Nicotine reinforcement and cognition restored by targeted expression of nicotinic receptors

Worldwide, 100 million people are expected to die this century from the consequences of nicotine addiction, but nicotine is also known to enhance cognitive performance. Identifying the molecular mechanisms involved in nicotine reinforcement and cognition is a priority and requires the development of new in vivo experimental paradigms. The ventral tegmental area (VTA) of the midbrain is thought to mediate the reinforcement properties of many drugs of abuse. Here we specifically re-expressed the beta2-subunit of the nicotinic acetylcholine receptor (nAChR) by stereotaxically injecting a lentiviral vector into the VTA of mice carrying beta2-subunit deletions. We demonstrate the efficient re-expression of electrophysiologically responsive, ligand-binding nicotinic acetylcholine receptors in dopamine-containing neurons of the VTA, together with the recovery of nicotine-elicited dopamine release and nicotine self-administration. We also quantified exploratory behaviours of the mice, and showed that beta2-subunit re-expression restored slow exploratory behaviour (a measure of cognitive function) to wild-type levels, but did not affect fast navigation behaviour. We thus demonstrate the sufficient role of the VTA in both nicotine reinforcement and endogenous cholinergic regulation of cognitive functions.     

Presynaptic glycine receptors enhance transmitter release at a mammalian central synapse

Glycine and GABAA (gamma-aminobutyric acid A) receptors are inhibitory neurotransmitter-gated Cl- channels localized in postsynaptic membranes. In some cases, GABAA receptors are also found presynaptically, but they retain their inhibitory effect as their activation reduces excitatory transmitter release. Here we report evidence for presynaptic ionotropic glycine receptors, using pre- and postsynaptic recordings of a calyceal synapse in the medial nucleus of the trapezoid body (MNTB). Unlike the classical action of glycine, presynaptic glycine receptors triggered a weakly depolarizing Cl- current in the nerve terminal. The depolarization enhanced transmitter release by activating Ca2+ channels and increasing resting intraterminal Ca2+ concentrations. Repetitive activation of glycinergic synapses on MNTB neurons also enhanced glutamatergic synaptic currents, indicating that presynaptic glycine receptors are activated by glycine spillover. These results reveal a novel site of action of the transmitter glycine, and indicate that under certain conditions presynaptic Cl- channels may increase transmitter release.     

Membrane potential has no direct role in evoking neurotransmitter release

Neurons communicate by secreting a transmitter that excites or inhibits other neurons at synapses. The role of presynaptic membrane potential in triggering transmitter release is still controversial. In one view, presynaptic action potentials trigger the release by the entry of calcium ions into presynaptic terminals through voltage-dependent calcium channels. Calcium acts at high local concentrations at release sites near channel mouths to cause neurosecretion. An opposing view is that, in addition to elevating presynaptic calcium, presynaptic potential stimulates transmitter release by a distinct direct action. The relative importance of depolarization and calcium entry in neurosecretion cannot be determined because the two events are tightly linked. To delineate the roles of presynaptic potential and calcium entry in transmitter release, we have used nitr-5, a photolabile calcium chelator, and a voltage-clamp technique to control intracellular calcium and membrane potential independently at a synapse formed between cell bodies of cultured neurons of the fresh water snail Helisoma trivolvis. We found transmitter release occurred when presynaptic calcium levels were elevated to concentrations of a few micromolar, and that presynaptic voltage had no direct effect on neurosecretion.     

Munc13-1 is essential for fusion competence of glutamatergic synaptic vesicles.

Neurotransmitter release at synapses between nerve cells is mediated by calcium-triggered exocytotic fusion of synaptic vesicles. Before fusion, vesicles dock at the presynaptic release site where they mature to a fusion-competent state. Here we identify Munc13-1, a brain-specific presynaptic phorbol ester receptor, as an essential protein for synaptic vesicle maturation. We show that glutamatergic hippocampal neurons from mice lacking Munc13-1 form ultrastructurally normal synapses whose synaptic-vesicle cycle is arrested at the maturation step. Transmitter release from mutant synapses cannot be triggered by action potentials, calcium-ionophores or hypertonic sucrose solution. In contrast, release evoked by alpha-latrotoxin is indistinguishable from wild-type controls, indicating that the toxin can bypass Munc13-1-mediated vesicle maturation. A small subpopulation of synapses of any given glutamatergic neuron as well as all synapses of GABA (gamma-aminobutyric acid)-containing neurons are unaffected by Munc13-1 loss, demonstrating the existence of multiple and transmitter-specific synaptic vesicle maturation processes in synapses.     

The amygdala modulates prefrontal cortex activity relative to conditioned fear

Animals learn that a tone can predict the occurrence of an electric shock through classical conditioning. Mice or rats trained in this manner display fear responses, such as freezing behaviour, when they hear the conditioned tone. Studies using amygdalectomized rats have shown that the amygdala is required for both the acquisition and expression of learned fear responses. Freezing to a conditioned tone is enhanced following damage to the dorsal part of the medial prefrontal cortex, indicating that this area may be involved in fear reduction. Here we show that prefrontal neurons reduce their spontaneous activity in the presence of a conditioned aversive tone as a function of the degree of fear. The depression in prefrontal spontaneous activity is related to amygdala activity but not to the freezing response itself. These data indicate that, in the presence of threatening stimuli, the amygdala controls both fear expression and prefrontal neuronal activity. They suggest that abnormal amygdala-induced modulation of prefrontal neuronal activity may be involved in the pathophysiology of certain forms of anxiety disorder.     

Dissociation of dopamine release in the nucleus accumbens from intracranial self-stimulation

Mesolimbic dopamine-releasing neurons appear to be important in the brain reward system. One behavioural paradigm that supports this hypothesis is intracranial self-stimulation (ICS), during which animals repeatedly press a lever to stimulate their own dopamine-releasing neurons electrically. Here we study dopamine release from dopamine terminals in the nucleus accumbens core and shell in the brain by using rapid-responding voltammetric microsensors during electrical stimulation of dopamine cell bodies in the ventral tegmental area/substantia nigra brain regions. In rats in which stimulating electrode placement failed to elicit dopamine release in the nucleus accumbens, ICS behaviour was not learned. In contrast, ICS was acquired when stimulus trains evoked extracellular dopamine in either the core or the shell of the nucleus accumbens. In animals that could learn ICS, experimenter-delivered stimulation always elicited dopamine release. In contrast, extracellular dopamine was rarely observed during ICS itself. Thus, although activation of mesolimbic dopamine-releasing neurons seems to be a necessary condition for ICS, evoked dopamine release is actually diminished during ICS. Dopamine may therefore be a neural substrate for novelty or reward expectation rather than reward itself.     

Endocannabinoid-mediated rescue of striatal LTD and motor deficits in Parkinson's disease models

The striatum is a major forebrain nucleus that integrates cortical and thalamic afferents and forms the input nucleus of the basal ganglia. Striatal projection neurons target the substantia nigra pars reticulata (direct pathway) or the lateral globus pallidus (indirect pathway). Imbalances between neural activity in these two pathways have been proposed to underlie the profound motor deficits observed in Parkinson's disease and Huntington's disease. However, little is known about differences in cellular and synaptic properties in these circuits. Indeed, current hypotheses suggest that these cells express similar forms of synaptic plasticity. Here we show that excitatory synapses onto indirect-pathway medium spiny neurons (MSNs) exhibit higher release probability and larger N-methyl-d-aspartate receptor currents than direct-pathway synapses. Moreover, indirect-pathway MSNs selectively express endocannabinoid-mediated long-term depression (eCB-LTD), which requires dopamine D2 receptor activation. In models of Parkinson's disease, indirect-pathway eCB-LTD is absent but is rescued by a D2 receptor agonist or inhibitors of endocannabinoid degradation. Administration of these drugs together in vivo reduces parkinsonian motor deficits, suggesting that endocannabinoid-mediated depression of indirect-pathway synapses has a critical role in the control of movement. These findings have implications for understanding the normal functions of the basal ganglia, and also suggest approaches for the development of therapeutic drugs for the treatment of striatal-based brain disorders.     

Anhedonia requires MC4R-mediated synaptic adaptations in nucleus accumbens

Chronic stress is a strong diathesis for depression in humans and is used to generate animal models of depression. It commonly leads to several major symptoms of depression, including dysregulated feeding behaviour, anhedonia and behavioural despair. Although hypotheses defining the neural pathophysiology of depression have been proposed, the critical synaptic adaptations in key brain circuits that mediate stress-induced depressive symptoms remain poorly understood. Here we show that chronic stress in mice decreases the strength of excitatory synapses on D1 dopamine receptor-expressing nucleus accumbens medium spiny neurons owing to activation of the melanocortin 4 receptor. Stress-elicited increases in behavioural measurements of anhedonia, but not increases in measurements of behavioural despair, are prevented by blocking these melanocortin 4 receptor-mediated synaptic changes in vivo. These results establish that stress-elicited anhedonia requires a neuropeptide-triggered, cell-type-specific synaptic adaptation in the nucleus accumbens and that distinct circuit adaptations mediate other major symptoms of stress-elicited depression.     

FMRFamide reverses protein phosphorylation produced by 5-HT and cAMP in Aplysia sensory neurons

Neurotransmitter can modulate neuronal activity through a variety of second messengers that act on ion channels and other substrate proteins. The most commonly described effector mechanism for second messengers in neurons depends on protein phosphorylation mediated by one of three sets of kinases: the cyclic AMP-dependent protein kinases, the Ca2+-calmodulin-dependent protein kinases, and the Ca2+-phospholipid-dependent protein kinases. In addition, some neurotransmitters and second messengers can also inhibit protein phosphorylation by lowering cAMP levels (either by inhibiting adenylyl cyclase or activating phosphodiesterases). This raises the question: can neurotransmitters also modulate neuronal activity by decreasing protein phosphorylation that is independent of cAMP? Various biochemical experiments show that a decrease in protein phosphorylation can arise through activation of a phosphatase or inhibition of kinases. In none of these cases, however, is the physiological role for the decrease in protein phosphorylation known. Here we report that in Aplysia sensory neurons, the presynaptic inhibitory transmitter FMRFamide decreases the resting levels of protein phosphorylation without altering the level of cAMP. Furthermore, FMRFamide overrides the cAMP-mediated enhancement of transmitter release produced by 5-hydroxytryptamine (5-HT), and concomitantly reverses the cAMP-dependent increase in protein phosphorylation produced by 5-HT. These findings indicate that a receptor-mediated decrease in protein phosphorylation may play an important part in the modulation of neurotransmitter release.     

Long-term heterosynaptic inhibition in Aplysia

Synaptic transmission between mechanosensory and motor neurons of the gill withdrawal reflex in Aplysia can undergo both short-term and long-term modulation. One form of short-term synaptic depression lasting minutes can be evoked by the peptide Phe-Met-Arg-Phe-amide (FMRFamide), and is mediated by the lipoxygenase pathway of arachidonic acid. We report here using cell culture, that the same monosynaptic sensory-to-motor component of the gill withdrawal reflex can also undergo long-term synaptic depression lasting 24 h after five applications of FMRFamide over a 2-h period. The long-term depression evoked by FMRFamide is transmitter-specific. Dopamine or low-frequency stimulation of sensory neurons, which also produce short-lasting synaptic depression in vivo, failed to evoke a long-term change. As is the case for long-term presynaptic facilitation of this connection with serotonin, the long-term depression, but not the short-term, can be blocked when applications of FMRFamide are given in the presence of anisomycin, a reversible inhibitor of protein synthesis. Thus, heterosynaptic depression parallels heterosynaptic facilitation in having a long-term as well as a short-term form, and in both cases the long-term modulation requires the synthesis of gene products not essential for the short-term changes.     

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation.

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.     

Glutamate spillover suppresses inhibition by activating presynaptic mGluRs

Metabotropic glutamate receptors (mGluRs) found on synaptic terminals throughout the brain are thought to be important in modulating neurotransmission. Activation of mGluRs by synaptically released glutamate depresses glutamate release from excitatory terminals but the physiological role of mGluRs on inhibitory terminals is unclear. We have investigated activation of mGluRs on inhibitory terminals within the cerebellar glomerulus, a structure in which GABA (gamma-aminobutyric acid)-releasing inhibitory terminals and glutamatergic excitatory terminals are in close apposition and make axo-dendritic synapses onto granule cells. Here we show that 'spillover' of glutamate, which is released from excitatory mossy fibres, inhibits GABA release from Golgi cell terminals by activating presynaptic mGluRs under physiological conditions. The magnitude of the depression of the inhibitory postsynaptic current is dependent on the frequency of mossy fibre stimulation, reaching 50% at 100 Hz. Furthermore, the duration of inhibitory postsynaptic current depression mirrors the time course of mossy fibre activity. Our results establish that mGluRs on inhibitory interneuron axons sense the activity of neighbouring excitatory synapses. This heterosynaptic mechanism is likely to boost the efficacy of active excitatory fibres by locally reducing the level of inhibition.     

Cortical remodelling induced by activity of ventral tegmental dopamine neurons.

Representations of sensory stimuli in the cerebral cortex can undergo progressive remodelling according to the behavioural importance of the stimuli. The cortex receives widespread projections from dopamine neurons in the ventral tegmental area (VTA), which are activated by new stimuli or unpredicted rewards, and are believed to provide a reinforcement signal for such learning-related cortical reorganization. In the primary auditory cortex (AI) dopamine release has been observed during auditory learning that remodels the sound-frequency representations. Furthermore, dopamine modulates long-term potentiation, a putative cellular mechanism underlying plasticity. Here we show that stimulating the VTA together with an auditory stimulus of a particular tone increases the cortical area and selectivity of the neural responses to that sound stimulus in AI. Conversely, the AI representations of nearby sound frequencies are selectively decreased. Strong, sharply tuned responses to the paired tones also emerge in a second cortical area, whereas the same stimuli evoke only poor or non-selective responses in this second cortical field in naive animals. In addition, we found that strong long-range coherence of neuronal discharge emerges between AI and this secondary auditory cortical area.     

Presynaptic induction of heterosynaptic associative plasticity in the mammalian brain

The induction of associative synaptic plasticity in the mammalian central nervous system classically depends on coincident presynaptic and postsynaptic activity. According to this principle, associative homosynaptic long-term potentiation (LTP) of excitatory synaptic transmission can be induced only if synaptic release occurs during postsynaptic depolarization. In contrast, heterosynaptic plasticity in mammals is considered to rely on activity-independent, non-associative processes. Here we describe a novel mechanism underlying the induction of associative LTP in the lateral amygdala (LA). Simultaneous activation of converging cortical and thalamic afferents specifically induced associative, N-methyl-D-aspartate (NMDA)-receptor-dependent LTP at cortical, but not at thalamic, inputs. Surprisingly, the induction of associative LTP at cortical inputs was completely independent of postsynaptic activity, including depolarization, postsynaptic NMDA receptor activation or an increase in postsynaptic Ca2+ concentration, and did not require network activity. LTP expression was mediated by a persistent increase in the presynaptic probability of release at cortical afferents. Our study shows the presynaptic induction and expression of heterosynaptic and associative synaptic plasticity on simultaneous activity of converging afferents. Our data indicate that input specificity of associative LTP can be determined exclusively by presynaptic properties.     

Repeated cocaine exposure in vivo facilitates LTP induction in midbrain dopamine neurons

Drugs of abuse are known to cause persistent modification of neural circuits, leading to addictive behaviours. Changes in synaptic plasticity in dopamine neurons of the ventral tegmental area (VTA) may contribute to circuit modification induced by many drugs of abuse, including cocaine. Here we report that, following repeated exposure to cocaine in vivo, excitatory synapses to rat VTA dopamine neurons become highly susceptible to the induction of long-term potentiation (LTP) by correlated pre- and postsynaptic activity. This facilitated LTP induction is caused by cocaine-induced reduction of GABA(A) (gamma-aminobutyric acid) receptor-mediated inhibition of these dopamine neurons. In midbrain slices from rats treated with saline or a single dose of cocaine, LTP could not be induced in VTA dopamine neurons unless GABA-mediated inhibition was reduced by bicuculline or picrotoxin. However, LTP became readily inducible in slices from rats treated repeatedly with cocaine; this LTP induction was prevented by enhancing GABA-mediated inhibition using diazepam. Furthermore, repeated cocaine exposure reduced the amplitude of GABA-mediated synaptic currents and increased the probability of spike initiation in VTA dopamine neurons. This cocaine-induced enhancement of synaptic plasticity in the VTA may be important for the formation of drug-associated memory.     

A cellular mechanism of reward-related learning.

Positive reinforcement helps to control the acquisition of learned behaviours. Here we report a cellular mechanism in the brain that may underlie the behavioural effects of positive reinforcement. We used intracranial self-stimulation (ICSS) as a model of reinforcement learning, in which each rat learns to press a lever that applies reinforcing electrical stimulation to its own substantia nigra. The outputs from neurons of the substantia nigra terminate on neurons in the striatum in close proximity to inputs from the cerebral cortex on the same striatal neurons. We measured the effect of substantia nigra stimulation on these inputs from the cortex to striatal neurons and also on how quickly the rats learned to press the lever. We found that stimulation of the substantia nigra (with the optimal parameters for lever-pressing behaviour) induced potentiation of synapses between the cortex and the striatum, which required activation of dopamine receptors. The degree of potentiation within ten minutes of the ICSS trains was correlated with the time taken by the rats to learn ICSS behaviour. We propose that stimulation of the substantia nigra when the lever is pressed induces a similar potentiation of cortical inputs to the striatum, positively reinforcing the learning of the behaviour by the rats.     

Direct modulation of synaptic vesicle priming by GABA(B) receptor activation at a glutamatergic synapse

Second messenger cascades involving G proteins and calcium are known to modulate neurotransmitter release. A prominent effect of such a cascade is the downmodulation of presynaptic calcium influx, which markedly reduces evoked neurotransmitter release. Here we show that G-protein-mediated signalling, such as through GABA (gamma-amino butyric acid) subtype B (GABA(B)) receptors, retards the recruitment of synaptic vesicles during sustained activity and after short-term depression. This retardation occurs through a lowering of cyclic AMP, which blocks the stimulatory effect of increased calcium concentration on vesicle recruitment. In this signalling pathway, cAMP (functioning through the cAMP-dependent guanine nucleotide exchange factor) and calcium/calmodulin cooperate to enhance vesicle priming. The differential modulation of the two forms of synaptic plasticity, presynaptic inhibition and calcium-dependent recovery from synaptic depression, is expected to have interesting consequences for the dynamic behaviour of neural networks.     

Neuron-type-specific signals for reward and punishment in the ventral tegmental area

Dopamine has a central role in motivation and reward. Dopaminergic neurons in the ventral tegmental area (VTA) signal the discrepancy between expected and actual rewards (that is, reward prediction error), but how they compute such signals is unknown. We recorded the activity of VTA neurons while mice associated different odour cues with appetitive and aversive outcomes. We found three types of neuron based on responses to odours and outcomes: approximately half of the neurons (type I, 52%) showed phasic excitation after reward-predicting odours and rewards in a manner consistent with reward prediction error coding; the other half of neurons showed persistent activity during the delay between odour and outcome that was modulated positively (type II, 31%) or negatively (type III, 18%) by the value of outcomes. Whereas the activity of type I neurons was sensitive to actual outcomes (that is, when the reward was delivered as expected compared to when it was unexpectedly omitted), the activity of type II and type III neurons was determined predominantly by reward-predicting odours. We 'tagged' dopaminergic and GABAergic neurons with the light-sensitive protein channelrhodopsin-2 and identified them based on their responses to optical stimulation while recording. All identified dopaminergic neurons were of type I and all GABAergic neurons were of type II. These results show that VTA GABAergic neurons signal expected reward, a key variable for dopaminergic neurons to calculate reward prediction error.     

学生的"习得性无助感"及其防治

“习得性无助感”现象最初是由心理学家塞利格曼研究动物行为时发现的,后又在人的身上得到证实。它是指个人经历了失败与挫折后,面临问题时产生的无能为力、丧失信心的心理状态与行为。形成习得性无助感的原因很复杂,有客观原因,但更主要的是学生本身的因素。当学生产生了习得性无助感时,会导致动机、认知和情绪上的缺陷,给学生一生的发展带来相当不利的影响。因此,教师要注意采取一定的措施以防治学生的习得性无助感。     

Polyamine-dependent facilitation of postsynaptic AMPA receptors counteracts paired-pulse depression.

At many glutamatergic synapses in the brain, calcium-permeable alpha - amino - 3 - hydro - 5 - methyl - 4 - isoxazolepropionate receptor (AMPAR) channels mediate fast excitatory transmission. These channels are blocked by endogenous intracellular polyamines, which are found in virtually every type of cell. In excised patches, use-dependent relief of polyamine block enhances glutamate-evoked currents through recombinant and native calcium-permeable, polyamine-sensitive AMPAR channels. The contribution of polyamine unblock to synaptic currents during high-frequency stimulation may be to facilitate currents and maintain current amplitudes in the face of a slow recovery from desensitization or presynaptic depression. Here we show, on pairs and triples of synaptically connected neurons in slices, that this mechanism contributes to short-term plasticity in local circuits formed by presynaptic pyramidal neurons and postsynaptic multipolar interneurons in layer 2/3 of rat neocortex. Activity-dependent relief from polyamine block of postsynaptic calcium-permeable AMPARs in the interneurons either reduces the rate of paired-pulse depression in a frequency-dependent manner or, at a given stimulation frequency, induces facilitation of a synaptic response that would otherwise depress. This mechanism for the enhancement of synaptic gain appears to be entirely postsynaptic.