Chris Jurgens

α1-Adrenergic Receptors Regulate Neurogenesis and Gliogenesis

The understanding of the function of α1-adrenergic receptors in the brain has been limited due to a lack of specific ligands and antibodies. We circumvented this problem by using transgenic mice engineered to overexpress either wild-type receptor tagged with enhanced green fluorescent protein or constitutively active mutant α1-adrenergic receptor subtypes in tissues in which they are normally expressed. We identified intriguing α1A-adrenergic receptor subtype-expressing cells with a migratory morphology in the adult subventricular zone that coexpressed markers of neural stem cell and/or progenitors. Incorporation of 5-bromo-2-deoxyuridine in vivo increased in neurogenic areas in adult α1A-adrenergic receptor transgenic mice or normal mice given the α1A-adrenergic receptor-selective agonist, cirazoline. Neonatal neurospheres isolated from normal mice expressed a mixture of α1-adrenergic receptor subtypes, and stimulation of these receptors resulted in increased expression of the α1B-adrenergic receptor subtype, proneural basic helix-loop-helix transcription factors, and the differentiation and migration of neuronal progenitors for catecholaminergic neurons and interneurons. α1-Adrenergic receptor stimulation increased the apoptosis of astrocytes and regulated survival of neonatal neurons through phosphatidylinositol 3-kinase signaling. However, in adult normal neurospheres, α1-adrenergic receptor stimulation increased the expression of glial markers at the expense of neuronal differentiation. In vivo, S100-positive glial and βIII tubulin neuronal progenitors colocalized with either α1-adrenergic receptor subtype in the olfactory bulb. Our results indicate that α1-adrenergic receptors can regulate both neurogenesis and gliogenesis that may be developmentally dependent. Our findings may lead to new therapies to treat neurodegenerative diseases.

α2A Adrenergic Receptor Activation Inhibits Epileptiform Activity in the Rat Hippocampal CA3 Region

Norepinephrine has potent antiepileptic properties, the pharmacology of which is unclear. Under conditions in which GABAergic inhibition is blocked, norepinephrine reduces hippocampal cornu ammonis 3 (CA3) epileptiform activity through α2 adrenergic receptor (AR) activation on pyramidal cells. In this study, we investigated which α2AR subtype(s) mediates this effect. First, α2AR genomic expression patterns of 25 rat CA3 pyramidal cells were determined using real-time single-cell reverse transcription-polymerase chain reaction, demonstrating that 12 cells expressed α2AAR transcript; 3 of the 12 cells additionally expressed mRNA for α2CAR subtype and no cells possessing α2BAR mRNA. Hippocampal CA3 epileptiform activity was then examined using field potential recordings in brain slices. The selective αAR agonist 6-fluoronorepinephrine caused a reduction of CA3 epileptiform activity, as measured by decreased frequency of spontaneous epileptiform bursts. In the presence of βAR blockade, concentration-response curves for AR agonists suggest that an α2AR mediates this response, as the rank order of potency was 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14304) ≥ epinephrine >6-fluoronorepinephrine > norepinephrine ⋙ phenylephrine. Finally, equilibrium dissociation constants (Kb) of selective αAR antagonists were functionally determined to confirm the specific α2AR subtype inhibiting CA3 epileptiform activity. Apparent Kb values calculated for atipamezole (1.7 nM), MK-912 (4.8 nM), BRL-44408 (15 nM), yohimbine (63 nM), ARC-239 (540 nM), prazosin (4900 nM), and terazosin (5000 nM) correlated best with affinities previously determined for the α2AAR subtype (r = 0.99, slope = 1.0). These results suggest that, under conditions of impaired GABAergic inhibition, activation of α2AARs is primarily responsible for the antiepileptic actions of norepinephrine in the rat hippocampal CA3 region.

Adrenergic receptor modulation of hippocampal CA3 network activity

Norepinephrine (NE) has demonstrated proconvulsant and antiepileptic properties; however, the specific pharmacology of these actions has not been clearly established. To address this, we studied the effect of NE on hippocampal CA3 epileptiform activity. Frequency changes of burst discharges in response to NE were biphasic; low concentrations increased the number of bursts, while higher concentrations reduced their frequency, suggesting the involvement of multiple adrenergic receptor (AR) types. This hypothesis was confirmed when, in the presence of betaAR blockade, increasing concentrations of NE caused a monophasic decrease in epileptiform activity. Antagonists selective for alpha1 or alpha2ARs were then used to determine which alphaAR type was involved. While discriminating concentrations of the alpha1AR antagonists prazosin and terazosin had no effect, selective amounts of the alpha2AR antagonists RS79948 and RX821002 significantly reduced the potency of NE in decreasing epileptiform activity. Furthermore, this antiepileptic action of NE persisted when all GABA-mediated inhibition was blocked. This data suggests that, under conditions of impaired GABAergic inhibition, the excitatory and inhibitory effects of NE on hippocampal CA3 epileptiform activity are mediated primarily via beta and alpha2ARs, respectively. Moreover, our results imply that the antiepileptic effect of alpha2AR activation in CA3 is not dependent on the GABAergic system.