Agmatine: an endogenous ligand at imidazoline receptors may be a novel neurotransmitter in brain

Manganese is essential for catalytic activity of Escherichia coli agmatinase

Purified Escherichia coli agmatinase (EC 3.5.3.11) expressed the same activity in the absence or presence of added Mn2+ (0-5mM). However, it was strongly inhibited by Co2+, Ni2+, and Zn2+ and almost half inactivated by EDTA. Partial inactivation by EDTA yielded enzyme species containing 0.85 +/- 0.1 Mn2+/subunit, and it was accompanied by a decrease in intensity of fluorescence emission and a red shift from the emission maximum of 340 nm to 346 nm, indicating the movement of tryptophane residues to a more polar environment. The activity and fluorescence properties of fully activated agmatinase were restored by incubation of dialysed species with Mn2+. Manganese-free species, obtained by treatment with EDTA and guanidinium chloride (3 M), were active only in the presence of added Mn2+. Results obtained, which represent the first demonstration of the essentiality of Mn2+ for agmatinase activity, are discussed in connection with a possible binuclear metal center in the enzyme.

Recent advances in imidazoline receptor research: ligands--localization and isolation--signaling--functional and clinical studies

In this article we outline the highlights of this special issue of the journal containing a series of articles covering many aspects of current interest in the field of imidazoline receptor research. This volume is the result of an international symposium held in September 1997 in Melbourne as an official satellite of the inaugural meeting of the International Society of Autonomic Neurosciences held in Cairns, Australia. A wide range of topics relating to imidazoline receptors were canvassed, including endogenous and synthetic ligands, identification and localisation of binding sites, putative transduction mechanisms and experimental and clinical functional studies.

Agmatine containing axon terminals in rat hippocampus form synapses on pyramidal cells

We examined the cellular and subcellular localization of agmatine in the hippocampal CA1 region by immunocytochemistry. By light microscopy, agmatine-like immunoreactivity (agmatine-LI) was found primarily in the perikarya and dendritic profiles of pyramidal cells and in punctate processes preponderantly in stratum radiatum. Electron microscopy revealed that agmatine-LI was cytoplasmic and concentrated in 'clusters' associated with mitochondria and tubular vesicles. In stratum radiatum, agmatine-LI was primarily in axons and axon terminals associated with small, synaptic vesicles. The terminals almost exclusively formed asymmetric synapses on the spines of dendrites, many of which originated from pyramidal cells. Some agmatine-LI also was present in shafts and spines of pyramidal cell dendrites and in astrocytic processes. The results demonstrate that agmatine in the hippocampus is found primarily in terminals forming excitatory (asymmetric) synapses on pyramidal cells, some of which contain agmatine-LI. These findings further implicate agmatine as an endogenous neurotransmitter which may be co-stored with L-glutamate and may act in part in the rat hippocampus as a blocker of the N-methyl-D-aspartate receptor and nitric oxide synthase.