OPIOID INHIBITION OF THE PULSATILE LUTEINIZING HORMONE RELEASE AS ASSESSED BY NALOXONE TREATMENT IN THE LACTATING RABBIT

Since in nursing rabbits sexual receptivity and fertility achieved after AI appear to be depressed during lactation, the effect of endogenous opioid peptides (EOP) on serum LH were investigated in lactating rabbits (n=30) by administration of the opiate antagonist naloxone (nal). Blood samples were collected every 15 min for 4 h via an indwelling catheter inserted in the central ear artery. After the first h of sampling, the rabbits received i.v. saline (SAL, n=10); nal (LN, 0.5 mg/kg, n=10); or nal (HN, 1.0 mg/kg, n=10). The pulsar algorithmic procedure for the study of pulsatile hormone secretion was used to calculate mean and basal LH concentrations, frequency and amplitude of LH pulses. Data from the statistical analysis are presented as mean ± SE of area under the curve (AUC) units of LH response. Mean LH concentration during the 60 min pre-challenge period was 0.24±0.03 ng/ml. The AUC of LH during that period did not differ among treatment groups (LN=16.6±4.1, HN=11.8±2.9 and SAL=14.3±4.0 area units). In the SAL group LH secretion remained low during the 4 h sampling period. Mean and basal LH concentrations were 0.21±0.05 ng/ml and 0.03±0.02 ng/ml (LN=0.59±0.13 ng/ml and 0.20±0.04 ng/ml; HN=0.38±0.09 ng/ml and 0.13±0.03 ng/ml). The mean pulse peak was 0.70±0.15 ng/ml (LN=1.95±0.37 ng/ml; HN=1.26±0.23 ng/ml), and the mean pulse amplitude wa