Postsynaptic integration of cholinergic and dopaminergic signals on medium-sized GABAergic projection neurons in the neostriatum

Activations of muscarinic M1 receptors in the anterior cingulate cortex contribute to the antinociceptive effect via GABAergic transmission

Background Cholinergic systems regulate the synaptic transmission resulting in the contribution of the nociceptive behaviors. Anterior cingulate cortex is a key cortical area to play roles in nociception and chronic pain. However, the effect of the activation of cholinergic system for nociception is still unknown in the cortical area. Here, we tested whether the activation of cholinergic receptors can regulate nociceptive behaviors in adult rat anterior cingulate cortex by integrative methods including behavior, immunohistochemical, and electrophysiological methods. Results We found that muscarinic M₁ receptors were clearly expressed in the anterior cingulate cortex. Using behavioral tests, we identified that microinjection of a selective muscarinic M₁ receptors agonist McN-A-343 into the anterior cingulate cortex dose dependently increased the mechanical threshold. In contrast, the local injection of McN-A-343 into the anterior cingulate cortex showed normal motor function. The microinjection of a selective M₁ receptors antagonist pirenzepine blocked the McN-A-343-induced antinociceptive effect. Pirenzepine alone into the anterior cingulate cortex decreased the mechanical thresholds. The local injection of the GABA_A receptors antagonist bicuculline into the anterior cingulate cortex also inhibited the McN-A-343-induced antinociceptive effect and decreased the mechanical threshold. Finally, we further tested whether the activation of M₁ receptors could regulate GABAergic transmission using whole-cell patch-clamp recordings. The activation of M₁ receptors enhanced the frequency of spontaneous and miniature inhibitory postsynaptic currents as well as the amplitude of spontaneous inhibitory postsynaptic currents in the anterior cingulate cortex. Conclusions These results suggest that the activation of muscarinic M₁ receptors in part increased the mechanical threshold by increasing GABAergic transmitter release and facilitating GABAergic transmission in the anterior cingulate cortex.

Pathophysiology of depression and innovative treatments: remodeling glutamatergic synaptic connections

Despite the complexity and heterogeneity of mood disorders, basic and clinical research studies have begun to elucidate the pathophysiology of depression and to identify rapid, efficacious antidepressant agents. Stress and depression are associated with neuronal atrophy, characterized by loss of synaptic connections in key cortical and limbic brain regions implicated in depression. This is thought to occur in part via decreased expression and function of growth factors, such as brain-derived neurotrophic factor (BDNF), in the prefrontal cortex (PFC) and hippocampus. These structural alterations are difficult to reverse with typical antidepressants. However, recent studies demonstrate that ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist that produces rapid antidepressant actions in treatment-resistant depressed patients, rapidly increases spine synapses in the PFC and reverses the deficits caused by chronic stress. This is thought to occur by disinhibition of glutamate transmission, resulting in a rapid but transient burst of glutamate, followed by an increase in BDNF release and activation of downstream signaling pathways that stimulate synapse formation. Recent work demonstrates that the rapid-acting antidepressant effects of scopolamine, a muscarinic receptor antagonist, are also associated with increased glutamate transmission and synapse formation. These findings have resulted in testing and identification of additional targets and agents that influence glutamate transmission and have rapid antidepressant actions in rodent models and in clinical trials. Together these studies have created tremendous excitement and hope for a new generation of rapid, efficacious antidepressants.

Scopolamine rapidly increases mammalian target of rapamycin complex 1 signaling, synaptogenesis, and antidepressant behavioral responses

BACKGROUND

Clinical studies report that scopolamine, an acetylcholine muscarinic receptor antagonist, produces rapid antidepressant effects in depressed patients, but the mechanisms underlying the therapeutic response have not been determined. The present study examines the role of the mammalian target of rapamycin complex 1 (mTORC1) and synaptogenesis, which have been implicated in the rapid actions of N-methyl-D-aspartate receptor antagonists.

METHODS

The influence of scopolamine on mTORC1 signaling was determined by analysis of the phosphorylated and activated forms of mTORC1 signaling proteins in the prefrontal cortex (PFC). The numbers and function of spine synapses were analyzed by whole cell patch clamp recording and two-photon image analysis of PFC neurons. The actions of scopolamine were examined in the forced swim test in the absence or presence of selective mTORC1 and glutamate receptor inhibitors.

RESULTS

The results demonstrate that a single, low dose of scopolamine rapidly increases mTORC1 signaling and the number and function of spine synapses in layer V pyramidal neurons in the PFC. Scopolamine administration also produces an antidepressant response in the forced swim test that is blocked by pretreatment with the mTORC1 inhibitor or by a glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor antagonist.

CONCLUSIONS

Taken together, the results demonstrate that the antidepressant actions of scopolamine require mTORC1 signaling and are associated with increased glutamate transmission, and synaptogenesis, similar to N-methyl-D-aspartate receptor antagonists. These findings provide novel targets for safer and more efficacious rapid-acting antidepressant agents.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Neurotransmitter receptor heteromers and their integrative role in 'local modules': the striatal spine module

'Local module' is a fundamental functional unit of the central nervous system that can be defined as the minimal portion of one or more neurons and/or one or more glial cells that operates as an independent integrative unit. This review focuses on the importance of neurotransmitter receptor heteromers for the operation of local modules. To illustrate this, we use the striatal spine module (SSM), comprised of the dendritic spine of the medium spiny neuron (MSN), its glutamatergic and dopaminergic terminals and astroglial processes. The SSM is found in the striatum, and although aspects such as neurotransmitters and receptors will be specific to the SSM, some general principles should apply to any local module in the brain. The analysis of some of the receptor heteromers in the SSM shows that receptor heteromerization is associated with particular elaborated functions in this local module. Adenosine A(2A) receptor-dopamine D(2) receptor-glutamate metabotropic mGlu(5) receptor heteromers are located adjacent to the glutamatergic synapse of the dendritic spine of the enkephalin MSN, and their cross-talk within the receptor heteromers helps to modulate postsynaptic plastic changes at the glutamatergic synapse. A(1) receptor-A(2A) receptor heteromers are found in the glutamatergic terminals and the molecular cross-talk between the two receptors in the heteromer helps to modulate glutamate release. Finally, dopamine D(2) receptor-non-alpha(7) nicotinic acetylcholine receptor heteromers, which are located in dopaminergic terminals, introduce the new concept of autoreceptor heteromer.

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.

ROLES OF .MU.-OPIOID RECEPTORS IN DEVELOPMENT OF TOLERANCE TO DIISOPROPYLFLUOROPHOSPHATE (DFP)

Anatomical evidence indicates that cholinergic and opioidergic systems are co-localized and acting on the same neuron. However, the regulatory mechanisms between cholinergic and opioidergic system have not been well characterized. In the present study, the potential involvement of mu-opioid receptors in mediating the changes of toxic signs and muscarinic receptor binding after administration of irreversible anti-acetylcholinesterase diisopropylfluorophosphate (DFP) was investigated. DFP (1 mg/kg/day, subcutaneous injection, s.c.)-induced tremors and chewing movements were monitored during the 28-day treatment period in mu-opioid receptor knockout and wild type mice. Autoradiographic studies of total, M1, and M2 muscarinic receptors were conducted using [(3)H]-quinuclidinyl benzilate, [(3)H]-pirenzepine, and [(3)H]-AF-DX384 as ligands, respectively. DFP-induced tremors in both mu-opioid receptor knockout and wild type mice showed tolerance development. However, DFP-induced tremors in mu-opioid receptor knockout mice showed delayed tolerance development than that of DFP-treated wild type controls. DFP-induced chewing movements in both mu-opioid receptor knockout and wild type mice failed to show development of tolerance after four weeks of treatment. M2 muscarinic receptor binding of DFP-treated mu-opioid receptor knockout mice was significantly decreased than that of the DFP-treated wild type controls in the striatum, but not in the cortex and hippocampus. However, there were no significant differences in total and M1 muscarinic receptor binding between DFP-treated mu-opioid receptor knockout and wild type mice in the cortex, striatum and hippocampus. These studies indicate that mu-opioid receptors play an important role through the striatal M2 muscarinic receptors to regulate the development of tolerance to DFP-induced tremors.

Increased diisopropylfluorophosphate-induced toxicity in ?-opioid receptor knockout mice

The potential involvement of mu-opioid receptors in mediating the changes of toxic signs and muscarinic receptor bindings after acute administration of irreversible antiacetylcholinesterase diisopropylfluorophosphate (DFP) was investigated. DFP-induced chewing movement and tremors were monitored for a period of 180 min in mu-opioid receptor knockout and wild-type mice. The autoradiographic studies of total, M1, and M2 muscarinic receptors were conducted using [(3)H]quinuclidinyl benzilate, [(3)H]pirenzepine, and [(3)H]AF-DX384 as ligands, respectively. Saline-treated mu-opioid receptor knockout and wild-type mice did not show chewing movement or tremors. Although DFP (1, 2, or 3 mg/kg, subcutaneous injection, s.c.)-induced chewing movement and tremors were shown in a dose-dependent manner, there were no significant differences in tremors induced by 1 or 2 mg/kg of DFP between mu-opioid receptor knockout and wild-type mice. There were also no significant differences in chewing movement induced by all doses of DFP between mu-opioid receptor knockout and wild-type mice. However, DFP (3 mg/kg)-induced tremors in mu-opioid receptor knockout mice were significantly increased over those in wild-type controls. Acetylcholinesterase activity in the striatum of saline-treated mu-opioid receptor knockout mice was significantly higher than that of the wild-type controls. After administration of DFP, acetylcholinesterase activity in the striatum of both mu-opioid receptor knockout and wild-type mice was significantly decreased (more than 36%, 58%, and 94% reduced at the doses of 1, 2, and 3 mg/kg, respectively) than that of their respective saline controls. M2 muscarinic receptor binding in saline-treated mu-opioid receptor knockout mice was significantly lower than that of the wild-type controls in the striatum. However, there were no significant differences in total, M1, or M2 muscarinic receptor binding in the cortex, striatum, or hippocampus of mu-opioid receptor knockout and wild-type mice after DFP administration. Our data show increased DFP-induced tremors, compensatory up-regulation of acetylcholinesterase activity, and compensatory down-regulation of M2 muscarinic receptors in the striatum of mice lacking mu-opioid receptor gene. These results suggest that the enhancement of DFP-induced tremors may be associated with the compensatory up-regulation of acetylcholinesterase activity and compensatory down-regulation of M2 muscarinic receptors in the striatum of mu-opioid receptor knockout mice.

Role of tonically active neurons in primate caudate in reward-oriented saccadic eye movement

Recent studies have suggested that the basal ganglia are essential for reward-oriented behavior. A popular proposal is that the interaction between sensorimotor and reward-related signals occurs in the striatal projection neurons. However, the role of interneurons remains unclear. Using the one-direction-rewarded version of the memory-guided saccade task (1DR), we examined the activity of tonically active neurons (TANs), presumed cholinergic interneurons, in the caudate. Many TANs (73/155, 47.1%) responded, usually with a pause, to a visual cue that indicated both the saccade goal and the presence or absence of reward. For most TANs (44/73, 60.3%), the response was spatially selective (contralateral dominant), but was not modulated by the reward significance. TANs are thus distinct from caudate projection neurons, which have responses to the cue that are both spatially selective and reward contingent, and from midbrain dopamine neurons, which have cue responses that are spatially nonselective and reward contingent. TANs were nonetheless sensitive to the reward schedule: in the all-directions-rewarded version (ADR) compared with 1DR, the cue responses of TANs were smaller, less frequent, and less spatially selective. In 1DR, it would first be detected that reward is not given regularly, and this process would then promote discrimination of individual stimuli in relation to reward. We propose that TANs would contribute to the detection of the context that requires discrimination, whereas dopamine neurons would contribute to the stimulus discrimination. These features of TANs might be explained by their cytoarchitecture, namely, as large aspiny neurons.

Subcellular localization of m2 muscarinic receptors in GABAergic interneurons of the olfactory bulb

We analysed the ultrastructural distribution of the m2 muscarinic receptor (m2R) in the rat olfactory bulb (OB) using immunohistochemical techniques and light and electron microscopy. m2R was differentially distributed within the cellular compartments of gamma-aminobutyric acid (GABA)ergic bulbar interneurons. It is located in the gemmules of granule cells and in the synaptic loci of the interneurons of the external plexiform layer, suggesting that m2R activation could modulate the release of GABA from these interneurons onto principal cells by a presynaptic mechanism. By contrast, the receptor appears in the somata and dendritic trunks of second-order short-axon interneurons located in the inframitral layers, suggesting that postsynaptic muscarinic activation in these cells could elicit the inhibition of granule cells, leading to a disinhibition of principal cells. We also detail the anatomical substrate for a new putative muscarinic modulation that has not been previously described, and that could influence the reception of sensory information within the olfactory glomeruli. m2R appears in a subset of GABAergic/dopaminergic juxtaglomerular cells innervated by olfactory axons but is absent in juxtaglomerular cells that do not receive sensory inputs. This finding suggests that m2R activation could modify, through dopaminergic local circuits, the strength of olfactory nerve inputs onto principal cells. Activation of the muscarinic receptor may modulate the olfactory information encoding within olfactory glomeruli and may facilitate the bulbar transmission to superior centres influencing the GABA release by presynaptic and postsynaptic mechanisms. Taken together, our data provide the neuroanatomical basis for a complex action of m2R at different levels in the mammalian OB.

The ratio of striatal D1 to muscarinic receptors changes in aging rats housed in an enriched environment

The enriched environment (EC) causes morphological plasticity in striatal cells that express D1 and D2 receptors. We used radioligand binding assays to examine whether EC produces plasticity in striatal receptor density and receptor density ratios. After 30 days of EC, 2-year-old rats had a higher ratio of D1 to muscarinic receptors in striatum relative to singly housed rats. Assays also showed trends for a greater ratio of D1 to cannabinoid receptors and a greater density of D1 receptors in striatum after EC. D2 receptor density was unaffected by the EC condition.

Negative allosteric modulators of AMPA-preferring receptors inhibit [(3)H]GABA release in rat striatum

The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective glutamate receptor agonist, on the release of previously incorporated [(3)H]GABA was examined in superfused striatal slices of the rat. The slices were loaded with [(3)H]GABA in the presence of beta-alanine (1 mM) and superfused with Krebs-bicarbonate buffer containing nipecotic acid (0.1 mM) and aminooxyacetic acid (0.1 mM) to inhibit GABA uptake and metabolism. AMPA (0.01 to 3 mM) increased basal [(3)H]GABA outflow and nipecotic acid potentiated this effect. The [(3)H]GABA releasing effect of AMPA was an external Ca(2+)-dependent process in the absence but not in the presence of nipecotic acid. Cyclothiazide (0.03 mM), a positive modulator of AMPA receptors, failed to evoke [(3)H]GABA release by itself, but it dose-dependently potentiated the [(3)H]GABA releasing effect of AMPA. The AMPA (0.3 mM)-induced [(3)H]GABA release was antagonized by NBQX (0.01 mM) in a competitive fashion (pA(2) 5.08). The negative modulator of AMPA receptors, GYKI-53784 (0.01 mM) reversed the AMPA-induced [(3)H]GABA release by a non-competitive manner (pD'(2) 5.44). GYKI-53784 (0. 01-0.1 mM) also decreased striatal [(3)H]GABA outflow on its own right, this effect was stereoselective and was not influenced by concomitant administration of 0.03 mM cyclothiazide. GYKI-52466 (0. 03-0.3 mM), another negative modulator at AMPA receptors, also inhibited basal [(3)H]GABA efflux whereas NBQX (0.1 mM) by itself was ineffective in alteration of [(3)H]GABA outflow. The present data indicate that AMPA evokes GABA release from the vesicular pool in neostriatal GABAergic neurons. They also confirm that multiple interactions may exist between the agonist binding sites and the positive and negative modulatory sites but no such interaction was detected between the positive and negative allosteric modulators. Since GYKI-53784, but not NBQX, inhibited [(3)H]GABA release by itself, AMPA receptors located on striatal GABAergic neurons may be in sensitized state and phasically controlled by endogenous glutamate. It is also postulated that these AMPA receptors are located extrasynaptically on GABAergic striatal neurons.

Presynaptically located CB1 cannabinoid receptors regulate GABA release from axon terminals of specific hippocampal interneurons

To understand the functional significance and mechanisms of action in the CNS of endogenous and exogenous cannabinoids, it is crucial to identify the neural elements that serve as the structural substrate of these actions. We used a recently developed antibody against the CB1 cannabinoid receptor to study this question in hippocampal networks. Interneurons with features typical of basket cells showed a selective, intense staining for CB1 in all hippocampal subfields and layers. Most of them (85.6%) contained cholecystokinin (CCK), which corresponded to 96.9% of all CCK-positive interneurons, whereas only 4.6% of the parvalbumin (PV)-containing basket cells expressed CB1. Accordingly, electron microscopy revealed that CB1-immunoreactive axon terminals of CCK-containing basket cells surrounded the somata and proximal dendrites of pyramidal neurons, whereas PV-positive basket cell terminals in similar locations were negative for CB1. The synthetic cannabinoid agonist WIN 55,212-2 (0.01-3 microM) reduced dose-dependently the electrical field stimulation-induced [3H]GABA release from superfused hippocampal slices, with an EC50 value of 0. 041 microM. Inhibition of GABA release by WIN 55,212-2 was not mediated by inhibition of glutamatergic transmission because the WIN 55,212-2 effect was not reduced by the glutamate blockers AP5 and CNQX. In contrast, the CB1 cannabinoid receptor antagonist SR 141716A (1 microM) prevented this effect, whereas by itself it did not change the outflow of [3H]GABA. These results suggest that cannabinoid-mediated modulation of hippocampal interneuron networks operate largely via presynaptic receptors on CCK-immunoreactive basket cell terminals. Reduction of GABA release from these terminals is the likely mechanism by which both endogenous and exogenous CB1 ligands interfere with hippocampal network oscillations and associated cognitive functions.

Cholinergic modulation of neostriatal output: a functional antagonism between different types of muscarinic receptors

It is demonstrated that acetylcholine released from cholinergic interneurons modulates the excitability of neostriatal projection neurons. Physostigmine and neostigmine increase input resistance (RN) and enhance evoked discharge of spiny projection neurons in a manner similar to muscarine. Muscarinic RN increase occurs in the whole subthreshold voltage range (-100 to -45 mV), remains in the presence of TTX and Cd2+, and can be blocked by the relatively selective M1,4 muscarinic receptor antagonist pirenzepine but not by M2 or M3 selective antagonists. Cs+ occludes muscarinic effects at potentials more negative than -80 mV. A Na+ reduction in the bath occludes muscarinic effects at potentials more positive than -70 mV. Thus, muscarinic effects involve different ionic conductances: inward rectifying and cationic. The relatively selective M2 receptor antagonist AF-DX 116 does not block muscarinic effects on the projection neuron but, surprisingly, has the ability to mimic agonistic actions increasing RN and firing. Both effects are blocked by pirenzepine. HPLC measurements of acetylcholine demonstrate that AF-DX 116 but not pirenzepine greatly increases endogenous acetylcholine release in brain slices. Therefore, the effects of the M2 antagonist on the projection neurons were attributable to autoreceptor block on cholinergic interneurons. These experiments show distinct opposite functions of muscarinic M1- and M2-type receptors in neostriatal output, i.e., the firing of projection neurons. The results suggest that the use of more selective antimuscarinics may be more profitable for the treatment of motor deficits.

Separation of carrier mediated and vesicular release of GABA from rat brain slices

In this study the temperature dependence of [3H]GABA release from brain slices evoked by electrical field stimulation and the Na+/K+ ATPase inhibitor ouabain was investigated. [3H]GABA has been taken up and released from hippocampal slices at rest and in response to electrical field stimulation (20 V, 10 Hz, 3 msec, 180 pulses) at 37 degrees C. When the bath temperature was cooled to 7 degrees C, during the sample collection period, the tissue uptake and the resting outflow of [3H]GABA were not significantly changed. In contrast, the stimulation-induced tritium outflow increased both in absolute amount (Bq/g) and in fractional release and the S2/S1 ratio was also higher at 7 degrees C. Perfusion of the slices with tetrodotoxin (TTX, 1 microM) inhibited stimulation-induced [3H]GABA efflux indicating that exocytotic release of vesicular origin is maintained under these conditions. 15 min perfusion with ouabain (10-20 microM) induced massive tritium release both in hippocampal and in striatal slices. However, the fraction of [3H]GABA outflow evoked by ouabain was much higher in the hippocampus than in the striatum. Sequential lowering the bath temperature from 37 degrees C to 17 degrees C completely abolished ouabain-induced [3H]GABA release in both brain regions, indicating that it is a temperature-dependent, carrier-mediated process. When the same experiments were repeated under Ca2+ free conditions, cooling the bath temperature to 17 degrees C, although substantially decreased the release but failed to completely abolish the tritium outflow evoked by ouabain, a significant part was maintained. Our results show that vesicular (field stimulation-evoked) and carrier-mediated (ouabain-induced) release of GABA is differentially affected by low temperature: while vesicular release is unaffected, carrier-mediated release is abolished at low bath temperature. Therefore, lowering the temperature offers a reliable tool to separate these two kinds of release and makes possible to study exclusively the pure neuronal release of GABA of vesicular origin.