To better understand physiological changes that accompany the neoplastic transition of astrocytes to become astrocytoma cells, we studied biopsies of low-grade, pilocytic astrocytomas. This group of tumors is most prevalent in children and the tumor cells maintain most antigenic features typical of astrocytes. Astrocytoma cells were studied with the use of whole cell patch-clamp recordings in acute biopsy slices from 4-mo- to 14-yr-old pediatric patients. Recordings from 53 cells in six cases of low-grade astrocytomas were compared to either noncancerous peritumoral astrocytes or astrocytes obtained from other surgeries. Astrocytoma cells almost exclusively displayed slowly activating, sustained, tetraethylammonium (TEA)-sensitive outward potassium currents (delayed rectifying potassium currents; IDR) and transient, tetrodotoxin (TTX)-sensitive sodium currents (INa). By contrast, comparison glial cells from peritumoral regions or other surgeries showed IDR and INa, but in addition these cells also expressed transient "A"-type K+ currents and inwardly rectifying K+ currents (IIR), both of which were absent in astrocytoma cells. IIR constituted the predominant conductance in comparison astrocytes and was responsible for a high-resting K+ conductance in these cells. Voltage-activated Na+ currents were observed in 37 of 53 astrocytoma cells. Na+ current densities in astrocytoma cells, on average, were three- to fivefold larger than in comparison astrocytes. Astrocytoma cells expressing INa could be induced to generate slow action potential-like responses (spikes) by current injections. The threshold for generating such spikes was -34 mV (from a holding potential of -70 mV). The spike amplitude and time width were 52.5 mV and 12 ms, respectively. No spikes could be elicited in comparison astrocytes, although some of them expressed Na+ currents of similar size. Comparison of astrocytes to astrocytoma cells suggests that the apparent lack of IIR, which leads to high-input resistance (>500 MOmega), allows glioma cells to be sufficiently depolarized to generate Na+ spikes, whereas the high resting K+ conductance in astrocytes prevents their depolarization and thus generation of spikes. Consistent with this notion, Na+ spikes could be induced in spinal cord astrocytes in culture when IIR was experimentally blocked by 10 microM Ba2+, suggesting that the absence of IIR in astrocytoma cells is primarily responsible for the unusual spiking behavior seen in these glial tumor cells. It is unlikely that such glial spikes ever occur in vivo.