Kazuho Abe

The possible role of hydrogen sulfide as an endogenous neuromodulator

Hydrogen sulfide (H2S), which is well known as a toxic gas, is produced endogenously from L-cysteine in mammalian tissues. H2S is present at relatively high levels in the brain, suggesting that it has a physiological function. Two other gases, nitric oxide and carbon monoxide, are also endogenously produced and have been proposed as neuronal messengers in the brain. In this work we show the following: (1) an H2S-producing enzyme, cystathionine beta-synthase (CBS), is highly expressed in the hippocampus; (2) CBS inhibitors hydroxylamine and amino-oxyacetate suppress the production of brain H2S; and (3) a CBS activator, S-adenosyl-L-methionine, enhances H2S production, indicating that CBS contributes to the production of endogenous H2S. We also show that physiological concentrations of H2S selectively enhance NMDA receptor-mediated responses and facilitate the induction of hippocampal long-term potentiation. These observations suggest that endogenous H2S functions as a neuromodulator in the brain.

Flavonoid fisetin promotes ERK-dependent long-term potentiation and enhances memory

Small molecules that activate signaling pathways used by neurotrophic factors could be useful for treating CNS disorders. Here we show that the flavonoid fisetin activates ERK and induces cAMP response element-binding protein (CREB) phosphorylation in rat hippocampal slices, facilitates long-term potentiation in rat hippocampal slices, and enhances object recognition in mice. Together, these data demonstrate that the natural product fisetin can facilitate long-term memory, and therefore it may be useful for treating patients with memory disorders.

Effects of saffron extract and its constituent crocin on learning behaviour and long-term potentiation.

Crocus sativus L., commonly known as saffron, is used in folk medicine for various purposes. Modern pharmacological studies have demonstrated that saffron extracts have antitumour effects, radical scavenger properties or hypolipaemic effects. Among the constituents of saffron extract, crocetin is mainly responsible for these pharmacological activities. In addition, recent behavioural and electrophysiological studies have demonstrated that saffron extract affects learning and memory in experimental animals. Saffron extract improved ethanol‐induced impairments of learning behaviours in mice, and prevented ethanol‐induced inhibition of hippocampal long‐term potentiation, a form of activity‐dependent synaptic plasticity that may underly learning and memory. This effect of saffron extract is attributed to crocin (crocetin di‐gentiobiose ester), but not crocetin. Saffron extract or its active constituents, crocetin and crocin, could be useful as a treatment for neurodegenerative disorders accompanying memory impairment. Copyright

Attenuated hippocampal long-term potentiation in basolateral amygdala-lesioned rats

Possible involvement of the amygdaloid input in long-term potentiation (LTP) in the medial perforant path-dentate gyrus granule cell synapses in vivo was investigated by evaluating the effects of lesions of the amygdaloid nucleus. The dentate gyrus synaptic potential evoked by low-frequency test stimulation did not change following lesions of the basolateral and central amygdala. However, when tetanic stimulation (30 pulses at 60 Hz) was applied 60 min after lesioning of the ipsilateral basolateral amygdala, the magnitude of LTP was significantly attenuated. Since lesions of the ipsilateral central amygdala and the contralateral basolateral amygdala did not affect the dentate gyrus LTP, the attenuation of the dentate gyrus LTP is a specific effect of acute lesions of the ipsilateral basolateral amygdala. The basolateral amygdaloid lesions significantly attenuated both LTP induced by weak (20 pulses at 60 Hz) and strong (100 pulses at 100 Hz) tetanus, indicating that the effect of the lesions does not depend on the strength of tetanus applied to induce LTP. When the ipsilateral basolateral amygdala was destroyed after application of tetanus, it did not affect the established LTP. The attenuation of LTP was also observed after the basolateral amygdala-lesioned rats were allowed a recovery period of 2 weeks. This is the first report providing evidence that the ipsilateral basolateral amygdala modulates hippocampal synaptic plasticity in vivo.

Measurement of cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide(MTT) reduction activity and lactate dehydrogenase release using MTT

3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay and lactate dehydrogenase (LDH) release assay have been widely used for evaluating cell viability in culture. MTT reduction assay measures the redox activity of living cells, while LDH assay measures the activity of LDH released into the medium from dead cells. In this paper, we introduce a quick and simple method of measuring cellular MTT reduction and LDH release with the same dye, MTT. The substrate mixture for measuring LDH activity contained lactate, beta-nicotinamide adenine dinucleotide, 1-methoxyphenazine methosulfate, MTT and Triton X-100. When the medium containing LDH was mixed with the substrates, MTT was converted into MTT formazan in proportion to LDH activity. This method was successfully applied for evaluating t-butyl hydroperoxide toxicity in cultured rat cortical astrocytes and glutamate toxicity in cultured rat hippocampal neurons. Our method is economical and convenient especially for measuring cellular MTT reduction and LDH release in the same culture.

High-frequency stimulation of the basolateral amygdala facilitates the induction of long-term potentiation in the dentate gyrus in vivo.

We investigated the effect of high-frequency stimulation of the basolateral amygdala (BLA) on the induction of long-term potentiation (LTP) in the medial perforant path (PP)-dentate gyrus (DG) synapses of anesthetized rats. A conditioning stimulation (100 pulses at 100 Hz) of the ipsilateral BLA did not change the DG synaptic potential. However, when the BLA conditioning stimulation was applied at the same time as a weak tetanic stimulation of PP (20 pulses at 20 Hz) which alone did not induce LTP, robust DG LTP was induced. Simultaneous application of contralateral BLA stimulation and PP weak tetanus did not induce LTP. Moreover, the ipsilateral BLA stimulation enhanced the magnitude of LTP induced by a moderate tetanic stimulation of PP (30 pulses at 60 Hz), but did not further enhance the LTP induced by a strong tetanic stimulation of PP (100 pulses at 100 Hz). These results suggest that the ipsilateral BLA neurons modulate the induction of DG LTP in vivo.

The Basomedial and Basolateral Amygdaloid Nuclei Contribute to the Induction of Long-term Potentiation in the Dentate Gyrus In Vivo

Recent behavioural studies have provided evidence that the amygdala modulates hippocampal‐dependent memory. To test the possibility that the amygdala modulates hippocampal synaptic plasticity, we investigated the effects of surgical lesions of the amygdaloid nuclei on the induction of long‐term potentiation (LTP) in the dentate gyrus of anaesthetized rats. Previously we reported that LTP in the dentate gyrus was attenuated by lesion of the basolateral amygdala, but was not affected by lesion of the central amygdala. In the present study, dentate gyrus LTP was significantly attenuated by basomedial amygdala lesion but not by medial amygdala lesion. These results suggest that, among the amygdaloid nuclei, the basomedial and basolateral nuclei are involved in the modulation of hippocampal plasticity. The roles of the basomedial and basolateral amygdala were further supported by experiments examining the effects of electrical stimulation of these nuclei. High‐frequency stimulation of the basomedial amygdala alone did not induce dentate gyrus LTP, but when applied at the same time as tetanic stimulation of the perforant path increased the magnitude of the dentate gyrus LTP. Similarly, high‐frequency stimulation of the basolateral amygdala enhanced LTP induced by tetanic stimulation of the perforant path. Furthermore, facilitation of dentate gyrus LTP by basomedial or basolateral amygdala stimulation was observed even in rats lesioned in either amygdala, suggesting that neurons in the basomedial and basolateral amygdala can modulate dentate gyrus LTP independently. Activity‐dependent facilitation of hippocampal plasticity by the basomedial and basolateral amygdala may underlie memory processing associated with emotion.

Epidermal growth factor and basic fibroblast growth factor promote the generation of long-term potentiation in the dentate gyrus of anaesthetized rats

The effects of epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and nerve growth factor (NGF) on long-term potentiation (LTP) of evoked potential were investigated in the dentate gyrus of anaesthetised rats. Tetanic stimulation of 100 pulses at 100 Hz was applied to induce complete LTP, and stimulation of 20 pulses at 60 Hz was used as a subthreshold stimulation in inducing LTP. Injection of 50 ng EGF, bFGF or NGF into the contralateral ventricle influenced neither the basal amplitude of the population spike nor the LTP induced by the tetanus of 100 pulses at 100 Hz. However, EGF or bFGF, but not NGF, significantly augmented the potentiation induced by the tetanus of 20 pulses at 60 Hz and facilitated the generation of LTP. Moreover, the effect of EGF was dose-dependent in the range 5-500 ng. These results suggest that EGF and bFGF promote the generation process of LTP.

Amygdala beta-noradrenergic influence on hippocampal long-term potentiation in vivo.

We have previously shown that hippocampal long-term potentiation (LTP), one form of synaptic plasticity that may underlie learning and memory, is attenuated by blocking neuron activity of the basolateral amygdala (BLA). In the present study we investigated the amygdala noradrenergic or cholinergic contribution to hippocampal LTP formation. When propranolol, a beta-adrenoceptor antagonist, was injected into the BLA 10 min before tetanus, the formation of LTP in the perforant path-dentate granule cell synapses was significantly impaired. Scopolamine, a muscarinic cholinergic receptor antagonist, did not affect the formation of LTP. These results suggest that amygdala beta-noradrenergic activity plays a critical role in modulation of hippocampal LTP.

Roles of GABAA, NMDA and muscarinic receptors in induction of long-term potentiation in the medial and lateral amygdala in vitro.

We have studied mechanisms underlying long-term potentiation (LTP) in the medial and lateral amygdala using in vitro slice preparations. In normal bathing medium, LTP was not induced by tetanic stimulation (100 pulses at 100 Hz). However, in the presence of a GABAA blocker, picrotoxin or bicuculline, LTP was reproducibly induced in both medial and lateral amygdala. In the medial amygdala, the LTP induced in the presence of picrotoxin was blocked by 2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, and was significantly reduced by scopolamine, a muscarinic receptor antagonist. On the other hand, the LTP in the lateral amygdala was not affected by APV, but was significantly reduced by scopolamine. These results suggest that both NMDA receptors and muscarinic receptors are involved in the induction of medial amygdala LTP, while muscarinic receptors, but not NMDA receptors, are involved in the induction of lateral amygdala LTP.