Development of GAD-immunoreactivity in the dorsal cochlear nucleus of the hamster and cat: light and electron microscopic observations

Inhibitory synaptogenesis in the rat anteroventral cochlear nucleus

Spherical cells in the anteroventral division of the cochlear nucleus, which relay excitatory inputs from the auditory nerve, also receive both GABAergic and glycinergic inhibitory synapses. Inhibition mediated by GABA and glycine fulfils essential roles in the processing abilities of these and other auditory neurons. However, the developmental program leading to a mature complement of GABAergic and glycinergic synapses and microcircuits is largely unknown. Because of their relatively simple geometry, spherical cells provide an excellent model for unraveling basic developmental patterns of inhibitory synaptogenesis. Using a combination of high resolution immunocytochemical methods, we report that, in the rat, synapses containing GABA or glycine are deployed on spherical cell bodies over a time period extending well beyond hearing onset. Such postnatal developmental recruitment of inhibitory endings is progressive, although there are two distinct leaps in their numbers. The first occurs by the end of the first postnatal week, prior to hearing onset, and the second, during the third postnatal week, after hearing onset. This pattern suggests that adjustments in inhibition could be driven by acoustic experience. While GABAergic and glycinergic endings are maturing and growing in number and size, their neurotransmitter content also appears to be developmentally regulated. Quantitative ultrastructural immunocytochemistry with colloidal gold suggests that GABA and glycine accumulation in synaptic endings follows a staggered pattern, with labeling stabilizing at adult levels by postnatal day 21. This may account for adjustments in synaptic efficacy and strength.

Sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis

The directional selectivity of auditory neurons is one of the essential response properties that underlie sound localization, an important task performed by the mammalian auditory system. Here we evaluated the sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis, at the age of 15 days under free-field stimulation conditions. Compared with those in adult bats, neurons in juvenile bats were broadly tuned to sound azimuth angles as indicated by both the type and width of the azimuth selectivity curves. Their best azimuth was distributed over a wide range and, moreover, the adult-like relationship between the best azimuth and the best frequency of these neurons was not developed. These data indicate that the directional selectivity of inferior collicular neurons, like other response properties examined previously, undergoes considerable change during postnatal maturation.

Metamorphic changes in GABA immunoreactivity in the brainstem of the bullfrog, Rana catesbeiana

We examined immunoreactivity for gamma-aminobutyric acid (GABA) in auditory and vestibular brainstem nuclei of the bullfrog, Rana catesbeiana, across metamorphosis, a developmental period featuring significant anatomical and functional remodeling of the nervous system. In the early larval period, GABA-immunoreactive cell somata were visible in the vestibular nucleus complex and the torus semicircularis, as well as in the spinal cord, cerebellum and optic tectum. Fiber bundles such as the medial longitudinal fasciculus and the lemnsical pathways also exhibited intense label at these early stages. In contrast, only diffuse neuropil label was visible in the dorsolateral nucleus and the superior olivary nucleus at the same stages. This diffuse immunoreactivity became progressively more reduced over larval development, and stained somata were visible in these medullary nuclei by metamorphic climax stages. In the torus semicircularis, the numbers of labeled somata in both the developing laminar and principal nuclei increased over metamorphic development, and became progressively more organized into distinct layers. The adult pattern of GABA-like immunoreactivity in the auditory brainstem was reached by metamorphic climax stages, coincident with the maturation of the opercularis system, and preceding the final development of the external tympanum and the tympanic conduction pathway. The relatively earlier maturation of vestibular, compared to auditory, areas in the medulla might reflect the behavioral importance of vestibular-mediated motor reactions during tadpole life. The distribution of GABA in auditory brainstem nuclei in both developing and adult frogs is comparable to that observed in mammals and birds.

Structure and function of gustatory neurons in the nucleus of the solitary tract. III. Classification of terminals using cluster analysis

In sensory systems, insight into synaptic arrangements on cells of known physiological response properties has helped our understanding of the structural basis for these properties. To carry out these types of studies, however, synaptic types in the region of interest must be defined. Unfortunately, defining synaptic types in the brainstem has proved to be a challenging enterprise. Our study was done to classify synapses in the gustatory part of the nucleus solitarius using objective quantitative criteria and a cluster analysis procedure. Cluster analysis allows classification of a population of objects, such as synaptic terminals, into groups that exhibit similar characteristics. Six terminal types were identified using cluster analysis and subsequent analyses of variance and post hoc tests. Unlike classification schemes used for the cerebral cortex, where synaptic apposition density thickness and shape of vesicles is useful (Gray's Type I and II synapses), the concentration of vesicles in a terminal was a more useful measurement with which to classify terminals in the nucleus solitarius. To validate that vesicle density (vesicles/microm2) is a useful defining characteristic to classify terminals in the nucleus solitarius, terminals of a known type were used. GABAergic terminals were identified using postembedding immunohistochemical techniques, and their vesicle density was determined. GABAergic terminals fall into the range of two of the terminal types defined by the cluster analysis and, based on vesicle density, two types of GABAergic terminals were identified. We conclude that vesicle density is a helpful means to identify synapses in this brainstem nucleus.

The development of glycine-like immunoreactivity in the dorsal cochlear nucleus

Both anatomical and physiological lines of evidence suggest that inhibitory influences are active early in the development of the dorsal cochlear nucleus (DCN). Data from physiological/pharmacological studies suggest that in the adult these inhibitory influences may be mediated at least in part by the neurotransmitter glycine. Using a polyclonal antibody to examine the development of glycine in the DCN, we have observed that glycine-like immunoreactive cell bodies and punctate label are present in the DCN by the day of birth in the hamster and in the kitten at least 10 days prior to birth. In contrast to the development of immunoreactivity for glutamic acid decarboxylase (GAD) (Schweitzer et al., 1993), glycine-like immunoreactivity shows a homogeneous distribution throughout the DCN from the day of birth through adulthood. In addition, glycine immunoreactivity is present earlier than GAD-immunoreactivity and is present well before these brain structures become responsive to air-borne sounds. Thus, glycine is present in the very young animal and may mediate inhibitory effects that occur early in development.

Activity-dependent responses of developing cochlear nuclear neurons to microionophoretically-applied amino acids

The experimental purpose of this investigation was to determine whether the efficacy of glutamate, N-methyl-D-aspartate (NMDA) and/or GABA is related to the activity state of neurons in the cochlear nuclear complex (CN). The hypothesis tested was that changes in discharge activity produced by glutamatergic and GABAergic ligands are, or may be, greater when neurons are stimulated at moderate to high acoustic levels compared to near threshold stimulation levels, when activity levels are high or low, respectively. All neurons from which discharge rate vs. sound pressure level curves were tested during simultaneous administration of amino acids exhibited characteristics commensurate with an activity-dependent system; at high sound levels, when discharge rates were elevated relative to rates produced by low level stimuli, both glutamate-induced increments and GABA-induced decrements in discharge rate were enhanced. The relationship between discharge rate and amino acid efficacy was a uniform property of neurons sampled throughout the first two postnatal weeks of development. In adults, preliminary indications are that activity-dependent neurotransmitter efficacy is characteristic of some cells, but not others. The activity-dependent nature of endogenous amino acid neurotransmission was demonstrated through the microionophoretic administration of NMDA and GABAA selective antagonists, D-alpha-aminoadipate (D alpha AA) and 2-amino-5-phosphonovalerate (APV), and bicuculline (BIC), respectively. These results suggest that postsynaptic actions elicited by membrane receptors subserving amino acid neurotransmission within the CN are activity-dependent.