Clément Léna

Acetylcholine receptors containing the β2 subunit are involved in the reinforcing properties of nicotine

Release of the neurotransmitter dopamine in the mesolimbic system of the brain mediates the reinforcing properties of several drugs of abuse, including nicotine. Here we investigate the contribution of the high-affinity neuronal nicotinic acetylcholine receptor to the effects of nicotine on the mesolimbic dopamine system in mice lacking the β2 subunit of this receptor. We found that nicotine stimulates dopamine release in the ventral striatum of wild-type mice but not in the ventral striatum of β2-mutant mice. Using patch-clamp recording, we show that mesencephalic dopaminergic neurons from mice without the β2 subunit no longer respond to nicotine, and that self-administration of nicotine is attenuated in these mutant mice. Our results strongly support the idea that the β2-containing neuronal nicotinic acetylcholine receptor is involved in mediating the reinforcing properties of nicotine.

Acetylcholine receptors containing the β2 subunit are involved inthe reinforcing properties of nicotine

Release of the neurotransmitter dopamine in the mesolimbic system of the brain mediates the reinforcing properties of several drugs of abuse, including nicotine. Here we investigate the contribution of the high-affinity neuronal nicotinic acetylcholine receptor to the effects of nicotine on the mesolimbic dopamine system in mice lacking the β2 subunit of this receptor. We found that nicotine stimulates dopamine release in the ventral striatum of wild-type mice but not in the ventral striatum of β2-mutant mice. Using patch-clamp recording, we show that mesencephalic dopaminergic neurons from mice without the β2 subunit no longer respond to nicotine, and that self-administration of nicotine is attenuated in these mutant mice. Our results strongly support the idea that the β2-containing neuronal nicotinic acetylcholine receptor is involved in mediating the reinforcing properties of nicotine.

Reduced antinociception in mice lacking neuronal nicotinic receptor subunits.

Nicotine exerts antinociceptive effects by interacting with one or more of the subtypes of nicotinic acetylcholine receptors (nAChRs) that are present throughout the neuronal pathways that respond to pain. To identify the particular subunits involved in this process, we generated mice lacking the α4 subunit of the neuronal nAChR by homologous recombination techniques and studied these together with previously generated mutant mice lacking the β2 nAChR subunit. Here we show that the homozygous α4−/− mice no longer express high-affinity [3H]nicotine and [3H]epibatidine binding sites throughout the brain. In addition, both types of mutant mice display a reduced antinociceptive effect of nicotine on the hot-plate test and diminished sensitivity to nicotine in the tail-flick test. Patch-clamp recordings further reveal that raphe magnus and thalamic neurons no longer respond to nicotine. The α4 nAChR subunit, possibly associated with the β2 nAChR subunit, is therefore crucial for nicotine-elicited antinociception.

Subunit Composition of Functional Nicotinic Receptors in Dopaminergic Neurons Investigated with Knock-Out Mice

Nicotinic acetylcholine receptors (nAChRs) expressed by dopaminergic (DA) neurons have long been considered as potential therapeutic targets for the treatment of several neuropsychiatric diseases, including nicotine and cocaine addiction or Parkinsons disease. However, DA neurons express mRNAs coding for most, if not all, neuronal nAChR subunits, and the subunit composition of functional nAChRs has been difficult to establish. Immunoprecipitation experiments performed on mouse striatal extracts allowed us to identify three main types of heteromeric nAChRs (α4β2*, α6β2*, and α4α6β2*) in DA terminal fields. The functional relevance of these subtypes was then examined by studying nicotine-induced DA release in striatal synaptosomes and recording ACh-elicited currents in DA neurons fromα4, α6, α4α6, and β2 knock-out mice. Our results establish that α6β2* nAChRs are functional and sensitive to α-conotoxin MII inhibition. These receptors are mainly located on DA terminals and consistently do not contribute to DA release induced by systemic nicotine administration, as evidenced by in vivo microdialysis. In contrast, (nonα6)α4β2* nAChRs represent the majority of functional heteromeric nAChRs on DA neuronal soma. Thus, whereas a combination of α6β2* and α4β2* nAChRs may mediate the endogenous cholinergic modulation of DA release at the terminal level, somato-dendritic (nonα6)α4β2* nAChRs most likely contribute to nicotine reinforcement.

Potentiation of nicotinic receptor response by external calcium in rat central neurons

Nicotinic acetylcholine receptor (nAChR) responses of rat medial habenular neurons are potentiated up to 3.5-fold by increasing the concentration of external Ca2+ in the millimolar range. This effect, independent of voltage, is probably due to the binding of Ca2+ to an external site. External Ca2+ decreases nAChR single-channel conductance at negative but not positive potentials, and it markedly enhances the frequency of opening of acetylcholine activated channels. The potentiating effect of Ca2+ is mimicked by Ba2+ and Sr2+, but barely by Mg2+. These data support the existence of positively acting allosteric sites for Ca2+, distinct from those involved in the decrease of single-channel conductance. A model in which external Ca2+ changes the properties of activation of the nAChR appears consistent with these data.

Coherent amygdalocortical theta promotes fear memory consolidation during paradoxical sleep

Brain activity in sleep plays a crucial role in memory consolidation, an offline process that determines the long-term strength of memory traces. Consolidation efficacy differs across individuals, but the brain activity dynamics underlying these differences remain unknown. Here, we studied how interindividual variability in fear memory consolidation relates to neural activity in brain structures that participate in Pavlovian fear learning. From the end of training to testing 24 h later, some rats showed increased and others decreased conditioned fear responses. We found that overnight bidirectional changes in fear memory were selectively correlated with modifications in theta coherence between the amygdala, medial prefrontal cortex, and hippocampus during paradoxical sleep. Thus, our results suggest that theta coordination in the limbic system may influence interindividual differences in memory consolidation of aversive experiences.

Allosteric modulations of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor behaves as an allosteric protein with multiple, interconvertible conformations: a resting state, an open channel state and several desensitized states. A variety of pharmacological agents and physiological ligands regulate the transitions between these states when they bind to sites topographically distinct from the acetylcholine binding site. The physiological significance of this type of regulation is discussed and its potential role in the modulation of synaptic efficacy suggested.

Lasting Syndrome of Depression Produced by Reduction in Serotonin Uptake during Postnatal Development: Evidence from Sleep, Stress, and Behavior

Dysfunction of the serotonin system is implicated in sleep and emotional disorders. To test whether these impairments could arise during development, we studied the impact of early-life, transient versus genetic, permanent alterations of serotonin reuptake on sleep–wakefulness patterns, depression-related behavior, and associated physiological features. Here, we show that female mice treated neonatally with a highly selective serotonin reuptake inhibitor, escitalopram, exhibited signs of depression in the form of sleep anomalies, anhedonia, increased helplessness reversed by chronic antidepressant treatment, enhanced response to acute stress, and increased serotoninergic autoinhibitory feedback. This syndrome was not reproduced by treatment in naive adults but resembled the phenotype of mutant mice lacking the serotonin transporter, except that these exhibited decreased serotonin autoreceptor sensitivity and additional anxiety-like behavior. Thus, alteration of serotonin reuptake during development, whether induced by external or genetic factors, causes a depressive syndrome lasting into adulthood. Such early-life impairments might predispose individuals to sleep and/or mood disorders.

Contribution of 5-HT2 Receptor Subtypes to Sleep–Wakefulness and Respiratory Control, and Functional Adaptations in Knock-Out Mice Lacking 5-HT2A Receptors

Serotonin (5-hydroxytryptamine; 5-HT) plays key roles in sleep–wakefulness regulation. Evidence indicates that 5-HT2 receptors are involved mainly in non-rapid eye movement sleep (NREMS) regulation and respiratory control. Here, we investigated the relative contribution of 5-HT2A, 5-HT2B, and 5-HT2C receptor subtypes to NREMS and breathing during sleep, using 5-HT2 subtype-selective ligands in wild-type (5-HT2A+/+) and knock-out (5-HT2A–/–) mice that do not express 5-HT2A receptors. Acute blockade of 5-HT2A receptors induced an increase in NREMS in 5-HT2A+/+ mice, but not 5-HT2A–/– mutants, which spontaneously expressed less NREMS than wild-type animals. In 5-HT2A+/+ mice, 5-HT2B receptor blockade produced a reduction of NREMS, whereas receptor activation induced an increase in this sleep stage. These effects were less pronounced in 5-HT2A–/– mice, indicating a lower sensitivity of 5-HT2B receptors in mutants, with no change in 5-HT2B mRNA. Blockade of 5-HT2C receptors had no effect on NREMS in both strains. In addition, an increase in EEG power density after sleep deprivation was observed in 5-HT2A+/+ mice but not in 5-HT2A–/– mice. Whole-body plethysmographic recordings indicated that 5-HT2A receptor blockade in 5-HT2A+/+ mice reduced NREMS apneas and bradypneas that occurred after sighs. In contrast, in 5-HT2A–/– mutants, NREMS apneas were not modified, and bradypnea after sighs were more pronounced. Our results demonstrate that 5-HT exerts a 5-HT2B-mediated facilitation of NREMS, and an influence respectively inhibitory on NREMS and facilitatory on sleep apnea generation, via 5-HT2A receptors. Moreover, 5-HT2A gene knock-out leads to functional compensations yielding adaptive changes opposite to those caused by pharmacological blockade of 5-HT2A receptors in 5-HT2A+/+ mice.

Electrical Coupling Mediates Tunable Low-Frequency Oscillations and Resonance in the Cerebellar Golgi Cell Network

Tonic motor control involves oscillatory synchronization of activity at low frequency (5-30 Hz) throughout the sensorimotor system, including cerebellar areas. We investigated the mechanisms underpinning cerebellar oscillations. We found that Golgi interneurons, which gate information transfer in the cerebellar cortex input layer, are extensively coupled through electrical synapses. When depolarized in vitro, these neurons displayed low-frequency oscillatory synchronization, imposing rhythmic inhibition onto granule cells. Combining experiments and modeling, we show that electrical transmission of the spike afterhyperpolarization is the essential component for oscillatory population synchronization. Rhythmic firing arises in spite of strong heterogeneities, is frequency tuned by the mean excitatory input to Golgi cells, and displays pronounced resonance when the modeled network is driven by oscillating inputs. In vivo, unitary Golgi cell activity was found to synchronize with low-frequency LFP oscillations occurring during quiet waking. These results suggest a major role for Golgi cells in coordinating cerebellar sensorimotor integration during oscillatory interactions.