Interaction between brain L-type calcium channels and alpha2-adrenoceptors in the inhibition of sodium appetite

Inhibition of salt appetite in sodium-depleted rats by carvacrol: Involvement of noradrenergic and serotonergic pathways

Carvacrol, a monoterpene phenol present in the essential oil of oregano, possesses several biological properties, such as antioxidant, anti-inflammatory, anxiolytic, anticonvulsive and antinociceptive. In vitro studies have shown that carvacrol inhibits serotonin, noradrenaline and dopamine transporters and the enzymes monoamine oxidase-A and B. Different brain functions are controlled by monoamines, including cardiovascular control, thirst and sodium appetite. In the present study we investigated the effects of intracerebroventricular (i.c.v.) injection of carvacrol on sodium appetite, and the participation of brain serotonergic and noradrenergic pathways on carvacrol effects. Neuronal activation in homeostasis-related brain areas induced by i.c.v. injection of carvacrol was also evaluated. Carvacrol dose-dependently inhibited hypertonic saline intake (1.5%) in sodium-depleted rats, and this antinatriorexigenic effect was reduced by brain serotonergic depletion and by alpha-adrenergic blockade. Furthermore, i.c.v. injections of carvacrol significantly increased the neuronal activation in brain areas involved in the control of salt appetite, such as MnPO, OVLT, PVN, SON, CeA and MeA. Taken together, our data show that carvacrol presents antinatriorexigenic activity through serotonin and noradrenaline pathways within brain circuits involved in the modulation of the body fluid homeostasis.

α2a adrenoceptors regulate phosphorylation of microtubule-associated protein-2 in cultured cortical neurons

Adrenoceptors have been suggested to mediate neuronal development. This study revealed the expression of alpha2A adrenoceptors in the cortical plate of fetal mouse cerebral wall. The effects of alpha2A adrenoceptor on dendrite growth were investigated in primary neuronal cultures. Application of alpha2 adrenoceptor agonists, BHT 933 or UK 14304 for 24 or 72 h resulted in a 1.5-2-fold increase in dendrite lengths. This effect was blocked by alpha2 adrenergic antagonists, RX 821002 or yohimbine, as well as a alpha2A selective antagonist, BRL 44408, but not by alpha2B/alpha2C selective antagonists ARC 239, imiloxan and rauwolscine. Guanfacine, a alpha2A selective agonists, also significantly increased the dendrite lengths in culture. These results suggest that the morphological effect is wholly attributable to alpha2A adrenoceptor activation. We further tested the hypothesis that alpha2A adrenoceptors act through altering the phosphorylation state of microtubule-associated protein 2. The results showed that the phosphorylation of microtubule-associated protein 2 was significantly reduced on both serine and threonine residues by over 40% after 2 h of application of guanfacine and was maintained at this low level for a prolonged time up to 96 h. These findings suggest that alpha2A adrenoceptors regulate the phosphorylation of microtubule-associated protein 2, which in turn mediates dendrite growth of cortical neurons.

Central serotonergic and adrenergic/imidazoline inhibitory mechanisms on sodium and water intake

Recent studies have shown the existence of two important inhibitory mechanisms for the control of NACL and water intake: one mechanism involves serotonin in the lateral parabrachial nucleus (LPBN) and the other depends on alpha(2)-adrenergic/imidazoline receptors probably in the forebrain areas. In the present study we investigated if alpha(2)-adrenergic/imidazoline and serotonergic inhibitory mechanisms interact to control NaCl and water intake. Male Holtzman rats with cannulas implanted simultaneously into the lateral ventricle (LV) and bilaterally into the LPBN were used. The ingestion of 0.3 M NaCl and water was induced by treatment with the diuretic furosemide (10 mg/kg of body weight)+the angiotensin converting enzyme inhibitor captopril (5 mg/kg) injected subcutaneously 1 h before the access of rats to water and 0.3 M NaCl. Intracerebroventricular (i.c.v.) injection of the alpha(2)-adrenergic/imidazoline agonist clonidine (20 nmol/1 microl) almost abolished water (1.6+/-1.2, vs. vehicle: 7.5+/-2.2 ml/2 h) and 0.3 M NaCl intake (0.5+/-0.3, vs. vehicle: 2.2+/-0.8 ml/2 h). Similar effects were produced by bilateral injections of the 5HT(2a/2c) serotonergic agonist 2,5-dimetoxy-4-iodoamphetamine (DOI, 5 microg/0.2 microl each site) into the LPBN on water (3.6+/-0.9 ml/2 h) and 0.3 M NaCl intake (0.4+/-0.2 ml/2 h). Injection of the alpha(2)-adrenergic/imidazoline antagonist idazoxan (320 nmol) i.c.v. completely blocked the effects of clonidine on water (8.4+/-1.5 ml/2 h) and NaCl intake (4.0+/-1.2 ml/2 h), but did not change the effects of LPBN injections of DOI on water (4.2+/-1.0 ml/2 h) and NaCl intake (0.7+/-0.2 ml/2 h). Bilateral injections of methysergide (4 microg/0.2 microl each site) into the LPBN increased 0.3 M NaCl intake (6.4+/-1.9 ml/2 h), not water intake. The inhibitory effect of i.c.v. clonidine on water and 0.3 M NaCl was still present after injections of methysergide into the LPBN (1.5+/-0.8 and 1.7+/-1.4 ml/2 h, respectively). The results show that the inhibitory effects of the activation of alpha(2)-adrenergic/imidazoline receptors in the forebrain are still present after blockade of the LPBN serotonergic mechanisms and vice versa for the activation of serotonergic mechanisms of the LPBN. Therefore, each system may act independently to inhibit NaCl and water intake.