Generation of slow postsynaptic potentials without increases in ionic conductance.

In sympathetic ganglia, slow surface positive and negative ganglionic potentials elicited by preganglionic impulses have now been shown to be independent responses and to be recordable intracellularly, in most ganglion cells, together with the well-known excitatory postsynaptic potential (EPSP). 1' 2 These two slow responses therefore represent a slow hyperpolarizing or inhibitory postsynaptic potential (IPSP) and a slow depolarizing one, i.e., a slow EPSP. Their unique features have important implications not only for the operations of the autonomic nervous system but also for the potentialities for monosynaptic mechanisms that may be available to the vertebrate nervous system in general. They could be especially significant in the brain, where slow processes are particularly prominent. The slow postsynaptic responses differ from the better-known fast PSP's (a) in their extraordinarily long synaptic delays (10's to 100's of msec instead of 1 msec or less);3 (b) in their duration of many seconds, instead of msecs; and (c) in their much greater dependence on repetition of input for their full development.4' It is now apparent from the present work that sources of the membrane currents that generate both the slow IPSP and the slow EPSP are also different, in principle, from those for the fast PSP's. In all cases analyzed, the fast EPSP's and IPSP's have been shown to involve an increase in ionic conductance of the postsynaptic membrane.6 The role of ionic conductance changes in generating slow PSP's was tested here by direct measurements of membrane resistance in the ganglion cells during the course of the slow PSP's. In addition, the effects produced on the size of the slow PSP's by varying the initial membrane potential were studied, for if a PSP is generated by currents due to ions moving down their electrochemical gradients toward their equilibrium potential, then shifting the initial membrane potential toward this equilibrium potential should reduce the response, and vice versa.7 In addition, tests of some possible alternative mechanisms for the electrogenesis of the slow PSP's are reported. (These findings have been outlined in abstracts for presentation at meetings.8' 9) Materials and Methods. -The sympathetic ganglia employed were the caudal ganglia of the paravertebral chain of the bullfrogs and the superior cervical ganglia of young rabbits.4 For intracellular study glass microelectrodes were filled with 3 M KCl (30-50 MU) for frog ganglia, or with 0.6 ML K2SO4 (50-80 MO) for rabbit ganglia. Changes in ohmic resistance of the membrane were determined from the change in membrane potential produced by passing a rectangular pulse of constant current across the membrane (see Figs. 1 and 2). The pulses had durations longer than the time constant of the membrane so that the plateau level of the change in the membrane potential could be measured. These pulses, as well as steady current for producing a maintained shift ill the initial membrane potential, were applied via the recording electrode by means of a bridge arrangement.11 The chamber and method for surface recordings and for soaking ganglia in