To address the role of D1 receptors in the medial prefrontal cortex, we combined pharmacological and genetic manipulations to examine long-term synaptic potentiation (LTP)͞long-term synaptic depression (LTD) in brain slices of rats and mice. We found that the D1 antagonist SCH23390 selectively blocked the maintenance but not the induction of LTP in the prefrontal cortex. Conversely, activation of D1 receptors facilitated the maintenance of LTP, and this effect is impaired in heterozygous D1 receptor knockout mice. Low-frequency stimulation induced a transient depression in the medial prefrontal cortex. This depression could be transformed into LTD by coapplication of dopamine. Coapplication of dopamine, however, shows no facilitating effect on LTD in heterozygous D1 receptor knockout mice. These results provide pharmacological and genetic evidence for a role of D1 receptors in the bidirectional modulation of synaptic plasticity in the medial prefrontal cortex. The absence of this modulation in heterozygous knockout mice shows that a dysregulation of dopamine receptor expression levels can have dramatic effects on synaptic plasticity in the prefrontal cortex.T he prefrontal cortex is thought to be the highest association area in the mammalian cortex and is required for proper executive control. In primates the dorsolateral prefrontal cortex, alone, has been implicated in several different, partly overlapping, cognitive processes, including inhibitory control, working memory, selective attention, attentional-set shifting, rule learning, and strategy switching. In rodents, where the architecture of the cortex is simpler than that of primates, the medial prefrontal cortex (mPFC), the cortex that encompasses the infralimbic and prelimbic areas, is considered to be homologous to the primate dorsolateral prefrontal cortex (1, 2).Consistent with this homology, a variety of studies have shown that dopamine modulates neuronal activity in the prefrontal cortex and affects working memory, attentional performance, attentional-set shifting, and strategy switching (3-9). Some functions of mPFC, such as strategy shifting or rule learning, may be affected by dopaminergic modulation of long-term synaptic plasticity. Dopamine indeed affects long-term plasticity in the prefrontal cortex. It facilitates long-term synaptic depression (LTD) via D1 and D2 receptors (10, 11), and D1 activation has been shown to be required for N-methyl-D-aspartate (NMDA) receptor-dependent long-term synaptic potentiation (LTP) at hippocampal-prefrontal cortex synapses (12).To further our understanding of how dopamine modulates long-term synaptic plasticity in opposite directions, and to develop a model for exploring the underlying molecular signaling pathways, we examined LTP and LTD in slices of the mPFC from rats, wild-type mice, and heterozygous dopamine D1 receptor knockout mice. Heterozygous D1 receptor knockout mice show a 50% reduction in receptor number. They allow us to address specifically the function of the D1 receptor as opposed to both D1 ...