Abstract
The effects of l-DOPA, a precursor of dopamine (DA), and sulpiride, a D2 -type DA receptor blocker, on growth hormone (GH) and prolactin (PRL) secretion were investigated in steers. Eight Holstein steers (212.8±7.8kg body weight) were used. Lighting conditions were 12:12 L:D (lights on: 06.00-18.00hours). Blood samplings were performed during the daytime (11.00-15.00hours) and nighttime (23.00-03.00hours). Intravenous injections of drugs or saline were performed at 12.00 hour for the daytime and 00.00hour for the nighttime, respectively. Plasma GH and PRL concentrations were determined by radioimmunoassay. l-DOPA did not alter the GH secretion when it was injected at 12.00hour (spontaneous GH level at its peak). On the other hand, l-DOPA increased GH secretion at 00.00hour (GH level at its trough). Injection of sulpiride suppressed GH secretion at 12.00hour but did not affect GH levels at 00.00hour. l-DOPA inhibited and sulpiride stimulated PRL release during both periods. These results suggest that dopaminergic neurons have stimulatory action on GH secretion and inhibitory action on PRL secretion in cattle. In addition, injection time should be considered to evaluate the exact effects on GH secretion due to its ultradian rhythm of GH secretion in cattle.
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