Adenosine modulates alpha2-adrenergic receptors within specific subnuclei of the nucleus tractus solitarius in normotensive and spontaneously hypertensive rats

Preeclampsia and the brain: neural control of cardiovascular changes during pregnancy and neurological outcomes of preeclampsia

Preeclampsia (PE) is a form of gestational hypertension that complicates ∼5% of pregnancies worldwide. Over 70% of the fatal cases of PE are attributed to cerebral oedema, intracranial haemorrhage and eclampsia. The aetiology of PE originates from abnormal remodelling of the maternal spiral arteries, creating an ischaemic placenta that releases factors that drive the pathophysiology. An initial neurological outcome of PE is the absence of the autonomically regulated cardiovascular adaptations to pregnancy. PE patients exhibit sympathetic overactivation, in comparison with both normotensive pregnant and hypertensive non-pregnant females. Moreover, PE diminishes baroreceptor reflex sensitivity (BRS) beyond that observed in healthy pregnancy. The absence of the cardiovascular adaptations to pregnancy, combined with sympathovagal imbalance and a blunted BRS leads to life-threatening neurological outcomes. Behaviourally, the increased incidences of maternal depression, anxiety and post-traumatic stress disorder (PTSD) in PE are correlated to low fetal birth weight, intrauterine growth restriction (IUGR) and premature birth. This review addresses these neurological consequences of PE that present in the gravid female both during and after the index pregnancy.

Alpha2-adrenoceptor and adenosine A1 receptor within the nucleus tractus solitarii in hypertension development

Alpha2-adrenoceptor and A1 adenosine receptor systems within the nucleus tractus solitarii (NTS) play an important role in cardiovascular control. Deregulation of these systems may result in an elevated sympathetic tone, one of the root causes of neurogenic hypertension. The dorsomedial/dorsolateral and subpostremal NTS subnuclei of spontaneously hypertensive rats (SHR) show density changes in both receptors, even at 15 days of age, prior to the onset of hypertension. In addition, adenosine A1 receptors have been specifically reported to modulate alpha2-adrenoceptors in several brain regions, including the NTS, via a PLC-dependent pathway involving cross regulation between sympathetic neurons and astrocytes. The physiological cross talk between these receptor systems is also deregulated in SHR suggesting that alpha2-adrenoceptor and A1 adenosine receptor might be germane to the development of hypertension. In this review, we will focus on these systems within the NTS during development, pointing out some interesting modulations in processes, and chemical changes within specific subnuclei of NTS circuitry, that might have implications for neurogenic hypertension.

The role of alpha-2 adrenoceptors in the anticonvulsant effects of adenosine on pentylenetetrazole-induced seizure threshold in mice

Adenosine has anticonvulsant effects in various models of seizures. Alpha-2 adrenoceptors have also demonstrated different effects in different models of epilepsy. In this study, the role of alpha-2 adrenoceptors in the anticonvulsant effects of adenosine in mice was determined according to the method of intravenous pentylenetetrazole-induced seizure. In this study, N(6)-cyclohexyladenosine (CHA) (a selective A1 receptor agonist), clonidine (an alpha-2 adrenoceptors agonist), yohimbine (an alpha-2 adrenoceptors antagonist) and 8-cyclopentyl-1,3-dimethylxanthine (8-CPT) (a selective A1 receptor antagonist) were used. CHA at doses of 0.5, 1 and 2mg/kg significantly increased seizure threshold with the maximum anticonvulsant effect at 2mg/kg. Yohimbine (0.1, 1 and 10mg/kg), clonidine (0.1, 0.5, 1 and 2mg/kg) and 8-CPT (0.5, 1, 2 and 4mg/kg) had no effect on seizure by itself. Combination of yohimbine (10mg/kg) and CHA (0.25mg/kg) increased clonic seizure latency showing that yohimbine and CHA have an additive effect. Increasing the seizure threshold created by combining ineffective doses of yohimbine (10mg/kg) and CHA (0.25mg/kg) was completely inhibited by 8-CPT (4mg/kg) or clonidine (1 and 2mg/kg). Clonidine (0.5, 1 and 2mg/kg) inhibited the anticonvulsant effects of CHA (2mg/kg). Combination of 8-CPT (1mg/kg) and clonidine (0.5mg/kg) which completely inhibited the anticonvulsant effect of CHA (2mg/kg) indicates that 8-CPT and clonidine have an additive effect. In conclusion, adenosine and yohimbine exhibit an additive effect on the enhancement of the pentylenetetrazole-induced seizure threshold in mice, indicating the interaction of alpha-2 adrenoceptors and A1 adenosine receptors.

Glutamate requires NMDA receptors to modulate alpha2 adrenoceptor in medulla oblongata cultured cells of newborn rats

α2 Adrenoceptors (α2-ARs) are important in regulating the central control of blood pressure in medulla oblongata. However, it is unclear how this receptor is modulated by different receptors, especially the glutamatergic. In the present study, we studied the influence of ionotropic glutamatergic receptors over the α2-ARs in cultured cells of the medulla oblongata of newborn rats. For this purpose, the protein level of the α2-ARs was assessed after administration to the cultured cells of glutamate (glu), the agonists NMDA and kainate (KA), the NMDA receptor antagonist MK801 and the KA receptor antagonist DNQX. Results indicate that the α2-AR protein levels were increased after the treatments with glu and NMDA, and the addition of MK801 to this treatment thwarted this increase. Notwithstanding the fact that KA did not alter the receptor protein level, the combined treatment of DNQX with glu prevented the α2-AR protein modulation. In conclusion, the present study suggests that ionotropic glutamatergic receptors could be related to the α2-AR protein regulation in the medulla oblongata.

Adenosine modulates alpha2-adrenergic receptors through a phospholipase C pathway in brainstem cell culture of rats

Adenosine acts in the nucleus tractus solitarii (NTS), one of the main brain sites related to cardiovascular control. In the present study we show that A(1) adenosine receptor (A(1R)) activation promotes an increase on alpha(2)-adrenoceptor (Alpha(2R)) binding in brainstem cell culture from newborn rats. We investigated the intracellular cascade involved in such modulatory process using different intracellular signaling molecule inhibitors as well as calcium chelators. Phospholipase C, protein kinase Ca(2+)-dependent, IP(3) receptor and intracellular calcium were shown to participate in A(1R)/Alpha(2R) interaction. In conclusion, this result might be important to understand the role of adenosine within the NTS regarding autonomic cardiovascular control.