Changes in postnatal norepinephrine alter alpha-2 adrenergic receptor development

Abstract

Alpha-2 adrenergic receptors (A2AR) regulate multiple brain functions and are enriched in developing brain. Studies demonstrate norepinephrine (NE) plays a role in regulating brain maturation, suggesting it is important in A2AR development. To investigate this we employed models of NE absence and excess during brain development. For decreases in NE we used N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), a specific noradrenergic neurotoxin. Increased noradrenergic terminal density was produced by methylazoxymethanol acetate (MAM) treatment. A2AR density was assayed with [3H]RX821002 autoradiography. DSP4 lesions on postnatal day (PND) 3 produce A2AR decreases in many regions by PND 5. A2AR recover to control levels by PND 15 and 25 and there is no further change in total receptor density. We also assayed A2AR in brains lesioned with DSP4 on PND 13, 23, 33 and 43 and harvested 22 days post-lesion. A2AR levels remain similar to control at each of these time points. We examined A2AR functionality and high affinity state with epinephrine-stimulated [35S]GTPγS and [125 I]p-iodoclonidine autoradiography, respectively. On PND 25, control animals and animals lesioned with DSP4 on PND 3 have similar levels of [35S]GTPγS incorporation and no change in high affinity state. This is in contrast to increases in A2AR high affinity state produced by DSP4 lesions of mature brain. We next investigated A2AR response to increases in norepinephrine levels produced by MAM. In contrast to DSP4 lesions, increasing NE results in a large increase in A2AR. Animals treated with MAM on gestational day 14 had cortical [3H]RX821002 binding 100-200% greater than controls on PND 25, 35, 45, 55 and 65. These data indicate that NE regulation of A2AR differs in developing and mature brain and support the idea that NE regulates A2AR development and this has long term effects on A2AR function.

Developmental DSP4 effects on cortical Arc expression

“…After injection, rats were returned to their home cage and brains harvested ~2-3 weeks later. This interval was chosen since this laboratory has confirmed a near complete loss of norepinephrine and noradrenergic innervation using a similar time frame [7]. Rats were taken to a separate room where they were killed by decapitation under isoflourane anesthesia and brains were removed, frozen on dry ice and stored at −80°C.…”

“…Noradrenergic receptors and transporters, for instance, are enriched in many specific brain areas during times of early brain development and dramatically decrease in expression with maturity, suggesting important developmental roles [5, 6]. More recently, the noradrenergic system has been characterized by differences in its regulation of the developing versus mature brain [7-9]. The regulation of adrenergic receptors by norepinephrine, for example, differs according to developmental stage [7].…”

“…More recently, the noradrenergic system has been characterized by differences in its regulation of the developing versus mature brain [7-9]. The regulation of adrenergic receptors by norepinephrine, for example, differs according to developmental stage [7]. …”

Elements toward novel therapeutic targeting of the adrenergic system

“…For instance, clonidine, an a2-AR agonist, is used as an anti-hypertensive agent that acts on cardiovascular regulatory centers in the brainstem (Yamazato et al, 2001). In psychiatry, other a2-AR agonists are used as antidepressants (Davis et al, 2001) including mirtazapine which is an a2-AR, 5 HT2 and 5-HT3 serotoninergic receptors' agonist (Sanders et al, 2011) used to treat social phobia (Mrakotsky et al, 2008) and to improve cognitive functions (Arnsten et al, 1996;Franowicz and Arnsten, 1999;Franowicz et al, 2002). Guanfacine, an a2A-AR selective agonist, has been used to treat patients suffering from attention deficit hyperactivity disorder (ADHD) (Sallee et al, 2009) and other psychiatric disorders (Faraone and Glatt, 2010;Sallee and Eaton, 2010;Taylor and Russo, 2001).…”

“…Herein, other aspects of the pharmacovigilance can be also illustrated, for instance a recent study has indicated that for agents targeting a2-AR, age-related differences in clinical response exist (Sanders et al, 2011). These new elements are consistent with the previous related data which show that tricyclic antidepressants, monoamine oxidase inhibitors, and venlafaxine, that exert their clinical effects mainly by acting on the noradrenergic system (Subhash et al, 2003), may not be recommended to treat depression in children, whereas they are efficacious in adults (Geller et al, 1999;Hazell et al, 2000;Varley, 2003;Whittington et al, 2004).…”