Coordinated expression of muscarinic receptor messenger RNAs in striatal medium spiny neurons

Molecular profiling of striatonigral and striatopallidal medium spiny neurons past, present, and future

Defining distinct molecular properties of the two striatal medium spiny neurons (MSNs) has been a challenging task for basal ganglia (BG) neuroscientists. Identifying differential molecular components in each MSN subtype is crucial for BG researchers to understand functional properties of these two neurons. The two MSN populations are morphologically identical except in their projections through the direct verses indirect BG pathways and they are heterogeneously dispersed throughout the dorsal striatum (dStr) and nucleus accumbens (NAc). These characteristics have made it difficult for researchers to distinguish and isolate these two neuronal populations thereby hindering progress toward a more comprehensive understanding of their differential molecular properties. Researchers began to investigate molecular differences in the striatonigral and striatopallidal neurons using in situ hybridization (ISH) techniques and single cell reverse transcription-polymerase chain reaction (scRT-PCR). Currently the field is utilizing more advanced techniques for large-scale gene expression studies including fluorescence activated cell sorting (FACS) of MSNs, from which RNA is purified, from fluorescent reporter transgenic mice or use of transgenic mice in which ribosomes from each MSN are tagged and can be immunoprecipitated followed by RNA isolation, a technique termed translating ribosomal affinity purification (TRAP). Additionally, the availability of fluorescent reporter mice for each MSN subtype is allowing, scientists to perform more accurate histology studies evaluating differential protein expression and signaling changes in each cell subtype. Finally, researchers are able to evaluate the role of specific genes in vivo by utilizing cell type-specific mouse models including Cre driver lines that can be crossed with conditional overexpression or knockout systems. This is a very exciting time in the BG field because researchers are well equipped with the most progressive tools to comprehensively evaluate molecular components in the two MSNs and their consequence on BG functional output in the normal, diseased, and developing brain.

Differential excitability and modulation of striatal medium spiny neuron dendrites

The loss of striatal dopamine (DA) in Parkinson's disease (PD) models triggers a cell-type-specific reduction in the density of dendritic spines in D(2) receptor-expressing striatopallidal medium spiny neurons (D(2) MSNs). How the intrinsic properties of MSN dendrites, where the vast majority of DA receptors are found, contribute to this adaptation is not clear. To address this question, two-photon laser scanning microscopy (2PLSM) was performed in patch-clamped mouse MSNs identified in striatal slices by expression of green fluorescent protein (eGFP) controlled by DA receptor promoters. These studies revealed that single backpropagating action potentials (bAPs) produced more reliable elevations in cytosolic Ca(2+) concentration at distal dendritic locations in D(2) MSNs than at similar locations in D(1) receptor-expressing striatonigral MSNs (D(1) MSNs). In both cell types, the dendritic Ca(2+) entry elicited by bAPs was enhanced by pharmacological blockade of Kv4, but not Kv1 K(+) channels. Local application of DA depressed dendritic bAP-evoked Ca(2+) transients, whereas application of ACh increased these Ca(2+) transients in D(2) MSNs, but not in D(1) MSNs. After DA depletion, bAP-evoked Ca(2+) transients were enhanced in distal dendrites and spines in D(2) MSNs. Together, these results suggest that normally D(2) MSN dendrites are more excitable than those of D(1) MSNs and that DA depletion exaggerates this asymmetry, potentially contributing to adaptations in PD models.

Muscarinic M(1) modulation of N and L types of calcium channels is mediated by protein kinase C in neostriatal neurons

In some neurons, muscarinic M(1)-class receptors control L-type (Ca(V)1) Ca(2+)-channels via protein kinase C (PKC) or calcineurin (phosphatase 2B; PP-2B) signaling pathways. Both PKC and PP-2B pathways start with phospholipase C (PLC) activation. In contrast, P/Q- and N-type (Ca(V)2.1, 2.2, respectively) Ca(2+)-channels are controlled by M(2)-class receptors via G proteins that may act, directly, to modulate these channels. The hypothesis of this work is that this description is not enough to explain muscarinic modulation of Ca(2+) channels in rat neostriatal projection neurons. Thus, we took advantage of the specific muscarinic toxin 3 (MT-3) to block M(4)-type receptors in neostriatal neurons, and leave in isolation the M(1)-type receptors to study them separately. We then asked what Ca(2+) channels are modulated by M(1)-type receptors only. We found that M(1)-receptors do modulate L, N and P/Q-types Ca(2+) channels. This modulation is blocked by the M(1)-class receptor antagonist (muscarinic toxin 7, MT-7) and is voltage-independent. Thereafter, we asked what signaling pathways, activated by M(1)-receptors would control these channels. We found that inactivation of PLC abolishes the modulation of all three channel types. PKC activators (phorbol esters) mimic muscarinic actions, whereas reduction of intracellular calcium virtually abolishes all modulation. As expected, PKC inhibitors prevented the muscarinic reduction of the afterhyperpolarizing potential (AHP), an event known to be dependent on Ca(2+) entry via N- and P/Q-type Ca(2+) channels. However, PKC inhibitors (bisindolylmaleimide I and PKC-1936) only block modulation of currents through N and L types Ca(2+) channels; while the modulation of P/Q-type Ca(2+) channels remains unaffected. These results show that different branches of the same signaling cascade can be used to modulate different Ca(2+) channels. Finally, we found no evidence of calcineurin modulating these Ca(2+) channels during M(1)-receptor activation, although, in the same cells, we demonstrate functional PP-2B by activating dopaminergic D(2)-receptor modulation.

Loss of muscarinic autoreceptor function impairs long-term depression but not long-term potentiation in the striatum

Muscarinic autoreceptors regulate cholinergic tone in the striatum. We investigated the functional consequences of genetic deletion of striatal muscarinic autoreceptors by means of electrophysiological recordings from either medium spiny neurons (MSNs) or cholinergic interneurons (ChIs) in slices from single M(4) or double M(2)/M(4) muscarinic acetylcholine receptor (mAChR) knock-out (-/-) mice. In control ChIs, the muscarinic agonist oxotremorine (300 nM) produced a self-inhibitory outward current that was mostly reduced in M(4)(-/-) and abolished in M(2)/M(4)(-/-) mice, suggesting an involvement of both M(2) and M(4) autoreceptors. In MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, muscarine caused a membrane depolarization that was prevented by the M(1) receptor-preferring antagonist pirenzepine (100 nM), suggesting that M(1) receptor function was unaltered. Acetylcholine has been involved in striatal long-term potentiation (LTP) or long-term depression (LTD) induction. Loss of muscarinic autoreceptor function is predicted to affect synaptic plasticity by modifying striatal cholinergic tone. Indeed, high-frequency stimulation of glutamatergic afferents failed to induce LTD in MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, as well as in wild-type mice pretreated with the M(2)/M(4) antagonist AF-DX384 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,1 1-dihydro-6H-pyrido[2,3b][1,4] benzodiazepin-6-one). Interestingly, LTD could be restored by either pirenzepine (100 nM) or hemicholinium-3 (10 microM), a depletor of endogenous ACh. Conversely, LTP induction did not show any difference among the three mouse strains and was prevented by pirenzepine. These results demonstrate that M(2)/M(4) muscarinic autoreceptors regulate ACh release from striatal ChIs. As a consequence, endogenous ACh drives the polarity of bidirectional synaptic plasticity.

Cholinergic modulation of Kir2 channels selectively elevates dendritic excitability in striatopallidal neurons

Dopamine-depleting lesions of the striatum that mimic Parkinson's disease induce a profound pruning of spines and glutamatergic synapses in striatopallidal medium spiny neurons, leaving striatonigral medium spiny neurons intact. The mechanisms that underlie this cell type-specific loss of connectivity are poorly understood. The Kir2 K(+) channel is an important determinant of dendritic excitability in these cells. Here we show that opening of these channels is potently reduced by signaling through M1 muscarinic receptors in striatopallidal neurons, but not in striatonigral neurons. This asymmetry could be attributed to differences in the subunit composition of Kir2 channels. Dopamine depletion alters the subunit composition further, rendering Kir2 channels in striatopallidal neurons even more susceptible to modulation. Reduced opening of Kir2 channels enhances dendritic excitability and synaptic integration. This cell type-specific enhancement of dendritic excitability is an essential trigger for synaptic pruning after dopamine depletion, as pruning was prevented by genetic deletion of M1 muscarinic receptors.

Differential involvement of M1-type and M4-type muscarinic cholinergic receptors in the dorsomedial striatum in task switching

Previous experiments have demonstrated that the rat dorsomedial striatum is one brain area that plays a crucial role in learning when conditions require a shift in strategies. Further evidence indicates that muscarinic cholinergic receptors in this brain area support adaptations in behavioral responses. Unknown is whether specific muscarinic receptor subtypes in the dorsomedial striatum contribute to a flexible shift in response patterns. The present experiments investigated whether blockade of M1-type and/or M4-type cholinergic receptors in the dorsomedial striatum underlie place reversal learning. Experiment 1 investigated the effects of the M1-type muscarinic cholinergic antagonist, muscarinic-toxin 7 (MT-7) infused into the dorsomedial striatum in place acquisition and reversal learning. Experiment 2 investigated the effects of the M4-type muscarinic cholinergic antagonist, muscarinic-toxin 3 (MT-3) injected into the dorsomedial striatum in place acquisition and reversal learning. All testing occurred in a modified cross-maze across two consecutive sessions. Bilateral injections of MT-7 into the dorsomedial striatum at 1 or 2 microg, but not 0.05 microg impaired place reversal learning. Analysis of the errors revealed that MT-7 at 1 and 2 microg significantly increased regressive errors, but not perseverative errors. An injection of MT-7 2 microg into the dorsomedial striatum prior to place acquisition did not affect learning. Experiment 2 revealed that dorsomedial striatal injections of MT-3 (0.05, 1 or 2 microg) did not affect place acquisition or reversal learning. The findings suggest that activation of M1-type muscarinic cholinergic receptors in the dorsomedial striatum, but not M4-type muscarinic cholinergic receptors facilitate the flexible shifting of response patterns by maintaining or learning a new choice pattern once selected.

The basal ganglia cholinergic neurochemistry of progressive supranuclear palsy and other neurodegenerative diseases

BACKGROUND

Progressive supranuclear palsy (PSP) is a progressive neurodegenerative disorder involving motor and cognitive dysfunction. Currently, there is no effective treatment either for symptomatic relief or disease modification. This relates, in part, to a lack of knowledge of the underlying neurochemical abnormalities, including cholinergic receptor status in the basal ganglia.

AIM

To measure muscarinic M2 and M4 receptors in the basal ganglia in PSP.

METHODS

The muscarinic M2 (presynaptic) and M4 (postsynaptic) receptors in the striatum, pallidum and adjacent insular cortex were autoradiographically measured in pathologically confirmed cases of PSP (n = 18), and compared with cases of Lewy body dementias (LBDs; n = 45), Alzheimer's disease (AD; n = 39) and controls (n = 50).

RESULTS

In cases of PSP, there was a reduction in M2 and M4 receptors in the posterior caudate and putamen compared to controls, but no significant changes in the pallidum. Cases with AD showed lower M2 receptors in the posterior striatum. Groups with LBD and AD showed higher M2 binding in the insular cortex compared with controls.

CONCLUSIONS

The results suggest loss of posterior striatal cholinergic interneurones in PSP, and reduction in medium spiny projection neurones bearing M4 receptors. These results should be taken in the context of more widespread pathology in PSP, but may have implications for future trials of cholinergic treatments.

Subcellular arrangement of molecules for 2-arachidonoyl-glycerol-mediated retrograde signaling and its physiological contribution to synaptic modulation in the striatum

Endogenous cannabinoids (endocannabinoids) mediate retrograde signals for short- and long-term suppression of transmitter release at synapses of striatal medium spiny (MS) neurons. An endocannabinoid, 2-arachidonoyl-glycerol (2-AG), is synthesized from diacylglycerol (DAG) after membrane depolarization and Gq-coupled receptor activation. To understand 2-AG-mediated retrograde signaling in the striatum, we determined precise subcellular distributions of the synthetic enzyme of 2-AG, DAG lipase-alpha (DAGLalpha), and its upstream metabotropic glutamate receptor 5 (mGluR5) and muscarinic acetylcholine receptor 1 (M1). DAGLalpha, mGluR5, and M1 were all richly distributed on the somatodendritic surface of MS neurons, but their subcellular distributions were different. Although mGluR5 and DAGLalpha levels were highest in spines and accumulated in the perisynaptic region, M1 level was lowest in spines and was rather excluded from the mGluR5-rich perisynaptic region. These subcellular arrangements suggest that mGluR5 and M1 might differentially affect endocannabinoid-mediated, depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE) in MS neurons. Indeed, mGluR5 activation enhanced both DSI and DSE, whereas M1 activation enhanced DSI only. Importantly, DSI, DSE, and receptor-driven endocannabinoid-mediated suppression were all abolished by the DAG lipase inhibitor tetrahydrolipstatin, indicating 2-AG as the major endocannabinoid mediating retrograde suppression at excitatory and inhibitory synapses of MS neurons. Accordingly, CB1 cannabinoid receptor, the main target of 2-AG, was present at high levels on GABAergic axon terminals of MS neurons and parvalbumin-positive interneurons and at low levels on excitatory corticostriatal afferents. Thus, endocannabinoid signaling molecules are arranged to modulate the excitability of the MS neuron effectively depending on cortical activity and cholinergic tone as measured by mGluR5 and M1 receptors, respectively.

D1 and D2 dopamine-receptor modulation of striatal glutamatergic signaling in striatal medium spiny neurons

Dopamine shapes a wide variety of psychomotor functions. This is mainly accomplished by modulating cortical and thalamic glutamatergic signals impinging upon principal medium spiny neurons (MSNs) of the striatum. Several lines of evidence suggest that dopamine D1 receptor signaling enhances dendritic excitability and glutamatergic signaling in striatonigral MSNs, whereas D2 receptor signaling exerts the opposite effect in striatopallidal MSNs. The functional antagonism between these two major striatal dopamine receptors extends to the regulation of synaptic plasticity. Recent studies, using transgenic mice in which cells express D1 and D2 receptors, have uncovered unappreciated differences between MSNs that shape glutamatergic signaling and the influence of DA on synaptic plasticity. These studies have also shown that long-term alterations in dopamine signaling produce profound and cell-type-specific reshaping of corticostriatal connectivity and function.

Tonic enhancement of endocannabinoid-mediated retrograde suppression of inhibition by cholinergic interneuron activity in the striatum

Tonically active cholinergic interneurons in the striatum modulate activities of striatal outputs from medium spiny (MS) neurons and significantly influence overall functions of the basal ganglia. Cellular mechanisms of this modulation are not fully understood. Here we show that ambient acetylcholine (ACh) derived from tonically active cholinergic interneurons constitutively upregulates depolarization-induced release of endocannabinoids from MS neurons. The released endocannabinoids cause transient suppression of inhibitory synaptic inputs to MS neurons through acting retrogradely onto presynaptic CB1 cannabinoid receptors. The effects were mediated by postsynaptic M(1) subtype of muscarinic ACh receptors, because the action of a muscarinic agonist to release endocannabinoids and the enhancement of depolarization-induced endocannabinoid release by ambient ACh were both deficient in M1 knock-out mice and were blocked by postsynaptic infusion of guanosine-5'-O-(2-thiodiphosphate). Suppression of spontaneous firings of cholinergic interneurons by inhibiting Ih current reduced the depolarization-induced release of endocannabinoids. Conversely, elevation of ambient ACh concentration by inhibiting choline esterase significantly enhanced the endocannabinoid release. Paired recording from a cholinergic interneuron and an MS neuron revealed that the activity of single cholinergic neuron could influence endocannabinoid-mediated signaling in neighboring MS neurons. These results clearly indicate that striatal endocannabinoid-mediated modulation is under the control of cholinergic interneuron activity. By immunofluorescent and immunoelectron microscopic examinations, we demonstrated that M1 receptor was densely distributed in perikarya and dendrites of dopamine D1 or D2 receptor-positive MS neurons. Thus, we have disclosed a novel mechanism by which the muscarinic system regulates striatal output and may contribute to motor control.

Dopaminergic control of corticostriatal long-term synaptic depression in medium spiny neurons is mediated by cholinergic interneurons

Long-term depression (LTD) of the synapse formed between cortical pyramidal neurons and striatal medium spiny neurons is central to many theories of motor plasticity and associative learning. The induction of LTD at this synapse is thought to depend upon D(2) dopamine receptors localized in the postsynaptic membrane. If this were true, LTD should be inducible in neurons from only one of the two projection systems of the striatum. Using transgenic mice in which neurons that contribute to these two systems are labeled, we show that this is not the case. Rather, in both cell types, the D(2) receptor dependence of LTD induction reflects the need to lower M(1) muscarinic receptor activity-a goal accomplished by D(2) receptors on cholinergic interneurons. In addition to reconciling discordant tracts of the striatal literature, these findings point to cholinergic interneurons as key mediators of dopamine-dependent striatal plasticity and learning.

M1 muscarinic receptors contribute to, whereas M4 receptors inhibit, dopamine D1 receptor-induced [3H]-cyclic AMP accumulation in rat striatal slices

In rat striatal slices labelled with [(3)H]-adenine and in the presence of 1 mM 3-isobutyl-1-methylxantine (IBMX), cyclic [(3)H]-AMP ([(3)H]-cAMP) accumulation induced by the dopamine D(1) receptor agonist SKF-81297 (1 microM; 177 +/- 13% of basal) was inhibited by the general muscarinic agonist carbachol (maximum inhibition 72 +/- 3%, IC(50) 0.30 +/- 0.06 microM). The muscarinic toxin 7 (MT-7), a selective antagonist at muscarinic M(1) receptors, reduced the effect of SKF-81297 by 40+/-7% (IC(50) 251+/- 57 pM) and enhanced the inhibitory action of a submaximal (1 microM) concentration of carbachol (69 +/- 4% vs. 40 +/- 7% inhibition, IC(50) 386 +/- 105 pM). The toxin MT-1, agonist at M(1) receptors, stimulated [(3)H]-cAMP accumulation in a modest but significant manner (137 +/- 11% of basal at 400 nM), an action additive to that of D(1) receptor activation and blocked by MT-7 (10 nM). The effects of MT-7 on D(1) receptor-induced [(3)H]-cAMP accumulation and the carbachol inhibition were mimicked by the PKC inhibitors Ro-318220 (200 nM) and Gö-6976 (200 nM). Taken together our results indicate that in addition to the inhibitory role of M(4) receptors, in rat striatum acetylcholine stimulates cAMP formation through the activation of M(1 )receptors and PKC stimulation.

The selective m1 muscarinic antagonist MT-7 blocks pilocarpine-induced striatal Fos expression

Systemic administration of the nonselective muscarinic agonist pilocarpine induces pronounced striatal Fos expression. Intrastriatal injections of the mamba snake toxin MT-7, a highly selective and irreversible m1 muscarinic antagonist, drastically attenuated this response when given 2, but not 8, days before pilocarpine. In contrast, MT-7 did not alter the response to amphetamine (5 mg/kg). These results suggest that pilocarpine induces Fos expression in the striatum as a result of stimulating m1 muscarinic receptors located within this structure and demonstrate the utility of the MT-7 for in vivo studies of cholinergic function.

Cholinergic suppression of KCNQ channel currents enhances excitability of striatal medium spiny neurons

In response to glutamatergic synaptic drive, striatal medium spiny neurons in vivo transition to a depolarized "up state" near spike threshold. In the up state, medium spiny neurons either depolarize enough to spike or remain below spike threshold and are silent before returning to the hyperpolarized "down state." Previous work has suggested that subthreshold K+ channel currents were responsible for this dichotomous behavior, but the channels giving rise to the current and the factors determining its engagement have been a mystery. To move toward resolution of these questions, perforated-patch recordings from medium spiny neurons in tissue slices were performed. K+ channels with pharmacological and kinetic features of KCNQ channels potently regulated spiking at up-state potentials. Single-cell reverse transcriptase-PCR confirmed the expression of KCNQ2, KCNQ3, and KCNQ5 mRNAs in medium spiny neurons. KCNQ channel currents in these cells were potently reduced by M1 muscarinic receptors, because the effects of carbachol were blocked by M1 receptor antagonists and lost in neurons lacking M1 receptors. Reversal of the modulation was blocked by a phosphoinositol 4-kinase inhibitor, indicating a requirement for phosphotidylinositol 4,5-bisphosphate resynthesis for recovery. Inhibition of protein kinase C reduced the efficacy of the muscarinic modulation. Finally, acceleration of cholinergic interneuron spiking with 4-aminopyridine mimicked the effects of exogenous agonist application. Together, these results show that KCNQ channels are potent regulators of the excitability of medium spiny neurons at up-state potentials, and they are modulated by intrastriatal cholinergic interneurons, providing a mechanistic explanation for variability in spiking during up states seen in vivo.

Mechanisms of long-lasting enhancement of corticostriatal neurotransmission by taurine

The long-lasting enhancement of corticostriatal neurotransmission by taurine, LLE-TAU represents a complex phenomenon requiring concurrent activation of glycine, DA and Ach receptors as well as taurine uptake. The data on the mechanisms of corticostriatal LLE-TAU can be integrated in the following scheme. Taurine interaction with glycine and GABAA receptors causes depolarization of striatal medium spiny cells (Chepkova et al., 2002) which is enhanced by taurine electrogenic uptake by TauT (Sarkar et al., 2003). This depolarization leads to Ca2+ entry via low voltage gated Ca2+ channels. Muscarinic M1 receptors are expressed in medium spiny neurons (Yan et al., 2001) and regulate their excitability mostly via phospholipase C (PLC)/PKC cascade (Lin et al., 2004). Concurrent activation of M1 and PLC-coupled D1 receptors (O'Sullivan et al., 2004) can amplify Ca2+ signal via IP3- stimulated Ca2+ release from intracellular stores and stimulate PKC.

G-protein-coupled receptor modulation of striatal CaV1.3 L-type Ca2+ channels is dependent on a Shank-binding domain

Voltage-gated L-type Ca2+ channels are key determinants of synaptic integration and plasticity, dendritic electrogenesis, and activity-dependent gene expression in neurons. Fulfilling these functions requires appropriate channel gating, perisynaptic targeting, and linkage to intracellular signaling cascades controlled by G-protein-coupled receptors (GPCRs). Surprisingly, little is known about how these requirements are met in neurons. The studies described here shed new light on how this is accomplished. We show that D2 dopaminergic and M1 muscarinic receptors selectively modulate a biophysically distinctive subtype of L-type Ca2+ channels (CaV1.3) in striatal medium spiny neurons. The splice variant of these channels expressed in medium spiny neurons contains cytoplasmic Src homology 3 and PDZ (postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1) domains that bind the synaptic scaffolding protein Shank. Medium spiny neurons coexpressed CaV1.3-interacting Shank isoforms that colocalized with PSD-95 and CaV1.3a channels in puncta resembling spines on which glutamatergic corticostriatal synapses are formed. The modulation of CaV1.3 channels by D2 and M1 receptors was disrupted by intracellular dialysis of a peptide designed to compete for the CaV1.3 PDZ domain but not with one targeting a related PDZ domain. The modulation also was disrupted by application of peptides targeting the Shank interaction with Homer. Upstate transitions in medium spiny neurons driven by activation of glutamatergic receptors were suppressed by genetic deletion of CaV1.3 channels or by activation of D2 dopaminergic receptors. Together, these results suggest that Shank promotes the assembly of a signaling complex at corticostriatal synapses that enables key GPCRs to regulate L-type Ca2+ channels and the integration of glutamatergic synaptic events.

The pathogenesis of clinical depression: stressor- and cytokine-induced alterations of neuroplasticity

Stressful events promote neurochemical changes that may be involved in the provocation of depressive disorder. In addition to neuroendocrine substrates (e.g. corticotropin releasing hormone, and corticoids) and central neurotransmitters (serotonin and GABA), alterations of neuronal plasticity or even neuronal survival may play a role in depression. Indeed, depression and chronic stressor exposure typically reduce levels of growth factors, including brain-derived neurotrophic factor and anti-apoptotic factors (e.g. bcl-2), as well as impair processes of neuronal branching and neurogenesis. Although such effects may result from elevated corticoids, they may also stem from activation of the inflammatory immune system, particularly the immune signaling cytokines. In fact, several proinflammatory cytokines, such as interleukin-1, tumor necrosis factor-alpha and interferon-gamma, influence neuronal functioning through processes involving apoptosis, excitotoxicity, oxidative stress and metabolic derangement. Support for the involvement of cytokines in depression comes from studies showing their elevation in severe depressive illness and following stressor exposure, and that cytokine immunotherapy (e.g. interferon-alpha) elicited depressive symptoms that were amenable to antidepressant treatment. It is suggested that stressors and cytokines share a common ability to impair neuronal plasticity and at the same time altering neurotransmission, ultimately contributing to depression. Thus, depressive illness may be considered a disorder of neuroplasticity as well as one of neurochemical imbalances, and cytokines may act as mediators of both aspects of this illness.

Cholinergic Control of Firing Pattern and Neurotransmission in Rat Neostriatal Projection Neurons: Role of CaV2.1 and CaV2.2 Ca2+ Channels

Besides a reduction of L-type Ca2+-currents (CaV1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through CaV2.1 (P/Q) and CaV2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.

Biphasic effects of oxotremorine-M on turning behavior induced by caffeine in 6-OHDA-lesioned rats

This work studied the interactions between cholinergic and adenosine systems in the denervated striatum. For that purpose, we evaluated the effects of an intrastriatal administration of the muscarincic receptor agonist, oxotremorine-M on turning behavior induced by systemic caffeine in unilaterally 6-hydroxydopamine-lesioned rats. Low doses of oxotremorine-M (0.1 ng/microl) enhanced, whereas high doses (100 ng/microl) attenuated contralateral turning induced by caffeine. These results support a functional link between muscarinic and adenosinergic systems in the denervated striatum and suggest opposite actions of muscarinic M2 and M1 receptors on caffeine-induced turning behavior.

The basal ganglia: anatomy, physiology, and pharmacology

The basal ganglia are perceived as important nodes in cortico-subcortical networks involved in the transfer, convergence, and processing of information in motor, cognitive, and limbic domains. How this integration might occur remains a matter of some debate, particularly given the consistent finding in anatomic and physiologic studies of functional segregation in cortico-subcortical loops. More recent theories, however, have raised the notion that modality-specific information might be integrated not spatially, but rather temporally, by coincident processing in discrete neuronal populations. Basal ganglia neurotransmitters, given their diverse roles in motor performance, learning, working memory, and reward-related activity are also likely to play an important role in the integration of cerebral activity. Further work will elucidate this to a greater extent, but for now, it is clear that the basal ganglia form an important nexus in the binding of cognitive, limbic, and motor information into thought and action.

Effects of muscarinic acetylcholine receptor activation on membrane currents and intracellular messengers in medium spiny neurones of the rat striatum

Acetylcholine, acting through muscarinic receptors, modulates the excitability of striatal medium spiny neurones. However, the underlying membrane conductances and intracellular signalling pathways have not been fully determined. Our aim was to characterize excitatory effects mediated by M1 muscarinic acetylcholine receptors in these neurones using whole-cell patch-clamp recordings in brain slices of postnatal rats. Under voltage-clamp, muscarine evoked an inward current associated with an increase in cell membrane resistance. The current, which reversed at -85 mV, was sensitive to the M1 receptor antagonist pirenzepine. Blocking the potassium conductance attenuated the response and the residual current was further reduced by ruthenium red (50 microm) and reversed at +15 mV. Simultaneous recordings from cholinergic interneurones and medium spiny neurones in conjunction with spike-triggered averaging revealed small unitary excitatory postsynaptic currents in four of 39 cell pairs tested. The muscarine-induced inward current was attenuated by a phospholipase C (PLC) inhibitor, U73122, but not by a protein kinase C inhibitor, chelerythrine, or by the intracellular calcium chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid, suggesting that the current was associated with PLC in a protein kinase C- and Ca2+ -independent manner. The phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) reduced the recovery of the inward current, indicating that the recovery process was dependent on the removal of diacylglycerol and/or inositol 1,4,5 triphosphate or resynthesis of phospholipid phosphatidylinositol 4,5-bisphophate. Ratiometric measurement of intracellular calcium after cell loading with fura-2 demonstrated a muscarine-induced increase in calcium signal that originated mainly from intracellular stores. Thus, the cholinergic excitatory effect in striatal medium spiny neurones, which is important in motor disorders associated with altered cholinergic transmission in the striatum such as Parkinson's disease, is mediated through M1 receptors and the PLC-dependent pathway.

Differential effects of M1 muscarinic receptor blockade and nicotinic receptor blockade in the dorsomedial striatum on response reversal learning

The present studies determined whether blockade of M(1)-like muscarinic or nicotinic cholinergic receptors in the dorsomedial striatum affects acquisition or reversal learning of a response discrimination. Testing occurred in a modified cross-maze across two consecutive sessions. In the acquisition phase, a rat learned to turn to the left or to the right. In the reversal learning phase, a rat learned to turn in the opposite direction as required during acquisition. Experiment 1 investigated the effects of the M(1)-like muscarinic receptor antagonist, pirenzepine infused into the dorsomedial striatum on acquisition and reversal learning. Experiment 2 examined the effects of the nicotinic cholinergic antagonist, mecamylamine injected into the dorsomedial striatum on acquisition and reversal learning. Bilateral injections of pirenzepine at 10 microg, but not 1 microg, selectively impaired reversal learning. Analysis of the errors indicated that pirenzepine treatment did not impair the initial shift, but increased reversions back to the original response choice following the initial shift. Bilateral injections of mecamylamine, 6 or 18 microg, did not affect acquisition or reversal learning. The results suggest that activation of M(1) muscarinic cholinergic receptors, but not nicotinic cholinergic receptors, in the dorsomedial striatum is important for facilitating the flexible shifting of response patterns.

M4 muscarinic receptors regulate the dynamics of cholinergic and dopaminergic neurotransmission: relevance to the pathophysiology and treatment of related CNS pathologies

Dopaminergic dysfunction is an important pathogenetic factor for brain pathologies such as Parkinson's disease, ADHD, schizophrenia, and addiction as well as for metabolic disorders and anorexia. Dopaminergic neurons projecting from the midbrain to forebrain regions, such as the nucleus accumbens and the prefrontal cortex, regulate motor and cognitive functions and coordinate the patterned response of the organism to sensory, affective, and rewarding stimuli. In this study, we showed that dopaminergic neurotransmission is highly dependent on M4 cholinergic muscarinic receptor function. Using in vivo microdialysis, we found elevated dopamine (DA) basal values and enhanced DA response to psychostimulants in the nucleus accumbens of M4 knockout mice. We also demonstrated impaired homeostatic control of cholinergic activity that leads to increased basal acetylcholine efflux in the midbrain of these animals. Thus, loss of M4 muscarinic receptor control of cholinergic function effectuates a state of dopaminergic hyperexcitability. This may be responsible for pathological conditions, in which appetitive motivation as well as affective and cognitive processing is impaired. We propose that M4 receptor agonists could represent an innovative strategy for the treatment of pathologies associated with hyperdopaminergia.

Acetylcholine actions in the dorsomedial striatum support the flexible shifting of response patterns

There is accumulating evidence that the dorsomedial striatum plays a significant role in the learning of a new response pattern and the inhibiting of old response patterns when conditions demand a shift in strategies. This paper proposes that activity of cholinergic neurons in the dorsomedial striatum is critical for enabling behavioral flexibility when there is a change in task contingencies. Recent experimental findings are provided supporting this idea. Measuring acetylcholine efflux from the dorsomedial striatum during the acquisition and reversal learning of a spatial discrimination shows that acetylcholine efflux selectively increases during reversal learning as a rat begins to learn a newly reinforced spatial location, but returns to near basal levels when a rat reliably executes the new choice pattern. Experimental findings are also described indicating that the blockade of muscarinic cholinergic receptors in the dorsomedial striatum does not impair acquisition of an egocentric response discrimination, but impairs reversal learning of an egocentric response discrimination. Based on these results, increased cholinergic activity at muscarinic receptors is part of a neurochemical process in the dorsomedial striatum that allows inhibition of a previously relevant response pattern while learning a new response pattern. In situations that demand behavioral flexibility, muscarinic cholinergic activity in the dorsomedial striatum may directly influence corticostriatal plasticity to produce changes in response patterns.

Altered Striatal Function and Muscarinic Cholinergic Receptors in Acetylcholinesterase Knockout Mice

Cholinesterase inhibitors are commonly used to improve cognition and treat psychosis and other behavioral symptoms in Alzheimer's disease, Parkinson's disease, and other neuropsychiatric conditions. However, mechanisms may exist that down-regulate the synaptic response to altered cholinergic transmission, thus limiting the efficacy of cholinomimetics in treating disease. Acetylcholinesterase knockout (AChE-/-) mice were used to investigate the neuronal adaptations to diminished synaptic acetylcholine (ACh) metabolism. The striatum of AChE-/- mice showed no changes in choline acetyltransferase activity or levels of the vesicular ACh transporter but showed striking 60% increases in the levels of the highaffinity choline transporter. This transporter takes choline from the synapse into the neuron for resynthesis of ACh. In addition, the striata of AChE-/- mice showed dramatic reductions in levels of the M1, M2, and M4 muscarinic ACh receptors (mAChRs), but no alterations in dopamine receptors or the beta2 subunit of nicotinic receptors. M1, M2, and M4 also showed decreased dendritic and cell surface distributions and enhanced intracellular localizations in striatal neurons of AChE-/- mice. mAChR antagonist treatment reversed the shifts in mAChR distribution, indicating that internalized receptors in AChE-/- mice can recover to basal distributions. Finally, AChE-/- mice showed increased sensitivity to mAChR antagonist-induced increases in locomotor activity, demonstrating functional mAChR down-regulation. mAChR downregulation in AChE-/- mice has important implications for the long-term use of cholinesterase inhibitors and other cholinomimetics in treating disorders characterized by perturbed cholinergic function.

Brain-derived neurotrophic factor activation of NFAT (nuclear factor of activated T-cells)-dependent transcription: a role for the transcription factor NFATc4 in neurotrophin-mediated gene expression

A member of the neurotrophin family, brain-derived neurotrophic factor (BDNF) regulates neuronal survival and differentiation during development. Within the adult brain, BDNF is also important in neuronal adaptive processes, such as the activity-dependent plasticity that underlies learning and memory. These long-term changes in synaptic strength are mediated through alterations in gene expression. However, many of the mechanisms by which BDNF is linked to transcriptional and translational regulation remain unknown. Recently, the transcription factor NFATc4 (nuclear factor of activated T-cells isoform 4) was discovered in neurons, where it is believed to play an important role in long-term changes in neuronal function. Interestingly, NFATc4 is particularly sensitive to the second messenger systems activated by BDNF. Thus, we hypothesized that NFAT-dependent transcription may be an important mediator of BDNF-induced plasticity. In cultured rat CA3-CA1 hippocampal neurons, BDNF activated NFAT-dependent transcription via TrkB receptors. Inhibition of calcineurin blocked BDNF-induced nuclear translocation of NFATc4, thus preventing transcription. Further, phospholipase C was a critical signaling intermediate between BDNF activation of TrkB and the initiation of NFAT-dependent transcription. Both inositol 1,4,5-triphosphate (IP3)-mediated release of calcium from intracellular stores and activation of protein kinase C were required for BDNF-induced NFAT-dependent transcription. Finally, increased expression of IP3 receptor 1 and BDNF after neuronal exposure to BDNF was linked to NFAT-dependent transcription. These results suggest that NFATc4 plays a crucial role in neurotrophin-mediated synaptic plasticity.

Impaired modulation of GABAergic transmission by muscarinic receptors in a mouse transgenic model of Alzheimer's disease

It has long been recognized that muscarinic acetylcholine receptors (mAChRs) are crucial for the control of cognitive processes, and drugs that activate mAChRs are helpful in ameliorating cognitive deficits of Alzheimer's disease (AD). On the other hand, GABAergic transmission in prefrontal cortex (PFC) plays a key role in "working memory" via controlling the timing of neuronal activity during cognitive operations. To test whether the muscarinic and gamma-aminobutyric acid (GABA) system are interconnected in normal cognition and dementia, we examined the muscarinic regulation of GABAergic transmission in PFC of an animal model of AD. Transgenic mice overexpressing a mutant gene for beta-amyloid precursor protein (APP) show behavioral and histopathological abnormalities resembling AD and, therefore, were used as an AD model. Application of the mAChR agonist carbachol significantly increased the spontaneous inhibitory postsynaptic current (sIPSC) frequency and amplitude in PFC pyramidal neurons from wild-type animals. In contrast, carbachol failed to increase the sIPSC amplitude in APP transgenic mice, whereas the carbachol-induced increase of the sIPSC frequency was not significantly changed in these mutants. Similar results were obtained in rat PFC slices pretreated with the beta-amyloid peptide (Abeta). Inhibiting protein kinase C (PKC) blocked the carbachol enhancement of sIPSC amplitudes, implicating the PKC dependence of this mAChR effect. In APP transgenic mice, carbachol failed to activate PKC despite the apparently normal expression of mAChRs. These results show that the muscarinic regulation of GABA transmission is impaired in the AD model, probably due to the Abeta-mediated interference of mAChR activation of PKC.

Muscarinic potentiation of GABA(A) receptor currents is gated by insulin signaling in the prefrontal cortex

Cholinergic neurotransmission and insulin signaling in cognitive areas, such as the prefrontal cortex (PFC), play a key role in regulating learning and memory. However, the cellular mechanisms by which this regulation occurs are unclear. Because GABAergic inhibition in the PFC controls the timing of neuronal activity during cognitive operations, we examined the potential regulation of GABA transmission by cholinergic and insulin signaling in PFC pyramidal neurons. Activation of muscarinic acetylcholine receptors (mAChRs) with carbachol produced an enhancement of GABA(A) receptor currents in acutely dissociated cells after a short treatment with insulin. Inhibiting phosphoinositide-3 kinase (PI3K), a downstream target of insulin signaling, eliminated this effect as well as the carbachol-induced enhancement of GABAergic miniature IPSC amplitudes in PFC slices. The muscarinic potentiation of GABA(A) currents was blocked by PKC inhibitors, broad-spectrum protein tyrosine kinase inhibitors, and specific inhibitors of the nonreceptor tyrosine kinase Src. Additionally, muscarinic receptors in PFC slices activated PKC and the focal adhesion kinase Pyk2 (a potential molecular link between PKC and Src) in a PI3K-dependent manner. Together, our results show that mAChR activation in PFC pyramidal neurons enhances GABA(A) receptor functions through a PKC-dependent, Src-mediated signaling cascade that is gated by an insulin/PI3K pathway. Given the significance of GABAergic transmission in regulating PFC functions, our results provide a novel mechanism for understanding the role of cholinergic systems and insulin signaling in learning and memory.

Muscarinic and nicotinic cholinergic mechanisms in the mesostriatal dopamine systems

The striatum and its dense dopaminergic innervation originating in the midbrain, primarily from the substantia nigra pars compacta and the ventral tegmental area, compose the mesostriatal dopamine (DA) systems. The nigrostriatal system is involved mainly in motor coordination and in disorders such as Tourette's syndrome, Huntington's disease, and Parkinson's disease. The dopaminergic projections from the ventral tegmental area to the striatum participate more in the processes that shape behaviors leading to reward, and addictive drugs act upon this mesolimbic system. The midbrain DA areas receive cholinergic innervation from the pedunculopontine tegmentum and the laterodorsal pontine tegmentum, whereas the striatum receives dense cholinergic innervation from local interneurons. The various neurons of the mesostriatal systems express multiple types of muscarinic and nicotinic acetylcholine receptors as well as DA receptors. Especially in the striatum, the dense mingling of dopaminergic and cholinergic constituents enables potent interactions. Evidence indicates that cholinergic and dopaminergic systems work together to produce the coordinated functioning of the striatum. Loss of that cooperative activity contributes to the dysfunction underlying Parkinson's disease.

The role of metabotropic glutamate receptors in regulation of striatal proenkephalin expression: implications for the therapy of Parkinson′s disease

Overactivity of the striatopallidal pathway, associated with an enhancement of enkephalin expression, has been suggested to contribute to the development of parkinsonian symptoms. The aim of the present study was to examine whether the blockade of group I metabotropic glutamate receptors: subtypes 1 and 5 (mGluR1/5), or stimulation of group II: subtypes 2 and 3 (mGluR2/3) may normalize enkephalin expression in the striatopallidal pathway in an animal model of parkinsonism. The proenkephalin mRNA level measured by in situ hybridization in the striatum was increased by pretreatments with haloperidol (1.5 mg/kg s.c., three times, 3 h apart). Triple (3 h apart), bilateral, intrastriatal administration of selective antagonists of mGluR1: (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (3 x 5 microg/0.5 microl) or 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (3 x 2.5 microg/0.5 microl), reversed the haloperidol-induced increases in proenkephalin mRNA levels in the rostral and central regions of the striatum. Similarly, repeated (6 times, 1.5 h apart), systemic injections of an antagonist of mGluR5, 2-methyl-6-(phenylethynyl)pyridine (6 x 10 mg/kg i.p.) counteracted an increase in the striatal proenkephalin mRNA expression elicited by haloperidol. None of the abovementioned antagonists of mGluR1 and mGluR5 per se influenced the proenkephalin expression. Differential effects were induced by agonists of the group II mGluRs, viz. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine administered intraventricularly (3 times at 0.1-0.2 microg/4 microl, 3 h apart) increased both the normal and haloperidol-increased proenkephalin mRNA level, whereas (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate injected intrastriatally (3 times at 15 microg/0.5 microl, 3 h apart) was ineffective. The present study indicates that the blockade of striatal glutamate receptors belonging to the group I (mGluR1 and mGluR5) but not stimulation of the group II mGluRs may normalize the function of the striatopallidal pathway in an animal model of parkinsonism, which may be important for future antiparkinsonian therapy in humans.

Muscarinic receptors involved in the subthreshold cholinergic actions of neostriatal spiny neurons

Administration of the peptide MT-1 (48 nM), a selective agonist of muscarinic M(1)-type receptors, mimicked the subthreshold actions of muscarine (1 microM) on neostriatal neurons, i.e., it produced a reduction in subthreshold inward rectification leading to an enhancement in input resistance (R(N)) and evoked discharge. In all recorded cells, MT-1 effects remained in the presence of the specific peptidergic antagonist of the M(4)-type receptor, MT-3 (10 nM), but were blocked by the specific M(1)-type receptor antagonist MT-7 (5 nM). These results suggest that most muscarinic facilitatory actions in the subthreshold voltage range occur through M(1)-type receptors. However, in a fraction of cells (40%) muscarine produced an excitability enhancement not blocked by MT-7. This additional facilitatory action, not present when using MT-1, was blocked by MT-3, suggesting it was mediated by M(4)-type receptor activation. This facilitation could not be blocked by Cs(+), TTX, or Cd(2+), but only by a reduction in extracellular sodium. This result is the first evidence that M(4)-type receptor activation enhances a cationic inward current in a fraction of neostriatal projection neurons.

Cholinergic interneuron characteristics and nicotinic properties in the striatum

The neostriatum (dorsal striatum) is composed of the caudate and putamen. The ventral striatum is the ventral conjunction of the caudate and putamen that merges into and includes the nucleus accumbens and striatal portions of the olfactory tubercle. About 2% of the striatal neurons are cholinergic. Most cholinergic neurons in the central nervous system make diffuse projections that sparsely innervate relatively broad areas. In the striatum, however, the cholinergic neurons are interneurons that provide very dense local innervation. The cholinergic interneurons provide an ongoing acetylcholine (ACh) signal by firing action potentials tonically at about 5 Hz. A high concentration of acetylcholinesterase in the striatum rapidly terminates the ACh signal, and thereby minimizes desensitization of nicotinic acetylcholine receptors. Among the many muscarinic and nicotinic striatal mechanisms, the ongoing nicotinic activity potently enhances dopamine release. This process is among those in the striatum that link the two extensive and dense local arbors of the cholinergic interneurons and dopaminergic afferent fibers. During a conditioned motor task, cholinergic interneurons respond with a pause in their tonic firing. It is reasonable to hypothesize that this pause in the cholinergic activity alters action potential dependent dopamine release. The correlated response of these two broad and dense neurotransmitter systems helps to coordinate the output of the striatum, and is likely to be an important process in sensorimotor planning and learning.

Multiple muscarinic acetylcholine receptor subtypes modulate striatal dopamine release, as studied with M1-M5 muscarinic receptor knock-out mice

A proper balance between striatal muscarinic cholinergic and dopaminergic neurotransmission is required for coordinated locomotor control. Activation of striatal muscarinic acetylcholine receptors (mAChRs) is known to modulate striatal dopamine release. To identify the mAChR subtype(s) involved in this activity, we used genetically altered mice that lacked functional M1-M5 mAChRs [knock-out (KO) mice]. In superfused striatal slices from wild-type mice, the non-subtype-selective muscarinic agonist oxotremorine led to concentration-dependent increases in potassium-stimulated [3H]dopamine release (by up to 60%). The lack of M1 or M2 receptors had no significant effect on the magnitude of these responses. Strikingly, oxotremorine-mediated potentiation of stimulated striatal [3H]dopamine release was abolished in M4 receptor KO mice, significantly increased in M3 receptor-deficient mice, and significantly reduced (but not abolished) in M5 receptor KO mice. Additional release studies performed in the presence of tetrodotoxin suggested that the dopamine release-stimulating M4 receptors are probably located on neuronal cell bodies, but that the release-facilitating M5 and the release-inhibiting M3 receptors are likely to be located on nerve terminals. Studies with the GABA(A) receptor blocker bicuculline methochloride suggested that M3 and M4 receptors mediate their dopamine release-modulatory effects via facilitation or inhibition, respectively, of striatal GABA release. These results provide unambiguous evidence that multiple mAChR subtypes are involved in the regulation of striatal dopamine release. These findings should contribute to a better understanding of the important functional roles that the muscarinic cholinergic system plays in striatal function.