Long-lasting Increase In Excitability Research Articles

Ketamine Produces a Long-Lasting Enhancement of CA1 Neuron Excitability.

Ketamine is a clinical anesthetic and antidepressant. Although ketamine is a known NMDA receptor antagonist, the mechanisms contributing to antidepression are unclear. This present study examined the loci and duration of ketamine’s actions, and the involvement of NMDA receptors. Local field potentials were recorded from the CA1 region of mouse hippocampal slices. Ketamine was tested at antidepressant and anesthetic concentrations. Effects of NMDA receptor antagonists APV and MK-801, GABA receptor antagonist bicuculline, and a potassium channel blocker TEA were also studied. Ketamine decreased population spike amplitudes during application, but a long-lasting increase in amplitudes was seen during washout. Bicuculline reversed the acute effects of ketamine, but the washout increase was not altered. This long-term increase was statistically significant, sustained for >2 h, and involved postsynaptic mechanisms. A similar effect was produced by MK-801, but was only partially evident with APV, demonstrating the importance of the NMDA receptor ion channel block. TEA also produced a lasting excitability increase, indicating a possible involvement of potassium channel block. This is this first report of a long-lasting increase in excitability following ketamine exposure. These results support a growing literature that increased GABA inhibition contributes to ketamine anesthesia, while increased excitatory transmission contributes to its antidepressant effects.

M1 muscarinic activation induces long-lasting increase in intrinsic excitability of striatal projection neurons

The dorsolateral striatum is critically involved in movement control and motor learning. Striatal function is regulated by a variety of neuromodulators including acetylcholine. Previous studies have shown that cholinergic activation excites striatal principal projection neurons, medium spiny neurons (MSNs), and this action is mediated by muscarinic acetylcholine subtype 1 receptors (M1) through modulating multiple potassium channels. In the present study, we used electrophysiology techniques in conjunction with optogenetic and pharmacological tools to determine the long-term effects of striatal cholinergic activation on MSN intrinsic excitability. A transient increase in acetylcholine release in the striatum by optogenetic stimulation resulted in a long-lasting increase in excitability of MSNs, which was associated with hyperpolarizing shift of action potential threshold and decrease in afterhyperpolarization (AHP) amplitude, leading to an increase in probability of EPSP-action potential coupling. The M1 selective antagonist VU0255035 prevented, while the M1 selective positive allosteric modulator (PAM) VU0453595 potentiated the cholinergic activation-induced persistent increase in MSN intrinsic excitability, suggesting that M1 receptors are critically involved in the induction of this long-lasting response. This M1 receptor-dependent long-lasting change in MSN intrinsic excitability could have significant impact on striatal processing and might provide a novel mechanism underlying cholinergic regulation of the striatum-dependent motor learning and cognitive function. Consistent with this, behavioral studies indicate that potentiation of M1 receptor signaling by VU0453595 enhanced performance of mice in cue-dependent water-based T-maze, a dorsolateral striatum-dependent learning task.

Dopamine D2 receptor controls hilar mossy cells excitability.

Hippocampal control of memory formation is regulated by dopaminergic signaling. Whereas the role of dopamine D1 receptors is well documented in such regulations, functions of dopamine D2 receptors (DRD2) are not fully understood. Using fluorescence in situ hybridization we demonstrate that Drd2 expression in the hippocampus of wild-type mice is limited to glutamatergic hilar mossy cells. Using whole cell electrophysiological recordings in hippocampal slice preparations, we provide evidence that unlike in basal ganglia, activation of DRD2 by the selective agonist, quinpirole, induces a long-lasting increase in excitability of hilar mossy cells, which can be blocked by the DRD2 antagonist raclopride. Such activity is mediated by the Akt/GSK pathway, as application of specific inhibitors such as A1070722 or SB216763 prevented quinpirole activity. Long-term effects of acute DRD2 activation in vitro suggest that volume transmission of dopamine may modulate mossy cell activities in vivo. This is supported by the presence of dense tyrosine hydroxylase positive varicosities in the hilus, which are rarely seen in the vicinity of mossy cell dendrites. From these data we discuss how dopamine could control mossy cell activity and thus dentate gyrus functions.

Protein kinase A regulates the long-term potentiation of intrinsic excitability in neonatal trigeminal motoneurons

Although much is known about neuronal plasticity in the mammalian hippocampus and other cortical neurons, the subcellular mechanisms underlying plasticity at the level of motor pools are less well characterized. Protein kinase A (PKA) activation plays an essential role in long-term potentiation of intrinsic excitability (LTP-IE) in layer V (LV) visual cortical neurons and may be involved in other systems as well. Trigeminal motoneurons (TMNs) participate in rhythmical motor behaviors, such as suckling, chewing, and swallowing. Using the whole-cell patch clamp method and various kinase inhibitors and activators, we investigated the mechanism of LTP-IE in neonatal rat TMNs. Ca2+ depletion using ACSF with 0mM Ca2+ or the Ca2+ chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) blocked the long-lasting increase in intrinsic excitability in TMNs, showing that intracellular Ca2+ during the induction protocol is necessary for the induction of LTP-IE. We next used specific inhibitors of PKA, protein kinase C, and calcium/calmodulin-dependent protein kinase II during the induction protocol. Only the PKA inhibitor H-89 blocked the increase in the firing rate induced by the induction protocol. In addition, forskolin, which activates PKA, induced a long-lasting increase in excitability that resembled the excitability produced by the induction protocol. Thus, we conclude that LTP-IE in TMNs is calcium-dependent, and PKA is the primary regulator of this process.

Plasticity of neuronal excitability in vivo

Plasticity of neuronal excitability <i>in vivo</i>

Role of neurotransmitters in sensitization of pain responses.

Injection of capsaicin into the skin results in pain, primary heat and mechanical hyperalgesia, and secondary mechanical allodynia and hyperalgesia. Sensory receptors in the area of secondary mechanical allodynia and hyperalgesia are unaffected, and so the sensory changes must be due to central actions of the initial intense nociceptive discharge that follows the capsaicin injection. Central sensitization of the responses of spinothalamic tract neurons lasts several hours, but can be prevented by spinal cord administration of non-NMDA and NMDA glutamate receptor antagonists or NK1 substance P receptor antagonists. The long-lasting increase in excitability of spinothalamic tract cells depends on the activation of several second messenger cascades (PKC, PKA, and NO/PKG signal transduction pathways). The excitability change also depends on activation of calcium/calmodulin-dependent kinase II, which is consistent with the proposal that this central sensitization response is a form of long-term potentiation.

Presynaptic control of transmission along the pathway mediating disynaptic reciprocal inhibition in the cat.

In cat lumbar motoneurones, disynaptic inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of antagonist motor nerves were depressed for at least 150 ms following conditioning stimulation of flexor (1.7-2 times threshold (T)) and ankle extensor (5T) nerves. The aim of the present study was to investigate the possibility that this depression is caused by presynaptic inhibitory mechanisms acting at the terminals of group I afferent fibres projecting to the Ia inhibitory interneurones and/or the terminals of these interneurones to the target motoneurones. Conditioning stimulation of flexor, but not ankle extensor, nerves evoked a depression of the monosynaptic Ia excitatory postsynaptic potentials (EPSPs) recorded intracellularly in Ia inhibitory interneurones. This depression lasted between 200 and 700 ms and was not accompanied by a depression of the monosynaptic EPSPs evoked by stimulation of descending pathways. These results suggest that flexor, but not ankle extensor, group I afferent fibres can modulate sensory transmission at the synapse between Ia afferent fibres and Ia inhibitory interneurones. Conditioning stimulation of flexor muscle nerves, extensor muscle nerves and cutaneous nerves produced a long-lasting increase in excitability of the terminals of the Ia inhibitory interneurones. The increase in the excitability of the terminals was not secondary to an electrotonic spread of synaptic excitation at the soma. Indeed, concomitant with the excitability increase of the terminals there were signs of synaptic inhibition in the soma. The unitary IPSPs induced in target motoneurones following the spike activity of single Ia inhibitory interneurones were depressed by conditioning stimulation of muscle and cutaneous nerves. Since the conditioning stimulation also evoked compound IPSPs in those motoneurones, a firm conclusion as to whether unitary IPSP depression involved presynaptic inhibitory mechanism of the terminals of the interneurones could not be reached. The possibility that the changes in excitability of the Ia interneuronal terminals reflect the presence of a presynaptic inhibitory mechanism similar to that operating at the terminals of the afferent fibres (presynaptic inhibition) is discussed.1. In cat lumbar motoneurones, disynaptic inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of antagonist motor nerves were depressed for at least 150 ms following conditioning stimulation of flexor (1.7-2 times threshold (T)) and ankle extensor (5T) nerves. The aim of the present study was to investigate the possibility that this depression is caused by presynaptic inhibitory mechanisms acting at the terminals of group I afferent fibres projecting to the Ia inhibitory interneurones and/or the terminals of these interneurones to the target motoneurones. Conditioning stimulation of flexor, but not ankle extensor, nerves evoked a depression of the monosynaptic Ia excitatory postsynaptic potentials (EPSPs) recorded intracellularly in Ia inhibitory interneurones. This depression lasted between 200 and 700 ms and was not accompanied by a depression of the monosynaptic EPSPs evoked by stimulation of descending pathways. These results suggest that flexor, but not ankle extensor, group I afferent fibres can modulate sensory transmission at the synapse between Ia afferent fibres and Ia inhibitory interneurones. Conditioning stimulation of flexor muscle nerves, extensor muscle nerves and cutaneous nerves produced a long-lasting increase in excitability of the terminals of the Ia inhibitory interneurones. The increase in the excitability of the terminals was not secondary to an electrotonic spread of synaptic excitation at the soma. Indeed, concomitant with the excitability increase of the terminals there were signs of synaptic inhibition in the soma. The unitary IPSPs induced in target motoneurones following the spike activity of single Ia inhibitory interneurones were depressed by conditioning stimulation

Visual potentiation of vestibular responses in lamprey reticulospinal neurons.

The lamprey normally swims with the dorsal side up. Illumination of one eye shifts the set-point of the vestibular roll control system, however, so that the animal swims with a roll tilt towards the source of light (the dorsal light response). A tilted orientation is often maintained for up to 1 min after the stimulation. In present study, the basis for this behaviour was investigated at the neuronal level. The middle rhombencephalic reticular nucleus (MRRN) is considered a main nucleus for the control of roll orientation in lampreys. Practically all MRRN neurons receive vestibular and visual input and project to the spinal cord. Earlier extracellular experiments had shown that optic nerve stimulation potentiates the response to vestibular stimulation in the ipsilateral MRRN. This most likely represents a neural correlate of the dorsal light response. Experiments were carried out in vitro on the isolated brainstem of the silver lamprey (Ichthyomyzon unicuspis). MRRN cells were recorded intracellularly, and the overall activity of descending systems was monitored with bilateral extracellular electrodes. The responses to 10 Hz optic nerve stimulation and 1 Hz vestibular nerve stimulation, and the influence of optic nerve stimulation on the vestibular responses, were investigated. In most preparations, optic nerve stimulation excited practically all ipsilateral MRRN cells. After stimulation, the cell was typically depolarized and showed an increased level of synaptic noise for up to 80 s. In contralateral MRRN neurons, optic nerve stimulation usually evoked hyperpolarization or no response. Vestibular nerve stimulation evoked compound excitatory postsynaptic potentials (EPSPs) or spikes in approximately 90% of the cells, both ipsilaterally and contralaterally. A smaller subpopulation of MRRN cells (approximately 10%) received vestibular inhibition. In 26 of 48 recorded MRRN cells, the response to vestibular stimulation was potentiated after ipsilateral optic nerve stimulation. The potentiation was seen in cells receiving either excitatory or inhibitory vestibular input as an increase in EPSP amplitude/spiking (85%) and a decrease in inhibitory postsynaptic potential amplitude (15%) respectively. In most cases the vestibular responses did not return to control levels during the testing period (10-30 min), and thus the visual stimulation most likely induced long-lasting changes in the functional connectivity of the role control network, in addition to the short-lasting afteractivity. In four of the 11 cells recorded contralateral to the stimulated optic nerve, a depression of the vestibular response could be seen. In potentiated cells, single vestibular pulses often evoked longer episodes of large synaptic noise and sometimes spiking. In the latter case, the action potentials appeared with highly variable latency after each stimulation pulse. This indicates that an important mechanism underlying the potentiation may be a long-lasting increase in excitability in a pool of unidentified interneurons located either upstream of the MRRN cells, relaying vestibular and visual inputs, or downstream, providing positive feedback.

Hippocampal plasticity following epileptiform bursting produced by GABA A antagonists

The effects of epileptiform bursts on hippocampal excitability were examined in the CA3 region of guinea-pig hippocampal slices. Partial blockade of γ-aminobutyric acid A (GABA A)-mediated inhibition by 500 IU/ml penicillin produced low frequency (2–4 Hz) “pro-convulsant” field potential oscillations. Normal spontaneous activity recovered less than 30 min after the penicillin was rinsed out providing bursting was prevented. Synchronized bursting rarely began on its own even after l h in penicillin 500 IU/ml, but could be initiated in most slices after one to eight all-or-none bursts were evoked by low-intensity, low-frequency (0.2–0.25 Hz) stimuli. Spontaneous bursting, once initiated, persisted for at least l h without further stimulation suggesting that a small number of bursts produced a long-lasting increase in excitability. Bursts disappeared more slowly than anticipated after convulsants were rinsed out and were followed by “post-burst” oscillations with different frequency characteristics than proconvulsant oscillations which persisted for at least 4 h. Selective augmentation of evoked N-methyl- d-aspartate excitatory postsynaptic potentials appeared to be the critical first step in the initiation of bursting. The specific N-methyl- d-aspartate antagonist, 2-amino-5-phosphonovaleric acid (50–100 μM), only partially suppressed pro-convulsant oscillations in partially disinhibited slices but completely prevented stimulus-triggered spontaneous bursting and prolonged hyperexcitability. Although N-methyl- d-aspartate receptors were necessary for the induction of bursting in partially disinhibited slices, they were not required to initiate bursting after more complete disinhibition. However, when 2-amino-5-phosphonovaleric acid was applied prior to and during perfusion with 2000 IU/ml penicillin, spontaneous bursts occurred at long, irregular intervals and lacked afterdischarges. These bursts rapidly disappeared upon penicillin washout and were not followed by persistent post-burst oscillations. N-methyl- d-aspartate antagonists applied only after bursts already established in penicillin blocked the afterdischarges but did not reduce the burst frequency. These observations indicate that epileptiform bursts can produce long-lasting, hippocampal hyperexcitability. The induction of these plastic changes requires N-methyl- d-aspartate receptor activation which then enhances both N-methyl- d-aspartate and non- N-methyl- d-aspartate receptor mechanisms. Furthermore, N-methyl- d-aspartate excitatory postsynaptic potentials can participate in triggering spontaneous bursts but this role is masked once plasticity has occurred. Partial disinhibition produces a pro-convulsant state which does not induce long-lasting changes in hippocampal excitability but renders the neuronal network vulnerable to develop persistent epileptiform bursting with small additional excitatory inputs. These bursts then induce long-lasting, N-methyl- d-aspartate-dependent changes in neuronal function which not only increase the probability of bursting but also produce network oscillations which persist long after removal of the convulsant. These post-burst oscillations reflect a previously undescribed prolonged hyper-excitability state of the hippocampus which is a direct consequence of seizure-like activity in vitro. Their similarity to electroencephalograph rhythms observed following seizures in patients with epilepsy suggest that this model of seizure-induced plasticity could be used to investigate the mechanisms by which seizures increase the susceptibility to further seizures and render anticonvulsants ineffective.

Extracellular potassium accumulation in the frog spinal cord induced by stimulation of the skin and ventrolateral columns.

1. Changes in extracellular K(+) concentration (Delta[K](e)), dorsal root potentials (DRPs) and single unit activity were studied in the frog spinal cord in response to stimulation of the skin of the hindlimb by touch, pressure, hot water (heat), single electrical pulses and to stimulation of the ventrolateral columns (LC).2. Single electrical pulses, various types of adequate stimulation applied to the skin of the hind limb for 1-2 s as well as single volleys of LC led to a Delta[K](e) of up to 0.2 mmol.l(-1). Stimuli which evoked larger Delta[K](e) also produced larger DRPs in the same frog preparation.3. The briefest heat stimuli, which lasted about 1-2 s, led to more prolonged activity in dorsal horn interneurones than did a single volley or a single tactile stimulus and the Delta[K](e) were longer and larger and had a slower rise time.4. The ;slow' second component of dorsal root depolarization (presumably mediated by K(+)) was observed after electrical and heat stimulation of the skin on the hind limb corresponding to the time course of Delta[K](e).5. The maximum Delta[K](e) induced by nociceptive stimulation occurred in the grey matter of the dorsal horn at a depth of 300-600 mum from the dorsal surface. The maximum response to single stimuli applied to the skin occurred at a depth of 400-800 mum, while that evoked by LC stimulation in the ventral horn at a depth of 1000-1400 mum.6. Repetitive and more prolonged nociceptive stimulation (5-20 s) produced a Delta[K](e) of up to 1 mmol.l(-1). The Delta[K](e) in response to repetitive tactile stimulation does not exceed 0.2 mmol.l(-1). Repetitive stimulation (100 Hz) of LC fibres led to an increase in [K](e) of up to 9-10 mmol.l(-1) in the ventral horn; this level was similar to that achieved in the intermediate region by electrical repetitive stimulation of the skin (100 Hz). Tetanic stimulation of the ventral root led to a Delta[K](e) of only about 0.05 mmol.l(-1) at a depth of 500-700 mum and no measurable Delta[K](e) within the ventral horn.7. Spontaneous Delta[K](e) associated with spontaneous DRPs and VRPs were observed during the decay phase of Delta[K](e) at various intervals from several seconds to one minute after nociceptive or electrical stimulation of the skin, suggesting the occurrence of a longlasting increase in excitability.8. The depolarization of dorsal root fibres evoked by nociceptive stimulation, tetanic stimulation of the LC and single or tetanic stimulation of the skin was followed by a dorsal root hyperpolarization. Its size, as well as that of Delta[K](e), was dependent on the frequency and duration of stimulation and its time course correlated with the dissipation of Delta[K](e) when stimulation was discontinued.9. It is suggested that the extracellular K(+) accumulation could, under physiological conditions, contribute to the modulation of spinal cord transmission, acting both pre-and post-synaptically. Low levels of increased [K](e) were associated with facilitation of impulse transmission while higher increases could result in its inhibition.

Neuropeptides: Effects on paradoxical sleep and theta rhythm in rats

In a search for the mechanism by which neuropeptides influence the maintenance of avoidance behavior, a study was done of the effects of ACTH 4–10, arginine 8-vasopressin (AVP), desglycinamide 9-arginine 8-vasopressin (DG-AVP), and of AVP antiserum on paradoxical sleep (PS) production and on theta activity generated during PS. Brattleboro rats which were homozygous (HO-DI) and heterozygous (HE-DI) for diabetes insipidus and homozygous normal (HO-NO) animals were used. The absence of AVP in HO-DI animals did not interfere with the production of PS during 8 hr recording sessions. However, it was found that the hippocampal theta activity of these animals exhibited significantly lower mean and peak frequencies than that of HE-DI or HO-NO rats. The deviation in the frequency content of the theta activity of HO-DI animals could be temporarily normalized by intraventricular injection of DG-AVP or ACTH 4–10. Inactivation of AVP in the brain of HO-NO rats by intraventricularly administered AVP antiserum induced hippocampal rhythmicity of the same frequency composition as that of HO-DI animals. Furthermore, intracerebroventricular administration of DG-AVP or ACTH 4–10 which both facilitate resistance to extinction of avoidance behavior [8,14], increased the proportion of high theta frequencies, whereas treatment with [D-phe 7] ACTH 4–10, which facilitates extinction of active avoidance behavior [8], decreased the proportion of high frequency components in hippocampal activity of HE-DI animals. In addition, [D-phe 7] ACTH 4–10 decreased the amount of high and increased the amount of low frequencies in the theta rhythm of HO-NO rats. None of the peptides tested appreciably affected the amount of PS. The experiments suggest that the increased resistance to extinction of avoidance behavior seen after the administration of neuropeptides related to vasopressin and ACTH could result from a longlasting increase in excitability induced by these substances in midbrain limbic circuits.