Abstract
The “ship” of the Arabian and North African deserts, the one-humped dromedary camel (Camelus dromedarius) has a remarkable capacity to survive in conditions of extreme heat without needing to drink water. One of the ways that this is achieved is through the actions of the antidiuretic hormone arginine vasopressin (AVP), which is made in a specialised part of the brain called the hypothalamo-neurohypophyseal system (HNS), but exerts its effects at the level of the kidney to provoke water conservation. Interestingly, our electron microscopy studies have shown that the ultrastructure of the dromedary HNS changes according to season, suggesting that in the arid conditions of summer the HNS is in an activated state, in preparation for the likely prospect of water deprivation. Based on our dromedary genome sequence, we have carried out an RNAseq analysis of the dromedary HNS in summer and winter. Amongst the 171 transcripts found to be significantly differentially regulated (>2 fold change, p value <0.05) there is a significant over-representation of neuropeptide encoding genes, including that encoding AVP, the expression of which appeared to increase in summer. Identification of neuropeptides in the HNS and analysis of neuropeptide profiles in extracts from individual camels using mass spectrometry indicates that overall AVP peptide levels decreased in the HNS during summer compared to winter, perhaps due to increased release during periods of dehydration in the dry season.
Highlights
Water balance is aggressively defended in all mammals [1], but this is all the more so in the homeostatic masterpiece that is the dromedary camel, which has a remarkable capacity to thrive in the hot, arid conditions of the Arabian and North African deserts [2,3,4,5,6], and to survive extended periods of dehydration during the summer months [7,8]
Both vasopressinergic (S4A Fig) and oxytocinergic (S4B Fig) magocellular neurones were identified in the dromedary supraoptic nucleus, intermingled throughout the nucleus
The supraoptic nucleus is the biosynthetic core of the hypothalamo-neurohypophyseal system (HNS), the specialized part of the central nervous system devoted to centrally overseeing the brain body-dialogue that ensures the regulation of water balance
Summary
Water balance is aggressively defended in all mammals [1], but this is all the more so in the homeostatic masterpiece that is the dromedary camel, which has a remarkable capacity to thrive in the hot, arid conditions of the Arabian and North African deserts [2,3,4,5,6], and to survive extended periods of dehydration during the summer months [7,8]. It is water economy that is vital for survival in the desert, and, in the dromedary camel, this is achieved by minimal evaporative cooling (camels rarely sweat), low urinary output, water extraction from undigested food residues, and variation in body temperature from 34 ̊C at night up to 42 ̊C during the day. This 8 ̊C variation in body temperature allows a 750 kg camel to store 3.9 kJ of heat energy per kg of body weight for each 1C increase in body temperature, which is dissipated at night [12,13]. This mechanism prevents insensible water loss through the secretion of sweat, and corresponds to a saving up to 5 L of water every day
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