Effects Of Agmatine On The Electrical Activity Of Hippocampal And Subfornical Organ Neurons

In 1994, Li et al isolated agmatine from cow brain and identified it as an endogenous clonidine–displacing substance (CDS). It has been well established that agmatine is an endogenous agonist at imidazoline receptors (IR) and noncatecholamine ligand at alpha - 2 adrenergic receptors (α2- AR). Agmatine has extensive distribution in the body. It can be synthesized from decarboxylation of L-arginine (L-Arg) by the enzyme arginine decarboxylase (ADC) and is hydrolysed to putrescine and urea by agmatine uryl hydrolase (agmatinase). ADC and agmatinase are also expressed in mammalian brain. Agmatine is coexisted with L-glutamate (L-Glu) in vesicles in axon terminals, which make excitatory synapses on pyramidal cells in hippocampus and can be released from synaptosomes in Ca2+-dependent manner. So it is believed that agmatine may serve as a novel neurotransmitter and/or neuromodulator. Agmatine has many functions in central nervous system. In primary cultured cerebellar neurons, agmatine can dose-dependently protect neurons from NMDA neurotoxicity. Furthermore, Yang et al investigated in cultured hippocampal neurons by whole-cell patch clamp, that agmatine directly binds at a site located within the NMDA channel pore and can blocks the NMDA currents but not non-NMDA currents. It has not been reported that effects of agmatine on discharges of hippocampal and subfornical organ neurons. The present study was designed to examine the effects of agmatine on the electrical activity of neurons of the rat hippocampus and subfornical organ in slice preparation using extracellular recording technique.ⅠEffects of agmatine on neuronal discharges in rat hippocampal CA1 areaThe effects of agmatine (Agm) on the discharges of neurons in CA1 area of hippocampal slices were examined by using extracellular recording technique. The results are as follows: (1) In response to the application of Agm (0.1 - 1.0 μmol/L) into the superfusate for 2 min, the spontaneous discharge rates (SDR) of 38/47 (80.9%) neurons were decreased significantly in a dose-dependent manner, while that of 9/47 (19.1%) neurons showed no change in discharge rate; (2) Pretreatment with L-glutamate (L-Glu, 0.2 mmol/L) led to a marked increase in SDR of 9/12 (75%) neurons in an epileptiform pattern and that of 2/12 (25%) neurons were not affected, then after Agm (1.0 μmol/L) was applied into the superfusate for 2 min, the epileptiform discharges were suppressed significantly; (3) In 7 neurons, perfusion of the selective L-type calcium channel agonist, Bay K 8644 (0.1 μmol/L), induced an increase in the SDR of 6/7 (85.7%) neurons, while that of 1/7 (14.3%) neuron showed no change, and the discharges also were decreased by application of Agm (1.0 μmol/L) into the superfusate; and (4) Application of NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 50 μmol/L) into the superfusate 5 min later also significantly increased the SDR in all 13 (100%) neurons, then Agm (1.0 μmol/L) applied into the superfusate inhibited the discharges of 11/13 (84.6%) neurons, while those of 2/13 (15.4%) neurons were not affected. These results suggested that agmatine can inhibit the spontaneous discharges and L-glutamate-, Bay K 8644- and L-NAME- induced discharges of hippocampal CA1 neurons. These inhibitory effects of agmatine may be related to the blockade of NMDA receptors and a reduction in calcium influx in hippocampal neurons.