Fig. 1.

Effects of central administration of naltrindole hydrochloride (NTI), a selective antagonist of δ-opioid receptors (DOR), on the suppression of luteinizing hormone (LH) pulses induced by suckling stimuli in ovariectomized (OVX) lactating rats during early lactation. Plasma LH profiles of individual OVX lactating rats, which were treated with the third ventricle (3V) injection of NTI (or vehicle) during early lactation (A). Blood samples were collected every 6 min for 3 h. Immediately after the first blood sampling, NTI or vehicle (timing indicated by arrows) was injected into the 3V. Arrowheads indicate the peaks of LH pulses identified by the PULSAR computer program. The mean plasma LH concentrations (B) and the baseline (C), frequency (D), and amplitude (E) of LH pulses in each group. Values are means ± SEM. The number in each column indicates the number of animals used. * Significant difference between NTI- and vehicle-treated rats (P < 0.05, Student’s t-test). N/A, not applicable.