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 afterdepolarizing potentials (ADPs). 3. Spontaneous activities were not abolished by the excitatory amino acid antagonists 6-cyano-7-nitroquinoxoline-2,3-dione (CNQX, 10-20 microM) and D-2-amino-5-phosphonovaleric acid (APV, 10-20 microM) nor by the inhibitory amino acid antagonists bicuculline (10-30 microM) and strychnine (1 microM). 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 microM)-containing Krebs solution. 6. The spontaneous spike afterhyperpolarization (AH) consisted of two components: a fast AHf and a slower AHS 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 Ca(2+)-free Krebs solution or Krebs solution containing tetraethylammonium (TEA, 10-30 mM) preferentially blocked the AHS, thereby increasing the spontaneous discharge frequency. 8. Caffeine (3 mM) initially prolonged the AHS, thereby slowing the discharge frequency; the spontaneous discharges were eventually obliterated in the continuous presence of caffeine.(ABSTRACT TRUNCATED AT 250 WORDS)
