Catecholamine synthesis and metabolism in the central nervous system of mice lacking alpha-adrenoceptor subtypes

Association between serum catecholamine levels and VNTR polymorphism in the promoter region of the monoamine oxidase A gene in forensic autopsy cases

Monoamine oxidase A (MAOA) catalyzes oxidative deamination of catecholamines. A functional variable number tandem repeat (VNTR) polymorphism in the promoter region of the MAOA gene has been previously reported. In the present study, we measured serum adrenaline (Adr), noradrenaline (Nad), and dopamine (DA) levels in 90 male and 34 female Japanese autopsy cases in which amphetamines or psychotropic drugs were not detected.We examined the frequencies of MAOA-uVNTR alleles in these cases and investigated the effects of the MAOA-uVNTR polymorphism on serum Adr, Nad, and DA levels. Evaluation indicated no significant association between MAOA-uVNTR polymorphism and serum Adr, Nad, or DA levels in males, although a significant association between MAOA-uVNTR polymorphism and serum Adr and DA levels were observed in females. Females with the 3/3 genotype had higher serum Adr and DA levels than those with a 4-repeat allele (3/4 and 4/4 genotypes) (p = 0.048 and 0.020, respectively). There was no significant association between MAOA-uVNTR polymorphism and serum Nad levels in females. The present study indicates that MAOA-uVNTR polymorphism influences serum Adr and DA levels only in females.

Melatonin mediated antidepressant-like effect in the hippocampus of chronic stress-induced depression rats: Regulating vesicular monoamine transporter 2 and monoamine oxidase A levels

The hippocampus is sensitive to stress which activates norepinephrine terminals deriving from the locus coeruleus. Melatonin exerts positive effects on the hippocampal neurogenic process and on depressive-like behaviour. Thus, in the present study, an examination was made of the effect of chronic melatonin treatment on norepinephrine content, synthesis, uptake, vesicular transport and degradation in the hippocampus of rats exposed to CUMS. This entailed quantifying the norephinephrine, mRNA and protein levels of DBH, NET, VMAT 2, MAO-A and COMT. The results show that CUMS evoked prolonged immobility. Melatonin treatment decreased immobility in comparison with the placebo group, reflecting an antidepressant-like effect. Compared with the placebo group, a dramatic decrease in norepinephrine content, decreased VMAT2 mRNA and protein and increased MAO-A protein levels in the hippocampus of the CUMS rats were observed. However, no significant differences in the levels of DBH, NET, COMT mRNA and protein and MAO-A mRNA levels between the placebo and the stressed groups were found. The results showed the restorative effects of melatonin on the stress-induced decline in the norepinephrine content of the hippocampus. It was observed that melatonin treatment in the CUMS rats prevented the stress-induced decrease in VMAT2 mRNA and protein levels, whereas it reduced the increase of the mRNA of COMT and protein levels of MAO-A. Chronic treatment with melatonin failed to alter the gene expression of DBH or NET in the hippocampus of the CUMS rats. Additionally, the results show that melatonin enhances VMAT2 expression and norepinephrine storage, whilst it reduces norepinephrine degrading enzymes.

GPCR dimerization in brainstem nuclei contributes to the development of hypertension

BACKGROUND AND PURPOSE

μ-Opioid receptors, pro-opiomelanocortin and pro-enkephalin are highly expressed in the nucleus tractus solitarii (NTS) and μ receptor agonists given to the NTS dose-dependently increased BP. However, the molecular mechanisms of this process remain unclear. In vitro, μ receptors heterodimerize with α2A -adrenoceptors. We hypothesized that α2A -adrenoceptor agonists would lose their depressor effects when their receptors heterodimerize in the NTS with μ receptors.

EXPERIMENTAL APPROACH

We microinjected μ-opioid agonists and antagonists into the NTS of rats and measured changes in BP. Formation of μ receptor/α2A -adrenoceptor heterodimers was assessed with immunofluorescence and co-immunoprecipitation methods, along with proximity ligation assays.

KEY RESULTS

Immunofluorescence staining revealed colocalization of α2A -adrenoceptors and μ receptors in NTS neurons. Co-immunoprecipitation revealed interactions between α2A -adrenoceptors and μ receptors. In situ proximity ligation assays confirmed the presence of μ receptor/α2A -adrenoceptor heterodimers in the NTS. Higher levels of endogenous endomorphin-1 and μ receptor/α2A -adrenoceptor heterodimers were found in the NTS of hypertensive rats, than in normotensive rats. Microinjection of the μ receptor agonist [D-Ala(2) , MePhe(4) , Gly(5) -ol]-enkephalin (DAMGO), but not that of the α2A -adrenoceptor agonist guanfacine, into the NTS of normotensive rats increased μ receptor/α2A -adrenoceptor heterodimer formation and BP elevation. The NO-dependent BP-lowering effect of α2A -adrenoceptor agonists was blunted following increased formation of μ receptor/α2A -adrenoceptor heterodimers in the NTS of hypertensive rats and DAMGO-treated normotensive rats.

CONCLUSIONS AND IMPLICATIONS

Increases in endogenous μ receptor agonists in the NTS induced μ receptor/α2A -adrenoceptor heterodimer formation and reduced the NO-dependent depressor effect of α2A -adrenoceptor agonists. This process could contribute to the pathogenesis of hypertension.

α2C-Adrenoceptors modulate L-DOPA uptake in opossum kidney cells and in the mouse kidney

Targeted deletion or selective pharmacological inhibition of α(2C)-adrenoceptors in mice results in increased brain tissue levels of dopamine and its precursor l-3,4-dihydroxyphenylalanine (l-DOPA), without significant changes in l-DOPA synthesis. l-DOPA uptake is considered the rate-limiting step in dopamine synthesis in the kidney. Since α(2C)-adrenoceptors may influence the transport of l-DOPA, we investigated the effect of α(2C)-adrenoceptor activation on l-DOPA uptake in a kidney cell line (opossum kidney cells). l-DOPA and dopamine kidney tissue levels in α(2C)-adrenoceptor knockout (α(2C)KO) mice and in mice treated with the selective α(2C)-adrenoceptor antagonist JP-1302 were also evaluated. The α(2)-adrenoceptor agonist medetomidine (0.1-1,000 nM) produced a concentration-dependent decrease in l-DOPA uptake in opossum kidney cells (IC(50): 2.5 ± 0.5 nM and maximal effect: 28 ± 5% of inhibition). This effect was abolished by a preincubation with JP-1302 (300 nM). Furthermore, the effect of medetomidine (100 nM) was abolished by a preincubation with U-0126 (10 μM), a MEK1/2 inhibitor. Kidney tissue levels of l-DOPA were significantly higher in α(2C)KO mice compared with wild-type mice (wild-type mice: 58 ± 2 pmol/g tissue and α(2C)KO mice: 81 ± 15 pmol/g tissue, P < 0.05) and in mice treated with JP-1302 (3 μmol/kg body wt) compared with control mice (control mice: 62 ± 2 pmol/g tissue and JP-1302-treated mice: 75 ± 1 pmol/g tissue, P < 0.05), both without significant changes in dopamine kidney tissue levels. However, mice treated with JP-1302 on a high-salt diet presented significantly higher dopamine levels in the kidney and urine compared with control animals on a high-salt diet. In conclusion, in a kidney cell line, α(2C)-adrenoceptor activation inhibits l-DOPA uptake, and in mice, deletion or blockade of α(2C)-adrenoceptors increases l-DOPA kidney tissue levels.

Adrenal α2-adrenergic receptors in the aging normotensive and spontaneously hypertensive rat

This study investigates α(2)-adrenergic receptor (α(2)AR) mediated feedback inhibition of catecholamine release from the adrenal medulla of adult (52 weeks) and old (98 weeks) spontaneously hypertensive rats (SHR) and normotensive controls Wistar Kyoto (WKY) rats. Adrenal epinephrine content as well as the spontaneous and the nicotinic-evoked release of epinephrine were similar between adult SHR and WKY rats. Aging produced a significant reduction in epinephrine synthesis in WKY rats. In contrast, in SHR aging produced a significant increase in epinephrine release without significant changes in epinephrine synthesis. The α(2)AR agonist medetomidine abolished (80-90% inhibition) the nicotinic-evoked release of epinephrine in adult SHR and WKY rats. With aging, this effect was unaltered in WKY rats but was significantly decreased in SHR (30% inhibition). Adrenal α(2A)AR mRNA levels were significantly reduced in old SHR compared with age matched WKY rats. In conclusion, in aging the α(2)AR mediated feedback inhibition of epinephrine release from the adrenal medulla is preserved in WKY rats but compromised in SHR, resulting in increased epinephrine release.