The ingestion of sucrose has been reported to produce analgesia in newly born rat [1], human [2,3], and adult rat models [4]. Sucrose ingestion in rat pups [1] immediately increases paw lift latency to thermal noxious stimulus, and in adult rats it decreases nociceptive behavioral rating to tonic formalin stimulus after 12 h [5]. In newborn infants, it reduces crying time, attenuates the increase in heart rate and blood pressure, and improves oxygen saturation, indicating analgesia [2,3,6]. This analgesic effect is clinically utilized to carry out minor surgical interventions such as immunization, circumcision, and venesection in newborn infants with no other anesthetic supplementation [2,3,6,7]. The analgesic action is possibly not secondary to distraction, but related to the hedonic qualities of sucrose [8]. The ingestion of palatable sweet substances such as candy is reported to decrease the levels of hypothalamic -endorphin and opioid receptor binding probably because of increased utilization [9]. Moreover, the involvement of opioid peptide is further supported by abolition of the sucrose-fed analgesia in naloxone pretreated infants [2].Medial hypothalamus is involved in the modulation of pain, since its lesion produces hyperalgesia [10,11] and stimulation produces analgesia [12,13]; its neurons have opioid receptors [14][15][16]. These neurons also respond to iontophoretic application of glucose. These glucose-responsive neurons respond to both the peripheral noxious stimulation and micro iontophoretic application of opioid peptides by attenuating their firing rate [14][15][16]. There are therefore indirect indications of a link between opioid, analgesia, and food ingestion. It is not known whether the sucrose-fed analgesia is also neurally mediated. Similarly, the VMH also governs the preference for palatable food and is also influenced by palatable food [9]. The VMH therefore exerts a significant effect in pain modulation and carbohydrate preference, although its Japanese Journal of Physiology Vol. 51, No. 1, 2001 63Japanese Journal of Physiology, 51, 63-69, 2001 Key words: sucrose-fed analgesia, formalin pain, tonic pain, VMH.
Abstract:The ingestion of sucrose (ad libitum) produces an immediate analgesic response to phasic noxious stimuli. The underlying mechanism for the analgesic effect of sucrose is attributed to its palatability, which mediates analgesia probably by the release of -endorphin in the hypothalamus. The present study was designed to explore the role of ventromedial hypothalamus in the mediation of sucrose-fed analgesia. Adult male albino rats each received (20%) sucrose solution orally through a separate bottle until they had ingested 4-5 ml. Their behavioral responses to tonic noxious stimulus in a formalin test were studied in pre-and postsucrose-fed rats of control and in the VMH lesion groups . The average pain rating of a 60-min session significantly ( pϽ0.01) decreased after sucrose feeding in control rats, from 1.94Ϯ0.13 to 1.45Ϯ0.14, but sucrose feeding by the VMH lesion rat...
