Inhibition of hippocampal excitability by citalopram.

PURPOSE: Preclinical data have suggested that selective serotonin reuptake inhibitors (SSRIs) may have anticonvulsant properties, and some SSRIs are known to modulate ion channels in vitro. We screened citalopram, fluoxetine, and sertraline for anticonvulsant actions in mouse hippocampal slices, and studied the effects of citalopram on active membrane properties and repetitive action potential firing. METHODS: To enable testing of antiepileptic effects and target modulation in a single experimental system, we used the simplistic low-Ca(2+) model, which is strongly dependent on the intrinsic excitability of CA1 pyramidal neurons. Field potentials and whole-cell currents were recorded from brain slices, and SSRIs were bath-applied. KEY FINDINGS: We found that citalopram, fluoxetine, and sertraline inhibited epileptiform activity recorded from area CA1. The effect of citalopram was more potent and less variable than that of fluoxetine and sertraline. The anticonvulsant action of citalopram was accompanied by marked slowing of action potential rise and decay, and robust inhibition of repetitive firing. This depression of membrane excitability appeared to be mediated in part by inhibition of a sustained potassium current. SIGNIFICANCE: These findings confirm that SSRIs can have anticonvulsant effects in the hippocampus, and further suggest that citalopram may exert these effects at least in part by inhibition of voltage-gated ion currents.

Low-magnesium medium induces epileptiform activity in mouse olfactory bulb slices.

Magnesium-free medium can be used in brain slice studies to enhance glutamate receptor function, but this manipulation causes seizure-like activity in many cortical areas. The rodent olfactory bulb (OB) slice is a popular preparation, and potentially ictogenic ionic conditions have often been used to study odor processing. We studied low Mg(2+)-induced epileptiform discharges in mouse OB slices using extracellular and whole cell electrophysiological recordings. Low-Mg(2+) medium induced two distinct types of epileptiform activity: an intraglomerular delta-frequency oscillation resembling slow sniff-induced activity and minute-long seizure-like events (SLEs) consisting of large negative-going field potentials accompanied by sustained depolarization of output neurons. SLEs were dependent on N-methyl-D-aspartate receptors and sodium currents and were facilitated by α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors. The events were initiated in the glomerular layer and propagated laterally through the external plexiform layer at a slow time scale. Our findings confirm that low-Mg(2+) medium should be used with caution in OB slices. Furthermore, the SLEs resembled the so-called slow direct current (DC) shift of clinical and experimental seizures, which has recently been recognized as being of great clinical importance. The OB slice may therefore provide a robust and unique in vitro model of acute seizures in which mechanisms of epileptiform DC shifts can be studied in isolation from fast oscillations.

A device for automated control of pipette internal pressure for patch-clamp recording.

Formation of a high-resistance seal between the tip of a glass recording pipette and the membrane of the recorded cell is the crucial step in patch clamping, or whole cell recording with patch pipettes. Formation of the seal, and subsequent rupture of the membrane for whole cell recording, requires a specific sequence of changes in pipette internal hydrostatic pressure. Generating this sequence of pressure changes adds to the complexity of setting up, gaining proficiency, and performing experiments. Automation of routine pipette pressure manipulations would simplify seal formation, and benefit productivity. Here we describe a device that automates control of patch pipette internal pressure. Solenoid valves sequentially operated by manual switching, or external electronic control, automatically provide the necessary sequence of connections to the pipette interior. This greatly simplifies the operations performed to obtain membrane seals and whole cell recordings and improves standardization and reproducibility in patch recording.

The antidepressant drug fluoxetine inhibits persistent sodium currents and seizure-like events.

The antidepressant drug fluoxetine (FLX) has been shown to exert antiepileptic effects in several animal models, but mixed preclinical findings and occasional reports of proconvulsant effects have led to hesitation towards its use in epileptic people. Despite being developed as a selective serotonin reuptake inhibitor, FLX has numerous other targets in the brain. One of the proposed targets is the neuronal sodium channel, which is inhibited by many existing antiepileptic drugs. In this study, we used electrophysiological methods in a brain slice model of seizures to test for anticonvulsant and Na(+) channel-blocking effects of FLX. This approach allowed us to use a single biological system to study the effects of FLX on (1) epileptiform activity, (2) Na(+)-dependent action potential generation, and (3) the persistent Na(+) current (I(NaP)). We found that FLX was anticonvulsant in a dose- and time-dependent manner, and that this action was accompanied by strong I(NaP) inhibition and impairment of repetitive firing. These findings suggest that the effect of FLX on active membrane properties is similar to that of many antiepileptic drugs, and that this action may contribute to anticonvulsant effects.

All-or-none population bursts temporally constrain surround inhibition between mouse olfactory glomeruli.

With each sniff, the olfactory bulbs of the brain generate a neural activity pattern representing the odour environment, transmitting this to higher brain centres in the form of mitral cell output. Inhibitory circuits in the olfactory bulb glomerular and external plexiform layers may amplify contrast in these patterns, through surround inhibition of mitral cells. These circuits may operate in series, but their respective roles are unclear. A single sniff is sufficient for odour discrimination, but is not clear that the inhibitory circuits act within this timeframe. We used microdissected slices of mouse olfactory bulb to study each circuit in isolation. We found that unlike surround inhibition mediated in the external plexiform layer, surround inhibition mediated in the glomerular layer was activated by sensory synaptic input, but not by mitral cell output. The results also suggest that interactions between olfactory glomeruli are exclusively inhibitory, unlike in antennal lobe, and that surround inhibition mediated within the external plexiform layer may involve neural circuit elements not preserved in slice preparations. Surround inhibition was effective only after an interval corresponding to a single sniff in vivo. Surplus excitation, initiated by sensory input but generated by collective all-or-none responses of mitral cells, may delay surround inhibition and allow the synchronous activation of multiple glomeruli without each suppressing the other. Surround inhibition in the glomerular layer may subsequently allow a fresh representation of the odour environment to be generated with each sniff. These findings are consistent with combinatorial odour coding based on all-or-none glomerular responses.

Lithium modulates cortical excitability in vitro.

The sometimes devastating mood swings of bipolar disorder are prevented by treatment with selected antiepileptic drugs, or with lithium. Abnormal membrane ion channel expression and excitability in brain neurons likely underlie bipolar disorder, but explaining therapeutic effects in these terms has faced an unresolved paradox: the antiepileptic drugs effective in bipolar disorder reduce Na(+) entry through voltage-gated channels, but lithium freely enters neurons through them. Here we show that lithium increases the excitability of output neurons in brain slices of the mouse olfactory bulb, an archetypical cortical structure. Treatment in vitro with lithium (1 to 10mM) depolarizes mitral cells, blocks action potential hyperpolarization, and modulates their responses to synaptic input. We suggest that Na(+) entry through voltage-gated channels normally directly activates K(+) channels regulating neuron excitability, but that at therapeutic concentrations, lithium entry and accumulation reduces this K(+) channel activation. The antiepileptic drugs effective in bipolar disorder and lithium may thus share a membrane target consisting of functionally coupled Na(+) and K(+) channels that together control brain neuron excitability.

Inhibition [corrected] of olfactory receptor neuron input to olfactory bulb glomeruli mediated by suppression of presynaptic calcium influx.

We investigated the cellular mechanism underlying presynaptic regulation of olfactory receptor neuron (ORN) input to the mouse olfactory bulb using optical-imaging techniques that selectively report activity in the ORN presynaptic terminal. First, we loaded ORNs with calcium-sensitive dye and imaged stimulus-evoked calcium influx in a slice preparation. Single olfactory nerve shocks evoked rapid fluorescence increases that were largely blocked by the N-type calcium channel blocker omega-conotoxin GVIA. Paired shocks revealed a long-lasting suppression of calcium influx with approximately 40% suppression at 400-ms interstimulus intervals and a recovery time constant of approximately 450 ms. Blocking activation of postsynaptic olfactory bulb neurons with APV/CNQX reduced this suppression. The GABA(B) receptor agonist baclofen inhibited calcium influx, whereas GABA(B) antagonists reduced paired-pulse suppression without affecting the response to the conditioning pulse. We also imaged transmitter release directly using a mouse line that expresses synaptopHluorin selectively in ORNs. We found that the relationship between calcium influx and transmitter release was superlinear and that paired-pulse suppression of transmitter release was reduced, but not eliminated, by APV/CNQX and GABA(B) antagonists. These results demonstrate that primary olfactory input to the CNS can be presynaptically regulated by GABAergic interneurons and show that one major intracellular pathway for this regulation is via the suppression of calcium influx through N-type calcium channels in the presynaptic terminal. This mechanism is unique among primary sensory afferents.