Control of a hippocampal recurrent excitatory circuit by cannabinoid receptor-interacting protein Gap43

The type-1 cannabinoid receptor (CB₁R) is widely expressed in excitatory and inhibitory nerve terminals, and by suppressing neurotransmitter release, its activation modulates neural circuits and brain function. While the interaction of CB₁R with various intracellular proteins is thought to alter receptor signaling, the identity and role of these proteins are poorly understood. Using a high-throughput proteomic analysis complemented with an array of in vitro and in vivo approaches in the mouse brain, we report that the C-terminal, intracellular domain of CB₁R interacts specifically with growth-associated protein of 43 kDa (GAP43). The CB₁R-GAP43 interaction occurs selectively at mossy cell axon boutons, which establish excitatory synapses with dentate granule cells in the hippocampus. This interaction impairs CB₁R-mediated suppression of mossy cell to granule cell transmission, thereby inhibiting cannabinoid-mediated anti-convulsant activity in mice. Thus, GAP43 acts as a synapse type-specific regulatory partner of CB₁R that hampers CB₁R-mediated effects on hippocampal circuit function.

Small molecules fail to induce direct reprogramming of adult rat olfactory ensheathing glia to mature neurons

An approach to generate new neurons after central nervous system injury or disease is direct reprogramming of the individual's own somatic cells into differentiated neurons. This can be achieved either by transduction of viral vectors that express neurogenic transcription factors and/or through induction with small molecules, avoiding introducing foreign genetic material in target cells. In this work, we propose olfactory ensheathing glia (OEG) as a candidate for direct reprogramming to neurons with small molecules due to its well-characterized neuro-regenerative capacity. After screening different combinations of small molecules in different culture conditions, only partial reprogramming was achieved: induced cells expressed neuronal markers but lacked the ability of firing action potentials. Our work demonstrates that direct conversion of adult olfactory ensheathing glia to mature, functional neurons cannot be induced only with pharmacological tools.

Endocannabinoid and Nitric Oxide-Dependent IGF-I-Mediated Synaptic Plasticity at Mice Barrel Cortex

Insulin-like growth factor-I (IGF-I) signaling plays a key role in learning and memory. IGF-I increases the spiking and induces synaptic plasticity in the mice barrel cortex (Noriega-Prieto et al., 2021), favoring the induction of the long-term potentiation (LTP) by Spike Timing-Dependent Protocols (STDP) (Noriega-Prieto et al., 2021). Here, we studied whether these IGF-I effects depend on endocannabinoids (eCBs) and nitric oxide (NO). We recorded both excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) evoked by stimulation of the basal dendrites of layer II/III pyramidal neurons of the Barrel Cortex and analyzed the effect of IGF-I in the presence of a CB₁R antagonist, AM251, and inhibitor of the NO synthesis, L-NAME, to prevent the eCBs and the NO-mediated signaling. Interestingly, L-NAME abolished any modulatory effect of the IGF-I-induced excitatory and inhibitory transmission changes, suggesting the essential role of NO. Surprisingly, the inhibition of CB1Rs did not only block the potentiation of EPSCs but reversed to a depression, highlighting the remarkable functions of the eCB system. In conclusion, eCBs and NO play a vital role in deciding the sign of the effects induced by IGF-I in the neocortex, suggesting a neuromodulatory interplay among IGF-I, NO, and eCBs.

IGF-1 facilitates extinction of conditioned fear

Insulin-like growth factor-1 (IGF-1) plays a key role in synaptic plasticity, spatial learning, and anxiety-like behavioral processes. While IGF-1 regulates neuronal firing and synaptic transmission in many areas of the central nervous system, its signaling and consequences on excitability, synaptic plasticity, and animal behavior dependent on the prefrontal cortex remain unexplored. Here, we show that IGF-1 induces a long-lasting depression of the medium and slow post-spike afterhyperpolarization (mAHP and sAHP), increasing the excitability of layer 5 pyramidal neurons of the rat infralimbic cortex. Besides, IGF-1 mediates a presynaptic long-term depression of both inhibitory and excitatory synaptic transmission in these neurons. The net effect of this IGF-1-mediated synaptic plasticity is a long-term potentiation of the postsynaptic potentials. Moreover, we demonstrate that IGF-1 favors the fear extinction memory. These results show novel functional consequences of IGF-1 signaling, revealing IGF-1 as a key element in the control of the fear extinction memory.

Nitric Oxide-Dependent LTD at Infralimbic Cortex

Dendritic calcium (Ca²⁺) spikes play a key role in the genesis of long-term synaptic plasticity. Although synaptic plasticity in the infralimbic cortex is critical for the extinction of fear-conditioned memory, the role of Ca²⁺-spikes in the induction of synaptic plasticity at this cortex has not been explored in depth. Here we show that Ca²⁺-spikes in layer 5 pyramidal neurons (L5 PNs) of the rat infralimbic cortex are crucial in the induction of long-term depression of the excitatory postsynaptic currents (EPSCs). The lack of effect on the postsynaptic currents evoked by puffing glutamate and the changes in the variance of the EPSC amplitude that paralleled its inhibition suggest that this LTD of the EPSCs is mediated presynaptically. However, its induction requires cytosolic calcium elevations because it is prevented when the recorded L5 PN is loaded with BAPTA. Moreover, it depends on the synthesis of nitric oxide (NO) because it is absent on slices incubated with nitric oxidase synthase inhibitor L-NAME. Therefore, Ca²⁺-spikes can trigger LTD of the ESPCs through the NO dependent presynaptic form of synaptic plasticity, thus providing a novel form of inducing synaptic plasticity at L5 PNs of the rat infralimbic cortex.

Endocannabinoid-Dependent Long-Term Potentiation of Synaptic Transmission at Rat Barrel Cortex

Brain-derived neurotrophic factor (BDNF) plays a critical role in modulating plasticity in sensory cortices. Indeed, a BDNF-dependent long-term potentiation (LTP) at distal basal excitatory synapses of Layer 5 pyramidal neurons (L5PNs) has been demonstrated in disinhibited rat barrel cortex slices. Although it is well established that this LTP requires the pairing of excitatory postsynaptic potentials (PSPs) with Ca2+ spikes, its induction when synaptic inhibition is working remains unexplored. Here we show that low-frequency stimulation at basal dendrites of L5PNs is able to trigger a PSP followed by an action potential (AP) and a slow depolarization (termed PSP-Ca2+ response) in thalamocortical slices without blocking synaptic inhibition. We demonstrate that AP barrage-mediated release of endocannabinoids (eCBs) from the recorded L5PNs induces PSP-Ca2+ response facilitation and BDNF-dependent LTP. Indeed, this LTP requires the type 1 cannabinoid receptors activation, is prevented by postsynaptic intracellular 1,2-bis(2-aminophenoxy) ethane-N,N,N,N'-tetraacetic acid (BAPTA) or the anandamide membrane transporter inhibitor AM404, and only occurs in L5PNs neurons showing depolarization-induced suppression of inhibition. Additionally, electrical stimulation at the posteromedial thalamic nucleus induced similar response and LTP. These results reveal a novel form of eCB-dependent LTP at L5PNs that could be relevant in the processing of sensory information in the barrel cortex.

Acetylcholine Facilitates a Depolarization-Induced Enhancement of Inhibition in Rat CA1 Pyramidal Neurons

Cholinergic mechanisms in the hippocampus regulate forms of synaptic plasticity linked with cognition and spatial navigation, but the underlying mechanisms remain largely unknown. Here, in rat hippocampal CA1 pyramidal cells under blockade of ionotropic glutamate receptors, we report that a single acetylcholine pulse and repeated depolarization activated a robust and enduring postsynaptic depolarization-induced enhancement of inhibition (DEI) that masked a presynaptic depolarization-induced suppression of inhibition (DSI). Increased cytosolic Ca2+ and M1-muscarinic receptor activation caused the rise in voltage-sensitive α5βγ2-containing γ-aminobutyric acid type-A receptors that generated DEI. In summary, this muscarinic-mediated activity-dependent plasticity rapidly transfers depolarization effects on inhibition from presynaptic suppression or DSI to postsynaptic enhancement or DEI, a change potentially relevant in behavior.

Bidirectional Hebbian Plasticity Induced by Low-Frequency Stimulation in Basal Dendrites of Rat Barrel Cortex Layer 5 Pyramidal Neurons

According to Hebb's original hypothesis (Hebb, 1949), synapses are reinforced when presynaptic activity triggers postsynaptic firing, resulting in long-term potentiation (LTP) of synaptic efficacy. Long-term depression (LTD) is a use-dependent decrease in synaptic strength that is thought to be due to synaptic input causing a weak postsynaptic effect. Although the mechanisms that mediate long-term synaptic plasticity have been investigated for at least three decades not all question have as yet been answered. Therefore, we aimed at determining the mechanisms that generate LTP or LTD with the simplest possible protocol. Low-frequency stimulation of basal dendrite inputs in Layer 5 pyramidal neurons of the rat barrel cortex induces LTP. This stimulation triggered an EPSP, an action potential (AP) burst, and a Ca²⁺ spike. The same stimulation induced LTD following manipulations that reduced the Ca²⁺ spike and Ca²⁺ signal or the AP burst. Low-frequency whisker deflections induced similar bidirectional plasticity of action potential evoked responses in anesthetized rats. These results suggest that both in vitro and in vivo similar mechanisms regulate the balance between LTP and LTD. This simple induction form of bidirectional hebbian plasticity could be present in the natural conditions to regulate the detection, flow, and storage of sensorimotor information.

Muscarinic Long-Term Enhancement of Tonic and Phasic GABAA Inhibition in Rat CA1 Pyramidal Neurons

Acetylcholine (ACh) regulates network operation in the hippocampus by controlling excitation and inhibition in rat CA1 pyramidal neurons (PCs), the latter through gamma-aminobutyric acid type-A receptors (GABA_ARs). Although, the enhancing effects of ACh on GABA_ARs have been reported (Dominguez et al., 2014, 2015), its role in regulating tonic GABA_A inhibition has not been explored in depth. Therefore, we aimed at determining the effects of the activation of ACh receptors on responses mediated by synaptic and extrasynaptic GABA_ARs. Here, we show that under blockade of ionotropic glutamate receptors ACh, acting through muscarinic type 1 receptors, paired with post-synaptic depolarization induced a long-term enhancement of tonic GABA_A currents (tGABA_A) and puff-evoked GABA_A currents (pGABA_A). ACh combined with depolarization also potentiated IPSCs (i.e., phasic inhibition) in the same PCs, without signs of interactions of synaptic responses with pGABA_A and tGABA_A, suggesting the contribution of two different GABA_A receptor pools. The long-term enhancement of GABA_A currents and IPSCs reduced the excitability of PCs, possibly regulating plasticity and learning in behaving animals.

Cholinergic-mediated response enhancement in barrel cortex layer V pyramidal neurons

Neocortical cholinergic activity plays a fundamental role in sensory processing and cognitive functions, but the underlying cellular mechanisms are largely unknown. We analyzed the effects of acetylcholine (ACh) on synaptic transmission and cell excitability in rat "barrel cortex" layer V (L5) pyramidal neurons in vitro. ACh through nicotinic and M1 muscarinic receptors enhanced excitatory postsynaptic currents and through nicotinic and M2 muscarinic receptors reduced inhibitory postsynaptic currents. These effects increased excitability and contributed to the generation of Ca(2+) spikes and bursts of action potentials (APs) when inputs in basal dendrites were stimulated. Ca(2+) spikes were mediated by activation of NMDA receptors (NMDARs) and L-type voltage-gated Ca(2+) channels. Additionally, we demonstrate in vivo that basal forebrain stimulation induced an atropine-sensitive increase of L5 AP responses evoked by vibrissa deflection, an effect mainly due to the enhancement of an NMDAR component. Therefore, ACh modified the excitatory/inhibitory balance and switched L5 pyramidal neurons to a bursting mode that caused a potent and sustained response enhancement with possible fundamental consequences for the function of the barrel cortex.

Role of AMPA and NMDA receptors and back-propagating action potentials in spike timing-dependent plasticity

The cellular mechanisms that mediate spike timing-dependent plasticity (STDP) are largely unknown. We studied in vitro in CA1 pyramidal neurons the contribution of AMPA and N-methyl-d-aspartate (NMDA) components of Schaffer collateral (SC) excitatory postsynaptic potentials (EPSPs; EPSP(AMPA) and EPSP(NMDA)) and of the back-propagating action potential (BAP) to the long-term potentiation (LTP) induced by a STDP protocol that consisted in pairing an EPSP and a BAP. Transient blockade of EPSP(AMPA) with 7-nitro-2,3-dioxo-1,4-dihydroquinoxaline-6-carbonitrile (CNQX) during the STDP protocol prevented LTP. Contrastingly LTP was induced under transient inhibition of EPSP(AMPA) by combining SC stimulation, an imposed EPSP(AMPA)-like depolarization, and BAP or by coupling the EPSP(NMDA) evoked under sustained depolarization (approximately -40 mV) and BAP. In Mg(2+)-free solution EPSP(NMDA) and BAP also produced LTP. Suppression of EPSP(NMDA) or BAP always prevented LTP. Thus activation of NMDA receptors and BAPs are needed but not sufficient because AMPA receptor activation is also obligatory for STDP. However, a transient depolarization of another origin that unblocks NMDA receptors and a BAP may also trigger LTP.

Selective shunting of the NMDA EPSP component by the slow afterhyperpolarization in rat CA1 pyramidal neurons

Pyramidal neuron dendrites express voltage-gated conductances that control synaptic integration and plasticity, but the contribution of the Ca(2+)-activated K(+)-mediated currents to dendritic function is not well understood. Using dendritic and somatic recordings in rat hippocampal CA1 pyramidal neurons in vitro, we analyzed the changes induced by the slow Ca(2+)-activated K(+)-mediated afterhyperpolarization (sAHP) generated by bursts of action potentials on excitatory postsynaptic potentials (EPSPs) evoked at the apical dendrites by perforant path-Schaffer collateral stimulation. Both the amplitude and decay time constants of EPSPs (tau(EPSP)) were reduced by the sAHP in somatic recordings. In contrast, the dendritic EPSP amplitude remained unchanged, whereas tau(EPSP) was reduced. Temporal summation was reduced and spatial summation linearized by the sAHP. The amplitude of the isolated N-methyl-D-aspartate component of EPSPs (EPSP(NMDA)) was reduced, whereas tau(NMDA) was unaffected by the sAHP. In contrast, the sAHP did not modify the amplitude of the isolated EPSP(AMPA) but reduced tau(AMPA) both in dendritic and somatic recordings. These changes are attributable to a conductance increase that acted mainly via a selective "shunt" of EPSP(NMDA) because they were absent under voltage clamp, not present with imposed hyperpolarization simulating the sAHP, missing when the sAHP was inhibited with isoproterenol, and reduced under block of EPSP(NMDA). EPSPs generated at the basal dendrites were similarly modified by the sAHP, suggesting both a somatic and apical dendritic location of the sAHP channels. Therefore the sAHP may play a decisive role in the dendrites by regulating synaptic efficacy and temporal and spatial summation.

Calcium-activated afterhyperpolarizations regulate synchronization and timing of epileptiform bursts in hippocampal CA3 pyramidal neurons

Calcium-activated potassium conductances regulate neuronal excitability, but their role in epileptogenesis remains elusive. We investigated in rat CA3 pyramidal neurons the contribution of the Ca(2+)-activated K(+)-mediated afterhyperpolarizations (AHPs) in the genesis and regulation of epileptiform activity induced in vitro by 4-aminopyridine (4-AP) in Mg(2+)-free Ringer. Recurring spike bursts terminated by prolonged AHPs were generated. Burst synchronization between CA3 pyramidal neurons in paired recordings typified this interictal-like activity. A downregulation of the medium afterhyperpolarization (mAHP) paralleled the emergence of the interictal-like activity. When the mAHP was reduced or enhanced by apamin and EBIO bursts induced by 4-AP were increased or blocked, respectively. Inhibition of the slow afterhyperpolarization (sAHP) with carbachol, t-ACPD, or isoproterenol increased bursting frequency and disrupted burst regularity and synchronization between pyramidal neuron pairs. In contrast, enhancing the sAHP by intracellular dialysis with KMeSO(4) reduced burst frequency. Block of GABA(A-B) inhibitions did not modify the abnormal activity. We describe novel cellular mechanisms where 1) the inhibition of the mAHP plays an essential role in the genesis and regulation of the bursting activity by reducing negative feedback, 2) the sAHP sets the interburst interval by decreasing excitability, and 3) bursting was synchronized by excitatory synaptic interactions that increased in advance and during bursts and decreased throughout the subsequent sAHP. These cellular mechanisms are active in the CA3 region, where epileptiform activity is initiated, and cooperatively regulate the timing of the synchronized rhythmic interictal-like network activity.

Cholinergic modulation of synaptic transmission and postsynaptic excitability in the rat gracilis dorsal column nucleus

Somatosensory information, conveyed through the gracilis nucleus (GN), is regulated by descending corticofugal (CF) glutamatergic fibers. In addition, the GN receives cholinergic inputs with still unclear source and functional significance. Using both the in vitro slice and intracellular recording with sharp and patch electrodes and in vivo extracellular single-unit recordings, we analyzed the effects of activation of cholinergic receptors on synaptic, intrinsic, and functional properties of rat GN neurons. The cholinergic agonist carbamilcholine-chloride [carbachol (CCh); 1-10 microM] in vitro (1) induced presynaptic inhibition of EPSPs evoked by both dorsal column and CF stimulation, (2) increased postsynaptic excitability, and (3) amplified the spike output of GN neurons. The inhibition by atropine (1 microM) and pirenzepine (10 microM) of all presynaptic and postsynaptic effects of CCh suggests actions through muscarinic M1 receptors. The above effects were insensitive to nicotinic antagonists. We searched the anatomical origin of the cholinergic projection to the GN throughout the hindbrain and forebrain, and we found that the cholinergic fibers originated mainly in the pontine reticular nucleus (PRN). Electrical stimulation of the PRN amplified sensory responses in the GN in vivo, an effect prevented by topical application of atropine. Our results demonstrate for the first time that cholinergic agonists induce both presynaptic and postsynaptic effects on GN neurons and suggest an important regulatory action of inputs from cholinergic neuronal groups in the pontine reticular formation in the functional control of somatosensory information flow in the GN.

1-Ethyl-2-benzimidazolinone (EBIO) suppresses epileptiform activity in in vitro hippocampus

Ca2+-activated K+ currents with medium (mI(AHP)) and slow (sI(AHP)) kinetics, that mediate the post-spike medium and slow after-hyperpolarization (AHP), respectively, play critical roles in regulating neuronal excitability and the spread of epileptiform activity and could provide new therapeutic targets for the management of epileptic patients. We tested if the enhancement of the mI(AHP) by 1-ethyl-2-benzimidazolinone (EBIO) could suppress epileptiform activity in two in vitro models of epileptogenesis induced in CA3 hippocampal pyramidal neurons by superfusion with 4-AP- and kainate-Mg2+-free solutions. Both interictal- and ictal-like epileptiform activities were reversibly suppressed by EBIO concentrations between 200 microM and 1 mM. EBIO predominantly acted by a strong reduction of excitability via an increase (approximately 450%) of the mI(AHP), without changing the sI(AHP). Glutamatergic excitatory synaptic transmission was also diminished (approximately 50%) by 1 mM EBIO. In contrast, EBIO concentrations <400 microM had no effect on synaptic excitation, consistent with a lesser sensitivity to the drug than the mI(AHP). Apamine (100 nM), a toxin that specifically inhibits the mI(AHP), rapidly and reversibly antagonized the blocking effects of EBIO on epileptiform activity. Our results suggest that manipulations that enhance the mI(AHP) may prove adequate in the treatment of epilepsies; they also suggest that an abnormal down regulation of the mI(AHP) may be a key factor in the genesis of hyperexcitable states.

Presynaptic inhibition of Schaffer collateral synapses by stimulation of hippocampal cholinergic afferent fibres

It has been known for decades that muscarinic agonists presynaptically inhibit Schaffer collateral synapses contacting hippocampal CA1 pyramidal neurons. However, a demonstration of the inhibition of Schaffer collateral synapses induced by acetylcholine released by cholinergic hippocampal afferents is lacking. We present original results showing that electrical stimulation at the stratum oriens/alveus with brief stimulus trains inhibited excitatory postsynaptic currents evoked by stimulation of Schaffer collaterals in CA1 pyramidal neurons of rat hippocampal slices. The increased paired-pulse facilitation and the changes in the variance of excitatory postsynaptic current amplitude that paralleled the inhibition suggest that it was mediated presynaptically. The effects of oriens/alveus stimulation were inhibited by atropine, and blocking nicotinic receptors with methyllycaconitine was ineffective, suggesting that the inhibition was mediated via the activation of presynaptic muscarinic receptors. The results provide a novel demonstration of the presynaptic inhibition of glutamatergic neurotransmission by cholinergic fibres in the hippocampus, implying that afferent cholinergic fibres regulate the strength of excitatory synaptic transmission.

Selective muscarinic regulation of functional glutamatergic Schaffer collateral synapses in rat CA1 pyramidal neurons

Analysis of the cholinergic regulation of glutamatergic neurotransmission is an essential step in understanding the hippocampus because it can influence forms of synaptic plasticity that are thought to underlie learning and memory. We studied in vitro the cholinergic regulation of excitatory postsynaptic currents (EPSCs) evoked in rat CA1 pyramidal neurons by Schaffer collateral (SC) stimulation. Using "minimal" stimulation, which activates one or very few synapses, the cholinergic agonist carbamylcholine (CCh) increased the failure rate of functional more (36 %) than of silent synapses (7 %), without changes in the EPSC amplitude. These effects of CCh were insensitive to manipulations that increased the probability of release, such as paired pulse facilitation, increases in temperature and increases in the extracellular Ca(2+) : Mg(2+) ratio. Using "conventional" stimulation, which activates a large number of synapses, CCh inhibited more the pharmacologically isolated non-NMDA (86 %) than the NMDA (47 %) EPSC. The changes in failure rate, EPSC variance and the increased paired pulse facilitation that paralleled the inhibition imply that CCh decreased release probability. Muscarine had similar effects. The inhibition by both CCh and by muscarine was prevented by atropine. We conclude that CCh reduces the non-NMDA component of SC EPSCs by selectively inhibiting transmitter release at functional synapses via activation of muscarinic receptors. The results suggest that SCs have two types of terminals, one in functional synapses, selectively sensitive to regulation through activation of muscarinic receptors, and the other in silent synapses less sensitive to that regulation. The specific inhibition of functional synapses would favour activity-dependent plastic phenomena through NMDA receptors at silent synapses without the activation of non-NMDA receptors and functional synapses.