Failure of Repeated Electroconvulsive Shock Treatment on 5-HT4-Receptor-Mediated Depolarization Due To Protein Kinase A System in Young Rat Hippocampal CA1 Neurons

Predominant Role of Serotonin at the Hippocampal Mossy Fiber Synapse with Redundant Monoaminergic Modulation

The hippocampal mossy fiber (MF) synapse has been implicated in the pathophysiology and treatment of psychiatric disorders. Alterations of dopaminergic and serotonergic modulations at this synapse are candidate mechanisms underlying antidepressant and other related treatments. However, these monoaminergic modulations share the intracellular signaling pathway at the MF synapse, which implies redundancy in their functions. We here show that endogenous monoamines can potentiate MF synaptic transmission in mouse hippocampal slices by activating the serotonin 5-HT₄ receptor. Dopamine receptors were not effectively activated by endogenous agonists, suggesting that the dopaminergic modulation is latent. Electroconvulsive treatment enhanced the 5-HT₄ receptor-mediated serotonergic synaptic potentiation specifically at the MF synapse, increased the hippocampal serotonin content, and produced an anxiolytic-like behavioral effect in a 5-HT₄ receptor-dependent manner. These results suggest that serotonin plays a predominant role in monoaminergic modulations at the MF synapse. Augmentation of this serotonergic modulation may mediate anxiolytic effects of electroconvulsive treatment.

Serotonergic modulation of neuronal activity in the nucleus accumbens following repeated methamphetamine administration in rats

Electrophysiological studies were performed to determine whether serotonergic modulation in the nucleus accumbens (NAcc) was affected after repeated methamphetamine (MAP) administration. NAcc slices (400 μm) from Wistar rats administered MAP (5 mg/kg) or saline once daily for 5 days were prepared 1, 5, or 10 days after the final injection. Population spikes (PS) induced by local stimulation of NAcc were recorded. PS inhibition by serotonin was significantly attenuated in the MAP group at 5 days but did not differ at 1 or 10 days. We next analyzed the effects of serotonin receptor subtype (5-HT1A,2,3,4,6,7)-selective agonists of PS. Differences between saline and MAP groups in 5-HT1A,2,3,4,6 receptor agonist-induced changes of PS were small or not significant. Interestingly, 5-HT7 receptor agonists significantly enhanced PS in the MAP group. Changes in the secondary messenger system related to 5-HT7 receptors were also investigated. Adenylate cyclase activator-induced PS enhancements were significantly larger in the MAP group. However, dibutyryl-cAMP-induced PS enhancement was not significantly different. In conclusion, 5-HT-induced inhibition of PS in NAcc was attenuated 5 days after repeated MAP treatment: the change in the effect of 5-HT was probably due to enhancement of the excitatory modulation via the 5-HT7 receptor with adenylate cyclase signal transduction systems.

Augmentation of Serotonin-Induced Inhibition of Neuronal Activity in the Hippocampus Following Repeated Treatment with Methamphetamine

Electrophysiological studies were performed to determine whether or not hippocampal serotonergic modulation was affected following repeated administration of methamphetamine (MAP). Rats were i.p. administered with MAP (5 mg/kg) or saline once a day for 5 days. Hippocampal slices were prepared at 24 h, 5 and 10 days after the final MAP or saline injection. The population spikes (PS) induced by stimulation of Schaffer collateral/commissural fibers were recorded in the hippocampal CA1 region. Application of serotonin (5-HT) via a bath perfusion system inhibited the PS in a concentration-dependent manner. At 24 h after the final injection, 10-microM 5-HT-induced inhibition of PS was not affected by MAP treatment. However, 5 days after the final injection, the inhibition by 5-HT (10 microM) of PS was significantly augmented in the MAP-treated group. Ten days after the final injection, this augmentation was not statistically significant compared with that of control group. 8-OH-DPAT, a 5-HT1A receptor agonist, inhibited PS, but the inhibition was not enhanced or reduced 5 days after the final MAP treatment. However, the enhancement of PS by RS 67333, a 5-HT4 receptor agonist, was attenuated 5 days after the MAP treatment. It was found that 5-HT-induced inhibition of PS in the hippocampal CA1 region was potentiated 5 days after cessation of MAP, and this effect was suggested to be due to reduction of excitatory 5-HT4 receptor functions.