PERIODICALLY MODULATED INHIBITION AND ITS POSTSYNAPTIC CONSEQUENCES .1. GENERAL FEATURES - INFLUENCE OF MODULATION FREQUENCY

Our aim was to examine the relation, or ''synaptic coding'', between s pike trains across a synapse with inhibitory postsynaptic potentials w hen the presynaptic rate is modulated periodically and the postsynapti c cell is a pacemaker. Experiments were on the synapse in crayfish str etch receptor organs. Spike trains were considered point processes alo ng time; the rime series of corresponding pre- and postsynaptic interv als were extracted. Analyses used displays of intervals along time and order (''basic graphs'', and ''rasters'', respectively), displays of differences between intervals along order (''recurrence plots''), cycl e histograms (as such and as Lissajous diagrams with presynaptic and p ostsynaptic histograms on the abscissae and ordinate, respectively), a nd correlation histograms. Cycle histograms and correlation histograms demonstrated that all presynaptic modulation frequencies (1/60-10 Hz) are reflected postsynaptically; novel frequencies may arise, not alwa ys relating simply to the pre- or postsynaptic ones. The transferred f requency domain is broad and physiologically meaningful. Indeed, vital ly important functions have strong periodicities in all portions of th e explored domain, and so do the discharges of participating neurons. Overall, pre- and postsynaptic discharges change oppositely, one accel erating while the other slows. Locally, however, pre- and postsynaptic discharges contrast clearly in other ways. The presynaptic evolution is everywhere smooth and orderly, half-cycles usually are symmetric, a nd there is a single kind of discharge, as expected because the presyn aptic axon follows well the controlling stimuli. The postsynaptic cycl e shows marked local distortions. These involve presynaptic domains ca lled ''congruent portions'' where changes are in the same sense (e.g., joint accelerations), ''saturated'' domains where postsynaptic discha rges are arrested, and asymmetric sensitivies to presynaptic change wi th hysteretic loops in the Lissajous diagrams; the postsynaptic discha rge is heterogeneous showing dissimilar forms in succession. Congruent portions are either ''positive segments'' with pre- to postsynaptic r ate ratios practically 1:1, 2:1, 1:1, or parts of Lissajous loops. Dif ferent modulation frequencies have different postsynaptic consequences . Differences involve the width and number of positive segments, the p roportion of the cycle with saturation, the sense, magnitude and lead- lag characteristics of the hysteretic loops, etc. Because their conseq uences are separable, frequencies are classified within categories lab elled ''low'' (under 0.5 Hz), ''high'' (between 0.5 and 5.0 Hz) and '' very high'' (over 5.0 Hz). Categories arise widely but each prevails i n different biological functions (e.g., low or high in, respectively, respiration or vibratory sensitivity). The refractoriness of the inhib itory fibre affects how it can be modulated: consequently, the very hi gh category resembles pacemaker discharges and was not analysed. The r elations between pre- and postsynaptic average rates using pacemaker a nd modulated low frequency drivings are very similar but, because of t he sensitivity to rate, become less so as frequency increases. The str ategy for identifying the underlying basic mechanisms in the membranes and at the microscopic level of synapses and neurons is to successive ly describe discharges fully, diagnose the different forms and, in sim ulations, analyse parametrically the variables of both levels. This pr ocess is incomplete, for so far core descriptions involve only average s within histogram bins, and not patterns. Hence, present understandin g cannot take us yet from channel dynamics to meaningful conjectures a bout the consequences of single arrivals or prolonged drivings. A tent ative conclusion is that novel properties in basic processes at membra nes are not needed to explain these results. This paper confirms and e xtends earlier publications on the synaptic coding of periodically mod ulated trains. Ubiquitous distortions demonstrate an essentially non-l inear mapping whose complexity far exceeds that of junctions used curr ently in simulations. A companion paper examines other parameters and discusses modulated inhibition more generally.(82)