Dopamine (DA) is the predominant catecholamine neurotransmitter in the mammalian brain, where it controls a variety of functions including locomotor activity, cognition, emotion, synaptic plasticity, learning and memory formation. In the hippocampus, D1/5 receptors (D1/5Rs) were reported to contribute to stabilizing late long-term potentiation (LTP) in the stratum radiatum (RAD) and memory formation, whereas activation of D4 receptors (D4Rs) was shown to inhibit early LTP in the stratum oriens (OR) and to depotentiate LTP in RAD. As novelty exploration rescued the memory impairment caused by D1/5Rs blockade in the hippocampus, here we explored whether dopamine-mediated effects influence synapses and/or coherently active neuronal clusters in behaving mice. We consider the latter possibility, since activation of D4Rs increased kainate-induced gamma oscillations in hippocampal slices by enhancing synchronization of fast-spiking interneurons. To investigate the role of D4Rs under physiological conditions, we set up a system to evoke local field potentials (LFPs) and to record spontaneous oscillatory activity in the hippocampus of freely behaving mice. We examined D4Rs’ function either in a novel environment or following a 12-hour habituation. In both paradigms, a systematic D4R agonist treatment slightly decreased the slope of evoked LFPs and postponed the rapid eye movement (REM) sleep in particular in the paradigm without habituation. The latency of non-rapid eye movement (NREM) sleep was comparable between the D4 agonist- and normal saline (NS)- treated group and was <30min in both paradigms. We are currently investigating the effects of the D4R agonist on LTP which we induce 30 min after the injection of either NS or D4 agonist. The prolonged NREM sleep before the first REM sleep in the presence of the D4 agonist may also affect memory formation that we are planning to test, as sleep facilitates memory consolidation.