Differential modulation of cerebellar climbing fiber and parallel fiber synaptic responses at high pressure

Selective pressure modulation of synaptic voltage-dependent calcium channels-involvement in HPNS mechanism

Exposure to hyperbaric pressure (HP) exceeding 100 msw (1.1 MPa) is known to cause a constellation of motor and cognitive impairments named high-pressure neurological syndrome (HPNS), considered to be the result of synaptic transmission alteration. Long periods of repetitive HP exposure could be an occupational risk for professional deep-sea divers. Previous studies have indicated the modulation of presynaptic Ca(2+) currents based on synaptic activity modified by HP. We have recently demonstrated that currents in genetically identified cellular voltage-dependent Ca(2+) channels (VDCCs), CaV 1.2 and CaV 3.2 are selectively affected by HP. This work further elucidates the HPNS mechanism by examining HP effect on Ca(2+) currents in neuronal VDCCs, CaV 2.2 and CaV 2.1, which are prevalent in presynaptic terminals, expressed in Xenopus oocytes. HP augmented the CaV 2.2 current amplitude, much less so in a channel variation containing an additional modulatory subunit, and had almost no effect on the CaV 2.1 currents. HP differentially affected the channels' kinetics. It is, therefore, suggested that HPNS signs and symptoms arise, at least in part, from pressure modulation of various VDCCs.

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.

Enduring medial perforant path short-term synaptic depression at high pressure

The high pressure neurological syndrome develops during deep-diving (>1.1 MPa) involving impairment of cognitive functions, alteration of synaptic transmission and increased excitability in cortico-hippocampal areas. The medial perforant path (MPP), connecting entorhinal cortex with the hippocampal formation, displays synaptic frequency-dependent-depression (FDD) under normal conditions. Synaptic FDD is essential for specific functions of various neuronal networks. We used rat cortico-hippocampal slices and computer simulations for studying the effects of pressure and its interaction with extracellular Ca²⁺ ([Ca²⁺]_o) on FDD at the MPP synapses. At atmospheric pressure, high [Ca²⁺]_o (4–6 mM) saturated single MPP field EPSP (fEPSP) and increased FDD in response to short trains at 50 Hz. High pressure (HP; 10.1 MPa) depressed single fEPSPs by 50%. Increasing [Ca²⁺]_o to 4 mM at HP saturated synaptic response at a subnormal level (only 20% recovery of single fEPSPs), but generated a FDD similar to atmospheric pressure. Mathematical model analysis of the fractions of synaptic resources used by each fEPSP during trains (normalized to their maximum) and the total fraction utilized within a train indicate that HP depresses synaptic activity also by reducing synaptic resources. This data suggest that MPP synapses may be modulated, in addition to depression of single events, by reduction of synaptic resources and then may have the ability to conserve their dynamic properties under different conditions.