Different effects of GABAergic anticonvulsants on 4-aminopyridine-induced spontaneous GABAergic hyperpolarizations of hippocampal pyramidal cells--implication for their potency in migraine therapy

Increasing GABA reverses age-related alterations in excitatory receptive fields and intensity coding of auditory midbrain neurons in aged mice

A key feature of age-related hearing loss is a reduction in the expression of inhibitory neurotransmitters in the central auditory system. This loss is partially responsible for changes in central auditory processing, as inhibitory receptive fields play a critical role in shaping neural responses to sound stimuli. Vigabatrin (VGB), an antiepileptic agent that irreversibly inhibits γ-amino butyric acid (GABA) transaminase, leads to increased availability of GABA throughout the brain. This study used multi-channel electrophysiology measurements to assess the excitatory frequency response areas in old CBA mice to which VGB had been administered. We found a significant post-VGB reduction in the proportion of V-type shapes, and an increase in primary-like excitatory frequency response areas. There was also a significant increase in the mean maximum driven spike rates across the tonotopic frequency range of all treated animals, consistent with observations that GABA buildup within the central auditory system increases spike counts of neural receptive fields. This increased spiking is also seen in the rate-level functions and seems to explain the improved low-frequency thresholds.

Comparison of intravenous valproate with intravenous lysine-acetylsalicylic acid in acute migraine attacks

OBJECTIVE

The study compared efficacy and tolerability of intravenous valproate (iVPA) with intravenous lysine-acetylsalicylic acid (iLAS) in acute migraine attacks. Background.-iLAS has been proven to be a highly effective treatment in acute migraine attacks, but it is not available in many countries and contraindicated in patients with asthma or peptic ulcers. Current data suggest that iVPA may be effective in the treatment of acute migraine attacks.

DESIGN/METHODS

In this randomized, double-blind, parallel-group phase-II study, 40 patients with acute migraine attacks (onset <5 hours, severe or moderate headache on a four-point IHS scale) alternately received iVPA 800 mg or iLAS 1000 mg. Primary outcome criteria were the percentage of patients reporting pain relief after 1 hour and patients who remained sustained pain free for 24 hours following drug administration. Secondary outcome criteria were relief of pain and associated migrainous symptoms (nausea, photophobia, and phonophobia) at 1, 2, 24, and 48 hours following drug administration.

RESULTS

There were no significant differences in demographic and clinical features between both treatment groups. Percentage of pain relief after 1 hour in the iVPA and iLAS groups were 25% and 30%, respectively, and of sustained pain free for 24 hours were 20% and 30%, respectively, without significant differences (P = 1 and P= .72, respectively). Both drugs improved associated migrainous symptoms without significant differences at the different time points, but again with a trend in favor of iLAS. No adverse events were observed.

CONCLUSION

Both drugs were effective in acute migraine attacks with a trend in favor of iLAS. As both drugs were well tolerated, further studies with higher doses of iVPA for the treatment of acute migraine attacks are recommended.

Somatosensory evoked high-frequency oscillations in migraine patients

OBJECTIVE

To explore additional evidence concerning generators of somatosensory evoked high-frequency oscillations (HFOs).

METHODS

We recorded HFOs in migraine patients. Subjects were 19 healthy normal subjects and 19 migraineurs. Electrical stimuli were delivered alternately to the right and left median nerves at their wrists. EEGs were recorded from C3'-Fz, C4'-Fz, Erb1-Erb2, Erb2-Erb1 and Cv6-Fz using a 0.3 Hz low-frequency filter and a 3000 Hz high-frequency filter. Responses to 5000 stimuli were averaged. For separation of HFOs from underlying N20, the digitized wide-band signals were digitally bandpass filtered (400-800 Hz) and averaged.

RESULTS

There were no significant differences in peak latencies and amplitudes for N9, N13, N20 and P25 components between normal controls and migraineurs. Root-mean-square amplitudes for HFOs in migraineurs were significantly diminished compared with normal controls.

CONCLUSIONS

A diminished inhibitory mechanism in the somatosensory system may exist in migraineurs. It remains to determine what cell populations contribute to generating HFOs.

SIGNIFICANCE

This indicates that there is a dysfunction in cortical information processing in the somatosensory cortex of migraineurs.

Topiramate modulates pH of hippocampal CA3 neurons by combined effects on carbonic anhydrase and Cl-/HCO3- exchange

Topiramate (TPM) is an anticonvulsant whose impact on firing activity and intracellular pH (pHi) regulation of CA3 neurons was investigated. Using the 4-aminopyridine-treated hippocampal slice model bathed in bicarbonate-buffered solution, TPM (25-50 microm) reduced the frequency of epileptiform bursts and action potentials without affecting membrane potential or input resistance. Inhibitory effects of TPM were reversed by trimethylamine-induced alkalinization. TPM also lowered the steady-state pHi of BCECF-AM-loaded neuronal somata by 0.18+/-0.07 pH units in CO(2)/HCO(3)(-)-buffered solution. Subsequent to an ammonium prepulse, TPM reduced the acidotic peak but clearly slowed pHi recovery. These complex changes were mimicked by the protein phosphatase inhibitor okadaic acid. Alkalosis upon withdrawal of extracellular Cl(-) was augmented by TPM. Furthermore, at decreased pHi due to the absence of extracellular Na(+), TPM reversibly increased pHi. These findings demonstrate that TPM modulates Na(+)-independent Cl(-)/HCO(3)(-) exchange. In the nominal absence of extracellular CO(2)/HCO(3)(-) buffer, both steady-state pHi and firing of epileptiform bursts remained unchanged upon adding TPM. However, pHi recovery subsequent to an ammonium prepulse was slightly increased, as was the case in the presence of the carbonic anhydrase (CA) inhibitor acetazolamide. Thus, a slight reduction of intracellular buffer capacity by TPM may be due to an inhibitory effect on intracellular CA. Together, these findings show that TPM lowers neuronal pHi most likely due to a combined effect on Na(+)-independent Cl(-)/HCO(3)(-) exchange and CA. The apparent decrease of steady-state pHi may contribute to the anticonvulsive property of TPM.

Effects of neurosteroids on epileptiform activity induced by picrotoxin and 4-aminopyridine in the rat hippocampal slice

The neurosteroids allopregnanolone (5alpha-pregnan-3alpha-ol-20-one; 5alpha,3alpha-P) and its 5beta-epimer pregnanolone (5beta,3alpha-P), and pregnenolone sulfate (PS) were examined for effects on spontaneous epileptiform discharges induced by 100 microM picrotoxin (PTX) and 55 microM 4-aminopyridine (4-AP) in the CA3 region of the rat hippocampal slice. At a concentration of 10 microM, 5alpha,3alpha-P partially reduced PTX-induced bursting and at 30 and 90 microM completely suppressed bursting. In contrast, 100 microM 5beta,3alpha-P failed to alter the discharge frequency. 5alpha,3alpha-P depressed 4-AP-induced bursting with similar potency as in the PTX model; 100 microM 5beta,3alpha-P was also partially effective. In the 4-AP model, 5alpha,3alpha-P inhibited both the more frequent predominantly positive-going potentials as well as the less frequent negative-going potentials that may be generated by synchronous GABAergic interneuron firing. PS enhanced the PTX bursting frequency and, in the 4-AP model, increased the frequency of negative potentials but did not alter the frequency of positive potentials. By itself, PS did not induce bursting. The effects of the steroids in the in vitro seizure models largely correspond with their activities on GABA(A) receptors; suppression of discharges may occur as a result of direct activation of these receptors rather than modulation of GABA-mediated synaptic responses. PTX and 4-AP-induced bursting in the hippocampal slice are useful models for directly assessing neurosteroid effects on seizure susceptibility under conditions that eliminate the factor of brain bioavailability.

Carbonic anhydrase inhibitor sulthiame reduces intracellular pH and epileptiform activity of hippocampal CA3 neurons

PURPOSE

Sulthiame is a carbonic anhydrase (CA) inhibitor with an anticonvulsant effect in the treatment of benign and symptomatic focal epilepsy in children. The aim of the study was to elucidate the mode of action of sulthiame with respect to possible changes of intracellular pH (pHi) that might develop along with sulthiame's anticonvulsant properties.

METHODS

The effects of sulthiame (a) on pHi of 2',7-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxymetyl ester (BCECF-AM) loaded CA3 neurones as well as (b) on epileptiform activity (induced by 50 microM 4-aminopyridine) were compared with those of the CA inhibitors acetazolamide and benzolamide.

RESULTS

In the majority of neurons, sulthiame (1.0-1.5 mM; n = 8) as well as the membrane permeant acetazolamide (0.5-1.0 mM; n = 6) reversibly decreased pHi by 0.18 +/- 0.05 (SD) and 0.17 +/- 0.10 (SD) pH units, respectively, within 10 min. The poor membrane permeant benzolamide (1.0-2.0 mM) had no influence on pHi (n = 8). Sulthiame (1.0-2.5 mM) and acetazolamide (1.0-2.0 mM) reversibly reduced the frequency of action potentials and epileptiform bursts after 10-15 min (n = 9, n = 7), whereas benzolamide (1.0-2.0 mM) had no effect (n = 6).

CONCLUSIONS

The results suggest that sulthiame acts as a membrane-permeant CA inhibitor whose beneficial effect on epileptiform activity results at least in part from a modest intracellular acidosis of central neurons.

Seizure-associated headache in epilepsy

PURPOSE

Headache is often ignored as a symptom of epileptic seizures. The purpose of this prospective study was to analyze frequency, classification, and characteristics of seizure-associated headache (SH) according to the criteria of the International Headache Society.

METHODS

Over a period of 15 months, 341 patients with epilepsy, consecutively evaluated at our outpatient clinic for SH, completed a standardized questionnaire.

RESULTS

Of the 341 epilepsy patients, 115 (34%) experienced SH with a pain intensity of 6.1 +/- 1.6 (SD) on the visual analogue scale and a duration of 12.8 +/- 15.7 (SD) h. Seizures were always accompanied by headache in 69 (60%) of these 115 patients. SH occurred in four (3%) of 115 patients only preictally, in 31 (27%) of 115 patients periictally, and in 80 (70%) of 115 patients only postictally. In the majority of the 115 patients (55.7%), SH could be classified as migraine headache, whereas in 36.5%, as tension-type headache. The type of SH was not correlated with sex, an epilepsy syndrome, or a seizure type. Migraine-like SH was significantly associated with a history of migraine (p < 0.001). In 20 (77%) of the 26 patients experiencing migraine-like SH with a history of migraine, the phenomenology of migraine-like SH and migraine attacks was identical.

CONCLUSIONS

SH is a frequent, long-lasting, and severe symptom of epileptic seizures, causing major impairment of daily living. A history of migraine significantly increases the risk for developing migraine-like SH.