Agmatine Is an Autocrine/Paracrine Inhibitor of Pre-Synaptic Ca++ Channels and Vasopressin Release in Vasopressinergic Neurons of the Hypothalamo-Hypophyseal Tract
Agmatine (decarboxylated arginine), an organic cation, endogenous ligand at imidazoline (I-) and α2-adrenergic receptors, antagonist of NMDA receptors and inhibitor of nitric oxide synthase, may be a novel neurotransmitter in mammalian CNS (Reis and Regunathan, TIPS, 2000). To further establish a cellular function of agmatine we investigated whether it is expressed in, and regulates the function of, magnocellular neurons of the hypothalamic paraventricular (PVH)/supraoptic (SON) nuclei in rat. By LM, agmatine-like immunoreactivity (Ag-LI) was present in virtually all magnocellular PVH and SON neurons, where it co-localized with arginine vasopressin (AVP) and oxytocin (OXY). By EM, Ag-LI was present in large dense core vesicles in neurohypophysial nerve terminals (NNTs), known storage sites of the peptides. NNTs acutely isolated from adult rats were dispersed and voltage-activated Na+, K+, and Ca++ currents recorded in whole terminals in amphotericin B perforated-patches. Agmatine (40-120 uM) did not affect the voltage-dependent, TTX-sensitive inward Na+, the 4-AP-sensitive, transient outward A-type K+ current, or the sustained, Ca++-activated, maxi-conductance K+ currents. In ∼ 60% of terminals 40 uM agmatine immediately, dose-dependently and reversibly blocked whole cell Ca++ currents evoked by 10 mM Ca++ with an IC50 ∼ 5 uM. Agmatine blocked the depolarization-induced release of AVP from isolated NNTs. We conclude that agmatine: (a) is co-stored with neuropeptides in hypothalamo-neurohypophysial neurons, and (b) inhibits pre-synaptic Ca++ channels to (c) inhibit release of AVP. Agmatine is a neurotransmitter in AVP neurons of PVH and SON, wherein it functions as a paracrine/autocrine inhibitor of AVP release by blocking Ca++ channels. Agmatine may regulate neurally integrated release of this important hormone, regulating its cardiovascular and renal functions.