The effects of photoperiod on median eminence dopamine and tuberoinfundibular neuronal activity in the male Syrian hamster (Mesocricetus auratus)

Seasonal changes in daylength affect reproductive physiology and behavior in Syrian hamsters. One of the systems affected by these changes in daylength is the tuberoinfundibular dopamine (TIDA) neuronal system. The experiments presented here examined the effects of daylength (photoperiod) on TIDA neurons to determine the mechanisms responsible for photoperiod-induced changes.

TIDA neurons are located in the arcuate nucleus of the hypothalamus and send projections to the median eminence (ME). In male hamsters, exposure to a short photoperiod (i.e., less than 12.5 hours of light/day) for 12 weeks resulted in a decrease in ME dopamine concentrations. This decrease in dopamine was not due to a decrease in dopamine synthesis or an increase in metabolism by TIDA neurons. The number of tyrosine hydroxylase producing neurons in the arcuate nucleus of male hamsters also was not affected by exposure to a short photoperiod. Time course experiments revealed that in males ME dopamine concentrations slowly decline and are significantly decreased after 4-6 weeks of short-photoperiod exposure, again without a concurrent reduction in synthesis.

In female hamsters housed in a short photoperiod for 12 weeks, there were no changes in either ME dopamine concentrations, or in dopamine metabolism by TIDA neurons. This suggests that there may be a gender difference in the photoperiodic regulation of TIDA neurons and in the hormones these neurons modulate.

Indirect estimates of dopamine release from TIDA neurons suggest that dopamine release from the ME is increased in males exposed to a short photoperiod. An increase in the release of dopamine from the ME without a compensatory increase in dopamine synthesis may be responsible for the depletion of dopamine stores seen when males are exposed to a short photoperiod. The increased release of dopamine from TIDA neurons may also be responsible for the decrease seen in a number of hormones normally inhibited by this transmitter.