Yugo Ikeda

Hippocampal Serotonin 5-HT1A Receptor Enhances Acetylcholine Release in Conscious Rats

Abstract: We investigated changes in the extracellular levels of acetylcholine (ACh) following local application of serotonergic agents to the dorsal hippocampus of freely moving rats by means of perfusion using a microdialysis technique. Perfusion of serotonin (5‐HT; 10 μM, for 30 min at a rate of 3 μl/min), dissolved in Ringers solution containing 10 μM eserine, showed no marked effect on the extracellular levels of ACh. 8‐Hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT; 20 μM), a 5‐HT1A agonist, increased ACh levels, whereas 7‐trifluoromethyl‐4‐(4‐methyl‐1 ‐piperazinyl)‐pymoto[1,2‐a]quinoxaline (CGS‐12066B; 100 μM), a 5‐HT1B agonist, decreased it. Clomipramine (2 μM), an uptake inhibitor of 5‐HT, had no effect on ACh levels. Following perfusion of 1‐(2‐methoxyphenyl)‐4‐[4‐ (2‐phthalimido)butyl]piperazine (NAN‐190; 10 μM), which is a selective 5‐HT1A antagonist, the effect of 8‐OH‐DPAT was totally abolished, whereas CGS‐12066B decreased extracellular ACh levels. 5‐HT, as well as Clomipramine, had a decreasing effect on ACh levels after pretreatment with NAN‐190. These results indicate that the 5‐HT1A receptor, which exists in the dorsal hippocampus, enhances the spontaneous ACh release, and that the mechanism of serotonergic modulation of ACh release partly depends on both the stimulatory control via the 5‐HT1A receptor and the suppressive one via the 5‐HT1B receptor in the dorsal hippocampus of rats.

Evidence for a depressive-like state induced by repeated saline injections in Fischer 344 rats.

We investigated the behavioral changes induced by mild stress in animals that may be relatively susceptible to a depressive-like state, the Fischer 344 rat strain. The mild stress of repeated handling and intraperitoneal (i.p.) injections with saline (2 ml/kg, twice a day for 14 days) elicited a moderate suppression of body weight gain, a decrease in open field activity, and a prolonged immobility during the tail suspension test in Fischer 344 rats compared with Sprague-Dawley rats. Chronic treatment of Fischer 344 rats with imipramine (10 mg/kg i.p., twice a day for 14 days) effectively suppressed open field activity and prolonged immobility. These results suggest that repeated saline injections may be a mild stressor in these rats. In the Fischer 344 strain, which may be vulnerable to the effects of mild stressors, repeated saline injections might induce a depressive-like state and could presumably represent an experimental model for depression.

Role of hippocampal serotonergic neurons in ischemic neuronal death

To clarify the serotonergic mechanisms involved in the protection against ischemic neuronal damage, ZD-211 (citalopram HBr), a serotonin (5-hydroxytryptamine; 5-HT) re-uptake inhibitor, or buspirone, a 5-HT1A agonist, was locally administered into the hippocampus of gerbils. Additionally, to clarify the role of the 5-HT nervous system in the hippocampus during ischemic neuronal damage, animals were subjected to the local administration of 5,7-dihydroxytryptamine (5,7-DHT), a 5-HT neurotoxin, before ischemia challenge. Gerbils received intrahippocampal administration of ZD-211 (200 nmol/animal) or buspirone (20 nmol/animal) before 5-min ischemia. 5,7-DHT was intrahippocampally administered 7 days before a 2-min non-lethal ischemia challenge. In vehicle-treated animals subjected to 5 min of ischemia, almost all hippocampal CA1 pyramidal neurons were lost. The treatment with ZD-211 or buspirone showed a significant protective effect, and the number of neurons was significantly increased compared to vehicle-treated animals. Pretreatment with NAN-190, a 5-HT1A antagonist, completely abolished the protective effect of ZD-211 or buspirone. In the 5,7-DHT-treated animals, the number of neurons was significantly reduced following 2 min of ischemia compared to vehicle-treated animals in which this period of ischemia is non-lethal. Thus, intrahippocampal treatment with ZD-211 or buspirone can protect neuronal damage following transient ischemia in gerbils. These effects of ZD-211 and buspirone were mediated through the 5-HT1A receptor in the hippocampus. Furthermore, the destruction of the 5-HT nervous system in the hippocampus aggravated ischemic neuronal damage. Therefore, this study showed that the enhanced activity of the 5-HT nervous system in the hippocampus may protect against neuronal damage following cerebral ischemia.

An attenuated alpha-1 potentiation of beta adrenoceptor-stimulated cyclic AMP formation after repeated saline injections in fischer 344 strain rats

We investigated the behavioral and neurochemical features of Fischer 344 strain rats in which a depressive state was induced by repeated handling and saline injections as a mild stressor. The repeated intraperitoneal injections of saline (2 ml/kg, twice a day for 14 days) elicited a moderate suppression of body weight gain, a decrease in their open field activity and a prolonged immobility in the tail suspension test. In the stress-exposed rats, the tissue content of norepinephrine (NE) was increased in the cerebral cortex and hypothalamus, whereas that of dopamine or serotonin was not affected. Although the stress exposure did not affect the binding properties of either the alpha-1 or beta adrenoceptors, it suppressed cAMP formation stimulated by NE, but not by isoproterenol or forskolin, in the cerebral cortical slices. In the presence of prazosin or phorbol ester, the difference in NE-stimulated cAMP formation between the control and the stress groups was totally abolished. Phenylephrine enhanced isoproterenol-stimulated cAMP formation in the control but not in the stress group. From these results, it is postulated that the alpha-1 potentiation of beta adrenoceptor- stimulated cAMP formation was attenuated in the stress group. These findings suggest that the manipulation of mild stressor with repeated handling and saline injections to Fischer 344 rats elicits a depressive state characterized by the behavioral changes and the attenuated alpha-1 potentiation in the cerebral cortex, and that this manipulation might be available for the study of the stress-induced depressive state as a generally acceptable mild stress model.

Protective effect of citalopram against the attenuation of the α1-potentiation of cAMP formation in Fischer 344 strain rats

We investigated the effects of citalopram, a selective serotonin reuptake inhibitor (SSRI), using an animal model for a depressive state. In Fischer 344 rats, known as emotional animals, repeated stress by twice-daily intraperitoneal (i.p.) saline injections for 14 days elicited a depressive state characterized by a decreased open-field activity and a prolonged immobility during the tail-suspension test. Concomitantly, suppression of norepinephrine (NE)-induced cAMP formation was found in the cerebral cortical slices of the stress-exposed rats without changes in adrenergic alpha 1- or beta-receptors. The difference in cAMP formation between the intact and the stress groups was totally abolished under the blockade of the alpha 1-receptor system or by the stimulation with isoproterenol or forskolin, whereas the suppressed response in the stress group was also observed in combination with isoproterenol and phenylephrine. From these results, we confirmed that the potentiation of the beta-receptor-stimulated cAMP formation by the alpha 1-receptor is attenuated following repeated stress. Chronic i.p. administration of citalopram dissolved in saline improved both the suppressed open-field activity and the prolonged immobility in the tail-suspension test. The animals treated with citalopram exhibited a comparable alpha 1-potentiation effect as observed in the intact rats. However, another SSRI, paroxetine, was less effective on the attenuation of the alpha 1-potentiation in spite of its behavioral improvement in the depressive state. These findings suggest that citalopram has a protective effect against the repeated stress-induced depressive state by mechanisms besides the serotonin reuptake inhibition.