Brain Research, 595 (1992) 291-294
© 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05.00
Effects of glucose and fat antimetabolites on norepinephrine turnover in rat hypothalamus and brainstem * Neil E. R o w l a n d Department of Psychology and Centersfor Nutritional and Neurobiological Sciences, Unicersity of Florida, Gainesrille, FL 32611-2065 (USA) (Accepted 16 June 1992)
Key words: 2-Deoxy-D-glucose; 2,5-Anhydro-D-mannitol; Methyipalmoxirate; Mercaptoacetate; Liver; Hypothalamus; Brainstem; Norepinephrine; Hemispheric asymmetry
Rats were injected acutely with antimetabolites of either glucose (2 deoxy-D-glucose, 2DG), fat (methylpalmoxirate, MP or mercaptoacetate, MAC), or the combination of these agents, in dosages known to stimulate food intake. Norepinephrine (NE) turnover in hypothalamus and brainstem was determined after these treatments by the method of synthesis inhibition. Glucoprivation (2DG) increased NE turnover in hypothalamus, confirming previous studies. Fat antimetabolites alone had no effect on NE turnover, nor did a peripherally-acting fructose antimetabolite. Combination of MP and 2DG, but not MAC and 2DG, produced a greater NE turnover than 2DG alone. These data suggest that peripheral signals of metabolic emergency do not activate brain NE systems, except when these systems are already activated by an ongoing cerebral metabolic emergency. The role of hypothalamic NE in metabolic integration of feeding is discussed, and possible hemispheric differences.
INTRODUCTION The circulating concentrations of metabolic fuels have a prominent role in many theories of the control of feeding behavior 7'9'l°'28. Alterations in the availability and/or utilization of these fuels can influence the activity of the brain both indirectly by acting at receptors in peripheral organs including liver, and directly by acting at cells in the brain that transduce their local metabolismS,9- t~,24. One classical neurotransmitter that has been implicated in feeding behavior is norpinephrine (NE). For example, microinjection of NE into the brain, and optimally into the paraventricular nucleus of the hypothalamus (PVN), induces vigorous feeding in sated rats and other mammals ~4. It has also been shown that decreased cerebral glucose utilization (glucoprivation) after peripheral administration of either insulin (hypoglycemia) or 2-deoxy-D-glucose (2DG; blockade of glycolysis) increases NE turnover in hypothalam u s 2'3'16'19'21'23. The so-inferred synaptic release of NE
may be important for the feeding response because pharmacological inhibition of central NE function blocks the feeding to glucoprivation4'15'25. More recently, it has been shown that inhibitors of fatty acid oxidation such as methylpalmoxirate (MP) and mercaptoacetate (MAC) themselves do not potently stimulate feeding but do potentiate the orexigenic and other effects of 2DG in rodents fed moderate levels of dietary fat 8'!1'!7'22. The oxidation of fatty acids, and thus the inhibition thereof, occurs in peripheral tissues. The peripheral effects of inhibitors such as MP and MAC must be integrated with those of 2DG in the brain to produce the synergistic feeding effect. It is unknown whether brain NE systems are part of this integrative process. In the present experiments we examine whether inhibitors of fatty acid oxidation, either alone or in combination with 2DG, increase NE turnover in rat brain. In addition, the effect of an orexigenic, peripherally-acting fructose anti-metabolite, 2,5-anhydro-omannitol (2,5-ADM) 27 on brain NE turnover is as-
Correspondence: N. Rowland, Dept. Psychology, University of Florida, Gainesville, FL 32611-2065, USA. Fax: (1) (90".,.) 392 7985. * These data were presented, in part, at the April 1991 Annual Meeting of the FASEB, Atlanta, GA, USA.
292 sessed. The dosages of the antimetabolites to be used are in the mid-range of those which yield either additive ~7 or synergistic ~ combination effects on feeding.
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MATERIALS AND METHODS
Animals and housing Male Sprague-Dawley rats, 3-6 months of age, were purchased from Harlan Industries. Prior to use in these experiments, most had served in unrelated behavioral studies that did not involve administration of pharmacologic agents; these rats were randomly reassigned to treatment groups. The rats were housed individually throughout with tap water available and, for at least 1 week prior to the present studies, all were fed Purina Rodent Chow (no. 5001; approx. 10% calories as fat) ad libitum. In Study D, to increase the level of dietary fat to which rats were adapted, Purina Mouse Chow (no. 5008, approx. 25% calories as fat) was used. The vivarium lights were on 06.00-18.00 h, with an ambient temperature of 23+2°C. All injections and procedures were conducted between 08.00 and 14.00 h, when spontaneous food intake is minimal.
| 80 50
STUDYA 1~3AMPT only r ~ 1 ~ + 2 ~
Fig. 1. Mean (_+ S.E.M.) norepinephrine (NE) concentration in the whole hypothalamus of rats of Studies A-C, given AMPT and the antimetabolites indicated (Antifat = MP in Studies A and C, and MAC in Study B) and killed 2 h later. All values differ from the levels ( = 100%) in untreated controls run simultaneously. * P < 0.05, + P