Some characteristics of glutamic acid decarboxylase of chick ampullary cristae

Modalities of GABA and glutamate neurotransmission in the vertebrate inner ear vestibule

GABA and glutamate have been postulated as afferent neurotransmitters at the sensory periphery inner ear vestibule in vertebrates. GABA has fulfilled the main criteria to act as afferent neurotransmitter but may also be a putative efferent neurotransmitter, mainly due to cellular localization of its synthesizing enzyme glutamate decarboxylase derived from biochemical, immunocytochemical, in situ hybridization and molecular biological techniques, whereas glutamate afferent neurotransmission role is supported mainly by pharmacological evidences. GABA and Glu could also act as afferent co-neurotransmitters based upon immunocytochemical techniques. This multiplicity was not considered earlier and postulates a peripheral modulation of afferent information being sent to higher vestibular centers. In order to make a definitive cellular assignation to these putative neurotransmitters it is necessary to have evidences derived from immunocytochemical and pharmacological experiments in which both substances are tested simultaneously.

Immunocytochemical evidence for an afferent GABAergic neurotransmission in the guinea pig vestibular system

To implicate gamma-aminobutyric acid (GABA) as an afferent neurotransmitter (AN), the localization of GABA synthesizing and degradation enzymes; L-glutamate decarboxylase (GAD) and GABA transaminase (GABA-T) was investigated by light and electron microscopy immunocytochemistry in guinea pig vestibular cristae and ganglion cells (GC). GAD-like immunoreactivity was exclusively confined to the sensory hair cell (HC) cytoplasm, suggesting that GAD synthesizes GABA in the HC. GABA-T like immunoreactivity was found within HC, nerve calyces, nerve fibers, and GC, suggesting its participation in terminating transmitter action. These results demonstrate the existence of a GABAergic system in the guinea pig vestibule and strongly support GABA as a vestibular AN.

Neurochemical evidence for afferent GABAergic and efferent cholinergic neurotransmission in the frog vestibule

Glutamate decarboxylase and choline acetyltransferase activities with magnitudes similar to those of their homologous enzymes in frog nervous tissue were found in homogenates of the frog labyrinth. Transection of the vestibular nerve resulted in a gradual diminution of choline acetyltransferase activity until it reached an 88% decrease 6 weeks after surgery. In contrast, glutamate decarboxylase activity did not suffer any alteration at any time after nerve excision. The presence of their enzymes of synthesis is evidence of the neurotransmitter participation of GABA and acetylcholine in the frog vestibule; the observed decrease of choline acetyltransferase following vestibule nerve excision supports the efferent synaptic bouton localization of choline acetyltransferase. The suggestion that glutamate decarboxylase is located in a cell type (or compartment) that may well be the hair cell is supported by the fact that this enzyme does not suffer any modification after surgery. These results are in accordance with an efferent cholinergic neurotransmission and a putative afferent role of GABA in the frog vestibule.

GABA-like immunoreactivity in the chick vestibular end organs

gamma-Aminobutyric acid (GABA)-like immunoreactivity in the chick vestibular endorgans was examined using an antiserum against GABA coupled with glutaraldehyde to bovine serum albumin. GABA-like immunoreactivity was confined to the cytoplasm of the hair cells in both cristae and maculae. GABA-like immunoreactive cells were evenly distributed throughout the sensory epithelia, and no difference existed between type I and type II hair cells. The results provide evidence that GABA-like immunoreactivity is localized to sensory cells and raises the possibility that GABA may serve as an afferent neurotransmitter in the chick vestibular end organs.

Is GABA an afferent transmitter in the vestibular system?

This study was undertaken to determine the possible role of GABA as an afferent transmitter in the vestibular system of the axolotl. We studied the effects of GABA, muscimol, bicuculline and picrotoxin on the spontaneous spike discharge of the afferent fibers of the sacculi lagena and anterior semicircular canal. It was found that GABA and muscimol produce a very weak excitatory effect which does not mimic either the temporal course or the amplitude of the response of vestibular afferents to physiological stimuli. The GABA antagonist bicuculline has no significant effect on these fibers, and picrotoxin partially blocks the spontaneous activity in 33% of the fibers studied. These results indicate that GABA is probably not an afferent transmitter in the vestibular system as has previously been proposed.

Ontogenetic approach to cellular localization of neurotransmitters in the chick vestibule

Gamma-aminobutyric acid (GABA) and acetylcholine (Ach) have been implicated in afferent and efferent neurotransmission, respectively, in the vestibular sensory periphery. Assuming that glutamate decarboxylase (GAD), the GABA-synthesizing enzyme, and choline acetyltransferase (ChAT), the enzyme for synthesis of acetylcholine, are located in distinct cell types of the inner ear whose maturation occurs at different times during ontogenesis, we measured these enzymes in the ampullary cristae of embryonic chicks at different stages of development. By making these measurements in parallel with electron-microscopic studies of the different cell elements of the chick vestibular sensory periphery, we found that the values of GAD activity were nearly the same from the earliest stage studied, i.e., the 13th day of ontogeny to day 18 of embryonic development, paralleling the morphologically mature appearance of the hair cells and their afferent synapses. A slight increase in enzymatic activity from day 19 of ontogeny to one day after hatching corresponded to a rise in the number of afferent synapses. In contrast, ChAT activity was practically undetectable up to day 17 of embryonic development, but rose suddenly on the 19th day, reaching 1-day-old levels by day 20 of ontogenesis in coincidence with an elevation in the number of well-developed efferent boutons. These results are in accord with the localization of GAD in the sensory cells and a localization of ChAT in the efferent nerve endings. These findings suggest that GABA and Ach are the respective neuromediators for the afferent and efferent systems.

Specific [3H]gamma-aminobutyric acid binding to vestibular membranes of the chick inner ear

To support a postulated neurotransmitter character of gamma-aminobutyric acid (GABA) in the vertebrate vestibule, [3H]GABA binding was measured in a crude membrane preparation of chick inner ear ampullary cristae. In the absence of divalent cations bound [3H]GABA was displaced by unlabeled GABA, muscimol or bicuculline, but it was not displaced by (+/-)-baclofen. A single population of [3H]GABA binding sites with an equilibrium constant of 19.4 nM and a maximum binding capacity of 0.58 pmol/mg protein was found. These results suggest the possible existence of a synaptic GABAA receptor in the chick inner ear membranes and sustain the neurotransmitter role of GABA in the chick vestibule.