Figure 7.

Proposed model for SST action within the hypothalamus that mediates the steroid-independent control of LH secretion in sheep between the anestrous (left) and the breeding seasons (right). The inhibitory photoperiod in anestrus stimulates serotonergic neurons that increase the activity of SST neurons in the VMN. These SST neurons in turn inhibit GnRH pulse frequency, which could be mediated directly on GnRH neurons, indirectly through KNDy neurons, or indirectly through some other neuron type. This system is distinct from the steroid-dependent actions of photoperiod, which act via A15 dopaminergic (DA) neurons. See text for a more detailed discussion.