Effects of RU 486 on hippocampal slice electrophysiology in normal and adrenalectomized BALB/c mice

Modulation of the electrophysiological properties (amplitude, latency, paired-pulse facilitation) of the CA1 hippocampal field by different concentrations of RU 486 was studied in hippocampal slice preparations from normal (sham) and adrenalectomized BALB/c mice. In the sham group as well as in the adrenalectomized group, RU 486 had no effect with the lower concentrations used (0.05 and 0.5 nM), but produced a decrease in the population spike amplitude for the higher concentrations used (5 and 50 nM). This decrease was delayed and sustained. Combined application of RU 486 at 5 nM and corticosterone at 0.5 or 5 nM displayed an excitatory effect, but no inhibitory effect. Combined application of RU 486 and dexamethasone showed no significant effect. These results are related to the antagonist and agonist properties of RU 486 and to the different steroid receptor systems in the hippocampus.

Administration of enoximone in cardiogenic shock

Thirteen patients in severe cardiogenic shock, persisting despite the use of adrenergic agents, were treated with enoximone, a recently available phosphodiesterase inhibitor. Cardiogenic shock was characterized by low cardiac output (less than 2.5 liter.min-1.m-2), elevated pulmonary artery balloon-occluded pressure (greater than or equal to 15 mm Hg), decreased urine output (less than 20 ml.hour-1) and increased blood lactate (greater than or equal to 2.0 mEq.liter-1). Ten patients were mechanically ventilated. A short-term intravenous infusion of 0.5 mg.kg-1 in 20 minutes of enoximone resulted in significant increases in cardiac index (from 1.8 +/- 0.3 to 2.9 +/- 0.3 liter.min-1.m-2, p less than 0.001) and stroke index (from 17.8 +/- 3.3 to 21.9 +/- 5.1 ml.m-2, p less than 0.001) and significant decrease in pulmonary artery balloon-occluded pressure (from 21.7 +/- 5.8 to 19.8 +/- 6.0 mm Hg, p less than 0.01) without a consistent change in mean arterial pressure (from 79 +/- 8 to 76 +/- 9 mm Hg, difference not significant). Enoximone administration decreased arterial oxygen tension (from 108 +/- 42 to 94 +/- 36 mm Hg, p less than 0.01) and increased venous admixture (from 12.8 +/- 6.5 to 16.0 +/- 8.0%, p less than 0.01). In 8 patients, a second infusion of 0.5 mg.kg-1 immediately thereafter amplified these changes. All patients but one survived the episode of cardiogenic shock and 5 patients left the hospital alive. These results indicate that the addition of enoximone to adrenergic agents in the treatment of cardiogenic shock can markedly increase cardiac output and stroke volume without substantial effects on arterial pressure.

Effects of corticosterone on hippocampal slice electrophysiology in normal and adrenalectomized BALB/c mice

Modulation of CA1 electrophysiological properties (amplitude, latency, paired-pulse facilitation) by different concentrations of corticosterone (CT) was studied in hippocampal slice preparations from normal (sham) and adrenalectomized (ADX) BALB/c mice. In the sham group, CT effects on the population spike (PS) amplitude were biphasic: at a low dose (200 pM) CT induced a delayed increase; at a moderate dose (2,000 pM), a rapid and partially reversible decrease was observed, and at higher doses (5,000 and 10,000 pM) this decrease was more pronounced after 30 min of washout. Diminution of latency and paired-pulse facilitation were obtained for the CT concentration inducing an increase of the PS amplitude. In the ADX group, similar CT effects were observed for lower doses: a decrease of the PS amplitude was observed for 40 and 100 pM and an increase for 5 pM, but variations of the PS amplitude were weaker than in the sham group. The role of the different CT receptor systems in this neuromodulatory effect is discussed.

A study of spontaneous and evoked activity in the rat hippocampus under helium-oxygen high pressure

High pressures affect the physiology of the central nervous system. For a better understanding of this effect, we examined the hippocampal activity in the rat under high pressures (91 bars) of helium-oxygen. Effects of high pressure on hippocampal physiology are: an abnormally sustained 5-8 Hz pattern of spontaneous activity, followed, in some cases, by seizures; a marked decrease in the responses of CA1 pyramidal cells to stimulation of their commissural afferents; and a 50% decrease in the afterdischarge threshold. On the basis of the relatively well understood hippocampal physiology in normobaric conditions, our observations suggest that high pressures induce hypoexcitability of afferents and/or target cells.

Sectioning the efferent bundle decreases cochlear frequency selectivity

Cochlear frequency selectivity was tested in rats and cats before and after sectioning the vestibular nerve which carries the efferent bundle to the cochlea. Compound action potential tuning curve were obtained by a simultaneous masking procedure. Transection of the efferent bundle resulted, in all group of animals, in an enlargement of the tip segment of the tuning curve. The Q10 dB value decreased by about 30% without any significant threshold change. These results taken together with previous developmental findings, suggest that efferents to outer hair cells could play a role in sharpening the frequency selectivity of the cochlea.

Ontogenetic approach to inner and outer hair cell function

Anatomo-functional relationships made during ontogenesis seem to offer possibilities of a better understanding of cochlear function. The asymmetrical kinetics of inner and outer hair cells (IHCs and OHCs) structural development can assist in determining their functional role. Light- and electron-microscopic observations of cochlear development in kittens, guinea pigs, hamsters and rats reveal that the maturation of IHC and their afferent and efferent innervation occurs first. The OHCs develop more slowly and the pattern of their innervation changes dramatically, the last stage involving the enlargement of efferent synapses. The timing of this maturation correlates well with the development of cochlear function. The findings suggest that IHCs are required for the onset of auditory function while mature OHCs are necessary for the more discriminative properties of the cochlea.

[Cellular detection of the in vivo incorporation of tritiated 2-deoxyglucose. Radioautographic study in the inner ear]

Inner ear injection of (3H) deoxyglucose was followed by in situ glutaraldehyde fixation and, after removal, by traditional preparation of the tissue for electron microscopy. Radioautography of semi-thin sections revealed preferential tracer accumulation in metabolically active structures such as the cochlear stria vascularis and the dark cells surrounding the basis of the vestibular cristae. Furthermore, in the vestibule, the perikaryon of some sensory cells was intensely labelled. These results show that an in vivo injection of deoxyglucose, followed by conventional histological processing can be used to mark active cells at both cellular and subcellular levels.

Supra-normal sensitivity to ototoxic antibiotic of the developing rat cochlea

A period of increased sensitivity to the ototoxic antibiotic amikacin was found in the cochlea of the developing rat. Starting either at birth, 10 or 30 days of age, rat were injected i.p. daily with 200 mg/kg of amikacine for 10 days. Only in the group injected with drug from 10 to 20 days of age was there evidence of a substantial (40-100 dB) permanent threshold shift (PTS). To characterize the period of supra-normal sensitivity more precisely, independent groups of pups were injected with 50 mg/kg of amikacin from 10 to 20, 15 to 25, 20 to 30, and 30 to 40 days of age. Only those pups injeced from 10 to 20 or 15 to 25 days of age showed PTS (30-60 dB). These data, taken together with those on rat cochlear maturation indicate that the sensitive period to ototoxicity starts at the age of onset of cochlear function (around day 10) and ends few days after the cochlea shows adult-like characteristics (3rd postnatal week).

Significance of presynaptic formations in early stages of cochlear synaptogenesis

Very early stages of cochlear synaptogenesis are described in inner (IHCs) and outer hair cells (OHCs) of cat foetuses. Ten days before birth (DBB) well-formed synaptic contacts were found between afferent dendrites and both IHCs and OHCs. At the IHC level a preliminary stage before functioning has been proposed. But at the OHC level where the adult cell membrane becomes mainly postsynaptic, we could only formulate hypotheses based on phylogenetic considerations (afferents precede efferents), or on embryology (presynaptic specializations play a role in the arrival and growth of fibers around the hair cell).

Different patterns of cochlear innervation during the development of the kitten

The postnatal development of neuro-epithelial junctions inside the kitten cochlea has been investigated by electron microscopy, and correlated with previous electrophysiological results. Two main stages of development are described. During the first postnatal days, outer hair cells look very immature with only a few afferent endings adjoining them. The inner hair cells, on the other hand, are surrounded by numerous endings with mature afferent and efferent synapses. Thus, when the efferent olivo-cochlear system begins to function during the first postnatal day s, it is able to modify only inner hair cell responses. The second postnatal week is characterized by maturation of the large efferent endings below the outer hair cells. At the same time, direct efferent connections become sparce at the level of inner hair cells. The maturation of hearing, at the receptor level, seems to proceed in two steps, one related to inner hair cells and corresponding to a gross and primitive hearing, the other related to outer hair cells and corresponding to more precise and discriminative hearing abilities.

Histo-physiological relationships in the deaf white cat auditory system

An investigation is underway in deaf white cats in order to clear up some problems about the timing of cochlear degeneration, the relationships between physiological and histological findings, and the consequential state of central auditory pathways. The first results reported here show that degeneration of sensory structures was much more advanced than physiological results lead us to believe. The possibility of direct fibre stimulation is discussed. Although degeneration of nervous structures was delayed compared with sensory ones, some early abnormalities in myelination were found. In the adult, completely deaf for years, the electrical stimulation of cochlear nuclei evoked normal-looking field responses at the cortical level. Thus some degree of auditory pathway functional integrity appeared even long after they were disconnected from receptors.

[Mauration of unitary responses to tonal stimulation in the cochlear nerve of the kitten]

The maturation of single auditory nerve fiber responses to long-duration (500 ms) tone-bursts was studied in kittens at various stages after birth. Spike discharges were examined as a function of three criteria. 10 Latency. Mean value of the "on" peak latency was about 25 ms at 1 to 3 days after birth. It then regressed, reaching 8 to 12 ms at the end of the first week, and 2 to 4 ms at 20 days. 20 Peristimulus histograms (PSTH). Evolution of PSTH revealed the characteristic sequence of unit reactivity to long duration stimuli (i.e. on, rhythmic, and continuous responses). Rhythmic responses is assumed up to now to be related to the immaturity of synaptic junctions below the hair cells. 30 interval histograms. The interspike interval histograms of the discharges showed a similar evolution. A bimodal distribution corresponding to the rhythmic mode of reactivity, appeared at the end of the first week.