Of particular interest was the inhibition of serotonin receptor signaling, due to the well-established connection between serotonin and anxiety.61 On this signaling pathway, genes downregulated in females withdrawn from morphine withdrawal included genes encoding multiple serotonin receptors, such as Htr2c and Htr5b, as well as the adrenergic receptor Adra2a. Here, ADRA2A is linked to Anxiety.