Block of NMDA receptor channels by endogenous neurosteroids: implications for the agonist induced conformational states of the channel vestibule

N-methyl-D-aspartate receptors (NMDARs) mediate synaptic plasticity, and their dysfunction is implicated in multiple brain disorders. NMDARs can be allosterically modulated by numerous compounds, including endogenous neurosteroid pregnanolone sulfate. Here, we identify the molecular basis of the use-dependent and voltage-independent inhibitory effect of neurosteroids on NMDAR responses. The site of action is located at the extracellular vestibule of the receptor's ion channel pore and is accessible after receptor activation. Mutations in the extracellular vestibule in the SYTANLAAF motif disrupt the inhibitory effect of negatively charged steroids. In contrast, positively charged steroids inhibit mutated NMDAR responses in a voltage-dependent manner. These results, in combination with molecular modeling, characterize structure details of the open configuration of the NMDAR channel. Our results provide a unique opportunity for the development of new therapeutic neurosteroid-based ligands to treat diseases associated with dysfunction of the glutamate system

Specificity of Connections Formed by Nociceptive Cells of the Leech in Tissue Culture

ABSTRACT Individual medial and lateral nociceptive (N) cells of the leech have been paired with a variety of different target cells to determine the patterns and the properties of the connections made in culture. Lateral nociceptive N cells did not make electrical or chemical connections with other N cells or with P sensory cells. N cells made electrical connections with Retzius cells, anterior pagoda cells (AP) and annulus erector (AE) and L motor neurones in culture. Lateral and medial N cells, which differ in the animal, exhibited consistent differences in culture. Well-established electrical junctions made by lateral N cells showed clear rectification. In contrast, medial N cells could make either non-rectifying or rectifying connections with target cells. In spite of the presence of strong electrical coupling, evidence for chemically mediated synaptic interactions between N cells and other neurones was obtained on occasion. These findings emphasize the specific nature of connections formed in culture. Unlike P sensory cells, N cells in culture did become electrically connected to Retzius cells and could make both rectifying and non-rectifying electrical junctions with appropriate targets.</jats:p