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The GABA synthesizing enzyme, glutamate decarboxylase (GAD), has been localized by light and electron microscopy in the rat lumbosacral spinal cord using a peroxidase-labeling antibody technique. The light microscopic localization shows heavy, punctate reaction product for GAD in the dorsal horn laminae I-III. Moderately heavy reaction product is also seen in the deeper dorsal horn laminae IV-VI, the medial aspect of the intermediate gray (lamina VII) and the region around the central canal (lamina X). A moderately light concentration of GAD reaction product is observed in the ventral horn, and punctate deposits of reaction product also are seen on motoneuron cell bodies. The punctate distribution of reaction product for GAD in both ventral and dorsal horns, as visualized by light microscopy, corresponds to GAD-containing synaptic terminals seen by electron microscopy in comparable regions of the spinal gray. Many more GAD-positive terminals are observed in dorsal horn laminae I-III than in deeper laminae IV-VI. GAD-containing terminals in the dorsal horn are presynpatic to dendrites and cell bodies. Gad-containing terminals presynaptic to other axon terminals are observed also, and they are more numerous in laminae II and III. In the ventral horn motor nuclei, GAD-positive knobs are presynaptic to large and small dendrites and motoneuror cell bodies. In addition, small GAD-containing terminals also are presynaptic to larger axonal terminals which are in turn presynaptic to motoneuron somata. The observation of GAD-containing terminals presynaptic to dendrites and cell bodies in both dorsal and ventral horns is compatible with the evidence suggesting that GABA terminals may mediate postsynaptic inhibition of spinal interneurons and motoneurons. The additional finding of GAD-positive terminals presynaptic to other axonal terminals in the dorsal horn and motor nuclei is consistent with the growing evidence that GABA also may be the transmises mediating presynaptic inhibition via axo-axond synapses in the spinal cord.

To test whether patches of papyrus swamp contribute to diversification of populations of non-air-breathing fishes, the gill morphology of Barbus neumayeri was compared between a papyrus swamp and several tributaries which differed in oxygen regime. Total gill filament length differed among sites and was negatively related to dissolved oxygen availability, supporting strong selection pressure for low-oxygen tolerance in the swamp interior. Among recaptures of marked B. neumayeri over a 4·5-year period among the focal swamp and connected stream and river sites, 93% of fish were recovered at the site of capture. Some of the individuals that moved crossed physicochemical gradients and traversed long distances within the swamp/stream system. This movement rate would theoretically be sufficient to homogenize gene frequencies among populations. However, randomly amplified polymorphic DNA (RAPD) markers indicated significant genetic differentiation among sites and no relationship between genetic differences and geographical distances among sites suggesting habitat-specific selection pressures on dispersers, rather than insufficient dispersal. 1999 The Fisheries Society of the British Isles

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