Effect of activity blockade on changes in vibrissae-related patterns in the rat's primary somatosensory cortex induced by serotonin depletion

Depletion of cortical serotonin (5-HT) during development results in a decr ease in the size of the patches of thalamocortical afferents representing t he mystacial vibrissae in lamina TV of the primary somatosensory cortex (SI ). We previously suggested that this change may be due to a reduction in 5- HT-induced suppression of thalamocortical activity in these animals. The pr esent experiments directly tested the role that modulation of activity may play in the morphologic changes observed after reducing cortical 5-HT conce ntrations. Serotonin was depleted from the cortex by systemic administratio n of 5,7-dihydroxytryptamine (5,7-DHT, 100 mg/kg) on the day of birth in an imals that also had either tetrodotoxin (TTX)-impregnated or control implan ts placed unilaterally over the developing SI on this day. Other rat pups w ere treated with TTX-impregnated or control implants alone. Administration of 5,7-DHT reduced cortical serotonin levels and this effect was not signif icantly modified by the presence of either control or TTX-impregnated corti cal implants. Administration of 5,7-DHT reduced the cross-sectional area of the cortical patches, demonstrated by acetylcholinesterase, corresponding to the vibrissae by 19.9% (P < 0.05). A similar reduction was observed in t he animals treated with both 5,7-DHT and TTX-impregnated implants. Treatmen t with TTX-impregnated implants alone resulted in a 3.1% increase in patch size (P > 0.05). None of the treatments significantly altered the overall a rea of the part of SI devoted to the representation of the long mystacial v ibrissae. These results suggest that the effects of 5-HT depletion on the s ize of the cortical patches representing the long vibrissae are independent of activity that can be blocked by administration of TTX. J. Comp. Neurol. 402:276-283, 1998. (C) 1998 Wiley-Liss, Inc.