Comp. Biochem. Physiol. Vol. 95B, No. 2, pp. 375 379, 1990 0305-0491/90 $3.00 + 0.00 Printed in Great Britain © 1990 Pergamon Press plc COMPARATIVE STUDIES ON GLUTAMATE DECARBOXYLASE AND CHOLINE ACETYLTRANSFERASE ACTIVITIES IN THE VERTEBRATE VESTIBULE IVAN L6PEZ and GRACIELA MEZA* Departamento de Neurociencias, Instituto de Fisiologia Celular, UNAM, Apdo Postal 70-600, 04510 M6xico (Tel: (905) 550-5215) (Received 17 May 1989) Abstract--1. Vestibular putative neurotransmitters GABA and acetylcholine synthesizing enzymes were quantified in four vertebrate species to find a correlation between all-vertebrate vestibular hair cell II (HCII) and synaptic contacts and appearance of hair cell I (HCI) and related synapses in terrestrial species. 2. Glutamate decarboxylase (GAD) and choline acetyltransferase (CHAT) values were: 3.76; 15.38; 21.68; 27.78 and 9.44; 450; 720; 970 n(pico)mol/mg protein/hr (min) in, respectively, frogs, guinea pigs, rats and chicks. 3. GAD and ChAT omnipresence may indicate constant GABAergic HCII and its cholinergic efferent synapses, their raised content, appearance of GABA-containing HCI and related cholinergic boutons in higher vertebrates. INTRODUCTION Scarcely modified in the course of evolution the main features of the inner ear can be recognized in all vertebrates (Vinnikov, 1982). It consists of a membra- nous labyrinth encased in a bony analogue located generally in the petrous part of the temporal bone or its analogue. In amphibians, birds and mammals the membranous labyrinth shows the following charac- teristics: three well defined semicircular canals, each with its own sensory surface (cristae); two or three distinct otolithic organs with separated sensory per- iphery areas (maculae) and one to three other organs lacking calciferous masses, the papillae, including the papilla neglecta and the organ of Corti. The apparent homologies among these organs in different species have led anatomists to assess a common name to each type of organ across species, even though there are substantial morphological and functional variations. The membranous labyrinth is generally divided into a superior and an inferior division. The superior division, which exhibits fewer evolutionary changes than the inferior one, includes the semicircular canal, the utricule and the papilla neglecta, which often shares a labyrinthine cavity with one of the other organs. Functionally, the organs of the superior division tend to be vestibular, i.e. equilibrial, with some exceptions. The inferior division, which changes dramatically in both form and function in the course of evolution, includes the saccule and its appendages: the lagena, the cochlea and the recesses containing the basilar and the amphibian papilae (Lewis et al., 1985). The labyrinthine wall is lined with a layer of epithelial cells which are specialized in the end organs to form the sensory epithelia. Each sensory epi- thelium in the inner ear of vertebrates contains two basic cell types (hair cells or sensory cells and *Author to whom correspondence should be addressed. supporting cells) as well as the nerve fibres inner- vating the hair cells (Lewis et al., 1985). The hair cell is the mechano-electric transducer element which is vital to all functional properties of vestibular end organs (Wers/ill, 1956). This type of sensory cell is found exclusively in vertebrates and has been described as a secondary cell of epithelial origin (Lowenstein, 1974). The hair cells are very similar in the macula of the utricule and of the sacule and in the cristae. There is, however, a great difference in the structure of the covering material (Engstrom and Engstrom, 1981). Wers~ill (1956) described two different kinds of sensory elements: type I and type II cells. Type I cells are flask-shaped with a rounded lower end. These cells are surrounded to a great extent by a nerve chalice which continues as an afferent nerve fibre through the basement membrane. The type II cell is more cylindrical but it also has a rounded lower end. The type II cell has contacts with many nerve endings which form afferent (centripetal) synapses along the plasma membrane at different levels both below and above the nucleus. This kind of cell is thought to be phylogenetically the oldest, i.e. it exists in the sensory periphery of all vertebrate classes whereas type I appears in terrestrial vertebrates (Wers/ill, 1956). The presence of synaptic endings filled with vesicles at the base of a type II hair cell and outside the calix of the type I hair cell of higher vertebrates supports the idea that hair cells are also contacted by terminals of efferent (centrifugal) origin (Lowenstein, 1974). There is ample evidence that communication, both centripetal and centrifugal, is chemical in nature (Furukawa, 1978; Schessel and Highstein, 1981). The identity of the neurotransmitters involved is presently a matter of controversy, although gamma-amino- butyric acid (GABA) and acetylcholine (Ach) have been implicated as afferent and efferent neuro- mediators, respectively (Gisselsson, 1960; Flock and Lam, 1974; Rossi et al., 1977, 1980; Felix and 375