The supraoptic nucleus as an osmoreceptor.

The "Progress' review is written to challenge the prevalent belief that neurones of the supraoptic nucleus are not involved in the reception of osmotic stimuli. Evidence is presented that these neurones are depolarised by physiological increases in the osmotic pressure of the extracellular fluid. However, generation of action potentials in these depolarised cells still requires the presence of synaptic input from currently unidentified neurones.

Relative efficiency of neural firing patterns for vasopressin release in vitro.

Single rat neurohypophyses were impaled on a platinum electrode, perifused with isotonic medium and fractions of perifusate collected each minute. Arginine vasopressin content of the fractions was measured by radioimmunoassay. The basal release of arginine vasopressin varied from 50 to 350 pg (0.

Effects of neural stalk stimulation on phasic discharge of supraoptic neurones in Brattleboro rats devoid of vasopressin.

Brattleboro rats, homozygous for diabetes insipidus, and Long-Evans rats were anaesthetized with urethane, and antidromically identified neurons were recorded from the supraoptic nucleus. Phasically firing neurones were studied during repeated electrical stimulation of the neural stalk, whereby most supraoptic neurones, but not the recorded neurone, were activated antidromically. Such stimulation consistently modified the discharge pattern of phasic neurones in Long-Evans rats, but was relatively ineffective in Brattleboro rats.

Threshold sensitivity of the auditory pathway: effects of worsening signal-to-noise ratio in the auditory nerve.

Perfusions of scala tympani with high potassium solutions increase spontaneous activity of auditory nerve fibres without affecting their threshold sensitivity. In these circumstances, however, the signal-to-noise ratio of the auditory nerve response to a given stimuli is worsened. Recordings from cochlear nucleus neurones during such perfusions indicate that this worsening is critical for threshold sensitivity at higher levels in the auditory pathway.

The effects of neural stalk stimulation upon firing patterns in rat supraoptic neurones.

Antidromically identified neurones were recorded from the supraoptic nucleus of urethane-anaesthetized rats. In test periods of about 10 min, each recorded action potential was followed 1 ms later by a shock to the neural stalk. In phasically firing cells, this stimulation consistently modified the observed firing pattern: burst length was shortened but intraburst activity was unchanged.

The effects of frusemide, bumetanide and piretanide on the quinea pig cochlea and auditory nerve.

Frusemide, bumetanide and piretanide were introduced into scala tympani of the guinea pig cochlea by perfusion. The short-term effects of these drugs on the cochlear microphonic, the compound action potential and on the activity of single auditory nerve fibres were studied. the compound action potential was more sensitive to the action of the above compounds than was the cochlear microphonic.

Kanamycin ototoxicity and pigmentation in the guinea pig.

Cochleas from albino and pigmented guinea pigs were examined histologically following chronic administration of Kanamycin. In pigmented animals, damage was concentrated amongst hair cells towards the base, but in albinos, hair cell damage was diffusely spread. These observations support suggestions that melanin is implicated in Kanamycin ototoxicity.

Rat supraoptic neurones: the effects of locally applied hypertonic saline.

1. Extracellular action potentials were recorded from supraoptic neurones in lactating, urethane-anaesthetized rats. A microtap was used to apply a very small volume (about 10(-7) ml.

The Davis theory: a review, and implications of recent electrophysiological evidence.

The Davis theory of mechano-electrical transduction asserts that the endocochlear potential and the hair cell resting potential summate to provide a driving force for current flow through the hair cell. However, while a variety of agents which depress the endocochlear potential simultaneously depress auditory nerve sensitivity and reduce the cochlear microphonic, recent reports suggest that the hair cells may be depolarised without such effects ensuing. The relevant literature is reviewed.

Effects of increased potassium in scala tympani on auditory nerve sensitivity.

Raising the K+ concentration in scala tympani of the guinea-pig cochlea generally caused a substantial increase in the spontaneous firing rate of single auditory nerve fibres. This effect was not accompanied by any observed reduction in the threshold sensitivity of these fibres. These findings cast doubt on current theories of cochlear transduction.

Action of putative neurotransmitters in the guinea pig cochlea.

Scala tympani of the guinea pig was perfused with artificial perilymph containing putative transmitter substances. Single auditory nerve fibre activity was recorded during these perfusions and the responses of these fibres were studied using computer-controlled routine. Acetylcholine significantly inhibited tone-induced responses of a large proportion of the fibres tested.

The effect of potassium on the activity of auditory nerve fibres of the guinea pig cochlea.

Scala tympani of the guinea pig cochlea was perfused with solutions having an increased potassium concentration. The response characteristics of single auditory nerve fibres in both normal and kanamycin-damaged cochleas were studied using computer controlled routines. The results indicate that these perfusions caused a marked increase in the spontaneous firing rate of auditory nerve fibres, without loss of threshold sensitivity.