1. Intracellular recordings were made from sympathetic preganglionic neurons (SPNs) in transverse thoracolumbar spinal cord slices of 12- to 16-day-old rats. 2. A population of SPNs exhibited spontaneous, rhythmic activities that can be grouped into regular firing, bursting, or tonic oscillatory mode; the spikes were interspersed with subthreshold after depolarizing potentials (ADPs). 3. Spontaneous activities were not abolished by the excitatory amino acid antagonists 6-cyano-7-nitroquinoxoline-2,3-dione (CNQX, 10-20 mu M) and D-2-amino-5-phosphonovaleric acid (APV, 10-20 mu M) nor by the inhibitory amino acid antagonists bicuculline (10-30 mu M) and strychnine (1 mu M). 4. Stimulation of lateral funiculus elicited a CNQX-sensitive excitatory postsynaptic potential(EPSP) in the majority of spontaneously active SPNs tested. Moreover, rhythmic activities could be triggered by stimulation of lateral funiculus in 8 of the 80 otherwise quiescent SPNs tested. 5. Spontaneous spikes and ADPs were abolished by either Na+- free or tetrodotoxin (TTX, 0.5 mu M)-containing Krebs solution. 6. The spontaneous spike afterhyperpolarization (AH) consisted of two components: a fast AH(f) and a slower AH(s) with a mean half decay time of 74.5 +/- 10.2 and 153.8 +/- 15.4 (SD) ms, respectively. 7. Superfusing the slices with a Ca2+-free Krebs solution or Krebs solution containing tetraethylammonium (TEA, 10-30 mM) preferentially blocked the AH(s), thereby increasing the spontaneous discharge frequency. 8. Caffeine (3 mM) initially prolonged the AH(s), thereby slowing the discharge frequency; the spontaneous discharges were even tually obliterated in the continuous presence of caffeine. 9. The putative transmitters norepinephrine (NE, 1-10 mu M); and 5-hydroxytryptamine (5-HT, 50-100 mu M) depolarized spontaneously active neurons and intensified their discharges; these agents could also induce rhythmic activities in otherwise quiescent neurons. 10. It is concluded that 1) a population of neonatal rat SPNs displays rhythmic activities, some of which can be initiated by synaptic inputs or by the putative transmitters NE and 5-HT, 2) Na+ are the principal ions underlying the spontaneous spike potentials, 3) a Ca2+-dependent K+ current, which underlies the AH(s), modulates the frequency and pattern of rhythmic activities,and 4) intracellular Ca2+ may play a pivotal role in modulating and/or terminating the rhythmic behavior.
