Osmolar changes and neural activity in frog vestibular organs

The effects of hypotonic and hypertonic solutions (the normal value was 240 mOsm) on posterior canal resting and evoked discharge were studied in isolated labyrinth preparations. Hypotonic solutions (60-180 mOsm) were obtained by reducing the perilymphatic NaCl content. Hypertonic solutions (300-420 mOsm) were obtained by adding to normal perilymphatic solutions suitable amounts of NaCl, glucose, sucrose, glycerol, mannitol and urea. The results demonstrated that any kind of receptor activity was inhibited by hypotonic solutions. On the contrary, hypertonic solutions produced different effects on resting and evoked activity. The resting discharge was, with the exception of urea, constantly increased whereas the evoked responses were constantly decreased by all the hypertonic solutions tested. The possible effects of media with changed osmolarity in Meniere's patients is also discussed.

NMDA receptors in frog semicircular canals

The effects of competitive (2-amino-5-phosphonovaleric acid) and noncompetitive (Mg2+, ketamine and kynurenic acid) NMDA receptor antagonists were tested in isolated semicircular canals of the frog. All these antagonists reduced the resting firing rate recorded from the whole ampullar nerve of the posterior canal. This suggests that NMDA receptors are present at the cytoneural junctions between hair cells and afferent nerve terminals in frog vestibular organs.