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 in 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.