Distinct Muscarinic Receptor Subtypes Suppress Excitatory and Inhibitory Synaptic Responses in Cortical Neurons

Abstract

Simultaneous whole cell recordings from monosynaptically connected cortical cells were performed with the use of two patch pipettes to determine the effect of acetylcholine (ACh) on both excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in cultured neurons from rat visual cortex. For 96% of EPSPs and 73% of IPSPs, ACh potently suppressed postsynaptic potentials in a dose-dependent manner. The estimated effective concentrations to produce half maximal response (EC50S) were 30 and 210 nM for EPSPs and IPSPs, respectively. To identify what subtypes of ACh receptors are involved in the suppression of postsynaptic potentials, three different, partially selective muscarinic receptor antagonists were used. According to the comparison of estimated Schild coefficients for each of the three antagonists against the suppression by ACh, EPSPs are most likely mediated by m4 receptors, and IPSPs by m1 receptors. When cells were treated with pertussis toxin, which inactivates m2 and m4 receptors while leaving m1, m3, and m5 receptors intact, 7 of 8 EPSPs were resistant to ACh whereas 8 of 12 IPSPs were still suppressed by ACh. This result supports the interpretation that the suppression of EPSPs was mediated by m4 receptors and that of IPSPs by m1 receptors. To obtain an indication as to whether ACh works presynaptically or postsynaptically, 1/CV2 analysis was carried out. The resultant diagonal alignment of the ratio of 1/CV2 plotted against the ratio of the amplitude of postsynaptic potentials suggests a presynaptic mechanism for the suppression of both EPSPs and IPSPs. In addition, in many cases a large synaptic suppression was observed without an obvious change in the input resistance. Furthermore, in one case where a single inhibitory driver cell was recorded with three different follower cells sequentially, none of the three IPSPs was suppressed by ACh, providing additional support for the presynaptic localization of ACh action. These results suggest that in cerebral cortex ACh has, in addition to its direct facilitatory effect via m3 pharmacology, a suppressive effect on EPSPs and IPSPs via m1 and m4 muscarinic receptors, respectively, probably with a presynaptic site of action. Separation of the actions of ACh into different receptor-second messenger pathways with potential for independent interactions with other neuromodulatory systems may be an important aspect of the mechanism of cholinergic regulation of functional state in cortex. Separation of cholinergic effects at different receptors might also offer a means for selective pharmacological intervention in disorders of sleep or memory.

Postsynaptic Cell Type–Dependent Cholinergic Regulation of GABAergic Synaptic Transmission in Rat Insular Cortex

“…Cholinergic agonists including ACh and carbachol effectively suppress evoked EPSPs/EPSCs recorded from pyramidal cells in the cerebral cortex (Kimura and Baughman 1997;Levy et al 2009;Murakoshi et al 2001). These studies have provided the evidence that a decrease in EPSP/EPSCs is accompanied by an increase in PPR and are blocked by atropine, suggesting that cholinergic suppression of EPSP/EPSCs is mediated by presynaptic mAChRs.…”

“…These suppressive effects of ACh are likely to be mediated by presynaptic mAChRs (Kimura and Baughman 1997;Murakoshi et al 2001), although Levy et al (2006) reported a nicotinic receptor contribution in addition to mAChRs. Similar results were obtained in the case of inhibitory postsynaptic potentials (IPSPs) or currents (IPSCs): thus ACh decreased the amplitude of evoked IPSPs/IPSCs recorded from pyramidal neurons via presynaptic mAChRs (Kimura and Baughman 1997;Xiao et al 2009). Another important role of ACh is modulation of neuronal excitability by depolarizing or hyperpolarizing the resting membrane potential.…”

“…Electrophysiological studies have revealed the suppressive effects of ACh on evoked excitatory postsynaptic potentials (EPSPs) recorded from pyramidal neuron in the neocortex (Kimura and Baughman 1997;Levy et al 2006). These suppressive effects of ACh are likely to be mediated by presynaptic mAChRs (Kimura and Baughman 1997;Murakoshi et al 2001), although Levy et al (2006) reported a nicotinic receptor contribution in addition to mAChRs.…”

Muscarinic Control of Long-Range GABAergic Inhibition within the Rhinal Cortices

“…The parameter 1/CV 2 is primarily determined by the presynaptic factors controlling transmitter release. By plotting the ratio of experimental to control 1/CV 2 against the ratio of experimental to control response amplitudes, the dependence of presynaptic versus postsynaptic function can be determined (Bekkers and Stevens, 1990;Manabe et al, 1993;Kimura and Baughman, 1997). A positive correlation between the two was shown to reflect a presynaptic mechanism, whereas a horizontal regression is indicative of a purely postsynaptic action.…”

Muscarinic suppression in stratum radiatum of CA1 shows dependence on presynaptic M1 receptors and is not dependent on effects at GABAB receptors

“…In addition to this enhancement of LTP, other eVects may also contribute to setting dynamics in the hippocampus appropriate for encoding information (Hasselmo & Schnell, 1994). In particular, a large body of research has demonstrated the involvement of mAChRs in the regulation of glutamatergic transmission, including suppression of transmission at the SchaVer collateral (SC) synapses in the hippocampus by a presynaptic mechanism ARTICLE IN PRESS (reviewed in Hasselmo, 1999b;Hounsgaard, 1978;Kimura and Baughman, 1997;Levey, 1996;Rasmusson, 2000;Valentino and Dingledine, 1981). Prior research from this laboratory has presented physiological data on ACh suppression (Hasselmo & Fehlau, 2001;Hasselmo & Schnell, 1994;Hasselmo, Schnell, & Barkai, 1995) as well as a neural model of how this suppression may be acting to regulate the dynamics of the hippocampus to enhance pattern separation and facilitate encoding (Hasselmo, 1999a(Hasselmo, , 1999bHasselmo, Bodelon, & Wyble, 2002;Hasselmo & McClelland, 1999;Hasselmo & Schnell, 1994;Hasselmo, Wyble, & Wallenstein, 1996;Wyble, Linster, & Hasselmo, 2000).…”