Effects of dynorphin A (1-17), dynorphin A (1-13) and D-ala2-D-leu5-enkephalin on the excitability of pyramidal cells in CA1 and CA2 of the rat hippocampus in vitro.

The effects of D-ala2-D-leu5-enkephalin (DADL), dynorphin (DYN) A (1-17), and DYN A (1-13) on the excitability of hippocampal (HC) pyramidal cells in CA1 and CA2 were compared in an in vitro preparation of the rat. DADL and DYN A (1-13) increased the size of the population spike (PS) evoked in CA1 and CA2 by stimulation of the stratum radiatum. Paired-pulse stimulation showed that both peptides decreased the inhibition produced by the conditioning stimulus. These effects were naloxone-reversible. DYN A (1-17) decreased the PS in CA2 and increased the inhibition evoked by the conditioning stimulus. When applied in the same concentration-range, the depressant effects of DYN A (1-17) were more pronounced in CA1 than in CA2 and not naloxone-reversible. DYN A (1-17) reduced the excitatory effects of DADL. Low concentrations of DYN A (1-17) (0.1 nM) produced, like DYN A (1-13) and DADL, an increase in the PS in CA1 in the majority of tests. The present data suggest that DYN A (1-17) shows the excitatory action of other opioids HC pyramidal cells only at low concentration.