The role of the mediobasal arcuate hypothalamus in relation to opioid systems in the control of ingestive behaviour in the rat.

Bilateral, radiofrequency lesions of the mediobasal arcuate hypothalamus (MBH) strongly depleted levels of immunoreactive (ir)-beta-endorphin (beta-EP) in the hypothalamus and other brain tissues: these changes reflect destruction of those beta-EP-containing perikarya which are located in the MBH. No change in plasma ir-beta-EP was seen. The ir-dynorphin (DYN) content of the hypothalamus was also depressed while that of ir-Met-enkephalin was unaffected. The fall in hypothalamic ir-beta-EP was correlated with the fall in that of ir-DYN. Lesioned rats displayed only a minor, transient reduction in rate of weight gain between days 3 and 9 postsurgery: this disappeared thereafter. Further, the lesion did not affect the pattern of weight loss and regain associated with 24 h food and water deprivation. Indeed, the total 24 h (daily) food intake (FI) and water intake (WI) of lesioned rats did not differ from that of sham animals while deprivation-induced hyperphagia and hyperdipsia was not attenuated by the lesions. Moreover, the ability of naltrexone to decrease FI and WI (during both dark and light phases of the daily cycle) was not altered by the lesions. These observations indicate that central beta-EP may not be essential for the maintenance of a normal 24 h FI and WI and that opioid antagonists do not act upon the MBH or upon central beta-EP neurones in their suppression of FI and WI. Further, they suggest that central beta-EP may not fulfil an essential role in the control of body weight in the rat. Lesioned rats did, however, reveal a shift in the diurnal rhythmicity of FI and WI reflected in a reduction in the dark:light ratios of these. An alteration in the diurnal rhythmicity of sleeping and core temperature, but not locomotor activity, was also seen. The shifts in hypothalamic ir-beta-EP and ir-DYN (but no other tissue levels of any peptide) were correlated with the magnitude of the shifts in diurnal rhythmicity of ingestive behaviour. Moreover, lesions caudal to the MBH (not affecting hypothalamic ir-beta-EP or ir-DYN) or dexamethasone treatment (which affects pituitary pools of ir-beta-EP and ir-DYN) did not modify these rhythms. Thus, in these respects, the effects are 'particular' to MBH lesions modifying hypothalamic ir-beta-EP and ir-DYN. The data suggest that the MBH may play a role in the modulation of the diurnal scheduling of ingestive behaviour in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)