The Role of Nutrition in the Regulation of Luteinizing Hormone Secretion by the Opioidergic, Dopaminergic, and Serotonergic Systems in Female Mediterranean Goats1

Abstract

This study examined which neural mechanism (opioid, dopaminergic, or serotonergic system) is involved in the regulation of luteinizing hormone (LH) secretion, with and without nutritional modulation, at different times of the photoperiodic cycle. Goats were randomly distributed into two experimental groups that received either 1.1 (high group; n = 18) or 0.7 (low group; n = 18) times the nutritional maintenance requirements. The goats were exposed to alternations of 3 mo of long days and 3 mo of short days. Plasma LH concentrations were measured twice a week. The effects of intravenous injections of naloxone (endogenous opioid receptor antagonist), pimozide (dopaminergic(2) receptor antagonist), and cyproheptadine (serotonin 5-hydroxytryptamine(2) receptor antagonist) on LH secretion were assessed during challenges in three different photoperiodic situations: the onset of LH stimulation by short days (OnsetSD), the onset of LH inhibition by long days (OnsetLD), and during the LH inhibition by long days (LateLD). The role of the different neural systems was clearly modified by the level of nutrition. In the low-nutrition group, only naloxone increased LH concentrations during onsetLD (P < 0.05). However, in the high-nutrition group, naloxone increased the concentration and pulsatility of LH (P < 0.05) in onsetSD and onsetLD. Pimozide increased LH concentration and pulsatility (P < 0.05) in onsetLD and LH concentration in lateLD (P < 0.001). Finally, cyproheptadine significantly increased LH concentration at all three times (P < 0.001). These results provide evidence that all three systems are involved in the inhibition of LH release in onsetLD, and that the opioid and serotonin mechanisms are involved during the onsetSD that were enhanced by a high plane of nutrition.

The effect of nutrition on the neural mechanisms potentially involved in melatonin-stimulated LH secretion in female Mediterranean goats

“…In this study, before the implantation of melatonin, the lower level of nutrition induced a reduction in the mean LH concentration independent of the photoperiod, as reported in previous work by our group (Zarazaga et al 2011a). These results help explain earlier findings that a low BCS induced by a low level of nutrition (De Santiago-Miramontes et al 2009), or a maintenance level of nutrition compared to a higher level of nutrition (Zarazaga et al 2005), induced a longer seasonal anoestrus as a result of an earlier onset of the anovulatory period.…”

“…Effect of nutrition and melatonin on LH level similar to those described in OVXCE Mediterranean goats (Zarazaga et al 2011a), confirming that, in caprines at this latitude, the photoperiod is the main environmental factor controlling pituitary activity but that nutrition is an important modulator of LH secretion. After melatonin implantation, the LH concentration increased and the onset of pituitary activity was earlier than in non-implanted females.…”

“…Continuing our previous research (Zarazaga et al 2011a), the present research assesses the interaction between the nutritional level and melatonin implants on the regulation of LH secretion by the endogenous opioid, dopaminergic, serotonergic and NMDA systems in melatonin-implanted Mediterranean goats maintained under a long-day photoperiod.…”

“…For the other drugs, doses similar to those used by Zarazaga et al (2011a) were given: 2 mg/kg, and originally 0 . 6 and 0 .…”

“…Further, the dopaminergic inhibitory pathways are responsible for a reduction in LH pulse frequency during the onset of LH inhibition by long days and during the inhibition of LH secretion by long days. Finally, the serotonergic system is involved during the onset of LH inhibition by long days, in the inhibition of LH secretion by long days, and in the onset of LH stimulation by short days, but always in well-nourished females (Zarazaga et al 2011a).…”