Polyamines are important regulators of basal cellular functions but also subserve highly specific tasks in the mammalian brain. With this respect, polyamines and the synthesizing and degrading enzymes are clearly differentially distributed in neurons versus glial cells and also in different brain areas. The synthesis of the diamine putrescine may be driven via two different pathways. In the “classical” pathway urea and carbon dioxide are removed from arginine by arginase and ornithine decarboxylase. The alternative pathway, first removing carbon dioxide by arginine decarboxlyase and then urea by agmatinase, may serve the same purpose. Furthermore, the intermediate product of the alternative pathway, agmatine, is an endogenous ligand for imidazoline receptors and may serve as a neurotransmitter. In order to evaluate and compare the expression patterns of the two gate keeper enzymes arginase and arginine decarboxylase, we generated polyclonal, monospecific antibodies against arginase-1 and arginine decarboxylase. Using these tools, we immunocytochemically screened the rat brain and compared the expression patterns of both enzymes in several brain areas on the regional, cellular and subcellular level. In contrast to other enzymes of the polyamine pathway, arginine decarboxylase and arginase are both constitutively and widely expressed in rat brain neurons. In cerebral cortex and hippocampus, principal neurons and putative interneurons were clearly labeled for both enzymes. Labeling, however, was strikingly different in these neurons with respect to the subcellular localization of the enzymes. While with antibodies against arginine decarboxylase the immunosignal was distributed throughout the cytoplasm, arginase-like immunoreactivity was preferentially localized to Golgi stacks. Given the apparent congruence of arginase and arginine decarboxylase distribution with respect to certain cell populations, it seems likely that the synthesis of agmatine rather than putrescine may be the main purpose of the alternative pathway of polyamine synthesis, while the classical pathway supplies putrescine and spermidine/spermine in these neurons.

Agmatinase, an ureohydrolase belonging to the arginase family, is widely expressed in mammalian tissues including the brain. Here, it may serve two different functions, the inactivation of the arginine derivative agmatine, a putative neurotransmitter, and the formation of the diamine putrescine. In order to identify the cellular sources of agmatinase expression in the brain, we generated a polyclonal monospecific antibody against recombinant rat agmatinase. With immunocytochemistry, selected areas of rat and human brain were screened. Clearly, in both species agmatinase-like immunoreactivity was predominantly detected in distinct populations of neurons, especially cortical interneurons. Also, principal neurons in limbic regions like the habenula and in the cerebellum robustly expressed agmatinase protein. When comparing the overall agmatinase expression with immunocytochemical data available for agmatine and polyamine biosynthetic enzymes, the observed pattern may argue in favor of an agmatine inactivating function rather than fueling the alternative pathway of polyamine synthesis. The putative neurotransmitter agmatine is seemingly involved with mental disorders. Therefore, agmatinase may be similarly important for pathogenesis. The normal expression profile of the protein as described here may therefore be altered under pathological conditions.

Light microscopic evidence suggested a synaptic role for agmatinase, an enzyme capable of inactivating the putative neurotransmitter and endogenous anti-depressant agmatine. Using electron microscopy and an alternative pre-embedding approach referred to as virtual pre-embedding, agmatinase was localised pre- and postsynaptically, to dendritic spines, spine and non-spine terminals, and dendritic profiles. In dendritic spines, labelling displayed a tendency towards the postsynaptic density. These results further strengthen a synaptic role for agmatine and strongly suggest a regulatory role for synaptically expressed agmatinase.

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