The astrocyte α1A-adrenoreceptor is a key component of the neuromodulatory system in mouse visual cortex.

Noradrenaline (norepinephrine) is known to modulate many physiological functions and behaviors. In this study, we tested to what extent astrocytes, a type of glial cell, participate in noradrenergic signaling in mouse primary visual cortex (V1). Astrocytes are essential partners of neurons in the central nervous system. They are central to brain homeostasis, but also dynamically regulate neuronal activity, notably by relaying and regulating neuromodulator signaling. Indeed, astrocytes express receptors for multiple neuromodulators, including noradrenaline, but the extent to which astrocytes are involved in noradrenergic signaling remains unclear. To test whether astrocytes are involved in noradrenergic neuromodulation in mice, we employed both short hairpin RNA mediated knockdown as well as pharmacological manipulation of the major noradrenaline receptor in astrocytes, the α1A-adrenoreceptor. Using acute brain slices, we found that the astrocytic α1A-adrenoreceptor subtype contributes to the generation of large intracellular Ca2+ signals in visual cortex astrocytes, which are generally thought to underlie astrocyte function. To test if reduced α1A-adrenoreceptor signaling in astrocytes affected the function of neuronal circuits in V1, we used both patch-clamp and field potential recordings. These revealed that noradrenergic signaling through the astrocyte α1A-adrenoreceptor is important to not only modulate synaptic activity but also to regulate plasticity in V1, through the potentiation of synaptic responses in circuits involved in visual information processing.

Locus coeruleus ablation in mice: protocol optimization, stereology and behavioral impact.

The Locus Coeruleus (LC) is in the brainstem and supplies key brain structures with noradrenaline, including the forebrain and hippocampus. The LC impacts specific behaviors such as anxiety, fear, and motivation, as well as physiological phenomena that impact brain functions in general, including sleep, blood flow regulation, and capillary permeability. Nevertheless, the short- and long-term consequences of LC dysfunction remain unclear. The LC is among the brain structures first affected in patients suffering from neurodegenerative diseases such as Parkinson's disease and Alzheimer's Disease, hinting that LC dysfunction may play a central role in disease development and progression. Animal models with modified or disrupted LC function are essential to further our understanding of LC function in the normal brain, the consequences of LC dysfunction, and its putative roles in disease development. For this, well-characterized animal models of LC dysfunction are needed. Here, we establish the optimal dose of selective neurotoxin N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (DSP-4) for LC ablation. Using histology and stereology, we compare LC volume and neuron number in LC ablated (LCA) mice and controls to assess the efficacy of LC ablation with different numbers of DSP-4 injections. All LCA groups show a consistent decrease in LC cell count and LC volume. We then proceed to characterize the behavior of LCA mice using a light-dark box test, Barnes maze test, and non-invasive sleep-wakefulness monitoring. Behaviorally, LCA mice differ subtly from control mice, with LCA mice generally being more curious and less anxious compared to controls consistent with known LC function and projections. We note an interesting contrast in that control mice have varying LC size and neuron count but consistent behavior whereas LCA mice (as expected) have consistently sized LC but erratic behavior. Our study provides a thorough characterization of an LC ablation model, firmly consolidating it as a valid model system for the study of LC dysfunction.

Inhibition of rat locus coeruleus neurons by prostaglandin E2 EP3 receptors: pharmacological characterization ex vivo.

Prostaglandin E2 (PGE2) is an inflammatory mediator synthesized by the brain constitutive cyclooxygenase enzyme. PGE2 binds to G protein-coupled EP1-4 receptors (EP1 to Gq, EP2,4 to Gs, and EP3 to Gi/o). EP2, EP3 and EP4 receptors are expressed in the locus coeruleus (LC), the main noradrenergic nucleus in the brain. EP3 receptors have been explored in the central nervous system, although its role regulating the locus coeruleus neuron activity has not been pharmacologically defined. Our aim was to characterize the function of EP3 receptors in neurons of the LC. Thus, we studied the effect of EP3 receptor agonists on the firing activity of LC cells in rat brain slices by single-unit extracellular electrophysiological techniques. The EP3 receptor agonist sulprostone (0.15 nM-1.28 µM), PGE2 (0.31 nM-10.2 µM) and the PGE1 analogue misoprostol (0.31 nM-2.56 µM) inhibited the firing rate of LC neurons in a concentration-dependent manner (EC50 = 15 nM, 110 nM, and 51 nM, respectively). The EP3 receptor antagonist L-798,106 (3-10 µM), but not the EP2 (PF-04418948, 3-10 µM) or EP4 (L-161,982, 3-10 µM) receptor antagonists, caused rightward shifts in the concentration-effect curves for the EP3 receptor agonists. Sulprostone-induced effect was attenuated by the Gi/o protein blocker pertussis toxin (pertussis toxin, 500 ng ml-1) and the inhibitors of inwardly rectifying potassium channels (GIRK) BaCl2 (300 µM) and SCH-23390 (15 µM). In conclusion, LC neuron firing activity is regulated by EP3 receptors, presumably by an inhibitory Gi/o protein- and GIRK-mediated mechanism.

Chronic ethanol alters adrenergic receptor gene expression and produces cognitive deficits in male mice.

Hyperkateifia and stress-induced alcohol cravings drive relapse in individuals with alcohol use disorder (AUD). The brain stress signal norepinephrine (also known as noradrenaline) tightly controls cognitive and affective behavior and was thought to be broadly dysregulated with AUD. The locus coeruleus (LC) is a major source of forebrain norepinephrine, and it was recently discovered that the LC sends distinct projections to addiction-associated regions suggesting that alcohol-induced noradrenergic changes may be more brain region-specific than originally thought. Here we investigated whether ethanol dependence alters adrenergic receptor gene expression in the medial prefrontal cortex (mPFC) and central amgydala (CeA), as these regions mediate the cognitive impairment and negative affective state of ethanol withdrawal. We exposed male C57BL/6J mice to the chronic intermittent ethanol vapor-2 bottle choice paradigm (CIE-2BC) to induce ethanol dependence, and assessed reference memory, anxiety-like behavior and adrenergic receptor transcript levels during 3-6 days of withdrawal. Dependence bidirectionally altered mouse brain α1 and ß receptor mRNA levels, potentially leading to reduced mPFC adrenergic signaling and enhanced noradrenergic influence over the CeA. These brain region-specific gene expression changes were accompanied by long-term retention deficits and a shift in search strategy in a modified Barnes maze task, as well as greater spontaneous digging behavior and hyponeophagia. Current clinical studies are evaluating adrenergic compounds as a treatment for AUD-associated hyperkatefia, and our findings can contribute to the refinement of these therapies by increasing understanding of the specific neural systems and symptoms that may be targeted.

Effects of Methylphenidate on the Dopamine Transporter and Beyond.

The dopamine transporter (DAT) is the main target of methylphenidate (MPH), which remains the number one drug prescribed worldwide for the treatment of Attention-Deficit Hyperactivity Disorder (ADHD). In addition, abnormalities of the DAT have been widely associated with ADHD. Based on clinical and preclinical studies, the direction of DAT abnormalities in ADHD are, however, still unclear. Moreover, chronic treatment of MPH has been shown to increase brain DAT expression in both animals and ADHD patients, suggesting that findings of overexpressed levels of DAT in ADHD patients are possibly attributable to the effects of long-term MPH treatment rather than the pathology of the condition itself. In this chapter, we will discuss some of the effects exerted by MPH, which are related to its actions on catecholamine protein targets and brain metabolites, together with genes and proteins mediating neuronal plasticity. For this purpose, we present data from biochemical, proton nuclear magnetic resonance spectroscopy (1H-NMR) and gene/protein expression studies. Overall, results of the studies discussed in this chapter show that MPH has a complex biological/pharmacological action well beyond the DAT.

Locus Coeruleus Norepinephrine in Learned Behavior: Anatomical Modularity and Spatiotemporal Integration in Targets.

The locus coeruleus (LC), a small brainstem nucleus, is the primary source of the neuromodulator norepinephrine (NE) in the brain. The LC receives input from widespread brain regions, and projects throughout the forebrain, brainstem, cerebellum, and spinal cord. LC neurons release NE to control arousal, but also in the context of a variety of sensory-motor and behavioral functions. Despite its brain-wide effects, much about the role of LC-NE in behavior and the circuits controlling LC activity is unknown. New evidence suggests that the modular input-output organization of the LC could enable transient, task-specific modulation of distinct brain regions. Future work must further assess whether this spatial modularity coincides with functional differences in LC-NE subpopulations acting at specific times, and how such spatiotemporal specificity might influence learned behaviors. Here, we summarize the state of the field and present new ideas on the role of LC-NE in learned behaviors.

Noradrenergic Hypothesis Linking Neurodegeneration-Based Cognitive Decline and Astroglia.

In the past, manipulation of the cholinergic system was seen as the most likely therapeutic for neurodegeneration-based cognitive decline in Alzheimer's disease (AD) (Whitehouse et al., 1982). However, targeting the noradrenergic system also seems a promising strategy, since more recent studies revealed that in post-mortem tissue from patients with AD and other neurodegenerative disorders there is a robust correlation between cognitive decline and loss of neurons from the Locus coeruleus (LC), a system with diffuse noradrenaline (NA) innervation in the central nervous system (CNS). Therefore, the hypothesis has been considered that increasing NA signaling in the CNS will prevent, or at least halt the progression of neurodegeneration and cognitive decline. A hallmark of the age- and neurodegeneration-related cognitive decline is reduced neurogenesis. We here discuss noradrenergic dysfunction in AD-related cognitive decline in humans and its potential involvement in AD pathology and disease progression. We also focus on animal models to allow the validation of the noradrenergic hypothesis of AD, including those based upon the immunotoxin-mediated ablation of LC based on saporin, a protein synthesis interfering agent, which offers selective and graded demise of LC neurons, Finally, we address how astrocytes, an abundant and functionally heterogeneous cell type of neuroglia maintaining homeostasis, may participate in the regulation of neurogenesis, a new strategy for preventing LC neuron loss.