Muscarinic receptor subtype-specific coupling to second messengers in neuronal systems

Differential regulation of primary afferent input to spinal cord by muscarinic receptor subtypes delineated using knockout mice

Stimulation of muscarinic acetylcholine receptors (mAChRs) inhibits nociceptive transmission at the spinal level. However, it is unclear how each mAChR subtype regulates excitatory synaptic input from primary afferents. Here we examined excitatory postsynaptic currents (EPSCs) of dorsal horn neurons evoked by dorsal root stimulation in spinal cord slices from wild-type and mAChR subtype knock-out (KO) mice. In wild-type mice, mAChR activation with oxotremorine-M decreased the amplitude of monosynaptic EPSCs in ∼67% of neurons but increased it in ∼10% of neurons. The inhibitory effect of oxotremorine-M was attenuated by the M2/M4 antagonist himbacine in the majority of neurons, and the remaining inhibition was abolished by group II/III metabotropic glutamate receptor (mGluR) antagonists in wild-type mice. In M2/M4 double-KO mice, oxotremorine-M inhibited monosynaptic EPSCs in significantly fewer neurons (∼26%) and increased EPSCs in significantly more neurons (33%) compared with wild-type mice. Blocking group II/III mGluRs eliminated the inhibitory effect of oxotremorine-M in M2/M4 double-KO mice. In M2 single-KO and M4 single-KO mice, himbacine still significantly reduced the inhibitory effect of oxotremorine-M. However, the inhibitory and potentiating effects of oxotremorine-M on EPSCs in M3 single-KO and M1/M3 double-KO mice were similar to those in wild-type mice. In M5 single-KO mice, oxotremorine-M failed to potentiate evoked EPSCs, and its inhibitory effect was abolished by himbacine. These findings indicate that activation of presynaptic M2 and M4 subtypes reduces glutamate release from primary afferents. Activation of the M5 subtype either directly increases primary afferent input or inhibits it through indirectly stimulating group II/III mGluRs.

Dynamic control of glutamatergic synaptic input in the spinal cord by muscarinic receptor subtypes defined using knockout mice

Activation of muscarinic acetylcholine receptors (mAChRs) in the spinal cord inhibits pain transmission. At least three mAChR subtypes (M(2), M(3), and M(4)) are present in the spinal dorsal horn. However, it is not clear how each mAChR subtype contributes to the regulation of glutamatergic input to dorsal horn neurons. We recorded spontaneous excitatory postsynaptic currents (sEPSCs) from lamina II neurons in spinal cord slices from wild-type (WT) and mAChR subtype knock-out (KO) mice. The mAChR agonist oxotremorine-M increased the frequency of glutamatergic sEPSCs in 68.2% neurons from WT mice and decreased the sEPSC frequency in 21.2% neurons. Oxotremorine-M also increased the sEPSC frequency in ∼50% neurons from M(3)-single KO and M(1)/M(3) double-KO mice. In addition, the M(3) antagonist J104129 did not block the stimulatory effect of oxotremorine-M in the majority of neurons from WT mice. Strikingly, in M(5)-single KO mice, oxotremorine-M increased sEPSCs in only 26.3% neurons, and J104129 abolished this effect. In M(2)/M(4) double-KO mice, but not M(2)- or M(4)-single KO mice, oxotremorine-M inhibited sEPSCs in significantly fewer neurons compared with WT mice, and blocking group II/III metabotropic glutamate receptors abolished this effect. The M(2)/M(4) antagonist himbacine either attenuated the inhibitory effect of oxotremorine-M or potentiated the stimulatory effect of oxotremorine-M in WT mice. Our study demonstrates that activation of the M(2) and M(4) receptor subtypes inhibits synaptic glutamate release to dorsal horn neurons. M(5) is the predominant receptor subtype that potentiates glutamatergic synaptic transmission in the spinal cord.

Activation of muscarinic receptors inhibits spinal dorsal horn projection neurons: role of GABAB receptors

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors produce efficacious analgesia. However, the mechanisms of the antinociceptive actions of muscarinic agonists in the spinal cord are not fully known. Previous in vitro studies have shown that muscarinic agonists increase GABA release and reduce the glutamatergic synaptic input to lamina II interneurons through GABAB receptors in the spinal cord. In the present study, we studied the effect of muscarinic agents on dorsal horn projection neurons and the role of spinal GABAB receptors in their action. Single-unit activity of ascending dorsal horn neurons was recorded in the lumbar spinal cord of anesthetized rats. The responses of dorsal horn neurons to graded mechanical stimuli were determined before and after topical spinal application of muscarine and neostigmine. We found that topical application of 0.1-5 microM muscarine or 0.5-5 microM neostigmine significantly suppressed the evoked response of dorsal horn neurons in a concentration-dependent manner. The inhibitory effect of muscarine or neostigmine on dorsal horn neurons was completely abolished in the presence of 1 microM atropine and by intrathecal pretreatment with 1 microg pertussis toxin to inactivate inhibitory G proteins. Furthermore, the inhibitory effect of both muscarine and neostigmine on the evoked response of dorsal horn neurons was significantly attenuated in the presence of 1 microM CGP55845, a GABAB receptor antagonist. Collectively, these data suggest that muscarinic agents inhibit dorsal horn projection neurons through muscarinic receptors coupled to pertussis toxin-sensitive Gi/o proteins. The inhibitory action of muscarinic agonists on these dorsal horn neurons is mediated in part by spinal GABAB receptors.

Up-regulation of spinal muscarinic receptors and increased antinociceptive effect of intrathecal muscarine in diabetic rats

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors produce effective pain relief. Intrathecal injection of a small dose of neostigmine produces a profound antiallodynic effect in rats with diabetic neuropathy. However, the mechanisms of increased antinociceptive effect of cholinergic agents on diabetic neuropathic pain are not clear. In the present study, we tested the hypothesis that spinal muscarinic receptors are up-regulated in diabetes. The withdrawal threshold of the hindpaw in response to noxious heat and pressure stimuli was determined in streptozotocin-induced diabetic and age-matched normal rats. Muscarine-stimulated guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding was used to assess the change of functional muscarinic receptors in the spinal cord in diabetes. The [3H]AF-DX 384 membrane binding was performed to determine the number and affinity of spinal cord M2 muscarinic receptors in normal and diabetic rats. We found that the antinociceptive effect of intrathecal 2 to 12 mug muscarine in diabetic animals was potentiated significantly compared with that in normal animals. The maximal muscarine-stimulated [35S]GTPgammaS binding was 112.5 +/- 8.3% in normal rats and 168.8 +/- 12.1% (P < 0.05) in diabetic rats. Although the KD value (2.9 nM) was similar in both groups, the Bmax of [3H]AF-DX 384 membrane binding was significantly higher in diabetic than in normal rats (255.2 +/- 5.9 versus 165.9 +/- 3.5 fmol/mg protein, P < 0.05). Collectively, these data strongly suggest that the muscarinic receptor is up-regulated in the dorsal spinal cord in diabetic rats. This finding probably accounts for the increased efficacy of the antinociceptive effect of intrathecal muscarinic agonists in diabetic neuropathic pain.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Age differences in phosphodiesterase type-IV and its functional response to dopamine D1 receptor modulation in the living brain: a PET study in conscious monkeys

The present study demonstrated the age-related changes in the striatal dopamine D1 receptor binding and its related cAMP second-messenger system in the living brains of conscious young (6.4 +/- 1.8 years old) and aged (19.5 +/- 3.3 years old) monkeys (Macaca mulatta) using positron emission tomography (PET). For quantitative analysis of D1 receptors, [11C]SCH23390 was used and phosphodiesterase type-IV (PDE-IV) activity, as an index of cAMP system, was estimated by two scans with R- and S-[11C]rolipram. Significant age-related decreases in D1 receptor binding were observed in the striatum and frontal cortex. Analysis of uptake of R- and S-[11C]rolipram indicated age-related decreases in PDE-IV activity showing 22.0 and 25.2% decreases in the striatum and frontal cortex, respectively, while no significant changes were observed in the cerebellum. With systemic preadministration of the dopamine D1 receptor antagonist SCH23390 (0.2, 0.6, and 2 mg/kg), the PDE-IV activities in the striatum and frontal cortex were dose-dependently suppressed in both age groups. However, the degree of suppression by SCH23390 was more marked in young than in aged monkeys. These results demonstrate that the striatal cAMP second-messenger system activity as well as its functional response to dopamine D1 antagonist showed age-related impairment in the brain.

Receptor reserve of phosphoinositide-coupled muscarinic receptors in mouse hippocampus in vivo

The ability of the partial muscarinic agonist pilocarpine to increase in vivo phosphoinositide (PI) hydrolysis in mouse brain was compared to two full agonists. Pilocarpine increased in vivo phosphoinositide (PI) hydrolysis in cortex, striatum, and to the greatest extent in the hippocampus. Pilocarpine injected either subcutaneously or intracerebroventricularly robustly increased in vivo PI hydrolysis in hippocampus up to 500% of control levels and the increases were blocked by the muscarinic antagonist scopolamine. The increases in vivo PI hydrolysis induced by pilocarpine were 60-75% of the magnitude of the full muscarinic agonists oxotremorine-M and cis-dioxolane. The muscarinic M(1) preferring antagonist pirenzepine potently blocked pilocarpine-induced increases in in vivo PI hydrolysis, consistent with the increase being mediated by M(1) receptors. Since pilocarpine is a relatively weak partial agonist, these data suggest a substantial level of receptor reserve for the PI response in mouse hippocampus.

Network and intrinsic contributions to carbachol-induced oscillations in the rat subiculum

Low-frequency network oscillations occur in several areas of the limbic system where they contribute to synaptic plasticity and mnemonic functions that are in turn modulated by cholinergic mechanisms. Here we used slices of the rat subiculum (a limbic area involved in cognitive functions) to establish how network and single neuron (intrinsic) membrane mechanisms participate to the rhythmic oscillations elicited by the cholinergic agent carbachol (CCh, 50-100 microM). We have found that CCh-induced network oscillations (intraoscillatory frequency = 5-16 Hz) are abolished by an antagonist of non-N-methyl-D-aspartate (NMDA) glutamatergic receptors (n = 6 slices) but persist during blockade of GABA receptors (n = 16). In addition, during application of glutamate and GABA receptor antagonists, single subicular cells generate burst oscillations at 2.1-6.8 Hz when depolarized with steady current injection. These intrinsic burst oscillations disappear during application of a Ca(2+) channel blocker (n = 6 cells), intracellular Ca(2+) chelation (n = 6), or replacement of extracellular Na(+) (n = 4) but persist in recordings made with electrodes containing a blocker of voltage-gated Na(+) channels (n = 7). These procedures cause similar effects on CCh-induced depolarizing plateau potentials that are contributed by a Ca(2+)-activated nonselective cationic conductance (I(CAN)). Network and intrinsic oscillations along with depolarizing plateau potentials were abolished by the muscarinic receptor antagonist atropine. In conclusion, our findings demonstrate that low-frequency oscillations in the rat subiculum rely on the muscarinic receptor-dependent activation of an intrinsic oscillatory mechanism that is presumably contributed by I(CAN) and are integrated within the network via non-NMDA receptor-mediated transmission.

Acetylcholine becomes the major excitatory neurotransmitter in the hypothalamus in vitro in the absence of glutamate excitation

Glutamate and GABA are two major fast neurotransmitters (excitatory and inhibitory, respectively) in the CNS, including the hypothalamus. They play a key role in the control of excitation/inhibition balance and determine the activity and excitability of neurons in many neuronal circuits. Using neuronal cultures, whole-cell recording, Ca(2+) imaging, and Northern blots, we studied the compensatory regulation of neuronal activity during a prolonged decrease in glutamate excitation. We report here that after a chronic (6-17 d) blockade of ionotropic glutamate receptors, neurons in hypothalamic cultures revealed excitatory electrical and Ca(2+) synaptic activity, which was not elicited in the control cultures that were not subjected to glutamate blockade. This activity was suppressed with acetylcholine (ACh) receptor antagonists and was potentiated by eserine, an inhibitor of acetylcholinesterase, suggesting its cholinergic nature. The upregulation of ACh receptors and the contribution of ACh to the control of the excitation/inhibition balance in cultures after a prolonged decrease in glutamate activity were also demonstrated. Enhanced ACh transmission was also found in chronically blocked cerebellar but not cortical cultures, suggesting the region-specific character of glutamate-ACh interactions in the brain. We believe that in the absence of glutamate excitation in the hypothalamus in vitro, ACh, a neurotransmitter normally exhibiting only weak activity in the hypothalamus, becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. The increase in excitatory ACh transmission during a decrease in glutamate excitation may represent a novel form of neuronal plasticity that regulates activity and excitability of neurons during the glutamate/GABA imbalance.

Laminar distribution of cholinergic- and serotonergic-dependent plasticity within kitten visual cortex

Both cholinergic and serotonergic modulatory projections to mammalian striate cortex have been demonstrated to be involved in the regulation of postnatal plasticity, and a striking alteration in the number and intracortical distribution of cholinergic and serotonergic receptors takes place during the critical period for cortical plasticity. As well, agonists of cholinergic and serotonergic receptors have been demonstrated to facilitate induction of long-term synaptic plasticity in visual cortical slices supporting their involvement in the control of activity-dependent plasticity. We recorded field potentials from layers 4 and 2/3 in visual cortex slices of 60--80 day old kittens after white matter stimulation, before and after a period of high frequency stimulation (HFS), in the absence or presence of either cholinergic or serotonergic agonists. At these ages, the HFS protocol alone almost never induced long-term changes of synaptic plasticity in either layers 2/3 or 4. In layer 2/3, agonist stimulation of m1 receptors facilitated induction of long-term potentiation (LTP) with HFS stimulation, while the activation of serotonergic receptors had only a modest effect. By contrast, a strong serotonin-dependent LTP facilitation and insignificant muscarinic effects were observed after HFS within layer 4. The results show that receptor-dependent laminar stratification of synaptic modifiability occurs in the cortex at these ages. This plasticity may underly a control system gating the experience-dependent changes of synaptic organization within developing visual cortex.

Inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release evoked by metabotropic agonists and backpropagating action potentials in hippocampal CA1 pyramidal neurons

We examined the properties of [Ca(2+)](i) changes that were evoked by backpropagating action potentials in pyramidal neurons in hippocampal slices from the rat. In the presence of the metabotropic glutamate receptor (mGluR) agonists t-ACPD, DHPG, or CHPG, spikes caused Ca(2+) waves that initiated in the proximal apical dendrites and spread over this region and in the soma. Consistent with previously described synaptic responses (Nakamura et al., 1999a), pharmacological experiments established that the waves were attributable to Ca(2+) release from internal stores mediated by the synergistic effect of receptor-mobilized inositol 1,4, 5-trisphosphate (IP(3)) and spike-evoked Ca(2+). The amplitude of the changes reached several micromoles per liter when detected with the low-affinity indicators fura-6F, fura-2-FF, or furaptra. Repetitive brief spike trains at 30-60 sec intervals generated increases of constant amplitude. However, trains at intervals of 10-20 sec evoked smaller increases, suggesting that the stores take 20-30 sec to refill. Release evoked by mGluR agonists was blocked by MCPG, AIDA, 4-CPG, MPEP, and LY367385, a profile consistent with the primacy of group I receptors. At threshold agonist concentrations the release was evoked only in the dendrites; threshold antagonist concentrations were effective only in the soma. Carbachol and 5-HT evoked release with the same spatial distribution as t-ACPD, suggesting that the distribution of neurotransmitter receptors was not responsible for the restricted range of regenerative release. Intracellular BAPTA and EGTA were approximately equally effective in blocking release. Extracellular Cd(2+) blocked release, but no single selective Ca(2+) channel blocker prevented release. These results suggest that IP(3) receptors are not associated closely with specific Ca(2+) channels and are not close to each other.

Involvement of calmodulin-dependent protein kinase II in carbachol-induced rhythmic activity in the hippocampus of the rat

The role of calcium and protein kinases in rhythmic activity induced by muscarinic receptor activation in the CA1 area in rat hippocampal slices was investigated. Extracellular recording showed that carbachol (20 microM) induced synchronized field potential activity with a dominant frequency of 7.39+/-0.68 Hz. Pretreatment with the membrane permeable Ca(2+) chelator BAPTA-AM (50 microM) or with thapsigargin (1 microM), a compound which depletes intracellular calcium stores, reduced the dominant power of carbachol-induced theta-like activity by 83% and 78%, respectively. Inhibition of calmodulin-dependent protein kinase II (CaMKII) by the cell permeable inhibitor KN-93 (10 microM) reduced the power of carbachol-induced theta-like activity by 80%. In contrast the protein kinase C (PKC) inhibitor calphostin C did not significantly (P>0.05) affect the effect of carbachol. Whole-cell recording indicated that KN-93 also blocked carbachol-induced suppression of slow I(AHP) and strongly inhibited the carbachol-induced plateau potential. Our data suggest that activation of CaMKII by carbachol is crucial for local theta-like activity in the CA1 area of the rat hippocampus in vitro. Furthermore, involvement of CaMKII in carbachol-induced suppression of the slow I(AHP) and the induction of plateau potentials could play a role in the induction of theta-like rhythmic activity by carbachol.

Design, synthesis and binding at cloned muscarinic receptors of N-[5-(1'-substituted-acetoxymethyl)-3-oxadiazolyl] and N-[4-(1'-substituted-acetoxymethyl)-2-dioxolanyl] dialkyl amines

Few muscarinic antagonists differentiate between the M4 and M2 muscarinic receptors. In a structure activity study, aimed at discovering leads for the development of a M4 muscarinic receptor-selective antagonist, we have synthesized and tested at cloned muscarinic receptors the binding of a group of dioxolane- or oxadiazole-dialkyl amines, and compared them to our compound 1, which contains the furan nucleus. Although none of these agents were particularly potent at M4 receptors (Kd values were typically 30-70 nM), furan derivatives (-)1 and (+)1 were significantly more potent at M4 receptors than at M2 receptors (approximately 3- and 4-fold, respectively). The dioxolane derivatives 12b and 12c were more than 10-fold selective for the M4 versus the M2 receptors, while the dioxolane derivative 12e was 15-fold more potent at M4 receptors than for M2 receptors. However, these agents bound to M3 receptors with potencies like that for the M4 receptor, so they are not M4-selective. The M4/M2 relative selectivities of some of our compounds are similar to the better hexahydrosiladifenidol derivatives, and may provide some important structural clues for the development of potent and selective M4 antagonists.

Interaction between alpha-MSH and acetylcholinergic system upon striatal cAMP and IP(3) levels

The interaction between the neuropeptide alpha-MSH and the acetylcholinergic system as reflected by changes in cAMP and inositol 1-3-5 triphosphate(IP(3))production was investigated in an in vitro model of striatal slices. The possible involvement of D(1) receptors in cholinergic and alpha-MSH- stimulated cAMP and IP(3) production in slices of rat striatum was also examined, because it has been demonstrated that acetylcholinergic drugs induce endogenous dopamine release in the striatum. alpha-MSH, pilocarpine(PL) and the selective muscarinic M1 agonist McN-A-343 increased cAMP and IP(3) striatal levels, effects blocked by the D(1) antagonist SCH-23390, except for the effects of alpha-MSH on IP(3). The muscarinic M(2) antagonist gallamine (GL) brought about an increase in cAMP levels, an effect blocked by SCH-23390. The M(1) antagonist pirenzepine (Pz) induced a decrease both in cAMP and IP(3) content, and the nicotinic antagonist di-hydro-beta-eritroidine(DBE) only diminished cAMP production. When alpha-MSH and cholinergic agents were simultaneously added, cAMP and IP(3) levels were modified with respect to the values reached when these agents were added alone. An interaction between the acetylcholinergic system and alpha-MSH through M(1) and nicotinic receptors was also observed. These results suggest that the intracellular signaling pathways related to cAMP and IP(3) production gated by alpha-MSH and these cholinergic receptors are probably related. alpha-MSH striatum cAMP IP(3) muscarinic and nicotinic receptors an in vitro model.

Facilitation of cholinergic transmission by substance P methyl ester in the mouse hippocampal slice preparation

Using sharp microelectrode recording from CA1 pyramidal neurons of the adult mouse hippocampal slice preparation, we studied the modulatory action of the selective neurokinin 1 (NK1) receptor agonist substance P methyl ester (SPME), a peptidase-resistant analogue of the peptide substance P (SP), on cholinergic responses. While SPME (0.1-1 microM) had only slight effects on membrane potential and input resistance of CA1 neurons, it largely and reversibly enhanced the membrane depolarization and oscillatory activity induced by the cholinergic agonist carbachol (CCh; 0.1-100 microM). This effect of SPME was prevented by the selective NK1 receptor antagonist SR 140333 (4 microM). In about half of the tested neurons the action of SPME was preserved in tetrodotoxin (TTX) solution, suggesting that it partly occurred at the level of pyramidal cells. Cholinergic slow excitatory postsynaptic potentials (sEPSPs) were reversibly enhanced by SPME which increased their amplitude and prolonged any associated bursting activity. This action was also blocked by SR 140333. The present results suggest that SPME largely enhances cholinergic activity in the mouse hippocampus, an effect which can help to explain, in this brain area, the recently reported facilitation of seizures by SP.

Muscarinic receptor activation induces depolarizing plateau potentials in bursting neurons of the rat subiculum

Acetylcholine functions as a neuromodulator in the mammalian brain by binding to specific receptors and thus bringing about profound changes in neuronal excitability. Activation of muscarinic receptors often results in an increased excitability of cortical cells. It is, however, unknown whether such an action is present in the subiculum, a limbic structure that may be involved in cognitive processes as well as in seizure propagation. Most rat subicular neurons are endowed of intrinsic membrane properties that make them fire action potential bursts. Using intracellular recordings from these bursting cells in a slice preparation, we report here that application of the cholinergic agonist carbachol (CCh, 30-100 microM) to medium containing ionotropic excitatory amino acid receptor antagonists reduces burst-afterhyperpolarizations (burst-AHPs) and discloses depolarizing plateau potentials that outlast the triggering current pulses by 140-2,800 ms. These plateau potentials appear with CCh concentrations >50 microM and are dependent on the resting membrane potential and on the intensity/duration of the triggering pulse; are recorded during application of tetrodotoxin (1 microM, n = 5 neurons); but are markedly reduced by replacing 82% of extracellular Na(+) with equimolar choline (n = 6). Plateau potentials also are abolished by Co(2+) (2 mM; n = 5) or Cd(2+) (1 mM; n = 2) application and by recording with electrodes containing the Ca(2+) chelator bis(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid (0.2 M; n = 6). CCh-induced burst-AHP reduction and plateau potentials are reversed by the muscarinic antagonist atropine (0.5 microM, n = 7). In conclusion, our findings demonstrate a powerful muscarinic modulation of the intrinsic excitability of subicular bursting cells that is predominated by the appearance of plateau potentials. These changes in excitability may contribute to physiological processes such as learning or memory and play a role in the generation of epileptiform depolarizations. We propose that, as in other limbic structures, muscarinic plateau potentials in the subiculum are mainly due to a Ca(2+)-dependent nonselective cationic conductance.

Muscarinic receptor modulation of GABA-mediated giant depolarizing potentials in the neonatal rat hippocampus

1. The whole-cell patch clamp technique was used to study the role of muscarinic receptors in regulating the frequency of giant depolarizing potentials (GDPs) in CA3 hippocampal neurones in slices from postnatal (P) P1-P8 rats. 2. Atropine (1 microM) reduced the frequency of GDPs by 64.2 +/- 2.9 %. The acetylcholinesterase inhibitor edrophonium (20 microM) increased the frequency of GDPs in a developmentally regulated way. This effect was antagonized by the M1 muscarinic receptor antagonist pirenzepine. 3. In the presence of edrophonium, tetanic stimulation of cholinergic fibres induced either an enhancement of GDP frequency (179 +/- 79 %) or a membrane depolarization (27 +/- 16 mV) associated with an increase in synaptic noise. These effects were prevented by atropine. 4. Application of carbachol (3 microM) produced an increase in GDP frequency that at P5-P6 was associated with a membrane depolarization and an increase in synaptic noise. These effects were prevented by atropine, pirenzepine (3 microM) and bicuculline (10 microM). 5. In the presence of pirenzepine, carbachol reduced GDP frequency by 50 +/- 4 %. Conversely, in the presence of methoctramine (3 microM), carbachol enhanced GDP frequency by 117 +/- 4 %. 6. It is concluded that endogenous acetylcholine, through the activation of M1 receptors, enhances the release of gamma-aminobutyric acid (GABA), in a developmentally regulated way. On the other hand, carbachol exerts both an up- and downregulation of GABA release through the activation of M1 and M2 receptors, respectively.

3-Alpha-chloro-imperialine, a potent blocker of cholinergic presynaptic modulation of glutamatergic afferents in the rat neostriatum

Cortico-thalamic glutamatergic afferents control neuronal activity in the neostriatum. Cholinergic interneurons modulate the activity of medium spiny neurons through both pre- and post-synaptic actions via the activation of muscarinic receptors. The muscarinic pre-synaptic modulation was analyzed electrophysiologically. The transmitter release, induced by 4-AP, was studied and the block of paired pulse facilitation (PPF) by different muscarinic receptor antagonists was analyzed. The GABA(A) antagonist bicuculline isolated the glutamatergic transmission. Muscarinic agonists decreased the frequency of random synaptic potentials induced by 4-AP in about 60% of the cases without changes in input resistance (RN) of the post-synaptic neuron or in the mean amplitude of the synaptic events; indicating a presynaptic action. The administration of both 1 microM carbachol or 20 nM muscarine increased PPF. Muscarinic receptor antagonists blocked this action with a potency order: 3-alpha-chloroimperialine > 4-DAMP>>AFDX-116 > or = gallamine >> pirenzepine. The IC50's for the first three antagonists were (nM): 0.65, 1.1, and 3.0. Their respective Hill coefficients were: 1.9, 1.4, and 1.3. 3-alpha-Chloroimperialine reduced the PPF almost completely. The M3 and the M2 muscarinic receptor antagonists 4-DAMP and AFDX-116, given at saturating concentrations, consistently blocked only a part of the PPF but had additive effects when given together. These data are consistent with the existence of both M2 and M3 muscarinic receptors in striatal glutamatergic afferents.

Properties of carbachol-induced oscillatory activity in rat hippocampus

Properties of carbachol-induced oscillatory activity in rat hippocampus. J. Neurophysiol. 78: 2631-2640, 1997. The recent resurgence of interest in carbachol oscillations as an in vitro model of theta rhythm in the hippocampus prompted us to evaluate the circuit mechanisms involved. In extracellular recordings, a regularly spaced bursting pattern of field potentials was observed in both CA3 and CA1 subfields in the presence of carbachol. Removal of the CA3 region abolished oscillatory activity observed in CA1, suggesting that the oscillatory generator is located in CA3. An alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), blocked carbachol oscillations, indicating that AMPA receptor-mediated synaptic currents are necessary for the population oscillation. Moreover, the spread of oscillatory activity into CA1 required intact N-methyl--aspartate receptors. These data are more consistent with epileptiform bursting than with theta rhythm described in vivo. In the presence of carbachol, individual CA3 pyramidal cells exhibited a slow, rhythmic intrinsic oscillation that was not blocked by DNQX and that was enhanced by membrane hyperpolarization. We hypothesize that this slower oscillation is the fundamental oscillator that participates in triggering the population oscillation by exciting multiple synaptically connected CA3 neurons. gamma-aminobutyric acid-A (GABAA) receptors are not necessary for carbachol to elicit synchronous CA3 field events but are essential to the bursting pattern observed. Neither GABAB nor metabotropic glutamate receptors appear to be necessary for carbachol oscillations. However, both nicotinic and M1 and M3 muscarinic cholinergic receptors contribute to the generation of this activity. These results establish the local circuit elements and neurotransmitter receptors that contribute to carbachol-induced oscillations and indicate that carbachol-induced oscillations are fundamentally distinct from theta rhythm in vivo.

mRNA for the m4 muscarinic receptor subtype is expressed in adult rat brain cholinergic neurons

A number of pharmacological, anatomical, and immunological studies have previously addressed the subtype identity of the hippocampal muscarinic pre-synaptic autoreceptor. A preponderance of findings indicate that it is of the M2 pharmacological type. Both the m2 and m4 molecular subtypes exhibit M2 pharmacology and there are few drugs that differentiate between these receptors. Pharmacological attempts at defining the hippocampal autoreceptor have yielded conflicting results. The basal forebrain is relatively enriched in m2 muscarinic receptor mRNA and protein, and lesions that denervate the hippocampus of its basal forebrain cholinergic input have shown a decrement in m2, but not m4, receptor protein in the hippocampus. Thus, the anatomical data obtained to date tend to support the view that the m2 subtype is expressed as the hippocampal autoreceptor. We have combined in situ hybridization histochemistry (ISHH) with immunocytochemistry to choline acetyltransferase to examine whether mRNA for the m4 subtype of muscarinic receptor is expressed in central cholinergic neurons. The m4 muscarinic mRNA was found at moderate levels in all subdivisions of the cholinergic basal forebrain, including the medial septum/diagonal band complex (MS/DB). The m4 mRNA was also found in striatal cholinergic interneurons, in the cholinergic reticular core of the upper brainstem, and in brainstem cholinergic motor neurons. Muscarinic m4 receptor mRNA was also found in many non-cholinergic cells in the brain. For example, the hippocampal pyramidal neurons, dentate gyrus granule cells, and entorhinal cortical pyramidal neurons express relatively high levels of m4 mRNA, while in the brainstem the dorsal raphe and pontine reticular nuclei express relatively high levels of this mRNA. The finding of m4 mRNA in the MS/DB cholinergic neurons suggests that this receptor protein might be expressed as an autoreceptor in hippocampal cholinergic terminals.

cAMP and protein kinase A modulate cholinergic rapid eye movement sleep generation

Cholinergic neurotransmission in the medial pontine reticular formation (mPRF) modulates rapid eye movement (REM) sleep generation. Microinjection of cholinergic agonists and acetylcholinesterase inhibitors into the mPRF induces a REM sleep-like state, and microdialysis data reveal increased mPRF levels of acetylcholine during REM sleep. Muscarinic cholinergic receptors (mAChRs) participate in REM sleep generation, and data suggest that mAChRs of a non-M1 subtype modulate REM sleep generation. The signal transduction pathway activated by m2 and m4 mAChRs involves a pertussis toxin-sensitive G protein, adenylate cyclase (AC), adenosine 3',5'-cyclic monophosphate (cAMP), and protein kinase A (PKA). Therefore, the present study tested the hypothesis that cAMP and PKA within the mPRF modulate the carbachol-induced REM sleep-like state. To test this hypothesis, the mPRF was microinjected with compounds known to facilitate the effects of cAMP (dibutyryl cAMP and 8-bromo-cAMP), stimulate PKA (Sp-cAMP[S]), and inhibit PKA (Rp-cAMP[S]). The results showed that compounds that fostered the intracellular effects of cAMP significantly decreased cholinergic REM sleep, while having no effect on spontaneously occurring REM sleep. These data are consistent with the recent finding that within the mPRF, AC and a pertussis toxin-sensitive G protein modulate cholinergic REM sleep generation. These new data suggest a modulatory role for pontine cAMP and PKA in cholinergic REM sleep regulation.

Ionic mechanisms of muscarinic depolarization in entorhinal cortex layer II neurons

The mechanisms underlying direct muscarinic depolarizing responses in the stellate cells (SCs) and non-SCs of medial entorhinal cortex layer II were investigated in tissue slices by intracellular recording and pressure-pulse applications of carbachol (CCh). Subthreshold CCh depolarizations were largely potentiated in amplitude and duration when paired with a short DC depolarization that triggered cell firing. During Na+ conductance block, CCh depolarizations were also potentiated by a brief DC depolarization that allowed Ca2+ influx and the potentiation was more robust in non-SCs than in SCs. Also, in non-SCs, CCh depolarizations could be accompanied by spikelike voltage oscillations at a slow frequency. In both SCs and non-SCs, the voltage-current (V-I) relations were similarly affected by CCh, which caused a shift to the left of the steady-state V-I relations over the entire voltage range and an increase in apparent slope input resistance at potentials positive to about -70 mV. CCh responses potentiated by Ca2+ influx demonstrated a selective increase in slope input resistance at potentials positive to about -75 mV in relation to the nonpotentiated responses. K+ conductance block with intracellular injection of Cs+ (3 M) and extracellular Ba2+ (1 mM) neither abolished CCh depolarizations nor resulted in any qualitatively distinct effect of CCh on the V-I relations. CCh depolarizations were also undiminished by block of the time-dependent inward rectifier Ih, with extracellular Cs . However, CCh depolarizations were abolished during Ca2+ conductance block with low-Ca2+ (0.5 mM) solutions containing Cd2+, Co2+, or Mn2+, as well as by intracellular Ca2+ chelation with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. Inhibition of the Na+-K+ ATPase with strophanthidin resulted in larger CCh depolarizations. On the other hand, when NaCl was replaced by N-methyl-D-glucamine, CCh depolarizations were largely diminished. CCh responses were blocked by 0.8 microM pirenzepine, whereas hexahydro-sila-difenidolhydrochloride,p-fluoroanalog (p-F-HHSiD) and himbacine were only effective antagonists at 5- to 10-fold larger concentrations. Our data are consistent with CCh depolarizations being mediated in both SCs and non-SCs by m1 receptor activation of a Ca2+-dependent cationic conductance largely permeable to Na+. Activation of this conductance is potentiated in a voltage-dependent manner by activity triggering Ca2+ influx. This property implements a Hebbian-like mechanism whereby muscarinic receptor activation may only be translated into substantial membrane depolarization if coupled to postsynaptic cell activity. Such a mechanism could be highly significant in light of the role of the entorhinal cortex in learning and memory as well as in pathologies such as temporal lobe epilepsy.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Muscarinic acetylcholine receptor expression in memory circuits: implications for treatment of Alzheimer disease

Cholinergic transmission at muscarinic acetylcholine receptors (mAChR) has been implicated in higher brain functions such as learning and memory, and loss of synapses may contribute to the symptoms of Alzheimer disease. A heterogeneous family of five genetically distinct mAChR subtypes differentially modulate a variety of intracellular signaling systems as well as the processing of key molecules involved in the pathology of the disease. Although many muscarinic effects have been identified in memory circuits, including a diversity of pre- and post-synaptic actions in hippocampus, the identities of the molecular subtypes responsible for any given function remain elusive. All five mAChR genes are expressed in hippocampus, and subtype-specific antibodies have enabled identification, quantification, and localization of the encoded proteins. The m1, m2, and m4 mAChR proteins are most abundant in forebrain regions and they have distinct cellular and subcellular localizations suggestive of various pre- and postsynaptic functions in cholinergic circuits. The subtypes are also differentially altered in postmortem brain samples from Alzheimer disease cases. Further understanding of the molecular pharmacology of failing synapses in Alzheimer disease, together with the development of new subtype-selective drugs, may provide more specific and effective treatments for the disease.

Acetylcholine release from frontal cortex in the waking rat measured by microdialysis without acetylcholinesterase inhibitors: effects of diisopropylfluorophosphate

Acetylcholine (ACh) release was measured in frontal cortex of awake quietly resting rats by microdialysis without using cholinesterase blockers in the perfusate. Resting release was 16.61 +/- 2.05 fmol/h (+/- S.E.M., n = 18). Injection of sublethal doses of the acetylcholinesterase blocker, diisopropylfluorophosphate produced dose-dependent increases in ACh release, reaching 79.9 fmol/h with a dose of 0.7-times the LD50. Although this irreversible inactivation of acetylcholinesterase increased ACh recovery to more than 700% of control values, levels of ACh in the perfusate never reached those seen in physostigmine-treated animals. The relationship between the amount of acetylcholinesterase inactivation and the quantity of ACh in the perfusate suggests that the extracellular ACh concentrations are controlled by simple enzyme kinetics. Within 2 h after enzyme inactivation, extracellular choline levels fell significantly, suggesting that ACh degradation by acetylcholinesterase plays an important role in regulating the amount of choline in the extracellular space.

Expression of m1-m4 muscarinic acetylcholine receptor immunoreactivity in septohippocampal neurons and other identified hippocampal afferents

Muscarinic cholinergic transmission plays an important role in modulating hippocampal activity and many higher brain functions. Many of the modulatory effects of acetylcholine on hippocampal function result from direct effects in the hippocampus or from actions on the hippocampal afferent neurons. At each site, the differential expression of a family of five distinct but related receptor subtypes governs the nature of the response. The aim of the present study was to identify the subtypes expressed in the hippocampal afferent neurons by combining retrograde tracing with immunocytochemistry. The retrograde tracer, wheat germ agglutinin conjugated to horseradish peroxidase, was injected into the hippocampus unilaterally to label afferent neurons, and was combined with muscarinic (m) acetylcholine (ACh) receptors (mAChRs) with immunocytochemistry to identify the m1-m4 subtypes expressed. The retrogradely labeled cells in the basal forebrain that contribute to the septohippocampal pathway were found to express m2, m3, and, to a lesser extent, m1. Commissural/associational pathway neurons, which were identified by retrogradely labeled cells in the ipsi- and contralateral dentate gyrus, expressed m1, m3, and m4. The retrogradely labeled cells in the entorhinal cortex of the perforant pathway expressed predominantly m1 and m3, with fewer neurons expressing m2 and m4. Raphe-hippocampal cells were found to express m1. Thus, this study provides evidence for the diversity of mAChR subtypes expressed in neurons that project to the hippocampus. The complex modulation by acetylcholine of hippocampal function, therefore, is governed not only by the variety of mAChRs expressed in the hippocampus but also by their differential expression in extrinsic hippocampal afferents.

Muscarinic agonist inhibition of rat striatal adenylate cyclase is enhanced by dopamine stimulation

Rat striatal adenylate cyclase stimulated with combinations of 100 microM dopamine plus 40-100 nM forskolin was inhibited to a significantly greater extent by the muscarinic agonist carbachol than was forskolin-stimulated activity in the absence of dopamine. In the presence of Ro 201724 as phosphodiesterase inhibitor, a 100 microM concentration of the adenosine agonist 2-chloroadenosine stimulated adenylate cyclase activity 6.6 -fold over basal activity. In contrast to dopamine-stimulated activity, carbachol did not significantly inhibit adenylate cyclase activity elevated by the adenosine agonist, indicating specificity of the muscarinic response for dopamine stimulation. The effects of muscarinic antagonists on striatal versus heart adenylate cyclase indicated that the striatal response was mediated primarily through M4 receptors. The present results suggest that muscarinic M4 and dopamine D1 receptors are co-localized and functionally coupled in rat striatum.