In addition to structural and functional alternations of neurons during their recruitment to be associative memory cells, these associative memory cells undergo the functional plasticity in response to their activity, called as recruitment-relevant plasticity and activity-dependent plasticity, respectively (Associative Memory Cells: Basic Units of Memory Trace by Jin-Hui Wang 2019). Here, we report the features of activity-dependent plasticity at associative memory cells studied by two-photon cell imaging. After the learning of triple associated signals (pair-stimulations by odor, whisker tactile and tail warming-up), secondary associative memory cells in response to these signals are recruited in the prefrontal cortex. The portion of functional associative memory cells is upregulated by intensive activity in one of three synaptic inputs or their combinations. When whisker, odor or tail stimulus is given intensively, the activity levels of these associative memory cells show low-to-high or highto-low in response to original cues, especially activity-dependent depression in response to homosynaptic signal and activity-dependent potentiation in response to heterosynaptic signal. When whisker, odor and tail stimulations are given simultaneously, these associative memory cells express activitydependent potentiation in response to all these input signals. The upregulated activities of norepinephrine and serotonin in the central nervous system strengthen the recruitment, the functional strength and the activity-dependent plasticity of associative memory cell in response to homosynaptic and heterosynaptic signals. As activity-dependent plasticity at associative memory cells occurs immediately after their activation, our results may be due to a possibility that the new synapses formed on associative memory cells during associative learning can be upregulated and downregulated between active state and inactive state, in which their intensive activities and monoamine neurotransmitters are critical modulators. The results can explain activity-related memory strengthening and the role of the arousal system through norepinephrine and serotonin in memory.