Elisa Mura

Awakenings and awareness recovery in disorders of consciousness: is there a role for drugs?

Disorders of consciousness (DOC) include coma, vegetative state (VS) and minimally conscious state (MCS). Coma is a condition of unarousability with a complete absence of wakefulness and awareness, whereas VS is characterized by a lack of awareness despite a preserved wakefulness. Patients in coma are unconscious because they lack both wakefulness and awareness. Patients in a VS are unconscious because, although they are wakeful, they lack awareness. Patients in a MCS show minimal but definite behavioural evidence of self and environmental awareness.Coma results from diffuse bilateral hemispheric lesions or selective damage to the ascending reticular system (which is functionally connected to the cerebral cortex by intralaminar thalamic nuclei). VS is a syndrome that is considered to be the result of a disconnection of different cortical networks rather than a dysfunction of a single area or a global reduction in cortical metabolism. As revealed by functional imaging studies, clinical recovery is often associated with a functional restoration of cortico-thalamo-cortical connections. Depending on the amount of network restored, patients may regain full consciousness or remain in a MCS. Molecular and neural mediators may indirectly contribute to the above restoration processes owing to their role in the phenomenon of neural synaptic plasticity. Therefore, there is growing interest in the possible effects of drugs that act at the level of the CNS in promoting emergence from DOC.Sporadic cases of dramatic recovery from DOC after the administration of various pharmacological agents, such as baclofen, zolpidem and amantadine, have been recently supported by intriguing scientific observations. Analysis of the reported cases of recovery, with particular attention paid to the condition of the patients and to the association of their improvement with the start of drug administration, suggests that these treatments might have promoted the clinical improvement of some patients. These drugs are from various and diverging classes, but can be grouped into two main categories, CNS stimulants and CNS depressants. Some of these treatments seem to directly encourage a consciousness restoration, while others play a more determinant role in improving cognitive domains, especially in patients with residual cognitive impairment, than in the field of consciousness.Given the great interest recently generated in the scientific community by the increasing number of papers addressing this issue, further investigation of the above treatments, with particular attention paid to their mechanisms of action, the neurotransmitters involved and their effects on cortico-thalamocortical circuitry, is needed.

Consequences of the 118A>G polymorphism in the OPRM1 gene: translation from bench to bedside?

The 118A>G single nucleotide polymorphism (SNP) in the μ-opioid receptor (OPRM1) gene has been the most described variant in pharmacogenetic studies regarding opioid drugs. Despite evidence for an altered biological function encoded by this variant, this knowledge is not yet utilized clinically. The aim of the present review was to collect and discuss the available information on the 118A>G SNP in the OPRM1 gene, at the molecular level and in its clinical manifestations. In vitro biochemical and molecular assays have shown that the variant receptor has higher binding affinity for β-endorphins, that it has altered signal transduction cascade, and that it has a lower expression compared with wild-type OPRM1. Studies using animal models for 118A>G have revealed a double effect of the variant receptor, with an apparent gain of function with respect to the response to endogenous opioids but a loss of function with exogenous administered opioid drugs. Although patients with this variant have shown a lower pain threshold and a higher drug consumption in order to achieve the analgesic effect, clinical experiences have demonstrated that patients carrying the variant allele are not affected by the increased opioid consumption in terms of side effects.

Dual effect of beta-amyloid on α7 and α4β2 nicotinic receptors controlling the release of glutamate, aspartate and GABA in rat hippocampus.

Background We previously showed that beta-amyloid (Aβ), a peptide considered as relevant to Alzheimers Disease, is able to act as a neuromodulator affecting neurotransmitter release in absence of evident sign of neurotoxicity in two different rat brain areas. In this paper we focused on the hippocampus, a brain area which is sensitive to Alzheimers Disease pathology, evaluating the effect of Aβ (at different concentrations) on the neurotransmitter release stimulated by the activation of pre-synaptic cholinergic nicotinic receptors (nAChRs, α4β2 and α7 subtypes). Particularly, we focused on some neurotransmitters that are usually involved in learning and memory: glutamate, aspartate and GABA. Methodology/Findings We used a dual approach: in vivo experiments (microdialysis technique on freely moving rats) in parallel to in vitro experiments (isolated nerve endings derived from rat hippocampus). Both in vivo and in vitro the administration of nicotine stimulated an overflow of aspartate, glutamate and GABA. This effect was greatly inhibited by the highest concentrations of Aβ considered (10 µM in vivo and 100 nM in vitro). In vivo administration of 100 nM Aβ (the lowest concentration considered) potentiated the GABA overflow evoked by nicotine. All these effects were specific for Aβ and for nicotinic secretory stimuli. The in vitro administration of either choline or 5-Iodo-A-85380 dihydrochloride (α7 and α4β2 nAChRs selective agonists, respectively) elicited the hippocampal release of aspartate, glutamate, and GABA. High Aβ concentrations (100 nM) inhibited the overflow of all three neurotransmitters evoked by both choline and 5-Iodo-A-85380 dihydrochloride. On the contrary, low Aβ concentrations (1 nM and 100 pM) selectively acted on α7 subtypes potentiating the choline-induced release of both aspartate and glutamate, but not the one of GABA. Conclusions/Significance The results reinforce the concept that Aβ has relevant neuromodulatory effects, which may span from facilitation to inhibition of stimulated release depending upon the concentration used.

Acute β -Amyloid Administration Disrupts the Cholinergic Control of Dopamine Release in the Nucleus Accumbens

The clinical presentation of Alzheimers disease is characterized by memory deficits but it also involves the impairment of several cognitive functions. Some of these cognitive and executive functions are mediated by limbic areas and are regulated by dopaminergic neurotransmission. Furthermore, literature data suggest that β-amyloid (Aβ) can influence synaptic activity in absence of neurotoxicity and in particular can impair cholinergic modulation of other neurotransmitter actions. In the present study, we evaluated whether small concentrations of Aβ could disrupt cholinergic control of dopamine (DA) release in nucleus accumbens using in vivo (brain dialysis) and in vitro (isolated synaptosomes) models. The cholinergic agonist carbachol (CCh) greatly enhanced DA release from dopaminergic nerve endings in nucleus accumbens both in vivo and in vitro. This effect was mainly exerted on muscarinic receptors because it was inhibited by the muscarinic antagonist atropine and it was unaffected by the nicotinic antagonist mecamylamine. Also the nicotinic agonists epibatidine and nicotine evoked a dopaminergic outflow in nucleus accumbens, which, however, was lower. Aβ 1–40 in absence of neurotoxicity fully inhibited the DA release evoked by CCh and only marginally affected the DA release evoked by epibatidine. The PKC inhibitor GF109203X mimicked the effect of Aβ on DA release and, in turn, Aβ impaired PKC activation by CCh. We can suggest that, in nucleus accumbens, Aβ disrupted in vivo and in vitro cholinergic control of DA release by acting on muscarinic transduction machinery.

β -Amyloid: A Disease Target or a Synaptic Regulator Affecting Age-Related Neurotransmitter Changes?

The amyloid cascade hypothesis sustains that beta-amyloid (Abeta) is the main pathogenetic factor of Alzheimers Disease (AD). Although the direct and indirect neurotoxic role of Abeta are unchallenged, recent findings suggest that the peptide may have so far unforeseen physiological roles. In this regard, the observations showing the ability of Abeta to exert synaptic activities in absence of neurotoxicity are very intriguing. In particular, the peptide is able to affect synaptic transmission of different neurotransmitter systems in key brain areas that regulate executive and cognitive functions, an observation that points Abeta as a new neuromodulator. However, in a pathological context, Abeta may drive functional alterations of several neurotransmitter systems in the early phases of the disease, in turn producing subtle cognitive and behavioural disturbances in addition and before the well known neurodegenerative events. On the other hand, advancing age is the most significant risk factor for the development of AD. In fact, during aging increased Abeta levels have been reported. Moreover, several neurotransmitter systems undergo age-related changes in parallel to a decline of cognitive functions. However, the putative neuromodulatory role of Abeta in the context of aging is nowadays unknown. For these reasons, future studies about the spectrum of action of Abeta (brain areas and neurotransmitter systems affected) are particularly interesting since may suggest new therapeutic targets in order to sustain those functions which may be altered during aging.

Specific Neuromodulatory Actions of Amyloid-β on Dopamine Release in Rat Nucleus Accumbens and Caudate Putamen

We previously demonstrated that amyloid-beta (Abeta) has a neuromodulatory action in the nucleus accumbens (NAc). In this area of the brain, the peptide disrupts the cholinergic control of dopamine (DA) release both in vivo and in vitro. The aim of the present work was to extend the research on the neuromodulatory effect of Abeta (1-40) on DA transmission to different release stimuli and to another dopaminergic brain area, the caudate putamen (CPu), in order to clarify whether the effect of the peptide is stimulus- or brain area-selective. We performed both in vivo (microdialysis associated to HPLC) and in vitro studies (synaptosomes in superfusion). Both in NAc and in CPu and both in vivo and in vitro, Abeta did not affect either basal or potassium-stimulated DA release. In CPu, the Abeta ability to impair the DA release evoked by the cholinergic agonist carbachol, observed in NAc, was confirmed only in vitro. Moreover, in vitro Abeta affected a specific component of the DA overflow evoked by the non-selective metabotropic glutamate receptors agonist t-ACPD. Altogether, these results show that Abeta may have different neuromodulatory actions depending upon the secretory stimulus and, in vivo, the brain area investigated.

Beta Amyloid Differently Modulate Nicotinic and Muscarinic Receptor Subtypes which Stimulate in vitro and in vivo the Release of Glycine in the Rat Hippocampus

Using both in vitro (hippocampal synaptosomes in superfusion) and in vivo (microdialysis) approaches we investigated whether and to what extent β amyloid peptide 1–40 (Aβ 1–40) interferes with the cholinergic modulation of the release of glycine (GLY) in the rat hippocampus. The nicotine-evoked overflow of endogenous GLY in hippocampal synaptosomes in superfusion was significantly inhibited by Aβ 1–40 (10 nM) while increasing the concentration to 100 nM the inhibitory effect did not further increase. Both the Choline (Ch; α7 agonist; 1 mM) and the 5-Iodo-A-85380 dihydrochloride (5IA85380, α4β2 agonist; 10 nM)-evoked GLY overflow were inhibited by Aβ 1–40 at 100 nM but not at 10 nM concentrations. The KCl evoked [3H]GLY and [3H]Acetylcholine (ACh) overflow were strongly inhibited in presence of oxotremorine; however this inhibitory muscarinic effect was not affected by Aβ 1–40. The effects of Aβ 1–40 on the administration of nicotine, veratridine, 5IA85380, and PHA543613 hydrochloride (PHA543613; a selective agonist of α7 subtypes) on hippocampal endogenous GLY release in vivo were also studied. Aβ 1–40 significantly reduced (at 10 μM but not at 1 μM) the nicotine-evoked in vivo release of GLY. Aβ 1–40 (at 10 μM but not at 1 μM) significantly inhibited the PHA543613 (1 mM)-elicited GLY overflow while was ineffective on the GLY overflow evoked by 5IA85380 (1 mM). Aβ 40–1 (10 μM) did not produce any inhibitory effect on nicotine-evoked GLY overflow both in the in vitro and in vivo experiments. Our results indicate that (a) the cholinergic modulation of the release of GLY occurs by the activation of both α7 and α4β2 nicotinic ACh receptors (nAChRs) as well as by the activation of inhibitory muscarinic ACh receptors (mAChRs) and (b) Aβ 1–40 can modulate cholinergic evoked GLY release exclusively through the interaction with α7 and the α4β2 nAChR nicotinic receptors but not through mAChR subtypes.

Dangerous Liaisons between Beta-Amyloid and Cholinergic Neurotransmission

The review examines the multifaceted interactions between cholinergic transmission and beta-amyloid suggesting a continuum in the action of the peptide that at low concentrations (picomolar-low nanomolar) may directly stimulate nicotinic cholinergic receptor while desensitizing them at increasing concentrations (high nanomolar-low micromolar). In addition high beta-amyloid concentrations may reduce the synaptic release of several neurotransmitters, including glutamate, aspartate, GABA, glycine and dopamine, when the release is elicited through cholinergic stimulation but not following depolarization. The effect of beta-amyloid has been observed both in vitro and in vivo in at least three different brain areas (nucleus accumbens, striatum, hippocampus) suggesting that the peptide may exert some general effects even if not all the brain areas have been evaluated. In turn the activation of cholinergic receptors may affect the amyloid precursor protein processing diverting the metabolism toward non-amyloidogenic products. These actions, dissociated from those described in the case of high beta-amyloid concentrations leading to neurotoxic oligomers, may participate to cause dysfunctions in the neurotransmitter activity, in turn leading, at least from a theoretical point of view, to early neuropsychiatric disturbances in the disease. Complexively these observations underscore novel relationships between two main players in Alzheimers disease pathogenesis that are beta-amyloid and cholinergic transmission. Also emerges the inherent difficulty of targeting beta-amyloid in a context in which the peptide exerts several actions beyond neurotoxicity.

Pharmacological Modulation of the State of Awareness in Patients with Disorders of Consciousness: An Overview

The neurobiological approach to consciousness moves from the assumption that all phenomenal experiences are based on neuronal activity in the brain. Consciousness has two main components: wakefulness and awareness. While it may be relatively easy to determine the neuronal correlates of wakefulness, it is not the same for awareness, of which the neural correlates are poorly understood. Knowledge of the circuitry and the neurochemistry of the sleep/wake condition is necessary but not sufficient to understand the circuitry and neurochemistry of consciousness. Disorders of consciousness (DOCs) include coma, vegetative state and minimally conscious state. The study of DOCs and of the electrophysiological changes underlying general anaesthesia-induced loss of consciousness may help in understanding the neuronal correlates of consciousness. In turn, the understanding of the neural bases of consciousness may help in designing interventions aimed at restoring consciousness in DOC patients. Sporadic cases of recovery from a DOC have been reported after the administration of various pharmacological agents (baclofen, zolpidem, amantadine etc.). This review provides an overview of such drugs, which are from various and diverging classes but can be grouped into two main categories: CNS stimulants and CNS depressants. The available data seem to suggest an awakening effect obtained with CNS depressants rather than stimulants, the latter being more effective at improving functional cognitive and behavioral recovery in patients who have spontaneously regained an appreciable level of consciousness. There is a need for more rigorous systematic trials and further investigation of the above treatments, with particular attention paid to their mechanisms of action and the neurotransmitters involved.

Specific inhibitory effect of amyloid-β on presynaptic muscarinic receptor subtypes modulating neurotransmitter release in the rat nucleus accumbens

In this study we investigate on the effect of amyloid-beta1-40 (A beta 1-40) on the oxotremorine (OXO)-induced release of [(3)H] dopamine (DA), [(3)H]GABA and [(3)H]acetylcholine (ACh) from synaptosomes in the rat nucleus accumbens (NAc). OXO in presence of himbacine (HIMBA) was able to increase the basal release of [(3)H]GABA. The OXO-elicited [(3)H]GABA overflow was significantly antagonized by atropine (A; 94%), by the M3 antagonists DAU5884 (96%) and 4-DAMP (70%), and by A beta 1-40 (65%). Exposure of NAc synaptosomes to OXO produced a dose-dependent increase of [(3)H]DA overflow which was antagonized by A, partially inhibited by A beta 1-40 (100 nM) but unaffected by DAU5884 and 4-DAMP. The K(+)-evoked [(3)H]ACh overflow was inhibited by OXO. This effect was counteracted by the M2 antagonist AFDX-116 but not by the selective M4 antagonist mamba toxin 3 (MT3). The K(+)-evoked [(3)H]GABA overflow was also inhibited by OXO but conversely, this effect was counteracted by MT3 and not by AFDX-116. A beta 1-40 (100 nM) did not modify the inhibitory effect of OXO both on the K(+)-evoked [(3)H]ACh and [(3)H]GABA overflow. The results show that in the rat NAc, A beta 1-40 selectively inhibits the function of the muscarinic subtypes which stimulate neurotransmitter release and not those which modulate negatively the stimulated release.