Proposed role of septohippocampal and pallido-habenulo-raphe systems in photoperiodic time measurement.

The neuroanatomic substrate for mammalian photoperiodic time measurement (PTM), a primary requirement for appropriately timed seasonal reproductive function, is unknown. It is suggested that the 'comparator' of successive day lengths previously postulated as necessary for PTM may be located within the septohippocampal system, output from which is then directed to reproductive-system response elements by way of several possible routes including the pallido-habenulo-raphe conduction line. PTM thus requires the integration of an invariant, context-free circadian function subserved by the well-known melatonin-generating system with a flexible, context-dependent extracircadian function subserved by the septohippocampal system. The context-dependent neuronal network endows reproductive-system responses with the ability to be modulated by previous photoperiodic history, an ability not possessed by the context-free circadian network proper. Several suggestions for testing the validity of the proposed model are offered.