The neurosteroid 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha, 5 alpha-THP) acts as a potent allosteric modulator and a direct activator of the GABA-chloride channel complex. This neurosteroid has also been found to protect against seizures that arise from blockade of the GABA-chloride channel complex. Because 3 alpha,5 alpha-THP protects against excitotoxin-induced seizure activity and because seizure activity has been found to be associated with aberrant hippocampal nerve cell growth, the rapid effect of the neurosteroid 3 alpha,5 alpha-THP upon nerve cell growth was investigated using videomicroscopy of hippocampal neurons in culture. Within 40 min of exposure 3 alpha,5 alpha-THP induced a significant decrease in the area and length of neurites. A concomitant decrement in the number and length of filopodia decorating neuritic extensions also occurred within the 40 min of 3 alpha,5 alpha-THP exposure. Both rapid and slow retrograde movement of intracellular organelles was observed in 3 alpha,5 alpha-THP-treated neurons. 3 alpha,5 alpha-THP-induced regression of neuritic extensions occurred only in nerve cells that had not yet established contact with other nerve or glial cells in culture. Established structural connections between neurons or glia did not erode during 3 alpha,5 alpha-THP exposure. Neither the inactive stereoisomer 3 beta-hydroxy-5 beta-pregnan-20-one nor progesterone had a significant effect upon any of the morphological parameters assessed. In approximately 25% of the cells in which 3 alpha,5 alpha-THP had induced regression, subsequent exposure to 17 beta-estradiol induced profuse filopodial growth within 60 sec of exposure. In cultures similar in age to those used in the morphological studies, 3 alpha,5 alpha-THP induced a significant increase in 36Cl- uptake within 10 sec. The magnitude of 36Cl- uptake was comparable to that induced by exposure to 100 microM GABA. In older, more mature cultures in which the nerve cells had established structural connections, 3 alpha,5 alpha-THP protected cells from picrotoxin-induced nerve cell death. These results demonstrate that 3 alpha,5 alpha-THP can induce regression of neuronal morphology within a relatively rapid time frame. 3 alpha,5 alpha-THP induction of 36Cl- uptake within 10 sec suggests that activation of neurosteroid-regulated chloride channels is an initial step in the biochemical mechanism underlying the retraction induced by this progesterone metabolite steroid. In select instances, 17 beta-estradiol induced an extremely rapid reversal of the filopodial regression produced by 3 alpha,5 alpha-THP.(ABSTRACT TRUNCATED AT 400 WORDS)