Age-dependent local modulation of hippocampal-evoked responses to perforant path stimulation.

Local modulation of hippocampal-evoked responses to perforant path stimulation was studied by leaking drugs from the recording pipette placed in the dentate gyrus of anesthetized young (3 months old), aging (17 months old) and old (28 months old) rats. In old rats, the excitatory postsynaptic potential (EPSP) slope was much reduced compared to young and aging rats. The population spike (PS) size was similar in all age groups. Bicuculline caused a marked increase in PS size relative to population EPSP, and reversed the response to the second pulse in a paired-pulse paradigm from inhibition to facilitation. The effect of bicuculline was only slightly reduced in old rats. The 5-HT1a agonist 8-OH-DPAT potentiated PSs in the dentate gyrus, while not affecting paired-pulse inhibition. The effect of 8-OH-DPAT was slightly reduced in old rats. Carbachol, a cholinergic agonist, reversed paired-pulse inhibition into facilitation in the young brain, but not in aging and old rats. These results demonstrate that age affects differentially the action of biogenic amines on hippocampal reactivity to afferent stimulation.