Increasing evidence suggests that postnatal events, such as handling or maternal separation, can produce long-term changes in brain function. These are often expressed as changes in the profile of endocrine or behavioral responses to stress. Changes in ␥-aminobutyric acid type A receptors (GABARs), which mediate the majority of fast synaptic inhibition in adult brain, have been proposed as one potential mediator of these behavioral effects. In the current article, we use a combination of single-cell electrophysiology and antisense mRNA amplification to demonstrate permanent molecular and functional differences in GABARs within hippocampal dentate granule neurons after as few as two episodes of neonatal handling with brief maternal separation. Adult animals that as pups experienced handling with maternal separation maintained a more immature GABAR phenotype and exhibited increased activity in response to swim stress. These findings demonstrate the exquisite sensitivity of the developing GABAergic system to even subtle environmental manipulations and provide an unique molecular mechanism by which postnatal handling with maternal separation may alter stress-related behavior.development ͉ glucocorticoid ͉ dentate granule neurons ͉ patch clamping ͉ single-cell antisense mRNA amplification T he developing nervous system can be exquisitely sensitive to even minor perturbations in the environment. It has been recognized for decades that handling of neonatal rats could produce profound effects on later neuroendocrine and behavioral responses to stress (1, 2). Repetitive brief handling in neonatal rats has been shown to result in permanent alterations in hippocampal glucocorticoid (GC) receptors, decreased GC responses to stress in adulthood, and a relative protection against age-related hippocampal neuronal death and cognitive impairments (3). Adult animals handled as pups also demonstrate decreased expression of fear-related behaviors under stressful conditions (4-6). The molecular mechanisms underlying handling-induced behavioral and cognitive changes are not fully understood. ␥-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in mammalian brain and regulates both endocrine and behavioral responses to stress (7-9). Benzodiazepines (BZs) and other drugs that potentiate GABA currents can be potent anxiolytics. Rats exposed to early-life handling have been found to have altered BZ receptor levels in several brain regions including brainstem nuclei, amygdala, and frontal cortex (5, 6), but handling effects on GABA type A receptor (GABAR) subunit expression and function in hippocampal neurons have not, to our knowledge, previously been reported.GABARs are chloride ion channel-associated ligand-gated heteromeric receptors composed of five subunits, which are modulated by BZs, barbiturates, zinc, and neurosteroids. Many genetically distinct subunit subtypes, including ␣1-6, 1-4, ␥1-3, ␦, , , , and 1-3, have been identified (10, 11). GABARs can be assembled in different subunit combinations, resulting in a st...