Opioidergic and dopaminergic effects on LH and prolactin release in pony mares at different times of the year.

The research explores the interplay of certain neurotransmitter systems (dopaminergic and opioidergic) on the release of hormones (Luteinizing Hormone/LH and prolactin) in pony mares. The experiments were carried out considering different factors such as the breeding and non-breeding seasons, and the hormonal state of the mares (intact, ovary-removed, or varying phases of the reproductive cycle). The study confirmed that opioidergic pathways can regulate LH release and do exhibit seasonal changes in pony mares. The same, however, was not confirmed for dopaminergic regulation.

Research Methodology

• The researchers carried out experiments on intact mares during both the non-breeding and breeding seasons. They also performed tests on mares with removed ovaries in November, March, and May.
• The mares underwent treatments with various substances, including the dopamine D2-antagonist sulpiride, the opioid antagonist naloxone, a combination of naloxone and sulpiride, and saline. These substances were used to understand the effects of the dopaminergic and opioidergic systems.

Key Findings

• Naloxone, both on its own and in combination with sulpiride, increased plasma LH concentrations in intact non-ovulating mares and in mares in the luteal phase of their reproductive cycle. Sulpiride alone, however, did not exhibit this effect.
• None of the treatments had any significant impact on LH release in mares in the follicular phase of their reproductive cycle.
• Naloxone administration consistently spurred LH release in ovariectomized mares throughout the year, but the effect was strongest in March.
• In terms of prolactin concentration, sulpiride increased it regardless of the time of year, and this increase was particularly pronounced in mares in their reproductive cycles. Naloxone, on the other hand, did not affect prolactin secretion.

Research Conclusions

• The researchers concluded that there are opioidergic pathways that can regulate LH release and these pathways undergo seasonal shifts in mares. This conclusion is drawn from the consistent influence of naloxone (an opioid antagonist) on LH levels across the year.
• No conclusive evidence was found for the involvement of the dopaminergic pathway in regulating LH release or for any significant interaction between dopamine and opioids in this context. This is based on the fact that the dopamine D2-antagonist sulpiride did not affect LH levels.