Effects of low frequency stimulation on spontaneous inhibitory and excitatory post-synaptic currents in hippocampal CA1 pyramidal cells of kindled rats

Objective: Low-frequency stimulation [LFS] exerts suppressive effects in kindled animals. It is believed that overstimulated glutamatergic and decreased GABAergic transmission have long been associated with seizure activity. In this study, we investigated the effect of electrical LFS on different parameters of spontaneous excitatory and inhibitory post-synaptic currents [sEPSCs and sIPSCs] in hippocampal CA1 pyramidal cells in kindled animals

Materials and Methods: In this experimental study, rats were kindled by electrical stimulation of the hippocampal CA1 area in a semi-rapid manner [12 stimulations/day]. The animals were considered fully kindled when they showed stage 5 seizures on three consecutive days. One group of animals received LFS 4 times at 30 seconds, 6 hours, 18 and 24 hours following the last kindling stimulation. Each LFS consisted of 4 packages at 5 minutes intervals. Each package of LFS consisted of 200 pulses at 1 Hz and each monophasic square wave pulse duration was 0.1 millisecond. At 2-3 hours post-LFS, acute hippocampal slices were prepared and a whole cell patch clamp recording was performed in all animals to measure the different parameters of sEPSCs and sIPSCs

Results: In kindled animals, the inter-event interval [as an index of occurrence] of sEPSCs decreased, whereas sIPSC increased. In addition, the decay time constant of sIPSCs as an index of the duration of its activity decreased compared to the control group. There was no significant difference in other parameters between the kindled and control groups. Application of LFS in kindled animals prevented the observed changes. There was no significant difference between the measured parameters in kindled+LFS and control groups

Conclusion: LFS application may prevent seizure-induced increase in the occurrence of sEPSCs and seizure-induced decrease in occurrence and activity duration of sIPSCs

Orexin-A modulates firing of rat rostral ventromedial medulla neurons: an in vitro study

The rostral ventromedial medulla [RVM] acts a key role in the descending inhibitory pain modulation. Neuropeptide orexin-A [ORXA] is confined to thousands of neurons in the lateral hypothalamus [LH]. While RVM gets the orexinergic projections, the orexin receptors are also expressed in this structure. The aim of this study was to specify the cellular effects of ORXA on RVM neurons in vitro by using the whole cell patch-clamp recording. RVM neurons were classified into three types based on their electrophysiological characteristics. Type 1 neurons exhibited an irregular spontaneous activity which was interrupted by periods of pause in 25% of recorded neurons. Type 2 neurons did not show any spontaneous baseline activity [53.8% of recorded neurons]. Type 3 neurons fired repetitively without interruption [51.2% of recorded neurons]. ORXA had either inhibitory or excitatory effects on 53.8% [7/13] of type 1 neurons. ORXA excited 46.4% [13/28] of type 2 neurons and 27.3% [3/11] of type 3 neurons. The excitatory effect of ORXA observed in type 2 neurons was suppressed by an orexin 1 receptor [OXR1] antagonist, SB-334867. Briefly, we hypothesized that the ORXA mediated excitation and/or inhibition in RVM neurons might work as a mechanism to modulate pain processing by orexinergic neurons

[ effect of chronic low Frequency magnetic stimulation on synaptic potentiation induced by electrical stimulation in the rat's dentate gyrus]

Background and purpose: Nowadays repeated transcranial magnetic stimulation [rTMS] is being used as a treatment for some neurological disorders, but its effect on neuronal activity and synaptic plasticity has not been completely determined. The purpose of this study was to evaluate the effect of chronic rTMS on the ability of synaptic plasticity.

Materials and Methods: rTMS was applied to the hippocampal region for 14 days. One week following termination of rTMS, the amount of synaptic long-term potentiation [LTP] in animals was investigated and compared with control group. High-frequency stimulation [HFS] was applied to the perforant path for LTP induction, andfield potentials were recorded from granular layer of the dentate gyrus. Baseline field potential was recorded 10 minutes before HFS. An increase of at least 20% in population spike amplitude was measured as an index of synaptic potentiation.To compare the effects of rTMS on measured parameters, we used t-test and two way ANOVA followed by Benferroni test [Prism 8 software].

Results: Obtained data showed that, following 14 days of rTMS application causeda reduction in population spike amplitude compared to the control group [P<0.05].In addition, the ability of neurons generating LTP was reduced compared to the control group [P<0.001]. Paired pulse index measurement also showed thatthe paired-pulse facilitationdid not change following LTP induction in animals whom had received rTMS. However, there was a decrease in paired pulse facilitation in control group.

Conclusion: Chronic rTMS application reducesthe amount of synaptic potentiation. Considering the important role of LTP in the occurrence of cognitive processes, patterns of rTMS that have less effects on the ability of synaptic plasticity should be found.

[Tolerance to anti-nociceptive effects of sodium-salicylate and morphine decreases adenosine deaminase activity in the rat hippocampus]

Adenosine has been considered as a fine-tuner of the neurotransmitters in the nerve system. Adaptive changes in the brain adenosine system occur in some patho-physiological situations such as chronic exposure to morphine. In this study, the adaptive changes in the adenosine deaminase activity as a key enzyme in the adenosine metabolism that converts adenosine to inosine and ammonia, irreversibly, due to morphine dependence and tolerance to anti-nociceptive effects of sodium-salicylate were investigated. Morphine dependence was induced by morphine administration in tap water [0.4mg/ml for 24 days]. Tolerance to sodium-salicylate was induced by 6 i.p. injection [1 injection/day] of sodium-salicylate. Tolerance to antinociceptive effects of sodium-salicylate was measured by tail flick [TF] and hot plate [HP] tests. Right hippocampus was dissected, homogenized at phosphate buffer, centrifuged and then the supernatant fraction was isolated. Protein content of the samples was measured by the Bradford method. Hippocampus adenosine deaminase activity was measured by a calorimetric method of enzyme assay which is based on the direct measurement of the produced ammonia from excessive adenosine degradation by adenosine deaminase. Daily injection of Sodium-salicylate produced antinociception in early days by latency increase rather than saline injection [P<0.0001] but in the following days this antinociceptive effect progressively decreased so at the day 5 and 6 following injection it was similar to saline [P>0.05]. Injection of morphine [5mg/Kg] at the day 7 showed more increase in the latency of saline injected rather than sadium-salicylate injected [P<0.001]. Adenosine deaminase activity was significantly higher in sucrose administrated rather than chronic morhine administrated [P<0.05] and in saline injected rather than sodium-salicylate injected [P<0.05]. Single injection of sodium-salicylate and its control shows the same activity. There was no significant difference in enzyme activity of morphine dependent with sadium-salicylate tolerance [P>0.05]. This decline in the adenosine deaminase activity may be related with adaptation in brain adenosine system subsequent of dependent to morphine or tolerance to sodium-salicylate