Establishment of a non-human primate model for menopausal hot flushes

Abstract

Menopause affects the quality of life of millions of women. With modern lifespan the postmenopausal attenuation of circulating estrogen levels can negatively impact a women's life for 30-40 years. The major hypoestrogenic consequence is hot flushes but decline in cognitive function, sleep disorders, depression/anxiety, cardiovascular disease, and osteoporosis are also characteristic for the menopause. Current treatments of hot flushes include estrogen therapy alone or in combination with progestins, soy products, and serotonin and norepinephrine reuptake inhibitors. However, with the exception of estrogens, none of these have satisfactory efficacy. But estrogens come with the unwanted side effects in the periphery, including stimulation of the uterus and breast leading to elevated cancer risk. Therefore, a tremendous effort has been devoted to developing safer therapies and the research has utilized classic rodent models of hot flush with considerable limitations. As hot flushes are primate-specific symptoms, the development of a non-invasive primate hot flush model would have a tremendous impact on drug development. Therefore, our aim was to develop such a non-human primate (NHP) model a hot flush that both recapitulates flushes women experience and is minimally invasive. We investigated if recent developments in thermal imaging have made it possible to accurately monitor skin temperature via camera imaging. In this study, the skin temperature of an ovariectomized rhesus monkey was measured continuously with an infrared camera in a freely moving animal over long time period. Following mapping skin temperatures of several areas of the neck and face we found that the nose of the monkeys showed that largest changes in skin temperature. In the ovariectomized monkey the temperature of the skin on the nose shows up to 9 °C elevations representing hot flushes. In the untreated monkey, hot flushes occurred more frequently in late afternoon/early evening hours than in the morning and last for several minutes. We observed 58 flushes in the 64 evenings of observation. The average number of hot flushes was 0.51 per evening. Oral administration of biotin (niacin) for seven days exaggerated the number of hot flushes to 2.43 per evening. Oral treatment with estradiol benzoate prevented hot flushes and only 2 flushes were detected in the 12 evenings after treatment, averaging 0.17 per evening. The development of this NHP model of hot flush provides great hope for utilizing it for future drug development and mechanistic studied.

Keywords: estrogen; hot flash; hot flush; infrared imaging; menopause; non-human primate.

Figure 1.

This figure shows the nose temperature (°C) over time on one evening (5:00 P.M. to 8:45 P.M.) prior to the monkey falling asleep.

Figure 2.

Thermal images of a freely moving rhesus monkey taken with and ICI 8640 Medical Infrared Camera. The face of the monkey can be seen between the horizontal and vertical bars of the cage. Before the initiation of estrogen treatment, the temperature of the nose of the ovariectomized monkey is 26.9 °C during a period when she does not flush (black color). During a flush, the nose temperature increases to 34.2 °C (white color). Following estrogen treatment, the monkey does not flush, i.e., her nose temperature goes back to a lower value (25.8 °C).