IMMUNOCYTOCHEMICAL IDENTIFICATION OF GABAERGIC NEURONS IN THE MAIN OLFACTORY-BULB OF THE RAT

  • 1 January 1984
    • journal article
    • research article
    • Vol. 122  (2) , 83-113
Abstract
With the aid of a sheep antiserum against rat brain glutamate decarboxylase (GAD), the endogenous marker for GABAergic neurons, to various types of nerve cells in the main olfactory bulb of rats, with and without topic injections of colchicine were immunocytochemically labeled. The peroxidase-antiperoxidase procedure was applied to floating Vibratome and frozen sections. A large part of the periglomerular cell population and practically all granule cells in the deep layers contain. GAD-like immunoreactivity in untreated rats, while tufted and mitral cells (the projection neurons) are unstained. This observation confirms a previous study with a rabbit antiserum against mouse brain GAD, which suggested that GABAergic neurons with presynaptic dendrites contain high somatal concentrations of GAD. Immunostaining of granule cell bodies decreases progressively from the internal plexiform layer to the deep portion of the granule cell layer. Many cell processes in the glomeruli are densely stained. They presumably represent synaptic gemmules of the numerous GAD-positive periglomerular cells, which could provide initial, inhibitory modulation of the afferent input. In the external plexiform layer immunostaining of the neuropil is substantially denser in the superficial half than in the deep half. This may reflect a corresponding gradient of inhibition related to unequal frequency of occurrence of synaptic gemmules of granule cell dendrites. Alternatively such a graded immunostaining of cell processes could be related to the corresponding gradient in the density of immunostaining of granule cell bodies in the deep layers, in accordane with recent data indicating that superficial and deep granule cells project their ascending dendrites respectively to superficial and deep portions of the external plexiform layer. The presence of additional classes of GAD-positive neurons, microneurons in the external plexiform layer, small neurons in the periglomerular region, the external plexiform layer, the mitral cell layer, the internal plexiform layer, and medium-size neurons in the granule layer and the white matter were demonstrated. The small- and medium-size GAD-positive neurons appear weakly immunoreactive in untreated rats, but become densely stained after topic colchicine injection. Such cells presumably lack presynaptic dendrites and may correspond to different types of short axon cells demonstrated by the Golgi method. The diversity of GAD-positive interneurons in the main olfactory bulb suggests that GABA mediated neuronal interactions have a more prominent role in the modulation of olfactory sensation than previously assumed.