THE EXPRESSION PATTERN OF SOMATOSTATIN AND CALRETININ BY POSTNATAL HIPPOCAMPAL INTERNEURONS IS REGULATED BY ACTIVITY-DEPENDENT AND ACTIVITY-INDEPENDENT DETERMINANTS

Hippocampal interneurons form distinct populations identified on the b asis of their projection pattern and neurochemical characteristics, wh ich includes the expression of specific neuropeptides and/or calcium-b inding proteins. The neurochemical maturation of hippocampal interneur ons is largely a postnatal event, and factors which govern this matura tion are presently unknown. Using slice cultures, we have investigated the role of neuronal activity in regulating the expression of somatos tatin and calretinin during the postnatal maturation of hippocampal in terneurons. Blocking inhibitory activity with bicuculline, or excitato ry activity with 6,7-dinitroquinoxaline-2,3-dione, for 14 days in slic e cultures from seven-day-old rat increased and decreased, respectivel y, the number of somatostatin-immunoreactive neurons. Withdrawal of th e blocking agents resulted in a reversal of the effects on somatostati n immunoreactivity. In addition, bicuculline slightly increased the nu mber of calretinin-positive neurons, while 6,7-dinitroquinoxaline-2,3- dione exerted no effect. However, bicuculline and 6,7-dinitroquinoxali ne-2,3-dione markedly increased and decreased, respectively, the numbe r of calretinin-labelled axons. Despite activity-linked modifications of immunoreactivity levels, no change in the organotypic location of s omatostatin-labelled neurons was observed, whatever the treatment. Dou ble labelling studies demonstrated that somatostatin and calretinin we re expressed by different neurons, even when the number of labelled ce lls was highly increased. These results show that the levels of expres sion of somatostatin and calretinin in maturing hippocampal interneuro ns are tuned to the endogenous balance of excitatory and inhibitory ac tivity. In contrast, the neurochemical specificity of each subtype of interneurons does not depend upon variations in neuronal activity. (C) 1997 IBRO. Published by Elsevier Science Ltd.