At present, more than 20 peptides are considered as putative neurotransmitters, based primarily on their localization in nerve terminals and their actions on nerve cells (H6kfelt et al. 1980). The list is almost certainly not exhaustive and it is likely that more will be found in the future. If many peptides turn out to be neurotransmitters, one might wonder why the nervous system utilizes so many different transmitters. A detailed analysis of the action of a number of neuropeptides may reveal characteristics unique to peptidergic transmission and thus shed light on the functional significance of peptide transmitters and perhaps explain the need for multiple transmitters.Frog sympathetic ganglia and parasympathetic cardiac ganglia are excellent preparations for this purpose (Langley and Orbeli 1910; McMahan and Kuffler 1971). Structurally, they are very simple. Neurons in these ganglia are unipolar and have no dendrites. Preganglionic fibers make synaptic contacts almost exclusively on the ganglionic cell bodies. This arrangement facilitates analysis of the physiological effects of neurotransmitters and allows accurate anatomical description of synaptic connections. Analysis of possible actions of peptides in these two preparations has already revealed interesting features. In the frog sympathetic ganglia, a luteinizing hormone releasing hormone (LHRH)-Iike peptide is almost certain to be the transmitter that mediates a slow excitatory postsynaptic potential, the late slow EPSP. Certain properties of this peptidergic transmission appear to be strikingly different from those of conventional cholinergic transmission. For instance, although the LHRH-like peptide and acetylcholine (ACh) seem to be released from the same nerve terminals that form synapses of conventional morphology, the peptide can diffuse far greater distances than ACh and activate sympathetic neurons nonsynaptically. In the frog heart septum, all of the terminals on the parasympathetic neurons are positively stained by antibodies to substance P (SP). However, little or no electrical effects on those neurons have been detected that may be attributed to the SP-like immunoreactivity.Our preliminary results indicate that one target of the SP-like material may be the mast cells situated near the neurons. These results prompted us to propose a hypothesis about the general mode of actions of neuropeptides and to suggest why the nervous system uses so many different chemical messengers. METHODSBoth preparations used here, the paravertebral sympathetic ganglia and the parasympathetic ganglia of the interartrial septum of the heart, were isolated from bullfrogs (Rana catesbiana) as previously described (McMahan and Kuffler 1971;Jan and Jan 1982).Electrophysiological experiments. The isolated ganglia were pinned to a thin layer of Sylgard in a petri dish and viewed with Nomarski optics. The bath was continuously perfused at room temperature with Ringer's solution containing 115 mM NaCI, 2 mM KCI, 3.6 mM CaCI2, and 5 mM HEPES (pH 7.2). Conventional recording t...