Brianna Goldenstein

Long-term α1A-adrenergic receptor stimulation improves synaptic plasticity, cognitive function, mood, and longevity.

The role of α1-adrenergic receptors (α1ARs) in cognition and mood is controversial, probably as a result of past use of nonselective agents. α1AAR activation was recently shown to increase neurogenesis, which is linked to cognition and mood. We studied the effects of long-term α1AAR stimulation using transgenic mice engineered to express a constitutively active mutant (CAM) form of the α1AAR. CAM-α1AAR mice showed enhancements in several behavioral models of learning and memory. In contrast, mice that have the α1AAR gene knocked out displayed poor cognitive function. Hippocampal brain slices from CAM-α1AAR mice demonstrated increased basal synaptic transmission, paired-pulse facilitation, and long-term potentiation compared with wild-type (WT) mice. WT mice treated with the α1AAR-selective agonist cirazoline also showed enhanced cognitive functions. In addition, CAM-α1AAR mice exhibited antidepressant and less anxious phenotypes in several behavioral tests compared with WT mice. Furthermore, the lifespan of CAM-α1AAR mice was 10% longer than that of WT mice. Our results suggest that long-term α1AAR stimulation improves synaptic plasticity, cognitive function, mood, and longevity. This may afford a potential therapeutic target for counteracting the decline in cognitive function and mood associated with aging and neurological disorders.

Regulator of G Protein Signaling Protein Suppression of Gαo Protein-Mediated α2A Adrenergic Receptor Inhibition of Mouse Hippocampal CA3 Epileptiform Activity

Activation of G protein-coupled α2 adrenergic receptors (ARs) inhibits epileptiform activity in the hippocampal CA3 region. The specific mechanism underlying this action is unclear. This study investigated which subtype(s) of α2ARs and G proteins (Gαo or Gαi) are involved in this response using recordings of mouse hippocampal CA3 epileptiform bursts. Application of epinephrine (EPI) or norepinephrine (NE) reduced the frequency of bursts in a concentration-dependent manner: (-)EPI > (-)NE >>> (+)NE. To identify the α2AR subtype involved, equilibrium dissociation constants (pKb) were determined for the selective αAR antagonists atipamezole (8.79), rauwolscine (7.75), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101; 6.87), and prazosin (5.71). Calculated pKb values correlated best with affinities determined previously for the mouse α2AAR subtype (r = 0.98, slope = 1.07). Furthermore, the inhibitory effects of EPI were lost in hippocampal slices from α2AAR-but not α2CAR-knockout mice. Pretreatment with pertussis toxin also reduced the EPI-mediated inhibition of epileptiform bursts. Finally, using knock-in mice with point mutations that disrupt regulator of G protein signaling (RGS) binding to Gα subunits to enhance signaling by that G protein, the EPI-mediated inhibition of bursts was significantly more potent in slices from RGS-insensitive GαoG184S heterozygous (Gαo+/GS) mice compared with either Gαi2G184S heterozygous (Gαi2+/GS) or control mice (EC50 = 2.5 versus 19 and 23 nM, respectively). Together, these findings indicate that the inhibitory effect of EPI on hippocampal CA3 epileptiform activity uses an α2AAR/Gαo protein-mediated pathway under strong inhibitory control by RGS proteins. This suggests a possible role for RGS inhibitors or selective α2AAR agonists as a novel antiepileptic drug therapy.

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.