Spatial Learning-Induced Changes at Agmatinergic and Glutamatergic Synapses in the Hippocampal CA1 Region

Agmatine, a metabolite of L-arginine, is considered to be a novel putative neurotransmitter and has been implicated in spatial learning and memory processes. Agmatine-immunoreactivity has been found in synaptic terminals of asymmetric excitatory synapses in the hippocampal CA1 region, which suggests that agmatine may be co-localized with glutamate, the primary excitatory neurotransmitter of hippocampal pyramidal cells. The first aim of the current project was to investigate if agmatine is co-localized with glutamate in the hippocampal pyramidal cells and within synaptic terminals at CA1 synapses, using immunofluorescence confocal microscopy and post-embedding electron microscopy-immunogold cytochemistry, respectively. Double immunofluorescence labelling indicated that agmatine and glutamate are co-localized in the majority of CA1-CA3 pyramidal cell bodies. Immunogold double labelling demonstrated that agmatine is also co-localized with glutamate in synaptic terminals in the CA1 stratum radiatum (SR) region. In the CA1 region of young male Sprague-Dawley rats, 97% of all agmatinergic profiles contained glutamate, and 92% of all glutamatergic profiles also contained agmatine (n = 6; 300 terminals). Having established co-localization of agmatine with glutamate within CA1 SR terminals, the second aim of the project was to investigate changes in co-localized agmatine and glutamate levels following 4 days Morris water maze training. The water maze trained rats (n = 3) showed significant increases in both agmatine and glutamate levels in the CA1 SR terminals (150 terminals; 78% increase in agmatine, p<0.01; ~41% increase in glutamate, p<0.05), compared to swim only control rats (n = 3; 150 terminals). The finding that water maze training induced increases in both agmatine and glutamate levels in the CA1 terminals, suggests that agmatine may play a role with glutamate in learning and memory processes. The final aim of the study was to examine possible spatial learning-induced structural changes at the postsynaptic site. The average thickness of the postsynaptic density at CA1 SR synapses (n = 240) was analysed. The water maze trained rats exhibited a 30% increase in their average postsynaptic density thickness (p < 0.001) compared to control rats. This suggests that there are alterations in the molecular components in the postsynaptic density following water maze training, which could lead to functional modifications of the postsynaptic neuron. Further studies are required to elucidate the molecular mechanisms by which agmatine may influence learning and memory processes, and to explore the functional significance of spatial learning induced increases in agmatine and glutamate levels and PSD thickness at CA1 SR synapses