Can seasonal timing in the pituitary be driven by day length without melatonin, challenging melatonin-centered models?

Day length really does reach the pituitary

The pituitary is not a passive bystander that waits for the brain to tell it what season it is. In sheep, comparing animals kept under short versus long days turned up a wave of gene activity in the pituitary's pars tuberalis, the region that acts as the regulating center for seasonal reproduction. The number of genes and long non-coding RNAs that differed between the two conditions grew as the long-day period was extended, and the list included EYA3 and TSHB, genes already tied to seasonal timing, alongside reproduction- and circadian-related pathways [2].

Fish show the same kind of responsiveness at the level of individual cells. In female medaka, long day length stimulated both the hormone output of gonadotrope cells, which produce the reproductive hormones FSH and LH, and their division by mitosis, so the pituitary was not just turning up production but adding more hormone-producing cells [5]. A separate body of work in the same fish found that long photoperiod boosts gonadotrope cell activity and number, and that the pituitary can remodel its cell populations through several routes, including cells changing from one endocrine type into another [3]. In female guanacos, blood measurements across short and long days showed melatonin lower and estradiol, FSH and LH higher during long days, with melatonin receptor and kisspeptin expression detected in the hypothalamus and pituitary [4].

Where melatonin keeps appearing in the relay

The reason the melatonin-free challenge is hard to sustain is that when researchers trace the pathway step by step, melatonin sits inside it. In medaka, long photoperiod was found to drive gonadotropin production and gonadotrope mitosis through an indirect route within the pituitary itself: pituitary Tsh cells pass a signal through folliculostellate cells, which are the only pituitary cells expressing Tsh receptors and which send projections to the gonadotropes. Melatonin, in that same study, suppressed pituitary tshba expression under long days, which the authors read as short days inhibiting gonadotropin synthesis via melatonin [5].

That is a pathway with melatonin as a brake rather than a trigger, but it is still a melatonin-dependent pathway. The broader review of mammals and teleosts describes melatonin released from the pineal gland at night as a key hormone in photoperiodic signaling, carrying rhythmic information to the whole organism and acting on the pituitary both directly and through brain centers upstream [1]. The guanaco data fit the same picture: melatonin varied with day length, and the authors concluded that the pineal gland is sensitive to seasonal changes in day length, with reproductive hormones likely modulated accordingly [4]. None of these studies removes melatonin and shows the seasonal response surviving intact.

What a real challenge to melatonin-centered models would require

A convincing challenge would need a design that takes melatonin out of the picture, for example by removing the pineal signal or blocking melatonin receptors, and then shows that day length still reshapes pituitary gene expression, cell number, or hormone output on the same seasonal schedule. The papers here do not report that experiment. The sheep work identifies photoperiod-induced genes and regulatory RNAs in the pars tuberalis but does not test whether melatonin is dispensable [2]. The medaka work identifies a melatonin-Tsh pathway and proposes that the intra-pituitary route involving folliculostellate cells might also exist in other vertebrates, which is a suggestion about where to look next rather than a demonstration that melatonin can be bypassed [5].

The review is explicit about the gap: it describes melatonin's involvement in photoperiodic signaling and does not provide evidence that seasonal timing can occur independently of melatonin [1]. So the current state of play is that day length is a genuine driver of pituitary seasonal change, and melatonin is a genuine part of how that drive is transmitted, with the possibility of additional local pituitary machinery layered on top.

Why the pituitary's own plasticity matters for the question

Part of what makes the question interesting is that the pituitary does more than relay a hormonal message. In medaka, the number of gonadotrope cells changes with photoperiod, and the route to that change differs between the sexes: in females, Fsh cell hyperplasia involved cells converting from Tsh to Fsh, while in males stem cell differentiation appeared to be involved [3]. That kind of cellular remodeling gives the pituitary its own capacity to respond to season, which is the sort of local machinery a melatonin-free model would need to explain.

But capacity is not the same as independence. The same medaka work places melatonin, Tsh and folliculostellate cells together in the pathway that stimulates gonadotrope activity and proliferation [3][5]. The guanaco study likewise found melatonin receptor expression in the hypothalamus and pituitary alongside seasonal hormone shifts, consistent with melatonin acting at those sites rather than being bypassed [4]. The open question is whether the intra-pituitary route can run on day length alone; the evidence so far shows it running with melatonin in the circuit.

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