Slow Afterhyperpolarization Research Articles

Age-related increase of sIAHP in prefrontal pyramidal cells of monkeys: relationship to cognition

Reduced excitability, due to an increase in the slow afterhyperpolarization (and its underlying current sIAHP), occurs in CA1 pyramidal cells in aged cognitively-impaired, but not cognitively-unimpaired, rodents. We sought to determine whether similar age-related changes in the sIAHP occur in pyramidal cells in the rhesus monkey dorsolateral prefrontal cortex (dlPFC). Whole-cell patch-clamp recordings were obtained from layer 3 and layer 5 pyramidal cells in dlPFC slices prepared from young (9.6 ± 0.7 years old) and aged (22.3 ± 0.7 years old) behaviorally characterized subjects. The amplitude of the sIAHP was significantly greater in layer 3 (but not layer 5) cells from aged-impaired compared with both aged-unimpaired and young monkeys, which did not differ. Aged layer 3, but not layer 5, cells exhibited significantly increased action potential firing rates, but there was no relationship between sIAHP and firing rate. Thus, in monkey dlPFC layer 3 cells, an increase in sIAHP is associated with age-related cognitive decline; however, this increase is not associated with a reduction in excitability.

Ca2+-Dependent and Na+-Dependent K+ Conductances Contribute to a Slow AHP in Thalamic Paraventricular Nucleus Neurons: A Novel Target for Orexin Receptors

Thalamic paraventricular nucleus (PVT) neurons exhibit a postburst apamin-resistant slow afterhyperpolarization (sAHP) that is unique to midline thalamus, displays activity dependence, and is abolished in tetrodotoxin. Analysis of the underlying sI(AHP) confirmed a requirement for Ca(2+) influx with contributions from P/Q-, N-, L-, and R subtype channels, a reversal potential near E(K)(+) and a significant reduction by UCL-2077, barium or TEA, consistent with a role for K(Ca) channels. sI(AHP) was significantly reduced by activation of either the cAMP or the protein kinase C (PKC) signaling pathway. Further analysis of the sAHP revealed an activity-dependent but Ca(2+)-independent component that was reduced in high [K(+)](o) and blockable after Na(+) substitution with Li(+) or in the presence of quinidine, suggesting a role for K(Na) channels. The Ca(2+)-independent sAHP component was selectively reduced by activation of the PKC signaling pathway. The sAHP contributed to spike frequency adaptation, which was sensitive to activation of either cAMP or PKC signaling pathways and, near the peak of membrane hyperpolarization, was sufficient to cause de-inactivation of low threshold T-Type Ca(2+) channels, thus promoting burst firing. PVT neurons are densely innervated by orexin-immunoreactive fibers, and depolarized by exogenously applied orexins. We now report that orexin A significantly reduced both Ca(2+)-dependent and -independent sI(AHP), and spike frequency adaptation. Furthermore orexin A-induced sI(AHP) inhibition was mediated through activation of PKC but not PKA. Collectively, these observations suggest that K(Ca) and K(Na) channels have a role in a sAHP that contributes to spike frequency adaptation and neuronal excitability in PVT neurons and that the sAHP is a novel target for modulation by the arousal- and feeding-promoting orexin neuropeptides.

VIP, CRF, and PACAP Act at Distinct Receptors to Elicit Different cAMP/PKA Dynamics in the Neocortex

The functional significance of diverse neuropeptide coexpression and convergence onto common second messenger pathways remains unclear. To address this question, we characterized responses to corticotropin-releasing factor (CRF), pituitary adenylate cyclase-activating peptide (PACAP), and vasoactive intestinal peptide (VIP) in rat neocortical slices using optical recordings of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) sensors, patch-clamp, and single-cell reverse transcription-polymerase chain reaction. Responses of pyramidal neurons to the 3 neuropeptides markedly differed in time-course and amplitude. Effects of these neuropeptides on the PKA-sensitive slow afterhyperpolarization current were consistent with those observed with cAMP/PKA sensors. CRF-1 receptors, primarily expressed in pyramidal cells, reportedly mediate the neocortical effects of CRF. PACAP and VIP activated distinct PAC1 and VPAC1 receptors, respectively. Indeed, a selective VPAC1 antagonist prevented VIP responses but had a minor effect on PACAP responses, which were mimicked by a specific PAC1 agonist. While PAC1 and VPAC1 were coexpressed in pyramidal cells, PAC1 expression was also frequently detected in interneurons, suggesting that PACAP has widespread effects on the neuronal network. Our results suggest that VIP and CRF, originating from interneurons, and PACAP, expressed mainly by pyramidal cells, finely tune the excitability and gene expression in the neocortical network via distinct cAMP/PKA-mediated effects.

Insulin facilitates repetitive spike firing in rat insular cortex via phosphoinositide 3-kinase but not mitogen activated protein kinase cascade

The insular cortex (IC) processes gustatory and visceral information, which functionally correlate to feeding behavior. Insulin, a well-known hormone controlling glucose metabolism, is released by elevation of blood glucose concentration following feeding behavior. The IC expresses dense insulin receptors and receives projection from the hypothalamus, which monitors changes in glucose concentration. Therefore, it is likely that insulin modulates neural properties in the IC. However, little is known about the effects of insulin on electrophysiological properties of the neocortex including the IC. To explore the effects of insulin on subthreshold responses and action potential properties in the IC, intracellular recording with sharp glass electrodes was performed from IC pyramidal cells using slice preparations. Although application of insulin (100 nM) had little effect on the resting membrane potential, input resistance and rheobase, insulin significantly increased the frequency of repetitive spike firing in response to a long depolarizing current pulse injection: the slope of the frequency-current curve was increased from 23.7±2.3 Hz/nA to 29.5±3.4 Hz/nA. Insulin slightly decreased the action potential threshold without affecting the amplitude of medium-duration and slow afterhyperpolarization (sAHP) s. The insulin-induced facilitation of repetitive spike firing was dose-dependent and blocked by pre-application of 200 nM lavendustin A, a tyrosine kinase inhibitor. Moreover, when combined with 200 nM wortmannin, a phosphoinositide 3-kinase (PI3-K) inhibitor, or 500 nM deguelin, an inhibitor of protein kinase B (PKB/Akt) downstream of PI3-K, insulin failed to increase the frequency of repetitive spike firing. In contrast, co-application of insulin and (10 μM) PD 98059, an inhibitor of mitogen activated protein kinase (MAPK), exerted facilitation of repetitive spike firing. These results suggest that acute insulin-induced facilitation of firing frequency is at least partially induced by hyperpolarizing effects on the action potential threshold, and that this facilitation is induced by activation of PI3-K but not MAPK cascade.

Plasma membrane sphingomyelin hydrolysis increases hippocampal neuron excitability by sphingosine‐1‐phosphate mediated mechanisms

Proteins that control the excitability of neurons, including voltage-dependent ion channels and neurotransmitter receptors, reside in a membrane lipid environment that includes sphingomyelin, but the influence of the metabolism of this lipid on excitability is unknown. Sphingomyelin in the plasma membrane can be cleaved by neutral sphingomyelinases (nSMase) to generate ceramides and sphingosine-1-phosphate (S1P) which have been shown to play a variety of roles in cellular signaling processes. We found that application of nSMase to hippocampal slices results in a selective enhancement in the population spike amplitude, resulting in fEPSP-PS potentiation of the CA3-CA1 schaeffer collateral synapse. Single cell recordings showed that nSMase activity increases action potential frequency in CA1 neurons in a reversible manner. Additional current clamp recordings showed that nSMase reduces the slow after-hyperpolarization after a burst of action potentials. Mass spectrometry-based measurements demonstrated that nSMase activity induces a rapid increase in the levels of ceramides and S1P in cells in hippocampal slices. The ability of nSMase to increase CA1 neuron excitability was blocked by an inhibitor of sphingosine kinase, the enzyme that converts ceramide to S1P. Moreover, direct intracellular application of S1P to CA1 neurons increased action potential firing. Our findings suggest roles for sphingomyelin metabolism and S1P in the positive regulation of the excitability of hippocampal neurons.

Physiological properties of cholinergic and non-cholinergic magnocellular neurons in acute slices from adult mouse nucleus basalis.

BackgroundThe basal forebrain is a series of nuclei that provides cholinergic input to much of the forebrain. The most posterior of these nuclei, nucleus basalis, provides cholinergic drive to neocortex and is involved in arousal and attention. The physiological properties of neurons in anterior basal forebrain nuclei, including medial septum, the diagonal band of Broca and substantia innominata, have been described previously. In contrast the physiological properties of neurons in nucleus basalis, the most posterior nucleus of the basal forebrain, are unknown.Methodology/Principal FindingsHere we investigate the physiological properties of neurons in adult mouse nucleus basalis. We obtained cell-attached and whole-cell recordings from magnocellular neurons in slices from P42-54 mice and compared cholinergic and non-cholinergic neurons, distinguished retrospectively by anti-choline acetyltransferase immunocytochemistry. The majority (70–80%) of cholinergic and non-cholinergic neurons were silent at rest. Spontaneously active cholinergic and non-cholinergic neurons exhibited irregular spiking at 3 Hz and at 0.3 to 13.4 Hz, respectively. Cholinergic neurons had smaller, broader action potentials than non-cholinergic neurons (amplitudes 64±3.4 and 75±2 mV; half widths 0.52±0.04 and 0.33±0.02 ms). Cholinergic neurons displayed a more pronounced slow after-hyperpolarization than non-cholinergic neurons (13.3±2.2 and 3.6±0.5 mV) and were unable to spike at high frequencies during tonic current injection (maximum frequencies of ∼20 Hz and >120 Hz).Conclusions/SignificanceOur results indicate that neurons in nucleus basalis share similar physiological properties with neurons in anterior regions of the basal forebrain. Furthermore, cholinergic and non-cholinergic neurons in nucleus basalis can be distinguished by their responses to injected current. To our knowledge, this is the first description of the physiological properties of cholinergic and non-cholinergic neurons in the posterior aspects of the basal forebrain complex and the first study of basal forebrain neurons from the mouse.

Luminal administrationex vivoof a liveLactobacillusspecies moderates mouse jejunal motility within minutes

Gut commensals modulate host immune, endocrine, and metabolic functions. They also affect peripheral and central neural reflexes and function. We have previously shown that daily ingestion of Lactobacillus reuteri (LR) for 9 d inhibits the pseudoaffective cardiac response and spinal single-fiber discharge evoked by visceral distension, and decreases intestinal motility and myenteric AH cell slow afterhyperpolarization (sAHP) by inhibiting a Ca-activated K (IK(Ca)) channel. We tested whether luminal LR could acutely decrease motility in an ex vivo perfusion model of naive Balb/c jejunum. Live LR dose dependently decreased motor complex pressure wave amplitudes with 9- to 16-min onset latency and an IC(50) of 5 × 10(7) cells/ml Krebs. Heat-killed LR or another live commensal, Lactobacillus salivarius, were without effect. The IK(Ca) channel blocker TRAM-34, but neither the opener (DCEBIO) nor the hyperpolarization-activated cationic channel inhibitor ZD7288 (5 μM) (or TTX 1 μM), mimicked the LR effect on motility acutely ex vivo. We provide evidence for a rapid, strain-specific, dose-dependent action of a live Lactobacillus on small intestinal motility reflexes that recapitulates the long-term effects of LR ingestion. These observations may be useful as a first step to unraveling the pathways involved in bacteria to the nervous system communication.

The KCNQ5 potassium channel mediates a component of the afterhyperpolarization current in mouse hippocampus

Mutations in KCNQ2 and KCNQ3 voltage-gated potassium channels lead to neonatal epilepsy as a consequence of their key role in regulating neuronal excitability. Previous studies in the brain have focused primarily on these KCNQ family members, which contribute to M-currents and afterhyperpolarization conductances in multiple brain areas. In contrast, the function of KCNQ5 (Kv7.5), which also displays widespread expression in the brain, is entirely unknown. Here, we developed mice that carry a dominant negative mutation in the KCNQ5 pore to probe whether it has a similar function as other KCNQ channels. This mutation renders KCNQ5(dn)-containing homomeric and heteromeric channels nonfunctional. We find that Kcnq5(dn/dn) mice are viable and have normal brain morphology. Furthermore, expression and neuronal localization of KCNQ2 and KCNQ3 subunits are unchanged. However, in the CA3 area of hippocampus, a region that highly expresses KCNQ5 channels, the medium and slow afterhyperpolarization currents are significantly reduced. In contrast, neither current is affected in the CA1 area of the hippocampus, a region with low KCNQ5 expression. Our results demonstrate that KCNQ5 channels contribute to the afterhyperpolarization currents in hippocampus in a cell type-specific manner.

Two Slow Calcium-Activated Afterhyperpolarization Currents Control Burst Firing Dynamics in Gonadotropin-Releasing Hormone Neurons

Gonadotropin-releasing hormone (GnRH) neurons release GnRH in a pulsatile manner to control fertility in all mammals. The mechanisms underlying burst firing in GnRH neurons, thought to contribute to pulsatile GnRH release, are not yet understood. Using minimally invasive, dual electrical-calcium recordings in acute brain slices from GnRH-Pericam transgenic mice, we find that the soma/proximal dendrites of GnRH neurons exhibit long-duration (approximately 10 s) calcium transients that are perfectly synchronized with their burst firing. These transients were found to be generated by calcium entry through voltage-dependent L-type calcium channels that was amplified by inositol-1,4,5-trisphosphate receptor-dependent store mechanisms. Perforated-patch current- and voltage-clamp electrophysiology coupled with mathematical modeling approaches revealed that these broad calcium transients act to control two slow afterhyperpolarization currents (sI(AHP)) in GnRH neurons: a quick-activating apamin-sensitive sI(AHP) that regulates both intraburst and interburst dynamics, and a slow-onset UCL2077-sensitive sI(AHP) that regulates only interburst dynamics. These observations highlight a unique interplay between electrical activity, calcium dynamics, and multiple calcium-regulated sI(AHP)s critical for shaping GnRH neuron burst firing.

Ictal activity induced by group I metabotropic glutamate receptor activation and loss of afterhyperpolarizations

Exposure to the group I metabotropic glutamate receptor (mGluR) agonist dihydroxyphenylglycine (DHPG) produces long-lasting changes in network excitability and epileptiform activity in the CA3 region of rat hippocampal slices that continues in the absence of the agonist and includes both interictal and more prolonged ictal-like activity. We evaluated the afterhyperpolarization (AHP) that follows repetitive neuronal firing in neurons exposed to DHPG and related the change in the AHP to the pattern of epileptiform activity. In contrast to neurons from control slices that had a robust AHP following neuronal depolarization and action potential generation, neurons that had been exposed to DHPG displayed a minimal AHP following depolarization. Whole-cell voltage-clamp recordings showed a small outward or transient inward current following a depolarizing pulse in neurons from slices that had been exposed to DHPG while control neurons had a long-lasting outward current. In slices that demonstrated ictal patterns after exposure to DHPG, bath application of 1-ethyl-2-benzimidazolinone (1-EBIO, 1 mM) or 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (DCEBIO, 100 microM) which enhance the AHP, suppressed ictal discharges. Whole-cell voltage-clamp recordings demonstrated the return of the medium and slow AHP current in neurons that had transiently been exposed to DHPG when 1-EBIO or DCEBIO was bath-applied. Co-application of either 1-EBIO or DCEBIO with DHPG blocked the induction of epileptiform activity. Transient DHPG exposure caused a long-term suppression of the AHP and ictal patterns of epileptiform activity. 1-EBIO or DCEBIO which re-established both the medium and slow AHP suppressed ictal discharges. These results support the hypothesis that the loss of the AHP contributes to the generation of ictal activity after transient DHPG exposure.

A new role for photoresponsive neurons called simple photoreceptors in the sea slug Onchidium verruculatum: Potentiation of synaptic transmission and motor response

The simple photoreceptors Ip-1 and Ip-2 are intrinsically light-sensitive neurons that exist in the abdominal ganglion of the sea slug Onchidium verruculatum. Using isolated ganglia and semi-intact or intact animal preparations, the present studies examined the light-sensing and physiological roles of Ip-1 and Ip-2 cells, which respond jointly to light by inducing a slow hyperpolarizing receptor potential. First, the synaptic inputs received by Ip-1 and Ip-2 and the axonal branches arising from their cell bodies were investigated. We found that these cells are not only first-order photosensory neurons, but also second-order neurons (interneurons), relaying inhibitory synaptic inputs such as water pressure and/or tactile senses. The amphibian Onchidium opens a pneumostome at low tide in order to aero-breathe. This pneumostome opening; i.e., aero-breathing behavior, was produced by spike discharges of Ip-1 and Ip-2 cells. Furthermore, the present results suggested that the hyperpolarizing photoresponse of Ip-1 and Ip-2 cells operates in the potentiation of inhibitory sensory synaptic transmission. Thus, we conclude that the simple photoreceptors of Onchidium play a role in the long-lasting potentiation of synaptic transmission and the subsequent behavioral response and so may be involved in a new photosensory modality, non-image-forming vision.

Lactobacillus reuteri ingestion and IKCa channel blockade have similar effects on rat colon motility and myenteric neurones

We have previously shown that ingestion of Lactobacillus reuteri may modulate colonic enteric neuron activity but with unknown effects on colon motility. The aim of the present report was to elucidate the neuronal mechanisms of action of the probiotic by comparing the effects on motility of L. reuteri ingestion with blockade of a specific ionic current in enteric neurons. We have used intraluminal pressure recordings from ex vivo rat colon segments and whole cell patch clamp recordings from neurons of rat longitudinal muscle myenteric plexus preparations to investigate the effects of L. reuteri and TRAM-34 on colon motility and neurophysiology. The effects of daily feeding of 10(9) L. reuteri bacteria or acute application of TRAM-34 on threshold fluid filling pressure or pulse pressure was measured. Lactobacillus reuteri increased intraluminal fluid filling pressure thresholds for evoking pressure pulses by 51% from 0.47 +/- 0.17 hPa; the probiotic also decreased the pulse pressure amplitudes, but not frequency, by 18% from 3.91 +/- 0.52 hPa. The intermediate conductance calcium-dependent potassium (IK(Ca)) channel blocker TRAM-34 (3 micromol L(-1)) increased filling threshold pressure by 43% from 0.52 +/- 0.22 hPa and reduced pulse pressure amplitude by 40% from 2.63 +/- 1.11 hPa; contraction frequency was unaltered. TRAM-34 (3 micromol L(-1)) reduced membrane polarization, leak conductance and the slow afterhyperpolarization current in 16/16 myenteric rat colon AH cells but 19/19 S cells were unaffected. The present results are consistent with L. reuteri enhancing tonic inhibition of colon contractile activity by acting via the IK(Ca) channel current in AH cells.

Pathophysiology of Locus Ceruleus Neurons in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder caused by loss-of-function mutations in the Methyl-CpG-binding protein-2 (MECP2) gene and is characterized by derangements in cognition, behavior, motor control, respiration and autonomic homeostasis, as well as seizures. Deficits in norepinephrine (NE) are thought to contribute to RTT pathogenesis, but little is known about how MeCP2 regulates function of noradrenergic neurons. We therefore characterized morphological, electrical, and neurochemical properties of neurons in the locus ceruleus (LC), the major source of noradrenergic innervation to the central neuraxis, in Mecp2 mutant mice. We found that MeCP2 null LC neurons are electrically hyperexcitable, smaller in size, and express less of the NE-synthesizing enzyme tyrosine hydroxylase (TH) compared with wild-type neurons. Increased excitability of mutant neurons is associated with reductions in passive membrane conductance and the amplitude of the slow afterhyperpolarization. Studies in Mecp2 heterozygotes, which are mosaic for the null allele, demonstrated that electrical hyperexcitability and reduced neuronal size are cell-autonomous consequences of MeCP2 loss, whereas reduced TH expression appears to reflect both cell-autonomous and non-autonomous influences. Finally, we found reduced levels of TH and norepinephrine in cingulate cortex, a forebrain target of the LC. Thus, genetic loss of MeCP2 results in a somewhat paradoxical LC neuron phenotype, characterized by both electrical hyperexcitability and reduced indices of noradrenergic function. Given the importance of the LC in modulating activity in brainstem and forebrain networks, we hypothesize that dysregulation of LC function in the absence of MeCP2 plays a key role in the pathophysiology of RTT.

High-Affinity Kainate Receptor Subunits Are Necessary for Ionotropic but Not Metabotropic Signaling

Kainate receptors signal through both ionotropic and metabotropic pathways. The high-affinity subunits, GluK4 and GluK5, are unique among the five receptor subunits, as they do not form homomeric receptors but modify the properties of heteromeric assemblies. Disruption of the Grik4 gene locus resulted in a significant reduction in synaptic kainate receptor currents. Moreover, ablation of GluK4 and GluK5 caused complete loss of synaptic ionotropic kainate receptor function. The principal subunits were distributed away from postsynaptic densities and presynaptic active zones. There was also a profound alteration in the activation properties of the remaining kainate receptors. Despite this, kainate receptor-mediated inhibition of the slow afterhyperpolarization current (I(sAHP)), which is dependent on metabotropic pathways, was intact in GluK4/GluK5 knockout mice. These results uncover a previously unknown obligatory role for the high-affinity subunits for ionotropic kainate receptor function and further demonstrate that kainate receptor participation in metabotropic signaling pathways does not require their classic role as ion channels.

Enhanced Calcium Buffering in F344 Rat Cholinergic Basal Forebrain Neurons Is Associated With Age-Related Cognitive Impairment

Alterations in neuronal Ca(2+) homeostasis are important determinants of age-related cognitive impairment. We examined the Ca(2+) influx, buffering, and electrophysiology of basal forebrain neurons in adult, middle-aged, and aged male F344 behaviorally assessed rats. Middle-aged and aged rats were characterized as cognitively impaired or unimpaired by water maze performance relative to young cohorts. Patch-clamp experiments were conducted on neurons acutely dissociated from medial septum/nucleus of the diagonal band with post hoc identification of phenotypic marker mRNA using single-cell RT-PCR. We measured whole cell calcium and barium currents and dissected these currents using pharmacological agents. We combined Ca(2+) current recording with Ca(2+)-sensitive ratiometric microfluorimetry to measure Ca(2+) buffering. Additionally, we sought changes in neuronal firing properties using current-clamp recording. There were no age- or cognition-related changes in the amplitudes or fractional compositions of the whole cell Ca(2+) channel currents. However, Ca(2+) buffering was significantly enhanced in cholinergic neurons from aged cognitively impaired rats. Moreover, increased Ca(2+) buffering was present in middle-aged rats that were not cognitively impaired. Firing properties were largely unchanged with age or cognitive status, except for an increase in the slow afterhyperpolarization in aged cholinergic neurons, independent of cognitive status. Furthermore, acutely dissociated basal forebrain neurons in which choline acetyltransferase mRNA was detected had the electrophysiological profiles of identified cholinergic neurons. We conclude that enhanced Ca(2+) buffering by cholinergic basal forebrain neurons may be important during aging.

Real-Time Measurements of Synaptic Autoinhibition Produced by Serotonin Release in Cultured Leech Neurons

We studied autoinhibition produced immediately after synaptic serotonin (5-HT) release in identified leech Retzius neurons, cultured singly or forming synapses onto pressure-sensitive neurons. Cultured Retzius neurons are isopotential, thus allowing accurate recordings of synaptic events using intracellular microelectrodes. The effects of autoinhibition on distant neuropilar presynaptic endings were predicted from model simulations. Following action potentials (APs), cultured neurons produced a slow hyperpolarization with a rise time of 85.4 +/- 5.2 ms and a half-decay time of 252 +/- 17.4 ms. These inhibitory postpotentials were reproduced by the iontophoretic application of 5-HT and became depolarizing after inverting the transmembranal chloride gradient by using microelectrodes filled with potassium chloride. The inhibitory postpotentials were reversibly abolished in the absence of extracellular calcium and absent in reserpine-treated neurons, suggesting an autoinhibition due to 5-HT acting on autoreceptors coupled to chloride channels. The autoinhibitory responses increased the membrane conductance and decreased subsequent excitability. Increasing 5-HT release by stimulating with trains of ten pulses at 10 or 30 Hz produced 23 +/- 6 and 47 +/- 2% of AP failures, respectively. These failures were reversibly abolished by the serotonergic antagonist methysergide (140 muM). Moreover, reserpine-treated neurons had only 5 +/- 4% of failures during trains at 10 Hz. This percentage was increased to 35 +/- 4% by iontophoretic application of 5-HT. Increases in AP failures correlated with smaller postsynaptic currents. Model simulations predicted that the autoinhibitory chloride conductance reduces the amplitude of APs arriving at neuropilar presynaptic endings. Altogether, our results suggest that 5-HT autoinhibits its subsequent release by decreasing the excitability of presynaptic endings within the same neuron.

Complex Intrinsic Membrane Properties and Dopamine Shape Spiking Activity in a Motor Axon

We studied the peripheral motor axons of the two pyloric dilator (PD) neurons of the stomatogastric ganglion in the lobster, Homarus americanus. Intracellular recordings from the motor nerve showed both fast and slow voltage- and activity-dependent dynamics. During rhythmic bursts, the PD axons displayed changes in spike amplitude and duration. Pharmacological experiments and the voltage dependence of these phenomena suggest that inactivation of sodium and A-type potassium channels are responsible. In addition, the "resting" membrane potential was dependent on ongoing spike or burst activity, with more hyperpolarized values when activity was strong. Nerve stimulations, pharmacological block and current clamp experiments suggest that this is due to a functional antagonism between a slow after-hyperpolarization (sAHP) and inward rectification through hyperpolarization-activated current (IH). Dopamine application resulted in modest depolarization and "ectopic" peripheral spike initiation in the absence of centrally generated activity. This effect was blocked by CsCl and ZD7288, consistent with a role of IH. High frequency nerve stimulation inhibited peripheral spike initiation for several seconds, presumably due to the sAHP. Both during normal bursting activity and antidromic nerve stimulation, the conduction delay over the length of the peripheral nerve changed in a complex manner. This suggests that axonal membrane dynamics can have a substantial effect on the temporal fidelity of spike patterns propagated from a spike initiation site to a synaptic target, and that neuromodulators can influence the extent to which spike patterns are modified.

The Fast and Slow Afterhyperpolarizations Are Differentially Modulated in Hippocampal Neurons by Aging and Learning

Normal aging is usually accompanied by increased difficulty learning new information. One contributor to aging-related cognitive decline is decreased intrinsic excitability in aged neurons, leading to more difficulty processing inputs and remodeling synapses to store new memories. Two measures of excitability known to be altered by learning are the slow afterhyperpolarization (sAHP) after a burst of action potentials and the fast AHP (fAHP) after individual action potentials. Using rats trained in trace eyeblink conditioning, we examined how these two measures of excitability were modulated in CA1 hippocampal neurons from young (3-4 months) and aged (29-31 months) animals. Although both the sAHP and the fAHP were reduced by successful learning in both age groups, only the sAHP showed aging-related increases. The dichotomy of learning-related and aging-related effects on two very similar calcium-dependent potassium-driven hyperpolarizations suggests several interesting hypotheses for how cellular excitability is modulated by aging and learning.

Properties of a T-Type Ca2+Channel–Activated Slow Afterhyperpolarization in Thalamic Paraventricular Nucleus and Other Thalamic Midline Neurons

Burst firing mediated by a low-threshold spike (LTS) is the hallmark of many thalamic neurons. However, postburst afterhyperpolarizations (AHPs) are relatively uncommon in thalamus. We now report data from patch-clamp recordings in rat brain slice preparations that reveal an LTS-induced slow AHP (sAHP) in thalamic paraventricular (PVT) and other midline neurons, but not in ventrobasal or reticular thalamic neurons. The LTS-induced sAHP lasts 8.9 +/- 0.4 s and has a novel pharmacology, with resistance to tetrodotoxin and cadmium and reduction by Ni(2+) or nominally zero extracellular calcium concentration, which also attenuate both the LTS and sAHP. The sAHP is inhibited by 10 mM intracellular EGTA or by equimolar replacement of extracellular Ca(2+) with Sr(2+), consistent with select activation of LVA T-type Ca(2+) channels and subsequent Ca(2+) influx. In control media, the sAHP reverses near E(K(+)), shifting to -78 mV in 10.1 mM [K(+)](o) and is reduced by Ba(2+) or tetraethylammonium. Although these data are consistent with opening of Ca(2+)-activated K(+) channels, this sAHP lacks sensitivity to specific Ca(2+)-activated K(+) channel blockers apamin, iberiotoxin, charybdotoxin, and UCL-2077. The LTS-induced sAHP is suppressed by a beta-adrenoceptor agonist isoproterenol, a serotonin 5-HT(7) receptor agonist 5-CT, a neuropeptide orexin-A, and by stimulation of the cAMP/protein kinase A pathway with 8-Br-cAMP and forskolin. The data suggest that PVT and certain midline thalamic neurons possess an LTS-induced sAHP that is pharmacologically distinct and may be important for information transfer in thalamic-limbic circuitry during states of attentiveness and motivation.

Lactobacillus reuteri enhances excitability of colonic AH neurons by inhibiting calcium‐dependent potassium channel opening

Probiotics are live non-pathogenic commensal organisms that exert therapeutic effects in travellers' diarrhea, irritable bowel syndrome and inflammatory bowel disease. Little is known about mechanisms of action of commensal bacteria on intestinal motility and motility-induced pain. It has been proposed that probiotics affect intestinal nerve function, but direct evidence for this has thus far been lacking. We hypothesized that probiotic effects might be mediated by actions on colonic intrinsic sensory neurons. We first determined whether sensory neurons were present in rat colon by their responses to chemical mucosal stimulation and identified them in terms of physiological phenotype and soma morphotype. Enteric neuron excitability and ion channel activity were measured using patch clamp recordings. We fed 10(9)Lactobacillus reuteri (LR) or vehicle control to rats for 9 days. LR ingestion increased excitability (threshold for evoking action potentials) and number of action potentials per depolarizing pulse, decreased calcium-dependent potassium channel (IK(Ca)) opening and decreased the slow afterhyperpolarization (sAHP) in sensory AH neurons, similar to the IK(Ca) antagonists Tram-34 and clotrimazole. LR did not affect threshold for action potential generation in S neurons. Our results demonstrate that LR targets an ion channel in enteric sensory nerves through which LR may affect gut motility and pain perception.