A facilitatory role of vasopressin in hypoxia/hypoglycemia-induced impairment of dopamine release from rat striatal slices

Arginine-vasopressin in nucleus of the tractus solitarius induces hyperglycemia and brain glucose retention

Hypothalamic arginine-vasopressin (AVP) plays an important role both as a neurotransmitter and hormone in the regulation of blood glucose and feeding behavior. AVP-containing axons from the parvocellular subdivision of paraventricular nucleus of the hypothalamus terminate in the nucleus of the tractus solitarius (NTS), but the function of this projection is not known. Interestingly, the NTS also receives afferent information from the carotid body and other peripheral receptors involved in glucose homeostasis. We have previously reported that stimulation of the carotid body receptors initiates a hyperglycemic reflex and increases brain glucose retention. Here we show that direct administration of micro-doses of AVP into the NTS of anesthetized or awake rats rapidly increased the levels of blood glucose concentration and brain arterio-venous (A-V) glucose difference. This effect was not observed when the same doses of AVP were injected in the brainstem outside NTS. Arginine-vasopressin antagonist microinjections alone produced a small but significant reduction in brain A-V glucose. Pre-administered VP1-receptor antagonist [beta-mercapto-beta,beta-cyclopentamethylene-propionyl(1),O-Me-Tyr(2),Arg(8)]vasopressin blocked the effects of AVP. These results indicate that AVP acting on its receptors locally within the NTS participates in glucose homeostasis, increasing both blood glucose concentration and brain A-V glucose differences. Hypothalamic AVP may facilitate hyperglycemic responses initiated by peripheral signals processed at the level of the NTS.

Long-term enhancement of dopamine release by high frequency tetanic stimulation via a N-methyl-D-aspartate-receptor-mediated pathway in rat striatum

We studied the effects of high frequency tetanic stimulation of the striatum on the KCl (20 mM)-evoked dopamine release in rat striatal slices. The KCl-evoked dopamine release was potentiated by high frequency tetanic stimulation (10-20 Hz) of the striatum including the corticostriatal fibers, and this potentiation was observed until 3 h after high frequency tetanic stimulation. Potentiation of dopamine release after high frequency tetanic stimulation was induced not only by KCl but also by glutamate in Mg(2+)-free medium, N-methyl-D-aspartate in Mg(2+)-free medium, and by DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid. 2-amino-5-phosphovalerate, 3-[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonate or dibenzocycloheptaneimine, N-methyl-D-aspartate receptor inhibitors, abolished enhancement by tetanus, whereas, 6,7-dinitroquinoxaline-2,3-dione, an antagonist of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid ionotropic receptors, or L-2-amino-4-phosphonobutyrate, an antagonist of glutamate metabotropic receptors, showed no effect. Moreover, pretreatment with glutamate or N-methyl-D-aspartate in the absence of Mg2+ also facilitated dopamine release evoked by KCl concentrations. When extracellular Ca2+ was removed from the medium during pretreatment, potentiation by glutamate disappeared. We conclude that activation of N-methyl-D-aspartate receptors on dopaminergic nerve terminals in the striatum produces the long-term changes in efficacy of the response to KCl or glutamatergic agents. That is, plastical phenomena could exist at presynaptic levels between glutamatergic neurons and dopaminergic neurons in striatum.