Galanin is a 30 amino acid peptide with widespread distribution in the CNS . Three distinct galanin receptor subtypes, GalR1, GalR2 and GalR3, have been described at the hypothalamic level , in neurons also expressing leptin receptor . Both galanin and galanin receptor have also been demonstrated in adipose tissue in rats .
The intracerebroventricular administration of galanin induces food intake . This effect has been suggested to be mediated by modulation of leptin expression and levels [51, 54]. Some studies affirm that galanin increases, in particular, fat ingestion . Galanin administration inhibits leptin levels and its expression in adipose tissue increases during fasting . However, the chronic administration of galanin does not induce hyperphagic behaviour or weight gain , in agreement with the observation that galanin knockout mice or galanin over-expressing mice maintain normal weight . In all, this peptide seems to be much more involved in the short-term regulation of feeding rather than in long-term metabolic balance [51,54].
Galanin-like peptide (GALP) is a 60 amino acid molecule recently isolated and structurally related to galanin . It binds GalR2 and, with lower affinity, GalR1 [51,57].
Similarly to POMC and unlike galanin, the central expression of GALP is negatively regulated by fasting and is reduced in transgenic models of leptin deficiency [51, 58]. Intracerebroventricular GALP administration reduces energy intake and increases energy expenditure in mice .
Moreover, GALP stimulates LH secretion in rodents, suggesting an involvement together with leptin in the functional relationship between energy metabolism and the reproductive system [51,59].
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