D-Amino acids influence ultrasonic calling in mice pups: effects of D-phenylalanine and D-leucine.

6-day-old mice pups were injected with D-amino acids (D-phenyl-alanine + D-leucine), and their ultrasonic distress vocalizations were measured. D-Amino acids, which exert opioid-like effects, reduce the number of ultrasonic calls without affecting the activity of the pups. This effect is reversed by naloxone, an opioid antagonist. The role of endogenous opioids in modulating early attachment is discussed.