DELAYED IONTOPHORETIC EJECTION OF SUBSTANCE-P FROM GLASS MICROPIPETS - CORRELATION WITH TIME-COURSE OF NEURONAL EXCITATION INVIVO

The microiontophoretic release of substance P (SP) and its carboxy-terminal octapeptide (SP8) was studied in vitro. At pH 5, SP and SP8 were released by a positive current. The rate of release was very low initially, increased with time, and then became constant. The duration of the initial period of low release was a function of the negative (retaining) current applied between release periods, and was unusually long even when relatively small retaining currents were used. In the presence of 17 mM sodium, the transport numbers of SP and SP8 were 0.016 and 0.0012, respectively. Addition of NaCl to the pipette produced a dramatic decrease in the transport number of SP (0.0008 in the presence of 170 mM NaCl). Substance P and SP8 were also applied in vivo in the vicinity of the noradrenaline-containing neurons of the rat locus coeruleus, using conventional five-barrel glass microelectrodes. Prior application of a 10nA retaining current for only 2 min delayed the response to iontophoresis of substance P by 30-40 sec. But when a very small retaining current (0.4 nA) was used between ejections, a very rapid excitatory response (within 5 sec) could be observed with as little as 5-15 nA of positive current. Following the delivery of SP or SP8 under such optimal conditions, recovery of neuronal firing rate was also rapid (10-30 sec). The low transport numbers and the long delay in the release of SP and of SP8 can be related to the physical and chemical characteristics of the molecules and the techniques of application. The low concentrations of these peptides present in the pipettes and their low diffusion constants are the main reasons for the differences of their iontophoretic behaviors from those of smaller neuroactive compounds. © 1979.