A method for activating neurons using endogenous synaptic waveforms

Neuronal input-output functions are traditionally studied using rectangular or ramp waveforms of injected current. These waveforms are easy to produce and responses to them easy to quantify; thus they have been central to our understanding of the roles that membrane properties play in controlling re petitive firing. However, since smooth rectangular step and ramp waveforms lack the dynamic features of endogenous synaptic input, their use has the p otential to underemphasize the importance of input patterns in controlling: physiological patterns of neuronal output. To activate neurons with curren t waveforms that replicate natural synaptic input, we developed a method fo r acquiring, digitally manipulating and reinjecting endogenous synaptic cur rents. We demonstrate, by applying this technique to phrenic motoneurons (P MNs) in rhythmically-active in vitro preparations from neonatal rats, that stimulation of neurons with endogenous current waveforms produces responses that mimic those produced by spontaneous synaptic inputs. Acquired wavefor ms can be reinjected repeatedly to produce consistent responses, and can al so be amplified or filtered prior to reinjection to yield a range of inform ation including standard descriptors of firing behavior such as frequency/c urrent plots. This technique provides a valuable tool for analysing charact eristics of the synaptic waveform important in generating neuronal output a nd how synaptic factors interact with membrane properties to control repeti tive firing. (C) 2000 Elsevier Science B.V. All rights reserved.