Pharmacological investigations of cochlear responses and of olivocochlear inhibition

Patterns of GABA-like immunoreactivity in efferent fibers of the human cochlea

Olivocochlear efferent neurons originate in the superior olivary complex of the brainstem and terminate within sensory cell regions of the organ of Corti. Components of this complex include the lateral olivocochlear bundle whose unmyelinated axons synapse with radial afferent dendrites below inner hair cells and the medial olivocochlear bundle, from which myelinated axons form a direct synaptic contact with outer hair cells. gamma-Aminobutyric acid (GABA), a major neurotransmitter of the central nervous system believed to be responsible for most fast-inhibitory transmissions, has been demonstrated with interspecies variation between mammal and primate auditory efferents. In the present study, we evaluate the immunocytochemical presence of GABA in 10 human cochleae using light and electron microscopy. GABA-like immunostaining could be observed in inner spiral fibers, tunnel spiral fibers, tunnel-crossing fibers, and at efferent endings synapsing with outer hair cells. To approximate medial efferent fiber quantifications, we counted labeled terminals at the base of each outer hair cell and then compared this sum with the number of tunnel crossing fibers. We found a 'branching ratio' of 1:2 implicating a doubling in quantifiable efferent fibers at the level of the outer hair cell. In human, the distribution of GABA-like immunoreactivity showed a consistent presence throughout all turns of the cochlea. A new method for application of immunoelectron microscopy on human cochleae using a pre-embedding technique is also presented and discussed.

The efferent modulation of mammalian inner hair cell afferents

The results of immunocytochemical, enzymatic and electrophysiological studies have indicated that acetylcholine and GABA may act as neurotransmitters in lateral olivocochlear efferent endings on inner hair cell afferent dendrites. Since spike activity can be recorded in the dendritic region of inner hair cells, microiontophoretic techniques were used testing the possible neurotransmitter candidates, acetylcholine and GABA, on spontaneous and induced firing of the afferent dendrites. The experiments were carried out in anaesthetised guinea-pigs, the third and fourth turns of the cochlea being exposed for electrode penetration. Ejection of acetylcholine resulted in a pronounced dose-dependent increase in subsynaptic spiking activity. Furthermore, acetylcholine enhanced glutamate-induced activity. In contrast, even at high doses, GABA had very little effect on the spontaneous cochlear firing rate. When the firing rate had first been enhanced by glutamate or N-methyl-D-aspartate, however, this activation could be reduced by the ejection of GABA. A similar reduction was observed when the firing rate had been enhanced with acetylcholine. The results of our studies support the hypothesis that these substances are involved in efferent neurotransmission on inner hair cell afferent fibres. It should be pointed out, however, that besides acetylcholine and GABA, several opioids such as enkephalins and dynorphins seem to be involved in efferent cochlear innervation.

Effect of elevated potassium concentration in the perilymph on the nonlinearity of cochlear microphonics in the guinea-pig cochlea

Various characteristics of cochlear microphonic responses (CM) were measured in the guinea-pig after perfusing scala tympani with KCl solutions. In addition to a decrease of CM magnitude, the nonlinearity and the symmetry of waveform showed typical modifications suggesting, in addition to a depolarization of hair cells, some mechanical alterations.

Dose-dependent effects of atropine sulfate on the brainstem and cortical auditory evoked potentials in the rat

Because brainstem auditory evoked potentials (BAEPs) are frequently recorded in anesthetized animals and humans, it is important to become familiar with the effects on the BAEP of drugs used during anesthesia, including pre-anesthetics. The dose-dependent and stimulus intensity-dependent effects on the BAEP of a pre-anesthetic, atropine sulfate, were studied in the unanesthetized rat. The animal subjects were 11 adult female Long-Evans rats. BAEPs in response to 0.1 ms clicks (12.5/s) were recorded from skull screw electrodes during a baseline period, as well as after saline and atropine treatments. Atropine sulfate was given i.p. in doses ranging from 0.250 to 40 mg/kg. Contrary to a prior report, doses in the standard pre-anesthetic range (i.e. 0.250-1.000 mg/kg) did not convincingly influence the BAEP. Only the highest dose (40 mg/kg) produced a significant and noteworthy change in the BAEP. This effect was characterized by significant amplitude increases in the P1, P2 and P3 components, but not in the P4, P5 and P6 components. This selective effect occurred at the highest stimulus intensity of 110 dB peak equivalent sound pressure level, but not at lower intensities. There were no convincing atropine-induced changes in BAEP latencies. Atropine-induced changes in the cortical auditory evoked potential (CAEP) were characterized by amplitude decrements. Thus, atropine seemed to have an excitatory effect on the BAEP and an inhibitory or depressive effect on the CAEP.

Immunoelectron microscopy identifies several types of GABA-containing efferent synapses in the guinea-pig organ of Corti

Using an immunoperoxidase technique, we have localized by light and electron microscopy GABA-immunostained fibers within a component of the efferent innervation of the organ of Corti. At the light microscopic level, GABA-immunostained fibers were observed within the inner spiral bundle (below the inner hair cells) and the tunnel spiral bundle. The immunostaining was clearly more intense in the upper turns than in the basal turns. Mostly in the upper turns, GABA-immunostained fibers were seen crossing the tunnel of Corti to reach the outer hair cells where they formed large immunostained patches at the base of the cells. Unevenly distributed throughout these upper turns, immunostained fibers were seen climbing along the outer hair cells and traveling near the non-sensorineural Hensen's cells. The electron microscopic observations of GABA-immunostained fibers in the upper turns allowed us to identify within the inner spiral bundle vesiculated varicosities synapsing with radial dendrites connected to the inner hair cells. In the outer hair cell area, the GABA-immunostained fibers made several kinds of synaptic contacts. They included a minor population of the large axosomatic synapses with the basal pole of the outer hair cells and many axodendritic synapses with the spiral dendrites connected to these cells. Occasionally, the GABA-immunostained climbing fibers also synapsed with the outer hair cells at a supranuclear level. These result confirm previous light microscopic data dealing with the projection of the GABA-immunostained fibers along the cochlear partition. Moreover, they extend them in characterizing several kinds of GABA-immunostained synapses. These latter findings agree with previous neurochemical electrophysiological data which suggests an efferent neurotransmitter role for GABA. Nevertheless, such an existence of an efferent innervation predominantly projecting to the upper turns of the cochlea adds another criterion distinguishing the "apical" from the "basal" cochlea.

Effects of divalent ions and drugs on synaptic transmission in phasic electroreceptors in a mormyrid fish

We recorded impulses in afferent nerve fibers innervating two kinds of phasic electroreceptors in a mormyrid fish. We used an isolated preparation of skin, receptors, and sensory nerves to estimate synaptic delays, and to change solution in contact with the receptor-nerve synapse. The minimum delays between stimuli and sensory nerve responses, which must be slightly larger than synaptic delays, are about 0.7 msec in medium receptors and about 0.25 msec in large receptors. This result supports previous suggestions that transmission is chemically mediated in medium receptors and electrically mediated in large receptors. Furthermore, Mg(+2) depresses synaptic transmission in medium receptors, and has little effect on transmission in large receptors. A complex dependence of response on both Mg(+2) and Ca(+2) masks divalent ion dependence of transmission, but a large excess of Mg(+2) cannot completely block transmission in medium electroreceptors. L-glutamate, and not cholinergic drugs, produces a sequence of excitation and depression of medium receptor response which indicates that a similar chemical is the transmitter in the afferent synapse.

Centrifugal inhibitory processes affecting neurones in the cat cochlear nucleus

1. Stimulation of the lateral part of the olivary S-segment in the cat inhibited neurones in the ipsilateral cochlear nucleus. A smaller number of neurones located in the ventral division of the cochlear nucleus were excited.2. It is suggested that inhibition in the ipsilateral cochlear nucleus may be mediated directly by fibres making synaptic connexions on the cochlear nucleus neurones, or indirectly by inhibitory fibres acting at the cochlea.3. The direct inhibitory process at the cochlear nucleus is unaffected by strychnine, whereas the inhibitory process at the cochlea is abolished by strychnine.4. A cochlear nucleus neurone can be influenced simultaneously by excitatory and inhibitory processes.

Acetylcholinesterase: method for demonstration in amacrine cells of rabbit retina

The activity of acetylcholinesterase in the inner plexiform layer of the rabbit retina was not affected detectably by prior section of the optic nerve. After the animals were treated with diisopropyl phosphorofluoridate, acetylcholinesterase reappeared in the somata of the amacrine cells and in certain cells of the ganglion cell layer before it reappeared in the inner plexiform fibers. This confirms the normal presence of acetylcholinesterase at the former site. The possible role of acetylcholine in intraretinal transmission is considered.

Inhibition of auditory nerve action potentials by acetylcholine and physostigmine

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Tetraethylammonium and tetrodotoxin: effects on cochlear potentials

Tetraethylammonium chloride, which is believed to decrease potassium conductance, and tetrodotoxin, which apparently decreases sodium conductance in nerve fibers, were introduced iontophoretically into the organ of Corti or the scala media of guinea pig cochlea. The former depressed the direct-current endocochlear potential and also the alternating-current cochlear microphonics (the receptor potential of the ear), but tetrodotoxin was ineffective except on the nerve impulses.