Developmental changes in serotonin actions in rat hippocampus.

Intracellular activity was recorded from neurons in immature rat hippocampal slices. The presence of intrinsic inhibitory synaptic potentials as well as responses to serotonin were assessed in slices of 1, 2 or 3 postnatal weeks of age. Young (1 week) cells had only a marginal hyperpolarizing response to serotonin and no detectable intrinsic inhibitory synaptic potentials. At 2 weeks of age neurons already expressed a fast IPSP (inhibitory postsynaptic potential). The responses to serotonin were different from those of adult cells in that they involved primarily a large decrease in input resistance with only small potential changes. In cells of this age serotonin caused a marked increase in spontaneous IPSP discharges and a blockade of a slow afterhyperpolarization. In 3-week-old rats the fast and slow components of the IPSP were present as in adult and the responses to serotonin included a large hyperpolarization associated with an increase in K conductance, a blockade of slow afterhyperpolarization and a blockade of a slow IPSP, as seen in adult cells. These results indicate that the complex pattern of reactivity to serotonin is differentially regulated in the developing brain.