Modulation of striatal dopamine (DA) neurotransmission plays a fundamental role in the reinforcing and ultimately addictive effects of nicotine. Nicotine, by desensitizing b2 subunit-containing (b2*) nicotinic acetylcholine receptors (nAChRs) on striatal DA axons, significantly enhances how DA is released by reward-related burst activity compared to nonreward-related tonic activity. This action provides a synaptic mechanism for nicotine to facilitate the DA-dependent reinforcement. The subfamily of b2*-nAChRs responsible for these potent synaptic effects could offer a molecular target for therapeutic strategies in nicotine addiction. We explored the role of a6b2*-nAChRs in the nucleus accumbens (NAc) and caudate-putamen (CPu) by observing action potential-dependent DA release from synapses in real-time using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in mouse striatal slices. The a6-specific antagonist a-conotoxin-MII suppressed DA release evoked by single and low-frequency action potentials and concurrently enhanced release by high-frequency bursts in a manner similar to the b2*-selective antagonist dihydro-b-erythroidine (DHbE) in NAc, but less so in CPu. The greater role for a6*-nAChRs in NAc was not due to any confounding regional difference in ACh tone since elevated ACh levels (after the acetylcholinesterase inhibitor ambenonium) had similar outcomes in NAc and CPu. Rather, there appear to be underlying differences in nAChR subtype function in NAc and CPu. In summary, we reveal that a6b2*-nAChRs dominate the effects of nicotine on DA release in NAc, whereas in CPu their role is minor alongside other b2*-nAChRs (eg a4*), These data offer new insights to suggest striatal a6*-nAChRs as a molecular target for a therapeutic strategy for nicotine addiction.

Striatal dopamine (DA) and acetylcholine (ACh) regulate motivated behaviors and striatal plasticity. Interactions between these neurotransmitters may be important, through synchronous changes in parent neuron activities and reciprocal presynaptic regulation of release. How DA signaling is regulated by striatal muscarinic receptors (mAChRs) is unresolved; contradictory reports indicate suppression or facilitation, implicating several mAChR subtypes on various neurons. We investigated whether mAChR regulation of DA signaling varies with presynaptic activity and identified the mAChRs responsible in sensorimotor-versus limbic-associated striatum. We detected DA in real time at carbon fiber microelectrodes in mouse striatal slices.Broad-spectrum mAChR agonists [oxotremorine-M, APET (arecaidine propargyl ester tosylate)] decreased DA release evoked by low-frequency stimuli (1-10 Hz, four pulses) but increased the sensitivity of DA release to presynaptic activity, even enhancing release by high frequencies (e.g., Ͼ25 Hz for four pulses). These bidirectional effects depended on ACh input to striatal nicotinic receptors (nAChRs) on DA axons but not GABA or glutamate input. In caudate-putamen (CPu), knock-out of M 2 -or M 4 -mAChRs (not M 5 ) prevented mAChR control of DA, indicating that M 2 -and M 4 -mAChRs are required. In nucleus accumbens (NAc) core or shell, mAChR function was prevented in M 4 -knock-outs, but not M 2 -or M 5 -knock-outs.These data indicate that striatal mAChRs, by inhibiting ACh release from cholinergic interneurons and thus modifying nAChR activity, offer variable control of DA release probability that promotes how DA release reflects activation of dopaminergic axons. Furthermore, different coupling of striatal M 2 /M 4 -mAChRs to the control of DA release in CPu versus NAc suggests targets to influence DA/ACh function differentially between striatal domains.

Studies in brain slices have provided a wealth of data on the basic features of neurons and synapses. In the intact brain, these properties may be strongly influenced by ongoing network activity. Although physiologically realistic patterns of network activity have been successfully induced in brain slices maintained in interface-type recording chambers, they have been harder to obtain in submerged-type chambers, which offer significant experimental advantages, including fast exchange of pharmacological agents, visually guided patch-clamp recordings, and imaging techniques. Here, we investigated conditions for the emergence of network oscillations in submerged slices prepared from the hippocampus of rats and mice. We found that the local oxygen level is critical for generation and propagation of both spontaneously occurring sharp wave-ripple oscillations and cholinergically induced fast oscillations. We suggest three ways to improve the oxygen supply to slices under submerged conditions: (i) optimizing chamber design for laminar flow of superfusion fluid; (ii) increasing the flow rate of superfusion fluid; and (iii) superfusing both surfaces of the slice. These improvements to the recording conditions enable detailed studies of neurons under more realistic conditions of network activity, which are essential for a better understanding of neuronal network operation.

The effects of nicotine on dopamine transmission from mesostriatal dopamine neurons are central to its reinforcing properties. Only recently however, has the influence of presynaptic nicotinic receptors (nAChRs) on dopaminergic axon terminals within striatum begun to be understood. Here, rather than simply enhancing (or inhibiting) dopamine release, nAChRs perform the role of a presynaptic filter, whose influence on dopamine release probability depends on presynaptic activity in dopaminergic as well as cholinergic neurons. Both mesostriatal dopaminergic neurons and striatal cholinergic interneurons play key roles in motivational and sensorimotor processing by the basal ganglia. Moreover, it appears that the striatal influence of dopamine and ACh cannot be fully appreciated without an understanding of their reciprocal interactions. We will review the powerful filtering by nAChRs of striatal dopamine release and discuss its dependence on activity in dopaminergic and cholinergic neurons. We will also review how nicotine, acting via nAChR desensitization, promotes the sensitivity of dopamine synapses to activity. This filtering action might provide a mechanism through which nicotine promotes how burst activity in dopamine neurons facilitates goal-directed behaviour and reinforcement processing. More generally, it indicates that we should not restrict our view of presynaptic nAChRs to simply enhancing neurotransmitter release. We will also summarize current understanding of the forms and functions of the diverse nAChRs purported to exist on dopaminergic axons. A greater understanding of nAChR form and function is imperative to guide the design of ligands with subtype-selective efficacy for improved therapeutic interventions in nicotine addiction as well as Parkinson's disease.

Nicotine is the primary psychoactive component of tobacco. Its reinforcing and addictive properties depend on nicotinic acetylcholine receptors (nAChRs) located within the mesolimbic axis originating in the ventral tegmental area (VTA). The roles and oligomeric assembly of subunit α4-and subunit α6-containing nAChRs in dopaminergic (DAergic) neurons are much debated. Using subunit-specific knockout mice and targeted lentiviral re-expression, we have determined the subunit dependence of intracranial nicotine self-administration (ICSA) into the VTA and the effects of nicotine on dopamine (DA) neuron excitability in the VTA and on DA transmission in the nucleus accumbens (NAc). We show that the α4 subunit, but not the α6 subunit, is necessary for ICSA and nicotine-induced bursting of VTA DAergic neurons, whereas subunits α4 and α6 together regulate the activity dependence of DA transmission in the NAc. These data suggest that α4-dominated enhancement of burst firing in DA neurons, relayed by DA transmission in NAc that is gated by nAChRs containing α4 and α6 subunits, underlies nicotine self-administration and its long-term maintenance.electrophysiology | lentivirus | nicotinic receptor | voltammetry | ventral striatum

Motor fatigue induced by physical activity is an everyday experience characterized by a decreased capacity to generate motor force. Factors in both muscles and the central nervous system are involved. The central component of fatigue modulates the ability of motoneurons to activate muscle adequately independently of the muscle physiology. Indirect evidence indicates that central fatigue is caused by serotonin (5-HT), but the cellular mechanisms are unknown. In a slice preparation from the spinal cord of the adult turtle, we found that prolonged stimulation of the raphespinal pathway-as during motor exercise-activated 5-HT 1A receptors that decreased motoneuronal excitability. Electrophysiological tests combined with pharmacology showed that focal activation of 5-HT 1A receptors at the axon initial segment (AIS), but not on other motoneuronal compartments, inhibited the action potential initiation by modulating a Na + current. Immunohistochemical staining against 5-HT revealed a high-density innervation of 5-HT terminals on the somatodendritic membrane and a complete absence on the AIS. This observation raised the hypothesis that a 5-HT spillover activates receptors at this latter compartment. We tested it by measuring the level of extracellular 5-HT with cyclic voltammetry and found that prolonged stimulations of the raphe-spinal pathway increased the level of 5-HT to a concentration sufficient to activate 5-HT 1A receptors. Together our results demonstrate that prolonged release of 5-HT during motor activity spills over from its release sites to the AIS of motoneurons. Here, activated 5-HT 1A receptors inhibit firing and, thereby, muscle contraction. Hence, this is a cellular mechanism for central fatigue.movement | spike genesis | input-output gain | movement control P rolonged physical activity leads to motor fatigue (1, 2). The force produced by muscles decreases in part because of the lack of glycogen (3) in the muscle and/or failures at the neuromuscular junctions (4). In addition to this well-described muscle fatigue, motor fatigue also involves an element originating in the CNS (5-8). This "central fatigue" is characterized by a decreased ability to contract the muscle fibers adequately during a motor activity and is observed independently of the muscle fatigue (6). It is often studied during maximal voluntary contractions (5, 6, 9). Nevertheless, it is also present during weak physical contraction (1,2,8,(10)(11)(12). Central fatigue secures rotation of motor units (13) and prevents hyperactivity of muscles (6). Elements of central fatigue involve the cortex (6), and the spinal cord at the level of motoneurons (MNs) (5).The cellular mechanisms responsible for a decrease of the activity of MNs remain unknown. However, evidence suggests that central fatigue correlates with increased levels of serotonin (5-HT) in the CNS. Human subjects that perform an intense motor task are faster exhausted after intake of a 5-HT 1A receptor agonist (14) or a selective serotonin reuptake inhibitor (15). In animals, inject...

Polymorphisms in the gene for the ␣5 nicotinic acetylcholine receptor (nAChR) subunit are associated with vulnerability to nicotine addiction. However, the underlying normal functions of ␣5-containing nAChRs in the brain are poorly understood. Striatal dopamine (DA) transmission is critical to the acquisition and maintenance of drug addiction and is modulated strongly by nicotine acting at heteromeric ␤2-containing (␤2*) nAChRs. We explored whether ␣5 subunits, as well as ␣4, ␣6, and ␤3 subunits, participate in the powerful regulation of DA release probability by ␤2* nAChRs in nucleus accumbens (NAc) core and in dorsal striatum [caudatoputamen (CPu)]. We detected evoked dopamine release using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in striatal slices from mice with deletions of ␣4, ␣5, ␣6, or ␤3 subunits. We show that the nAChR subtypes that dominantly regulate dopamine transmission depend critically upon ␣5 subunits in the dorsal CPu in ␣4␣5(non-␣6)␤2-nAChRs but not in NAc core, where ␣4␣6␤2␤3-nAChRs are required. These data reveal the distinct populations of nAChRs that govern DA transmission in NAc core versus dorsal CPu. Furthermore, they indicate that ␣5 subunits are critical to the regulation of DA transmission by ␣4␤2* nAChRs in regions of striatum associated with habitual and instrumental responses (dorsal CPu) rather than pavlovian associations (NAc).

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