Primary afferent-evoked EPSCs were recorded from medullary dorsal horn neurons of rat horizontal brain stem slices using a conventional whole-cell patch clamp technique under a voltage-clamp condition. KEY RESULTSCP93129, a selective 5-HT1B receptor agonist, reversibly and concentration-dependently decreased the amplitude of glutamatergic EPSCs and increased the paired-pulse ratio. In addition, CP93129 reduced the frequency of spontaneous miniature EPSCs without affecting the current amplitude. The CP93129-induced inhibition of EPSCs was significantly occluded by GR55562, a 5-HT1B/1D receptor antagonist, but not LY310762, a 5-HT1D receptor antagonist. Sumatriptan, an anti-migraine drug, also decreased EPSC amplitude, and this effect was partially blocked by either GR55562 or LY310762. On the other hand, primary afferent-evoked EPSCs were mediated by the Ca 2+ influx passing through both presynaptic N-type and P/Q-type Ca 2+ channels. The CP93129-induced inhibition of EPSCs was significantly occluded by w-conotoxin GVIA, an N-type Ca 2+ channel blocker. CONCLUSIONS AND IMPLICATIONSThe present results suggest that the activation of presynaptic 5-HT1B receptors reduces glutamate release from primary afferent terminals onto medullary dorsal horn neurons, and that 5-HT1B receptors could be, at the very least, a potential target for the treatment of pain from orofacial tissues. Abbreviations w-AgTx, w-agatoxin IVA; APV, DL-2-amino-5-phosphonovaleric acid; w-CgTx, w-conotoxin GVIA; CP93129, 3-(1,2,3,6-tetrahydropyridin-4-yl)-1,4-diydropyrrolo[3,2-b]pyridine-5-one; GIRK, G-protein-coupled inwardly rectifying K LINKED ARTICLE

BACKGROUND AND PURPOSE Although 3α‐hydroxy, 5α‐reduced pregnane steroids, such as allopregnanolone (AlloP) and tetrahydrodeoxycorticosterone, are endogenous positive modulators of postsynaptic GABAA receptors, the functional roles of endogenous neurosteroids in synaptic transmission are still largely unknown. EXPERIMENTAL APPROACH In this study, the effect of AlloP on spontaneous glutamate release was examined in mechanically isolated dentate gyrus hilar neurons by use of the conventional whole‐cell patch‐clamp technique. KEY RESULTS AlloP increased the frequency of glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in a dose‐dependent manner. The AlloP‐induced increase in sEPSC frequency was completely blocked by a non‐competitive GABAA receptor blocker, tetrodotoxin or Cd2+, suggesting that AlloP acts on presynaptic GABAA receptors to depolarize presynaptic nerve terminals to increase the probability of spontaneous glutamate release. On the other hand, γ‐cyclodextrin (γ‐CD) significantly decreased the basal frequency of sEPSCs. However, γ‐CD failed to decrease the basal frequency of sEPSCs in the presence of a non‐competitive GABAA receptor antagonist or tetrodotoxin. In addition, γ‐CD failed to decrease the basal frequency of sEPSCs after blocking the synthesis of endogenous 5α‐reduced pregnane steroids. Furthermore, γ‐CD decreased the extent of muscimol‐induced increase in sEPSC frequency, suggesting that endogenous neurosteroids can directly activate and/or potentiate presynaptic GABAA receptors to affect spontaneous glutamate release onto hilar neurons. CONCLUSIONS AND IMPLICATIONS The modulation of presynaptic GABAA receptors by endogenous neurosteroids might affect the excitability of the dentate gyrus‐hilus‐CA3 network, and thus contribute, at least in part, to some pathological conditions, such as catamenial epilepsy and premenstrual dysphoric disorder.

Reductions in neurotrophic factors are implicated in synaptic dysfunction in the central nervous system, but exogenous neurotrophic factors with potential effects on neuritic regeneration and synaptic reconstruction could offer therapeutic and preventive strategies for treating memory-related neurological disorders. In an earlier effort to identify natural neurotrophic agents, we found that the ethanol extract of the edible marine alga Undaria pinnatifida (UPE) had promising effects on the neuritogenesis of cultured hippocampal neurons. Here, we further investigated the ability of UPE to promote spinogenesis and synaptogenesis in primary cultures of hippocampal neurons. It was found that UPE triggered significant increase in numbers of dendritic filopodia and spines, promoted the formation of excitatory and inhibitory synapses, and potentiated synaptic transmission by increasing the sizes of reserve vesicle pools at presynaptic terminals. These findings indicate a substantial role for UPE in the morphological and functional maturation of neurons and suggest that UPE is a possible therapeutic preventative measure and treatment for neurodegenerative diseases, such as those involving cognitive disorders and memory impairments.