Jun-Ichiro Oka

Docosahexaenoic acid provides protection from impairment of learning ability in Alzheimer's disease model rats

Docosahexaenoic acid (C22:6, n‐3), a major n‐3 fatty acid of the brain, has been implicated in restoration and enhancement of memory‐related functions. Because Alzheimers disease impairs memory, and infusion of amyloid‐β (Aβ) peptide (1–40) into the rat cerebral ventricle reduces learning ability, we investigated the effect of dietary pre‐administration of docosahexaenoic acid on avoidance learning ability in Aβ peptide‐produced Alzheimers disease model rats. After a mini‐osmotic pump filled with Aβ peptide or vehicle was implanted in docosahexaenoic acid‐fed and control rats, they were subjected to an active avoidance task in a shuttle avoidance system apparatus. Pre‐administration of docosahexaenoic acid had a profoundly beneficial effect on the decline in avoidance learning ability in the Alzheimers disease model rats, associated with an increase in the cortico‐hippocampal docosahexaenoic acid/arachidonic acid molar ratio, and a decrease in neuronal apoptotic products. Docosahexaenoic acid pre‐administration furthermore increased cortico‐hippocampal reduced glutathione levels and glutathione reductase activity, and suppressed the increase in lipid peroxide and reactive oxygen species levels in the cerebral cortex and hippocampus of the Alzheimers disease model rats, suggesting an increase in antioxidative defence. Docosahexaenoic acid is thus a possible prophylactic means for preventing the learning deficiencies of Alzheimers disease.

Hydrogen sulfide induces calcium waves in astrocytes

Hydrogen sulfide (H2S) modifies hippocampal long‐term potentiation (LTP) and functions as a neuromodulator. Here, we show that H2S increases intracellular Ca2+ and induces Ca2+ waves in primary cultures of astrocytes as well as hippocampal slices. H2S increases the influx of Ca2+ and to a lesser extent causes the release from intracellular Ca2+ stores. Ca2+ waves induced by neuronal excitation as well as responses to exogenously applied H2S are potently blocked by La3+ and Gd3+, inhibitors of Ca2+ channels. These observations suggest that H2S induces Ca2+ waves that propagate to neighboring astrocytes.

Glucagon-like peptide-1 modulates neuronal activity in the rat's hippocampus.

In order to assess effects of glucagon-like peptide-1 (GLP-1) in the brain excluding the hypothalamus, the effects of GLP-1 (7-36) amide, a naturally produced active fragment, on the electroencephalogram and hippocampal single unit activities of anesthetized male Wistar rats were examined. I.c.v. injection of GLP-1 (7-36) amide decreased the hippocampal theta wave duration. Juxtacellular administration of GLP-1 (7-36) amide first increased and then decreased single unit activities recorded in the hippocampal CA1, which effects were antagonized by exendin (9-39) amide, a GLP-1 receptor antagonist, or 6-cyano-7-nitroquinoxaline-2,3-dione, a non-NMDA type glutamate receptor antagonist. These results indicate that GLP-1 receptors exist in the hippocampus and are involved in modulating hippocampal activity through an increase in the release of excitatory amino acid transmitters.

Effects of four diltiazem stereoisomers on binding of d-cis-[^3H]diltiazem and (+)-[^3H]PN200-110 to rabbit T-tubule calcium channels

Diltiazem, a benzothiazepine Ca2+ antagonist, has four stereoisomeric forms: d- and l-isomers of the cis and trans forms. All four isomers were shown to completely inhibit the binding of d-cis-[3H]diltiazem to rabbit T-tubule Ca2+ channels. The potency of the inhibition was in the order: d-cis greater than l-cis greater than d-trans = l-trans. The Hill slope for each inhibition was close to unity. The l-cis, d-trans and l-trans isomers had no effect on dissociation of the d-cis[3H]diltiazem-benzothiazepine receptor complex. These results indicate that all four isomers bind to benzothiazepine receptors. Furthermore, all of the isomers modulated (+)-[3H]PN200-110 binding. At 37 degrees C, only the d-cis isomer stimulated the binding, whereas the others showed inhibition. At 2 degrees C, all of them inhibited the binding. Both trans isomers exerted virtually the same, weak effects on the binding. It is concluded that the effects of diltiazem on radioligand binding to Ca2+ channels are highly stereospecific for the d-cis isomer.

Endogenous GLP-1 modulates hippocampal activity in β-amyloid protein-treated rats

We examined the relationship between the histochemical distribution of endogenous GLP-1 (7-36) amide in the hippocampus and memory impairment assessed by a step-through type passive avoidance task in rats continuously infused with beta-amyloid protein (1-40) into the lateral cerebroventricle using a mini-osmotic pump. Our results demonstrated that endogenous GLP-1 (7-36) amide appears in the hippocampus accompanied by progression of memory impairment. Electrophysiological studies also indicated that endogenous GLP-1 inhibits hippocampal neuronal activity in beta-amyloid protein-treated rats.

Effects of tizanidine, a centrally acting muscle relaxant, on motor systems

Effects of tizanidine were studied with special reference to the effect on motor systems. The drug effectively reduced the intercollicular decerebrate rigidity and gamma-activity indirectly recorded from muscle spindle afferent discharges without showing the direct inhibitory effect on muscle spindles in rats. The drug dose-dependently inhibited the phasic responses of alpha-rigidity in anemic decerebrate rats without showing marked inhibition of the tonic response. Tizanidine effectively depressed the crossed extensor reflex in chicks and depressed mono- and polysynaptic reflex potentials in rats; dorsal root reflex was increased transiently. Tizanidine had no effect on the neuromuscular junction in rats and [3H]diazepam binding in rat brain membrane. It is suggested that the depression by tizanidine of gamma-system and spinal reflexes contribute to muscle relaxation and anti-spastic effects and that mechanisms of action are different from those of other centrally acting muscle relaxants such as mephenesin and benzodiazepines.

Properties of the depolarization induced by TRH in the isolated frog spinal cord

We observed the excitatory effects of thyrotropin-releasing hormone (TRH) on isolated, intraarterially perfused spinal cords of frogs. TRH (10(-7)-10(-3) M) produced a transient depolarization and potentiated the spontaneous activities in both the ventral and dorsal roots. The TRH effects were related to both direct and indirect actions, and Na+ deficiency attenuated the direct actions. Serotonin was the only neurotransmitter candidate with which TRH exhibited an interaction. Iproniazid potentiated the TRH effects, in normal Ringers solution. These results indicate that the TRH depolarization due to direct actions is Na+ dependent, and that indirect actions involve an interaction with a monoaminergic pathway.

NMDA receptors mediate neuronal burst firing in rat somatosensory cortex in vivo

Two patterns of neuronal firing, bursting and regular spiking, are observed in the somatosensory cortex of anesthetized rats. The effects of excitatory amino acid receptor agonists and antagonists on these discharge types have been examined in vivo, in order to assess their involvement in the generation of such firing patterns. The analysis of interspike interval histograms indicates that N-methyl-D-aspartate (NMDA) receptors are involved in the generation of bursting responses while non-NMDA receptors mediate the regular spiking pattern of firing in individual neurons. It is also suggested that a basal level of ongoing neuronal excitation via the activation of non-NMDA receptors is required for bursting to be evoked through NMDA receptors.

Rigidity in rats due to radio frequency decerebration and effects of chlorpromazine and mephenesin.

Bilateral radio frequency (RF) lesions of the midbrain were produced in an attempt to establish a non-bleeding method of producing decerebrate rigidity in rats. Marked extensor rigidity occurred with a high reproducibility in hindlimbs without the appearance of voluntary movement. Stretch reflex tension induced by repetitive dorsiflexion of the hindfeet was employed as a measure of the intensity of rigidity. Chlorpromazine-HCl (0.1-1 mg/kg, i.v.) and mephenesin (10-50 mg/kg, i.v.) which depress intercollicular decerebrate rigidity, reduced the RF decerebrate rigidity. These results suggest that RF decerebrate rigidity can be used instead of intercollicular transection rigidity in the pharmacological and physiological studies.

Electrophysiological and pharmacological properties of single spinal neurons isolated from adult bullfrogs

1. We developed an isolated spinal cell preparation from adult bullfrogs. 2. The average resting membrane potential was -60 mV, and an action potential was activated by positive current injection. 3. The cells retained their tetrodotoxin-sensitive Na+ channels and at least two kinetically different types of K+ channel. 4. Under K(+)-free conditions, responses to GABA were blocked by bicuculline while responses to glycine, taurine or beta-alanine were blocked by strychnine. 5. The potency of excitatory amino acids decreased in the order: kainic acid greater than glutamate greater than NMDA. 6. These studies demonstrated that the isolated cells are applicable for electrophysiological and pharmacological investigations.