In aged laboratory Norway rats (the Sprague-Dawley strain, where this was measured), a pituitary adenoma is the most common spontaneous brain-adjacent tumor in aging females.[1] In roof rats the picture may be different: “I have never seen a roof rat with a verified pituitary tumor. I do not have a definitive medical diagnosis of the cause of death for every female, but none of my females died symptomatically to my knowledge.” (the breeder, from his own account). Most are prolactinomas: they arise from a specific kind of pituitary cell called a lactotroph that normally secretes the hormone prolactin. The tumor cells themselves secrete prolactin in large quantities, which is why an intact non-pregnant female with a prolactinoma may inappropriately lactate. That single sign is nearly diagnostic.
The signs to watch for
As the tumor grows, it presses on the adjacent brain. Central neurological signs follow over weeks or months:
Wobbly, wide-based stance. She sits or walks with her hind legs splayed for balance.
Ataxia. She has trouble coordinating her movements; she may stumble or fall when she tries to turn quickly.
Knuckling. The top of her paw drags on the ground as she walks, because she has lost proprioceptive awareness of where her paw actually is. This is a CENTRAL neurological sign and is one of the strongest indicators that the cause is tumor rather than inner-ear infection.
Inability to grasp food. She approaches food and licks it directly off the floor instead of lifting it with her forepaws. This is a hallmark sign of forelimb proprioceptive deficit.
Central blindness with intact pupillary light reflex. Wave a hand near her face: she doesn’t blink. Shine a small light at her eye: the pupil constricts normally. The light pathway is intact; the seeing pathway isn’t. Classic optic-chiasm compression from an expanding pituitary mass.
Behavioral changes. Sudden aggression, stupor, withdrawal, or just looking “not herself.”
Inappropriate lactation. Milk in the mammary chain of an intact, non-pregnant female is a reason to have her checked for a pituitary tumor. It is not by itself a diagnosis: of the spontaneous pituitary adenomas found in aged rats, 53 percent are prolactin-producing and only 33 percent are pure prolactinomas.[6]
Treatment: cabergoline
Cabergoline is a dopamine D2 agonist. In a pet rat with a prolactinoma, cabergoline binds D2 receptors on the tumor cells themselves and shuts down their prolactin secretion. Without prolactin signaling driving them, the tumor cells undergo apoptosis and the tumor shrinks.[2]
The drug concentrates strongly in pituitary tissue relative to plasma (reported at well over 100-fold), which is why it can be dosed infrequently (every 72 hours, typically). Its half-life is long, on the order of 60 hours. The rat’s response is often dramatic. Improvement within 7 to 14 days. Tumor shrinkage documented on follow-up imaging in case reports. She may be able to grasp food again, ataxia may improve, the head tilt may resolve partially or fully.
The honest prognosis
This is a tumor, and the treatment is palliative, not curative. Typical extension of good-quality life is 3 to 6 months by general expectation per the Rat Guide. A single peer-reviewed pet rat case report documented about 8 months.[3] Population survival statistics in pet rats don’t exist; what we have is clinical experience and the published case literature.
The win is the quality of the time bought, not a cure. A few months of her being able to eat properly, groom herself, snuggle into your hand again. That’s what cabergoline buys, and it can be genuinely good time.
Prevention: spay her young
The single most consequential thing an owner of female pet rats can do to reduce lifetime pituitary tumor risk is to spay them young, before about 5 to 7 months of age.[4][5] In laboratory rats, hormone-driven tumors (both mammary and pituitary) are very common in intact aging females, and early spaying substantially reduces that risk.
The window is wide. In the one controlled study, removing the ovaries at any point before middle age onset (about 5 to 7 months) still cut spontaneous mammary tumors by 95 percent, from 73.8 percent of intact females to 5.3 percent.[4] See the spaying entry.
Walk through it interactively
Our JUSTICE LEAGUE vs THE HEAD TILT text adventure includes the pituitary-tumor case as one of three random villain options. The FLAWLESS ending honestly preserves the clinical reality: even with perfect play, the win is the quality of time bought, not a cure.
References
- [1] Nakazawa M, Tawaratani T, Uchimoto H, et al. “Spontaneous neoplastic lesions in aged Sprague-Dawley rats.” Experimental Animals 2001;50(2):99-103. PubMed.
- [2] Lin SJ et al. “Suppression of mTOR pathway and induction of autophagy-dependent cell death by cabergoline.” Oncotarget 2015;6(36):39329-39341. PubMed.
- [3] Mayer J et al. “Extralabel use of cabergoline in the treatment of a pituitary adenoma in a rat.” Journal of the American Veterinary Medical Association 2011;239(5):656-660. PubMed.
- [4] Planas-Silva MD, Rutherford TM, Stone MC. “Prevention of age-related spontaneous mammary tumors in outbred rats by late ovariectomy.” Cancer Detection and Prevention 2008;32(1):65-71. PubMed.
- [5] Vergneau-Grosset C, Keel MK, Goldsmith D, et al. “Description of the prevalence, histologic characteristics, concomitant abnormalities, and outcomes of mammary gland tumors in companion rats (Rattus norvegicus): 100 cases (1990-2015).” JAVMA 2016. PubMed.
- [6] Carretero J, Burks DJ, Vazquez G, et al. “Expression of aromatase P450 is increased in spontaneous prolactinomas of aged rats.” Pituitary 2002;5(1):5-10. PubMed.