Inf luence of leptin on neurotransmitter overf low from the rat brain in vitro Jacqueline A. Hastings a, * , Glen Wiesner a , Gavin Lambert a , Margaret J. Morris b , Geoff Head a , Murray Esler a a Baker Medical Research Institute, Human Neurotransmitter Laboratory, P.O. Box 6492, Melbourne, Victoria 8008, Australia b Department of Pharmacology, The University of Melbourne, Victoria 3010, Australia Received 13 June 2001; received in revised form 20 September 2001; accepted 10 October 2001 Abstract The 16-kDa polypeptide hormone, leptin along with the neurotransmitters noradrenaline and serotonin (5-HT) have important physiological roles in the regulation of a number of neuroendocrine actions particularly feeding. Leptin receptor mRNA and immunoreactivity has been reported in various brain regions, while recent studies suggest that leptin is released from the human brain. This study investigated the interactions between leptinergic and neurotransmitter systems of the rat brain in vitro. Techniques were established to simultaneously monitor the release of endogenous noradrenaline and its metabolite 3,4 dihydroxyphenylglycol (DHPG), and 5- HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) from the rat brain. The neuromodulatory action of leptin (0.2 and 3 nM) on the overflow of noradrenaline and DHPG from the medulla and hypothalamus was examined. The effect of leptin on 5-HT and 5-HIAA overflow from the hypothalamus was also investigated. Administration of 0.2 and 3 nM leptin significantly increased medullary noradrenaline overflow to 172% and 174% of basal levels, respectively. Leptin had no significant effect on hypothalamic noradrenaline overflow, while leptin perfusion induced a significant increase in 5-HIAA overflow from the hypothalamus. This study lends support to the notion of a complex interaction of the leptinergic and brain neurotransmitters involved in the control of feeding and energy metabolism. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Noradrenaline; Serotonin (5-HT); Dihydroxyphenylglycol (DHPG); 5-Hydroxyindoleacetic acid (5-HIAA) 1. Introduction In 1953, the hypothesis that energy stores in adipose tissue may produce signaling molecules capable of modu- lating energy expenditure and food intake was proposed [1]. The discovery of leptin in 1994 by Zhang et al. [2] has been a major landmark in the field of obesity research. Administration of leptin to ob/ob and normal mice in- duces weight reduction by decreasing food intake and increasing energy expenditure [3–5]. In obese, aged rats, an increase in basal food intake and endogenous leptin mRNA in perirenal white adipose tissue has been reported [6]. In humans, plasma leptin levels are usually propor- tional to the fat mass and are also, therefore, elevated with obesity. Leptin, the ob/ob gene product is largely secreted by mature adipocytes and some believe that it provides feedback to the receptors in the central nervous system that control body weight homeostasis [7]. While obese humans [8] and rats [6] were thought to suffer from ‘‘leptin resis- tance’’, which is also supported by the observation of a reduced cerebrospinal fluid/serum leptin ratio [8], recent observations from our laboratory suggest that leptin is released from the human brain and that the brain is a nonadipose source of leptin [9,10]. Leptin receptor mRNA has been identified in various rat brain regions such as the hypothalamus and cerebellum [11]. Leptin receptor im- munoreactivity has been reported in various regions of the hypothalamus [12]. C-fos-like immunoreactivity studies have shown a link between the magnocellular PVN and the dorsomedial nucleus, which may be involved in the reduction of neuropeptide Y (NPY)-induced feeding by lep- tin [13]. Leptin gene expression has been reported in the rat pituitary gland [14] while leptin immunoreactivity has been observed in the rat cerebral cortex, hypothalamus and cerebellum [14]. 0167-0115/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-0115(01)00332-9 * Corresponding author. Tel.: +61-3-9522-4331; fax: +61-3-9521-1362. E-mail address: Jacqueline.Hastings@baker.edu.au (J.A. Hastings). www.elsevier.com/locate/regpep Regulatory Peptides 103 (2002) 67 – 74