Chlorotoxin-mediated disinhibition of noradrenergic locus coeruleus neurons using a conditional transgenic approach

Multiple regulatory actions of 2-guanidine-4-methylquinazoline (GMQ), an agonist of acid-sensing ion channel type 3, on ionic currents in pituitary GH3 cells and in olfactory sensory (Rolf B1.T) neurons

GMQ (2-guanidine-4-methylquinazoline or N-(4-methyl-2-quinazolinyl)-guanidine hydrochloride), an agonist of acid-sensing ion channel type 3, has been increasingly used for in vivo studies of alternations in nociceptic behavior. In this study, we tried to investigate whether GMQ has any possible effect on other types of ion channels. Addition of GMQ to pituitary GH3 cells raised the amplitude of Ca²⁺-activated K⁺ currents (I_K(Ca)), which was reversed by verruculogen or PF1022A, but not by TRAM-39. Under inside-out current recordings, addition of GMQ into bath enhanced the probability of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with an EC₅₀ value of 0.95 µM. The activation curve of BK_Ca channels during exposure to GMQ shifted to a lower depolarized potential, with no change in the gating charge of the curve; however, there was a reduction of free energy for channel activation in its presence. As cells were exposed to GMQ, the amplitude of ion currents were suppressed, including delayed rectifying K⁺ current, voltage-gated Na⁺ and L-type Ca²⁺ currents. In Rolf B1.T olfactory sensory neuron, addition of GMQ was able to induce inward current and to suppress peak I_Na. Taken together, findings from these results indicated that in addition to the activation of ASIC3 channels, this compound might directly produce additional actions on various types of ion channels. Caution should be taken in the interpretation of in vivo experimental results when GMQ or other structurally similar compounds are used as targets to characterize the potential functions of ASIC3 channels.

A Molecular Window on Sleep: Changes in Gene Expression between Sleep and Wakefulness

Sleep is thought to be “by the brain and for the brain,” but despite decades of behavioral and neurophysiologic research, we still do not know why the brain actually needs to sleep. Recently, gene expression studies have allowed researchers to investigate the molecular correlates of sleep and wakefulness and to gain new insights into the benefits that sleep may bring at the cellular level. In the latest series of studies, a genome-wide screening of brain gene expression was performed in rats that had been asleep, spontaneously awake, or sleep deprived for 8 hours. It was found that of ~15,000 transcripts expressed in the cerebral cortex, about 5% change their expression levels depending on behavioral state but independently of time of day. Half of the modulated genes increase in wakefulness and half in sleep. Moreover, wakefulness-related and sleep-related transcripts belong to different functional categories. Waking-related transcripts are involved in energy metabolism, excitatory neurotransmission, transcriptional activation, synaptic potentiation and memory acquisition, and the response to cellular stress. Sleep-related transcripts are involved in brain protein synthesis, synaptic consolidation/depression, and membrane trafficking and maintenance, including cholesterol metabolism, myelin formation, and synaptic vesicle turnover.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Retrograde viral vector-mediated inhibition of pontospinal noradrenergic neurons causes hyperalgesia in rats

Pontospinal noradrenergic neurons form a component of an endogenous analgesic system and represent a potential therapeutic target. We tested the principle that genetic manipulation of their excitability can alter nociception using an adenoviral vector (AVV-PRS-hKir(2.1)) containing a catecholaminergic-selective promoter (PRS) to retrogradely transduce and inhibit the noradrenergic neurons projecting to the lumbar dorsal horn through the expression of a potassium channel (hKir(2.1)). Expression of hKir(2.1) in catecholaminergic PC12 cells hyperpolarized the membrane potential and produced a barium-sensitive inward rectification. LC neurons transduced by AVV-PRS-hKir(2.1) in slice cultures also showed barium-sensitive inward rectification and reduced spontaneous firing rate (median 0.2 Hz; n = 19 vs control 1.0 Hz; n = 18, p < 0.05). Pontospinal noradrenergic neurons were retrogradely transduced in vivo by injection of AVV into the lumbar dorsal horn (L4-5). Rats transduced with AVV-PRS-hKir(2.1) showed thermal but not mechanical hyperalgesia. Similar selective augmentation of thermal hyperalgesia was seen in the CFA-inflammatory pain model after AVV-PRS-hKir(2.1). In the formalin test, rats transduced with hKir(2.1) showed enhanced nocifensive behaviors (both Phase I and II, p < 0.05, n = 11/group) and increased c-Fos-positive cells in the lumbar dorsal horn. Transduction with AVV-PRS-hKir(2.1) before spared nerve injury produced no change in tactile or cold allodynia. Thus, the selective genetic inhibition of approximately 150 pontospinal noradrenergic neurons produces a modality-specific thermal hyperalgesia, increased nocifensive behaviors, and spinal c-Fos expression in the formalin test, but not in the spared nerve injury model of neuropathic pain, indicating that these neurons exert a selective tonic restraining influence on in vivo nociception.

Locus ceruleus control of slow-wave homeostasis

Sleep intensity is regulated by the duration of previous wakefulness, suggesting that waking results in the progressive accumulation of sleep need (Borbely and Achermann, 2000). In mammals, sleep intensity is reflected by slow-wave activity (SWA) in the nonrapid eye movement (NREM) sleep electroencephalogram, which increases in proportion to the time spent awake. However, the mechanisms responsible for the increase of NREM SWA after wakefulness remain unclear. According to a recent hypothesis (Tononi and Cirelli, 2003), the increase in SWA occurs because during wakefulness, many cortical circuits undergo synaptic potentiation, as evidenced by the widespread induction of long-term potentiation (LTP)-related genes in the brain of awake animals. A direct prediction of this hypothesis is that manipulations interfering with the induction of LTP-related genes should result in a blunted SWA response. Here, we examined SWA response in rats in which cortical norepinephrine (NA) was depleted, a manipulation that greatly reduces the induction of LTP-related genes during wakefulness (Cirelli and Tononi, 2004). We found that the homeostatic response of the lower-range SWA was markedly and specifically reduced after NA depletion. These data suggest that the wake-dependent accumulation of sleep need is causally related to cellular changes dependent on NA release, such as the induction of LTP-related genes, and support the hypothesis that sleep SWA homeostasis may be related to synaptic potentiation during wakefulness.

The neuro-biomolecular basis of alertness in sleep disorders

Sleep is present in all species in which it has been studied, but its functions remain unknown. Identification of the molecular correlates of sleep and wakefulness is essential if we are to understand the restorative processes that occur during sleep, the cellular mechanisms that underlie sleep regulation, and the functional consequences of sleep loss and poor quality sleep. To address the questions of how we know whether sleep has performed its functions and whether treatment has improved sleep quality, we have proposed a synaptic homeostasis hypothesis about the significance of slow wave activity (SWA) during sleep and its homeostatic regulation. Briefly, the hypothesis states that (1) wakefulness is associated with potentiation in several cortical circuits; (2) synaptic potentiation is then tied to the homeostatic regulation of SWA; (3) SWA is associated with synaptic downscaling; and (4) synaptic downscaling is tied to the beneficial effects of sleep on performance. According to this hypothesis, the potentiation of neural circuits that results from synaptic plasticity during alert wakefulness is responsible for SWA homeostasis. Increasing noradrenergic activity increases the expression of long-term potentiation (LTP)-related genes, and interference with these changes block the induction of markers of synaptic potentiation during alert wakefulness. Inducing local LTP-like changes during alert wakefulness also results in increased local slow wave homeostasis. Thus, as SWA homeostasis can be induced on a local level or can be triggered by a learning task, and is strongly correlated with postsleep performance enhancement, plasticity during alert wakefulness depends on good sleep, which, in turn, depends on efficient synaptic downscaling.