Abstracts

Rationale: Homocarnosine is synthesized from ATP, GABA and histidine and is present in relatively high concentrations in human and primate brain. Drugs that increase intracellular GABA also increase intracellular and extracellular homocarnosine levels in human subjects. Studies of patients with epilepsy suggest that below normal intracellular homocarnosine levels are associated with frequent seizures. Our observations suggest, but do not prove, that homocarnosine modulates human cortical excitability. We report our initial experiments designed to determine whether homocarnosine is an inhibitory neuromodulator in human brain slices using standard electrophysiological techniques.Methods: All tissue used was resected for clinically relevant reasons and these studies were approved by the Yale University Human Investigations Committee. The tissue was resected en bloc and a 5-10 mm section was placed immediately into ice cold, oxygenated cutting medium. The cutting medium was made up of artificial cerebrospinal fluid in which the NaCl is replaced iso-osmotically with sucrose. The standard ACSF contains: (in mM) NaCl 124, KCl 3,MgSO4 2, NaH2PO4 1.2, NaHCO3 26, CaCl2 2.0, and dextrose 10 (pH 7.4). The tissue was blocked to ensure that the slices (400 µm) uniformly. The time between resection in the operating room and tissue slicing was approximately 10 minutes. After one hour incubation in warm (32 C), oxygenated ACSF, the tissue was held at room temperature for the balance of the experiments. The tissue was allowed to recover for at least two hours prior to recording.Results: We obtained synchronized bursting in non-sclerotic human hippocampus (hemispherectomy for treatment of a porencephalic cyst) by bath applying 0 Mg ACSF. We examined the relative effects of homocarnosine and histidine on both spontaneous and evoked activity using intracellular recordings from CA3 pyramidal cells. We found that there was a significant decrease in several aspects of the excitatory activity within 10 minutes of washing on 10 µM homocarnosine. First, the mean duration of the spontaneous events decreased in homocarnosine from 223.3 ± 1.8 ms to 136.4 ± 5.7 ms (n= 3 cells, 30 seconds each condition). This effect of homocarnosine on the duration of the spontaneous synaptic events was consistent between the three cells examined and was statistically significant (p<0.05). The effect of homocarnosine on the frequency of the spontaneous activity was more variable and ranged from a -62.5% to -20% with a mean effect of -32.4 ± 17.8%. These effects washed within 20 minutes. We also noted a 34.3 ± 16% decrease in the duration and 24.6 ± 16.7% decrease in the area of the evoked synaptic responses. These effects were also reversible following a 20 minute wash. Finally, we examined the effect of 20 µM histidine on bursting activity in two additional cells. There were no apparent changes in any of the variables studied between control and in histidine.Conclusions: These findings support the hypothesis that either homocarnosine has direct effects on excitability or that these effects are mediated by GABA.