Cholinergic Activation of Corticofugal Circuits in the Adult Mouse Prefrontal Cortex

Acetylcholine (ACh) promotes neocortical output to the thalamus and brainstem by preferentially enhancing the postsynaptic excitability of layer 5 pyramidal tract (PT) neurons relative to neighboring intratelencephalic (IT) neurons. Less is known about how ACh regulates the excitatory synaptic drive of IT and PT neurons. To address this question, spontaneous excitatory postsynaptic potentials (sEPSPs) were recorded in dual recordings of IT and PT neurons in slices of prelimbic cortex from adult female and male mice. ACh (20 µM) enhanced sEPSP amplitudes, frequencies, rise-times, and half-widths preferentially in PT neurons. These effects were blocked by the muscarinic receptor antagonist atropine (1 µM). When challenged with pirenzepine (1 µM), an antagonist selective for M1-type muscarinic receptors, ACh instead reduced sEPSP frequencies, suggesting that ACh may generally suppress synaptic transmission in the cortex via non-M1 receptors. Cholinergic enhancement of sEPSPs in PT neurons was not sensitive to antagonism of GABA receptors with gabazine (10 µM) and CGP52432 (2.5 µM) but was blocked by tetrodotoxin (1 µM), suggesting that ACh enhances action-potential-dependent excitatory synaptic transmission in PT neurons. ACh also preferentially promoted the occurrence of synchronous sEPSPs in dual recordings of PT neurons relative to IT-PT and IT-IT parings. Finally, selective chemogenetic silencing of hM4Di-expressing PT, but not commissural IT, neurons blocked cholinergic enhancement of sEPSP amplitudes and frequencies in PT neurons. These data suggest that, in addition to selectively enhancing the postsynaptic excitability of PT neurons, M1 receptor activation promotes corticofugal output by amplifying recurrent excitation within networks of PT neurons.

Cholinergic activation of corticofugal circuits in the adult mouse prefrontal cortex

In layer 5 of the neocortex, ACh promotes cortical output to the thalamus and brainstem by preferentially enhancing the postsynaptic excitability of pyramidal tract (PT) neurons relative to neighboring intratelencephalic (IT) neurons. Less is known about how ACh regulates the excitatory synaptic drive of IT and PT neurons. To address this question, spontaneous excitatory postsynaptic potentials (sEPSPs) were recorded in pairs of IT and PT neurons in slices of prelimbic cortex from adult female and male mice. ACh (20 µM) enhanced sEPSP amplitudes, frequencies, rise-times, and half-widths preferentially in PT neurons. These effects were blocked by the muscarinic acetylcholine receptor antagonist atropine (1 µM). When challenged with pirenzepine (1 µM), an antagonist selective for M1-type muscarinic receptors, ACh instead reduced sEPSP frequencies. The cholinergic increase in sEPSP amplitudes and frequencies in PT neurons was not sensitive to blockade of GABAergic receptors with gabazine (10 µM) and CGP52432 (2.5 µM), but was blocked by tetrodotoxin (1 µM), suggesting that ACh enhances action-potential-dependent excitatory synaptic transmission in PT neurons. ACh also preferentially promoted the occurrence of synchronous sEPSPs in pairs of PT neurons relative to IT-PT and IT-IT pairs. Finally, selective chemogenetic silencing of hM4Di-expressing PT, but not IT, neurons with clozapine-N-oxide (5 µM) blocked cholinergic enhancement of sEPSP amplitudes and frequencies in PT neurons. These data suggest that, in addition to enhancing the postsynaptic excitability of PT neurons, M1 receptor activation promotes corticofugal output by preferentially amplifying recurrent excitation within networks of PT neurons.

Abstract 905: Targeting muscarinic acetylcholine receptors in glioma stem like cells

Primary gliomas arising within the brain remain the deadliest form of brain cancer and account for 78% of all malignant brain tumors. Glioblastoma patients have a 5 percent five-year survival rate and drug-resistant tumors often recur following surgical resection and treatment with radiation or chemotherapy. The cancer stem cell hypothesis suggests the presence of a subset of undifferentiated cells, namely glioma stem cells (GSCs), in the heterogenous tumor mass that are likely responsible for tumor initiation and recurrence of tumors post resection and give rise to drug resistant tumors. Determining a way to suppress these malignant characteristics and/or depleting the GSC population could improve current cancer treatments and the survival of glioma patients. Some GSCs are similar to oligodendrocyte precursor cells (OPCs), found during neural development and also residing in the adult brain. OPCs are prone to malignant transformation and believed to be a cellular origin for glioma. Recent findings indicate that the well-characterized neurotransmitter acetylcholine (ACh) maintains the primitive state of normal OPCs via muscarinic ACh receptors (mAChRs) preventing maturation and cell cycle exit. We investigated the functional characterization of ACh and mAChRs in modulating malignancy in OPC-like cells. We studied cultures of primary mouse OPC-like GSCs. We used the primary cells to culture three-dimensional tumor organoids in vitro to better represent tumor heterogeneity. We also used patient derived xenografts (PDX) of glioma to grow flank xenografts in NSG mice for an in vivo model. Publicly available data and studies in our lab show high levels of expression of CHRM3 (M3mAChR) in glioma patients and in primary OPC like GSCs. A drug screen conducted in the context of multiple sclerosis determined that benztropine (BZT), an anti muscarinic drug targeting M3mAChR, causes normal OPCs to exit the cell cycle, lose stem cell like characteristics, and differentiate. In our lab, electrophysiological studies demonstrated that activation of mAChRs in OPC-like cells from mouse and patient tumors generates rapid (< 1 second) increases in cytosolic calcium. Pharmacologic studies indicated that treatment with FDA approved anti muscarinic benztropine suppresses proliferation in cultured glioma cells. We also observed that serially passaging flank tumors treated with BZT into new host mice slowed down recurrence of new tumors. In order to determine key receptors mediating cholinergic responses in OPC-like cells we performed protein quantification, which displayed lowered phosphorylation of ERK. Electrophysiological studies are being conducted to dissect the mechanisms by which ACh evokes calcium release. These studies improve understanding of how cholinergic microenvironment influences stem-like glioma cells, providing a platform for repositioning available small molecule modifiers for treatment of glioma.

Citation Format: Sumyuktha V. Anand, Alexander G. Skorput, Allan T. Gulledge, Isabella B. Fox, Damian A. Bonnin, Alison L. Young, Matthew C. Havrda. Targeting muscarinic acetylcholine receptors in glioma stem like cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 905.

Activity-mediated accumulation of potassium induces a switch in firing pattern and neuronal excitability type

During normal neuronal activity, ionic concentration gradients across a neuron’s membrane are often assumed to be stable. Prolonged spiking activity, however, can reduce transmembrane gradients and affect voltage dynamics. Based on mathematical modeling, we investigated the impact of neuronal activity on ionic concentrations and, consequently, the dynamics of action potential generation. We find that intense spiking activity on the order of a second suffices to induce changes in ionic reversal potentials and to consistently induce a switch from a regular to an intermittent firing mode. This transition is caused by a qualitative alteration in the system’s voltage dynamics, mathematically corresponding to a co-dimension-two bifurcation from a saddle-node on invariant cycle (SNIC) to a homoclinic orbit bifurcation (HOM). Our electrophysiological recordings in mouse cortical pyramidal neurons confirm the changes in action potential dynamics predicted by the models: (i) activity-dependent increases in intracellular sodium concentration directly reduce action potential amplitudes, an effect typically attributed solely to sodium channel inactivation; (ii) extracellular potassium accumulation switches action potential generation from tonic firing to intermittently interrupted output. Thus, individual neurons may respond very differently to the same input stimuli, depending on their recent patterns of activity and/or the current brain-state.

Ionic concentrations in the brain are not constant. We show that during intense neuronal activity, they can change on the order of seconds and even switch neuronal spiking patterns under identical stimulation from a regular firing mode to an intermittently interrupted one. Triggered by an accumulation of extracellular potassium, such a transition is caused by a specific, qualitative change in of the neuronal voltage dynamics—a so-called bifurcation—which affects crucial features of action-potential generation and bears consequences for how information is encoded and how neurons behave together in the network. Also, changes in intracellular sodium can induce measurable effects, like a reduction of spike amplitude that occurs independently of the fast amplitude effects attributed to sodium channel inactivation. Taken together, our results demonstrate that a neuron can respond very differently to the same stimulus, depending on its previous activity or the current brain state. This finding may be particularly relevant when other regulatory mechanisms of ionic homeostasis are challenged, for example, during pathological states of glial impairment or oxygen deprivation. Finally, categorization of cortical neurons as intrinsically bursting or regular spiking may be biased by the ionic concentrations at the time of the observation, highlighting the non-static nature of neuronal dynamics.

Loss of Neurofascin-186 Disrupts Alignment of AnkyrinG Relative to Its Binding Partners in the Axon Initial Segment

The axon initial segment (AIS) is a specialized region within the proximal portion of the axon that initiates action potentials thanks in large part to an enrichment of sodium channels. The scaffolding protein ankyrinG (AnkG) is essential for the recruitment of sodium channels as well as several other intracellular and extracellular proteins to the AIS. In the present study, we explore the role of the cell adhesion molecule (CAM) neurofascin-186 (NF-186) in arranging the individual molecular components of the AIS in cultured rat hippocampal neurons. Using a CRISPR depletion strategy to ablate NF expression, we found that the loss of NF selectively perturbed AnkG accumulation and its relative proximal distribution within the AIS. We found that the overexpression of sodium channels could restore AnkG accumulation, but not its altered distribution within the AIS without NF present. We go on to show that although the loss of NF altered AnkG distribution, sodium channel function within the AIS remained normal. Taken together, these results demonstrate that the regulation of AnkG and sodium channel accumulation within the AIS can occur independently of one another, potentially mediated by other binding partners such as NF.

Serotonergic Regulation of Corticoamygdalar Neurons in the Mouse Prelimbic Cortex

Neuromodulatory transmitters, such as serotonin (5-HT), selectively regulate the excitability of subpopulations of cortical projection neurons to gate cortical output to specific target regions. For instance, in the mouse prelimbic cortex, 5-HT selectively excites commissurally projecting (COM) intratelencephalic neurons via activation of 5-HT2A (2A) receptors, while simultaneously inhibiting, via 5-HT1A (1A) receptors, corticofugally projecting pyramidal neurons targeting the pons. Here we characterize the physiology, morphology, and serotonergic regulation of corticoamygdalar (CAm) projection neurons in the mouse prelimbic cortex. Layer 5 CAm neurons shared a number of physiological and morphological characteristics with COM neurons, including higher input resistances, smaller HCN-channel mediated responses, and sparser dendritic arbors than corticopontine neurons. Across cortical lamina, CAm neurons also resembled COM neurons in their serotonergic modulation; focally applied 5-HT (100 μM; 1 s) generated 2A-receptor-mediated excitation, or 1A- and 2A-dependent biphasic responses, in ipsilaterally and contralaterally projecting CAm neurons. Serotonergic excitation depended on extrinsic excitatory drive, as 5-HT failed to depolarize CAm neurons from rest, but could enhance the number of action potentials generated by simulated barrages of synaptic input. Finally, using dual tracer injections, we identified double-labeled CAm/COM neurons that displayed primarily excitatory or biphasic responses to 5-HT. Overall, our findings reveal that prelimbic CAm neurons in layer 5 overlap, at least partially, with COM neurons, and that neurons projecting to either, or both targets, exhibit 2A-dependent serotonergic excitation. These results suggest that 5-HT, acting at 2A receptors, may promote cortical output to the amygdala.

Preferential cholinergic excitation of corticopontine neurons

KEY POINTS

Phasic activation of M1 muscarinic receptors generates transient inhibition followed by longer lasting excitation in neocortical pyramidal neurons. Corticopontine neurons in the mouse prefrontal cortex exhibit weaker cholinergic inhibition, but more robust and longer lasting excitation, than neighbouring callosal projection neurons. Optogenetic release of endogenous ACh in response to single flashes of light (5 ms) preferentially enhances the excitability of corticopontine neurons for many tens of seconds. Cholinergic excitation of corticopontine neurons involves at least three ionic mechanisms: suppression of K_V 7 currents, activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Preferential cholinergic excitation of prefrontal corticopontine neurons may facilitate top-down attentional processes and behaviours.

ABSTRACT

Pyramidal neurons in layer 5 of the neocortex comprise two broad classes of projection neurons: corticofugal neurons, including corticopontine (CPn) neurons, and intratelencephalic neurons, including commissural/callosal (COM) neurons. These non-overlapping neuron subpopulations represent discrete cortical output channels contributing to perception, decision making and behaviour. CPn and COM neurons have distinct morphological and physiological characteristics, and divergent responses to modulatory transmitters such as serotonin and acetylcholine (ACh). To better understand how ACh regulates cortical output, in slices of mouse prefrontal cortex (PFC) we compared the responsivity of CPn and COM neurons to transient exposure to exogenous or endogenous ACh. In both neuron subtypes, exogenous ACh generated qualitatively similar biphasic responses in which brief hyperpolarization was followed by longer lasting enhancement of excitability. However, cholinergic inhibition was more pronounced in COM neurons, while excitatory responses were larger and longer lasting in CPn neurons. Similarly, optically triggered release of endogenous ACh from cholinergic terminals preferentially and persistently (for ∼40 s) enhanced the excitability of CPn neurons, but had little impact on COM neurons. Cholinergic excitation of CPn neurons involved at least three distinct ionic mechanisms: suppression of K_V 7 channels (the 'M-current'), activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Our findings demonstrate projection-specific selectivity in cholinergic signalling in the PFC, and suggest that transient release of ACh during behaviour will preferentially promote corticofugal output.

Mechanisms Underlying Serotonergic Excitation of Callosal Projection Neurons in the Mouse Medial Prefrontal Cortex

Serotonin (5-HT) selectively excites subpopulations of pyramidal neurons in the neocortex via activation of 5-HT_2A (2A) receptors coupled to G_q subtype G-protein alpha subunits. G_q-mediated excitatory responses have been attributed primarily to suppression of potassium conductances, including those mediated by K_V7 potassium channels (i.e., the M-current), or activation of non-specific cation conductances that underlie calcium-dependent afterdepolarizations (ADPs). However, 2A-dependent excitation of cortical neurons has not been extensively studied, and no consensus exists regarding the underlying ionic effector(s) involved. In layer 5 of the mouse medial prefrontal cortex, we tested potential mechanisms of serotonergic excitation in commissural/callosal (COM) projection neurons, a subpopulation of pyramidal neurons that exhibits 2A-dependent excitation in response to 5-HT. In baseline conditions, 5-HT enhanced the rate of action potential generation in COM neurons experiencing suprathreshold somatic current injection. This serotonergic excitation was occluded by activation of muscarinic acetylcholine (ACh) receptors, confirming that 5-HT acts via the same G_q-signaling cascades engaged by ACh. Like ACh, 5-HT promoted the generation of calcium-dependent ADPs following spike trains. However, calcium was not necessary for serotonergic excitation, as responses to 5-HT were enhanced (by >100%), rather than reduced, by chelation of intracellular calcium with 10 mM BAPTA. This suggests intracellular calcium negatively regulates additional ionic conductances gated by 2A receptors. Removal of extracellular calcium had no effect when intracellular calcium signaling was intact, but suppressed 5-HT response amplitudes, by about 50%, when BAPTA was included in patch pipettes. This suggests that 2A excitation involves activation of a non-specific cation conductance that is both calcium-sensitive and calcium-permeable. M-current suppression was found to be a third ionic effector, as blockade of K_V7 channels with XE991 (10 μM) reduced serotonergic excitation by ∼50% in control conditions, and by ∼30% with intracellular BAPTA present. Together, these findings demonstrate a role for at least three distinct ionic effectors, including K_V7 channels, a calcium-sensitive and calcium-permeable non-specific cation conductance, and the calcium-dependent ADP conductance, in mediating serotonergic excitation of COM neurons.

A unifying hypothesis for M1 muscarinic receptor signalling in pyramidal neurons

KEY POINTS

Phasic release of acetylcholine (ACh) in the neocortex facilitates attentional processes. Acting at a single metabotropic receptor subtype, ACh exerts two opposing actions in cortical pyramidal neurons: transient inhibition and longer-lasting excitation. Cholinergic inhibitory responses depend on calcium release from intracellular calcium stores, and run down rapidly at resting membrane potentials when calcium stores become depleted. We demonstrate that cholinergic excitation promotes calcium entry at subthreshold membrane potentials to rapidly refill calcium stores, thereby maintaining the fidelity of inhibitory cholinergic signalling. We propose a 'unifying hypothesis' for M1 receptor signalling whereby inhibitory and excitatory responses to ACh in pyramidal neurons represent complementary mechanisms governing rapid calcium cycling between the endoplasmic reticulum, the cytosol and the extracellular space.

ABSTRACT

G_q -coupled M1-type muscarinic acetylcholine (ACh) receptors (mAChRs) mediate two distinct electrophysiological responses in cortical pyramidal neurons: transient inhibition driven by calcium-dependent small conductance potassium ('SK') channels, and longer-lasting and voltage-dependent excitation involving non-specific cation channels. Here we examine the interaction of these two cholinergic responses with respect to their contributions to intracellular calcium dynamics, testing the 'unifying hypothesis' that rundown of inhibitory SK responses at resting membrane potentials (RMPs) reflects depletion of intracellular calcium stores, while mAChR-driven excitation acts to refill those stores by promoting voltage-dependent entry of extracellular calcium. We report that fidelity of cholinergic SK responses requires the continued presence of extracellular calcium. Inhibitory responses that diminished after repetitive ACh application at RMPs were immediately rescued by pairing mAChR stimulation with subthreshold depolarization (∼10 mV from RMPs) initiated with variable delay (up to 500 ms) after ACh application, but not by subthreshold depolarization preceding mAChR stimulation. Further, rescued SK responses were time-locked to ACh application, rather than to the timing of subsequent depolarizing steps, suggesting that cholinergic signal transduction itself is not voltage-sensitive, but that depolarization facilitates rapid cycling of extracellular calcium through the endoplasmic reticulum to activate SK channels. Consistent with this prediction, rescue of SK responses by subthreshold depolarization required the presence of extracellular calcium. Our results demonstrate that, in addition to gating calcium release from intracellular stores, mAChR activation facilitates voltage-dependent refilling of calcium stores, thereby maintaining the ongoing fidelity of SK-mediated inhibition in response to phasic release of ACh.

Activity-dependent serotonergic excitation of callosal projection neurons in the mouse prefrontal cortex

Layer 5 pyramidal neurons (L5PNs) in the mouse prefrontal cortex respond to serotonin (5-HT) according to their long-distance axonal projections; 5-HT_1A (1A) receptors mediate inhibitory responses in corticopontine (CPn) L5PNs, while 5-HT_2A (2A) receptors can enhance action potential (AP) output in callosal/commissural (COM) L5PNs, either directly (in “COM-excited” neurons), or following brief 1A-mediated inhibition (in “COM-biphasic” neurons). Here we compare the impact of 5-HT on the excitability of CPn and COM L5PNs experiencing variable excitatory drive produced by current injection (DC current or simulated synaptic current) or with exogenous glutamate. 5-HT delivered at resting membrane potentials, or paired with subthreshold depolarizing input, hyperpolarized CPn and COM-biphasic L5PNs and failed to promote AP generation in COM-excited L5PNs. Conversely, when paired with suprathreshold excitatory drive generating multiple APs, 5-HT suppressed AP output in CPn L5PNs, enhanced AP generation in COM-excited L5PNs, and generated variable responses in COM-biphasic L5PNs. While COM-excited neurons failed to respond to 5-HT in the presence of a 2A receptor antagonist, 32% of CPn neurons exhibited 2A-dependent excitation following blockade of 1A receptors. The presence of pharmacologically revealed 2A receptors in CPn L5PNs was correlated with the duration of 1A-mediated inhibition, yet biphasic excitatory responses to 5-HT were never observed, even when 5-HT was paired with strong excitatory drive. Our results suggest that 2A receptors selectively amplify the output of COM L5PNs experiencing suprathreshold excitatory drive, while shaping the duration of 1A-mediated inhibition in a subset of CPn L5PNs. Activity-dependent serotonergic excitation of COM L5PNs, combined with 1A-mediated inhibition of CPn and COM-biphasic L5PNs, may facilitate executive function by focusing network activity within cortical circuits subserving the most appropriate behavioral output.

Electrical advantages of dendritic spines

Many neurons receive excitatory glutamatergic input almost exclusively onto dendritic spines. In the absence of spines, the amplitudes and kinetics of excitatory postsynaptic potentials (EPSPs) at the site of synaptic input are highly variable and depend on dendritic location. We hypothesized that dendritic spines standardize the local geometry at the site of synaptic input, thereby reducing location-dependent variability of local EPSP properties. We tested this hypothesis using computational models of simplified and morphologically realistic spiny neurons that allow direct comparison of EPSPs generated on spine heads with EPSPs generated on dendritic shafts at the same dendritic locations. In all morphologies tested, spines greatly reduced location-dependent variability of local EPSP amplitude and kinetics, while having minimal impact on EPSPs measured at the soma. Spine-dependent standardization of local EPSP properties persisted across a range of physiologically relevant spine neck resistances, and in models with variable neck resistances. By reducing the variability of local EPSPs, spines standardized synaptic activation of NMDA receptors and voltage-gated calcium channels. Furthermore, spines enhanced activation of NMDA receptors and facilitated the generation of NMDA spikes and axonal action potentials in response to synaptic input. Finally, we show that dynamic regulation of spine neck geometry can preserve local EPSP properties following plasticity-driven changes in synaptic strength, but is inefficient in modifying the amplitude of EPSPs in other cellular compartments. These observations suggest that one function of dendritic spines is to standardize local EPSP properties throughout the dendritic tree, thereby allowing neurons to use similar voltage-sensitive postsynaptic mechanisms at all dendritic locations.

Selective serotonergic excitation of callosal projection neurons

Serotonin (5-HT) acting as a neurotransmitter in the cerebral cortex is critical for cognitive function, yet how 5-HT regulates information processing in cortical circuits is not well understood. We tested the serotonergic responsiveness of layer 5 pyramidal neurons (L5PNs) in the mouse medial prefrontal cortex (mPFC), and found three distinct response types: long-lasting 5-HT_1A (1A) receptor-dependent inhibitory responses (84% of L5PNs), 5-HT_2A (2A) receptor-dependent excitatory responses (9%), and biphasic responses in which 2A-dependent excitation followed brief inhibition (5%). Relative to 5-HT-inhibited neurons, those excited by 5-HT had physiological properties characteristic of callosal/commissural (COM) neurons that project to the contralateral cortex. We tested whether serotonergic responses in cortical pyramidal neurons are correlated with their axonal projection pattern using retrograde fluorescent labeling of COM and corticopontine-projecting (CPn) neurons. 5-HT generated excitatory or biphasic responses in all 5-HT-responsive layer 5 COM neurons. Conversely, CPn neurons were universally inhibited by 5-HT. Serotonergic excitation of COM neurons was blocked by the 2A antagonist MDL 11939, while serotonergic inhibition of CPn neurons was blocked by the 1A antagonist WAY 100635, confirming a role for these two receptor subtypes in regulating pyramidal neuron activity. Selective serotonergic excitation of COM neurons was not layer-specific, as COM neurons in layer 2/3 were also selectively excited by 5-HT relative to their non-labeled pyramidal neuron neighbors. Because neocortical 2A receptors are implicated in the etiology and pathophysiology of schizophrenia, we propose that COM neurons may represent a novel cellular target for intervention in psychiatric disease.

Serotonin and prefrontal cortex function: neurons, networks, and circuits

Higher-order executive tasks such as learning, working memory, and behavioral flexibility depend on the prefrontal cortex (PFC), the brain region most elaborated in primates. The prominent innervation by serotonin neurons and the dense expression of serotonergic receptors in the PFC suggest that serotonin is a major modulator of its function. The most abundant serotonin receptors in the PFC, 5-HT1A, 5-HT2A and 5-HT3A receptors, are selectively expressed in distinct populations of pyramidal neurons and inhibitory interneurons, and play a critical role in modulating cortical activity and neural oscillations (brain waves). Serotonergic signaling is altered in many psychiatric disorders such as schizophrenia and depression, where parallel changes in receptor expression and brain waves have been observed. Furthermore, many psychiatric drug treatments target serotonergic receptors in the PFC. Thus, understanding the role of serotonergic neurotransmission in PFC function is of major clinical importance. Here, we review recent findings concerning the powerful influences of serotonin on single neurons, neural networks, and cortical circuits in the PFC of the rat, where the effects of serotonin have been most thoroughly studied.

M1 and M4 receptors modulate hippocampal pyramidal neurons

Acetylcholine (ACh), acting at muscarinic ACh receptors (mAChRs), modulates the excitability and synaptic connectivity of hippocampal pyramidal neurons. CA1 pyramidal neurons respond to transient ("phasic") mAChR activation with biphasic responses in which inhibition is followed by excitation, whereas prolonged ("tonic") mAChR activation increases CA1 neuron excitability. Both phasic and tonic mAChR activation excites pyramidal neurons in the CA3 region, yet ACh suppresses glutamate release at the CA3-to-CA1 synapse (the Schaffer-collateral pathway). Using mice genetically lacking specific mAChRs (mAChR knockout mice), we identified the mAChR subtypes responsible for cholinergic modulation of hippocampal pyramidal neuron excitability and synaptic transmission. Knockout of M1 receptors significantly reduced, or eliminated, most phasic and tonic cholinergic responses in CA1 and CA3 pyramidal neurons. On the other hand, in the absence of other G(q)-linked mAChRs (M3 and M5), M1 receptors proved sufficient for all postsynaptic cholinergic effects on CA1 and CA3 pyramidal neuron excitability. M3 receptors were able to participate in tonic depolarization of CA1 neurons, but otherwise contributed little to cholinergic responses. At the Schaffer-collateral synapse, bath application of the cholinergic agonist carbachol suppressed stratum radiatum-evoked excitatory postsynaptic potentials (EPSPs) in wild-type CA1 neurons and in CA1 neurons from mice lacking M1 or M2 receptors. However, Schaffer-collateral EPSPs were not significantly suppressed by carbachol in neurons lacking M4 receptors. We therefore conclude that M1 and M4 receptors are the major mAChR subtypes responsible for direct cholinergic modulation of the excitatory hippocampal circuit.

Phasic cholinergic signaling in the hippocampus: functional homology with the neocortex?

Acetylcholine (ACh) acts as a neurotransmitter in both the hippocampus and neocortex to facilitate learning, memory, and cognitive function. Here we show that transient muscarinic ACh receptor (mAChR) activation inhibits action potential generation in CA1, but not in CA3, pyramidal neurons via activation of an SK-type calcium-activated potassium conductance. Hyperpolarizing responses generated by focal ACh application near the somata of CA1 pyramidal neurons were blocked by atropine or the M1-like mAChR antagonist pirenzepine, but not by the M2-like mAChR antagonist methoctramine. Inhibitory cholinergic responses required intracellular calcium signaling, as evidenced by their sensitivity to depletion of internal calcium stores or internal calcium chelation. Cholinergic inhibition did not require GABAergic synaptic transmission, but was blocked by apamin, an SK channel antagonist. In contrast to inhibitory effects in CA1 neurons, ACh was primarily depolarizing, and enhanced action potential firing in CA3 pyramidal neurons. These results, when combined with recent data in neocortical neurons, suggest a functional homology in phasic cholinergic signaling in the hippocampus and neocortex whereby ACh preferentially inhibits those neurons in the lower cortical layers (CA1 and layer 5 neurons) that provide the majority of extracortical efferent projections.

Heterogeneity of phasic cholinergic signaling in neocortical neurons

Acetylcholine (ACh) is a neurotransmitter critical for normal cognition. Here we demonstrate heterogeneity of cholinergic signaling in neocortical neurons in the rat prefrontal, somatosensory, and visual cortex. Focal ACh application (100 muM) inhibited layer 5 pyramidal neurons in all cortical areas via activation of an apamin-sensitive SK-type calcium-activated potassium conductance. Cholinergic inhibition was most robust in prefrontal layer 5 neurons, where it relies on the same signal transduction mechanism (M1-like receptors, IP(3)-dependent calcium release, and SK-channels) as exists in somatosensory pyramidal neurons. Pyramidal neurons in layer 2/3 were less responsive to ACh, but substantial apamin-sensitive inhibitory responses occurred in deep layer 3 neurons of the visual cortex. ACh was only inhibitory when presented near the somata of layer 5 pyramidal neurons, where repetitive ACh applications generated discrete inhibitory events at frequencies of up to approximately 0.5 Hz. Fast-spiking (FS) nonpyramidal neurons in all cortical areas were unresponsive to ACh. When applied to non-FS interneurons in layers 2/3 and 5, ACh generated mecamylamine-sensitive nicotinic responses (38% of cells), apamin-insensitive hyperpolarizing responses, with or without initial nicotinic depolarization (7% of neurons), or no response at all (55% of cells). Responses in interneurons were similar across cortical layers and regions but were correlated with cellular physiology and the expression of biochemical markers associated with different classes of nonpyramidal neurons. Finally, ACh generated nicotinic responses in all layer 1 neurons tested. These data demonstrate that phasic cholinergic input can directly inhibit projection neurons throughout the cortex while sculpting intracortical processing, especially in superficial layers.

Cholinergic inhibition of neocortical pyramidal neurons

Acetylcholine (ACh) is a central neurotransmitter critical for normal cognitive function. Here we show that transient muscarinic acetylcholine receptor activation directly inhibits neocortical layer 5 pyramidal neurons. Using whole-cell and cell-attached recordings from neurons in slices of rat somatosensory cortex, we demonstrate that transient activation of M1-type muscarinic receptors induces calcium release from IP3-sensitive intracellular calcium stores and subsequent activation of an apamin-sensitive, SK-type calcium-activated potassium conductance. ACh-induced hyperpolarizing responses were blocked by atropine and pirenzepine but not by methoctramine or GABA receptor antagonists (picrotoxin, SR 95531 [2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide], and CGP 55845 [(2S)-3-[[(15)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid]). Responses were associated with a 31 +/- 5% increase in membrane conductance, had a reversal potential of -93 +/- 1 mV, and were eliminated after internal calcium chelation with BAPTA, blockade of IP3 receptors, or extracellular application of cadmium but not by sodium channel blockade with tetrodotoxin. Calcium-imaging experiments demonstrated that ACh-induced hyperpolarizing, but not depolarizing, responses were correlated with large increases in intracellular calcium. Surprisingly, transient increases in muscarinic receptor activation were capable of generating hyperpolarizing responses even during periods of tonic muscarinic activation sufficient to depolarize neurons to action potential threshold. Furthermore, eserine, an acetylcholinesterase inhibitor similar to those used therapeutically in the treatment of Alzheimer's disease, disproportionately enhanced the excitatory actions of acetylcholine while reducing the ability of acetylcholine to generate inhibitory responses during repeated applications of ACh. These data demonstrate that acetylcholine can directly inhibit the output of neocortical pyramidal neurons.

Synaptic integration in dendritic trees

Most neurons have elaborate dendritic trees that receive tens of thousands of synaptic inputs. Because postsynaptic responses to individual synaptic events are usually small and transient, the integration of many synaptic responses is needed to depolarize most neurons to action potential threshold. Over the past decade, advances in electrical and optical recording techniques have led to new insights into how synaptic responses propagate and interact within dendritic trees. In addition to their passive electrical and morphological properties, dendrites express active conductances that shape individual synaptic responses and influence synaptic integration locally within dendrites. Dendritic voltage-gated Na(+) and Ca(2+) channels support action potential backpropagation into the dendritic tree and local initiation of dendritic spikes, whereas K(+) conductances act to dampen dendritic excitability. While all dendrites investigated to date express active conductances, different neuronal types show specific patterns of dendritic channel expression leading to cell-specific differences in the way synaptic responses are integrated within dendritic trees. This review explores the way active and passive dendritic properties shape synaptic responses in the dendrites of central neurons, and emphasizes their role in synaptic integration.

Action potential initiation and propagation in layer 5 pyramidal neurons of the rat prefrontal cortex: absence of dopamine modulation

Somatic and dendritic whole-cell recording was used to examine action potential (AP) initiation and propagation in layer 5 pyramidal neurons of the rat prelimbic prefrontal cortex. APs generated by somatic current injection, or via antidromic stimulation, were reliably recorded at apical dendritic locations as far as 480 microm from the soma. Although the backpropagation of single APs into the apical dendrite was robust, frequency-dependent attenuation was observed during AP trains delivered at 10-100 Hz. APs were usually initiated close to the soma (presumably in the axon); however, strong depolarizing input to the apical dendrite could generate dendritic spikes that preceded somatic APs. AP backpropagation was dependent solely on activation of dendritic voltage-gated sodium channels and did not require activation of dendritic calcium channels. Despite not playing a role in AP backpropagation, calcium-imaging experiments demonstrated that dendritic calcium channels are activated by backpropagating APs, leading to transient increases in intracellular calcium. In addition, calcium imaging revealed that AP backpropagation into the distal apical tuft was frequency dependent. Finally, we tested whether dopamine, a prominent neuromodulator associated with prefrontal activity, could alter AP initiation or backpropagation. Bath-applied dopamine (10 or 100 microm) did not effect AP backpropagation, frequency-dependent depression, local dendritic spike initiation, or AP-induced calcium signaling. These data indicate that AP backpropagation in prefrontal layer 5 pyramidal neurons is robust but frequency dependent in the distal tuft, requires dendritic sodium rather than calcium channel activation, and, unlike other aspects of neuronal excitability, insensitive to modulation by dopamine.

Excitatory actions of GABA in the cortex

Little is known about how GABAergic inputs interact with excitatory inputs under conditions that maintain physiological concentrations of intracellular anions. Using extracellular and gramicidin perforated-patch recording, we show that somatic and dendritic GABA responses in mature cortical pyramidal neurons are depolarizing from rest and can facilitate action potential generation when combined with proximal excitatory input. Dendritic GABA responses were excitatory regardless of timing, whereas somatic GABA responses were inhibitory when coincident with excitatory input but excitatory at earlier times. These excitatory actions of GABA occur even though the GABA reversal potential is below action potential threshold and largely uniform across the somato-dendritic axis, and arise when GABAergic inputs are temporally or spatially isolated from concurrent excitation. Our findings demonstrate that under certain circumstances GABA will have an excitatory role in synaptic integration in the cortex.

Multiple effects of dopamine on layer V pyramidal cell excitability in rat prefrontal cortex

The mechanisms underlying the inhibitory effects of dopamine (DA) on layer V pyramidal neuron excitability in the prelimbic region of the rat medial prefrontal cortex were investigated. Under control conditions, DA depressed both action potential generation (driven by somatic current injection) and input resistance (R(N)). The presence of GABA(A) receptor antagonists blocked DA-induced depression of action potential generation and revealed a delayed increase in excitability that persisted for the duration of experimental recording, up to 20 min following the washout of DA. In contrast to spike generation, disinhibition did not affect the transient depression of R(N) produced by DA, suggesting independent actions of DA on spike generation and R(N). Consistent with the hypothesis that DA acts to decrease pyramidal cell output via a GABAergic mechanism, DA increased the frequency of spontaneous inhibitory postsynaptic currents in both the absence and presence of TTX. Furthermore focal application of GABA to a perisomatic region mimicked the inhibitory effect of DA on spike production without affecting R(N). Focal application of bicuculline to the same location reversed the inhibitory effect of bath-applied DA on spike generation, while again having no effect on R(N). The depression of R(N) by DA was both occluded and mimicked by the Na(+) channel blocker TTX, suggesting the involvement of a Na(+) conductance in reducing pyramidal cell R(N) during the acute presence of DA. Together these data demonstrate that the acute presence of DA decreases pyramidal neuron excitability by two independent mechanisms. At the same time DA triggers a delayed and longer-lasting increase in excitability that is partially masked by synaptic inhibition.

Dopamine decreases the excitability of layer V pyramidal cells in the rat prefrontal cortex

In both primates and rodents, the prefrontal cortex (PFC) is highly innervated by dopaminergic fibers originating from the ventral tegmental area, and activation of this mesocortical dopaminergic system decreases spontaneous and evoked activity in the PFC in vivo. We have examined the effects of dopamine (DA), over a range of concentrations, on the passive and active membrane properties of layer V pyramidal cells from the rat medial PFC (mPFC). Whole-cell and perforated-patch recordings were made from neurons in rat mPFC. As a measure of cell excitability, trains of action potentials were evoked with 1-sec-long depolarizing current steps. Bath application of DA (0.05-30 microM) produced a reversible decrease in the number of action potentials evoked by a given current step. In addition, DA reversibly decreased the input resistance (RN) of these cells. In a subset of experiments, a transient increase in excitability was observed after the washout of DA. Control experiments suggest that these results are not attributable to changes in spontaneous synaptic activity, age-dependent processes, or strain-specific differences in dopaminergic innervation and physiology. Pharmacological analyses, using D1 agonists (SKF 38393 and SKF 81297), a D1 antagonist (SCH 23390), a D2 receptor agonist (quinpirole), and a D2 antagonist (sulpiride) suggest that decreases in spiking and RN are mediated by D2 receptor activation. Together, these results demonstrate that DA, over a range of concentrations, has an inhibitory effect on layer V pyramidal neurons in the rat mPFC, possibly through D2 receptor activation.