure effect to higher pressures. 4. Higher pressure (10.40-15.55 MPa) produces a small increase in the time constant of decay (tau D) of m.e.p.c.s with no effect on the growth phase. A pressure of 15.55 MPa increases tau D to 1.35 of the control value. 5. The possible actions of high pressure on both the pre- and post-synaptic processes are briefly discussed.

The action of high hydrostatic pressure on the membrane currents of Helix neurones

1. The actions of high hydrostatic pressure (10.4, 20.8 MPa) on the membrane currents of Helix neurones were examined under voltage clamp. 2. High hydrostatic pressure (20.8 MPa) reduced the maximum inward current to 0.78 and the delayed outward current, measured at the inward current reversal potential, to 0.75 of their value at atmospheric pressure. 3. High hydrostatic pressure shifted the curve relating the inward current conductance to membrane potential to more positive values but the maximum conductance was altered. 4. The rates of activation of the inward and delayed outward currents were slowed by pressure. 5. The steady-state level and time course of inactivation of the inward current was unaffected by high pressure over the investigated range. 6. The effects of high hydrostatic pressure on the fast outward current identified in gastropod neurones by Connor & Stevens (1971) were also examined. 20.8 MPa reduced the current measured at -30 mV to 0.71 of its control value. 7. The rate of activation of the fast outward current was slowed by high pressure but the time constant of inactivation was unchanged. 8. The majority of the effects of high hydrostatic pressure were completely reversible upon decompression. 9. These results are discussed with reference to the known effects of high hydrostatic pressure on the action potential and discharge frequency of gastropod neurones. Possible sites and mechanisms of pressure action on the excitable cell are briefly discussed.

Spinal cord seizures elicited by high pressures of helium

Rats with complete spinal transections were compressed in helium-oxygen to 120 bars. Tremors and increased EMG activity in limbs rostral as well as caudal to the lesions were observed beginning at 30 bars. Spinal seizures occurred at 95 bars, similar to cortical seizure thresholds of intact rats. Denervated limbs remained flaccid throughout the dives. No rostro-caudal progression of symptoms was evident in normal animals, but fluctuation of symptoms with increasing pressure was frequently observed. These findings are consistent with the hypothesis that the effects of pressure on aggregates of neurons exceed those on isolated components.