Topical application of penicillin into hippocampal in vitro slices: A methodological study using benzyl (14C) penicillin.

A gas pressure system is employed for topical application of pico/nanolitre volumes of 9 mmol/l benzyl (14C) penicillin into guinea-pig in vitro transverse hippocampal slices. Control of pressure pulse parameters enables ejectates, computed after liquid scintillation, from 10 micrometer, 5 micrometer and 2 micrometer pipettes with high reliability of ejection. A detailed study was done on the ejection performance of 5 micrometer and 2 micrometer pipettes. It shows that for the 5 micrometer tips, with the chosen ejection parameters, feeding pressure = 0.5 MPa, feeding pulse duration 50 ms, a 99% ejection incidence was obtained (n = 200). The mean volumes ejected were 0.12 and 0.5 nl (n = 50) for one and four pulses delivered respectively. The corresponding ejectate means obtained from five 2 micrometer pipettes given a feeding pressure of 1.5 MPa, and a feeding pulse duration of 70 ms, were 0.04 and 0.13 nl. Statistical evaluation of the individual pipette ejection performances gave characteristic regression slopes within each pipette group. A comparison between visually controlled injection into oil and injection into slice, showed a close correspondence. The applicability of the ejection method is discussed in relation to functional studies on the conversion of single neurones into "epileptic" ones.