Diurnal variation in plasma ir-beta-endorphin levels and experimental pain thresholds in the horse

Chronobiological characteristics of painful diabetic neuropathy and postherpetic neuralgia: Diurnal pain variation and effects of analgesic therapy

“…Various preclinical studies in laboratory animals have demonstrated diurnal variations in pain sensitivity precipitating numerous hypotheses to account for this phenomenon. For example, endorphins, endogenous analgesic peptides that activate mu opioid receptors, have demonstrated circadian fluctuations (Hamra et al, 1993;Kerdeldhue et al, 1983) and, also, hyperalgesia induced by naloxone, a nonselective opioid receptor antagonist, was shown to follow a diurnal rhythm (Frederickson et al, 1977). Preclinical data have suggested a pro-nociceptive role for the pineal hormone, melatonin (Perissin et al, 2004), which exhibits a circadian rhythm such that secretion starts to increase at 21:00 with peak levels occurring at 03:00 (Kennaway and Voultios, 1998), although John et al (1994) reported analgesic effects of melatonin and its ability to restore pain threshold variations in pinealectomized rats.…”

Tail docking in horses: a review of the issues

“…Pain in foals is a subject in current development, as most pain measurements have been carried out on adults, using behavioural (Price et al, 2003) and physiological parameters (Hamra et al, 1993;McCarthy et al, 1993); the same for stress measurements (McGreevy and Nicol, 1998). In a similar way, pain in human newborns was ignored for a long time.…”

The role of beta-endorphin in horses: a review

“…The highest values of opioid were noted in morning blood samples (peak level at 9.00) (Hamra et al, 1993). Different results were obtained by Mehl et al (1999), who did not note statistically significant differences in the levels of beta-endorphin examined in mares every two hours between 8 a.m. and 8 p.m. Owing to the inconsistency of research results so far, the data published by Pell and McGreevy (1999) concerning the activity of beta-endorphin in horses with stereotypy, and the results of the study on foals by Fazio et al (2009) should be interpreted with caution, taking into consideration such factors as seasonality.…”

“…They also exhibit pain relieving properties and regulate pain threshold in horses, similarly to the way they do in people (Hamra et al, 1993). The pain relieving function of opioid receptors evolved as an element of the organismal defence system against stress.…”