Background & Aims
Acromegaly is a disorder caused by a prolonged excess secretion of growth hormone (GH), usually due to a pituitary adenoma. Excess GH induces morphological changes including enlargement of the hands, feet, forehead, jaw, and nose (1). Pain symptoms in acromegaly are particularly debilitating as 60-70% of patients experience painful conditions such as migraines, joint pain, neck pain and abdominal pain (2). Moreover, current treatments that aim to lower GH levels result in incomplete pain relief causing pain symptoms to persist even after long-term treatment of the pituitary disease. The underlying pain mechanisms are not known and animal models are needed to understand pain pathophysiology in acromegaly to discover novel therapeutic approaches.
Methods
All experiments were performed in 8-week old wild-type and transgenic mice overexpressing bGH (bGH mice). Mechanical sensitivity of the hindpaw was assessed with the von Frey filament testing using the up-down method. Thermal sensitivity of the hindpaw was assessed using the Hargreaves test. Trigeminal ganglia were collected and immunostained for CGRP IB4 and NF200. Trigeminal sensory neurons were isolated and cultured for 3h before imaging of intracellular calcium concentration using Fura2-AM. Calcium response of sensory neuron was evaluated after stimulation with acyl-isothiocyanate (AITC, 250µM) or capsaicin (0.1µM).
Results
The evaluation of mechanical sensitivity, shows a decrease in pressure threshold to trigger hindpaw withdrawal in both male (-38%) and female (-34%) bGH mice compared to wild-type mice. The evaluation of thermal sensitivity, shows a decrease in latency to trigger hindpaw withdrawal in male bGH mice (-29% compared to male wild-type). However, no difference in thermal sensitivity was observed in female bGH mice versus wild type (wt) mice.
Immunostaining experiments showed a fewer number of CGRP immunoreactive neurons in bGH mice compared to wt mice but no difference in the number of neurons immunoreactive for NF200 and IB4.
In trigeminal ganglion neuron culture, we show that AITC treatment induced neuronal activation through the increase of intra-cellular calcium concentration in wild-type mice. In male bGH mice the proportion of AITC-activated neurons (+24%) and the intensity of the response (+25%) were higher than in male wild-type mice. In female bGH mice only the proportion of AITC
Conclusions
We show that the overexpression of bGH induces hypersensitivity to mechanical and thermal stimulation associated with sensory neuron sensitization. This mouse model shows some characteristics of pain symptoms in acromegaly which make it a good model to study the pathophysiology of pain associated with acromegaly.
References
1.Ayuk J, Sheppard MC. Growth hormone and its disorders. Postgrad Med J. 2006 Jan;82(963):24-30. doi: 10.1136/pgmj.2005.036087. PMID: 16397076; PMCID: PMC2563724.
2.Dimopoulou C, Athanasoulia AP, Hanisch E, Held S, Sprenger T, Toelle TR, Roemmler-Zehrer J, Schopohl J, Stalla GK, Sievers C. Clinical characteristics of pain in patients with pituitary adenomas. Eur J Endocrinol. 2014 Nov;171(5):581-91. doi: 10.1530/EJE-14-0375. Epub 2014 Aug 12. PMID: 25117460.