OSMOTICALLY INDUCED CHANGES IN BRAIN STEADY POTENTIAL AND AUDITORY EVOKED RESPONSE

Cerebral steady potential (SP), ECoG and auditory evoked responses were recorded simultaneously incident to i.v. injection of hypertonic solutions of NaCl, KCl and sucrose in forty-seven cats (acute experiments). When the surface cortical electrode was led against an extracerebral reference, NaCl and sucrose solutions produced an SP shift. No significant change was seen in transcortical records. The effect with KCl differed in that the polarity of the SP change was opposite to that observed with NaCl and sucrose. Also, with KCl isotonic as well as hypertonic solutions produced a shift. Similar shifts were recorded from cerebellum following hypertonic NaCl; only minor SP departures were observed in spinal cord and none were recorded from muscle. Evoked responses (to click) and ECoG were first depressed (1 min) and then augmented (70 min or more) by all hypertonic solutions tested (NaCl, KCl and sucrose). The same changes in response were observed when auditory cortex was activated by stimulation of the auditory radiation, proving that cortical excitability is altered directly by the changes in osmotic pressure. Augmentation of the cortical responses occurred with either positive or negative shifts and could remain altered long after SP had recovered to the baseline. Such findings suggest that osmotically induced SP changes and altered cortical excitability are separate and unrelated phenomena. In ten additional animals, auditory evoked responses were studied incident to topical application of sucrose and NaCl (hypertonic and hypotonic solutions). The results differed from those observed following i.v. injection. Both hypertonic and hypotonic solutions of sucrose and hypotonic NaCl caused response augmentation; hypertonic NaCl depressed the response. It appears that with surface application the chloride ion exerts an important effect and overpowers any changes that might occur from local altered osmolality. © 1969.