Lauro Alejandro de Santiago and Dr. Gregory N. Ervin, Psychology
The effects of injections of some compounds on feeding behavior have been studied thoroughly in rats. However, the simultaneous pattern of feeding and drinking after the injection of either anorexic (feeding suppressor) or orexigenic (feeding inducer) compounds have been observed only infrequently or not at all. Only the feeding effects of a few compounds, like Neuropeptide Y (an orexigenic compound), have been studied in conjunction with drinking behavior. Observing the patterns of drinking and feeding behavior is important because it can provide valuable neuropharmacological and psychobiological information about the mechanisms that mediate the anorexic and orexigenic effects of these compounds. Also, the difference in sensitivity of feeding and drinking in different rat strains has not been studied. The objective of the research was to observe simultaneously the feeding and drinking effects of Melanin-Concentrating Hormone (MCH), a ligand of the endogenous neuropeptide galanin, in two different rat strains.
The simultaneous measurement of feeding and drinking in rats was made possible by an apparatus called an “Eatometer/Drinkometer.” For days at a time, rats can live in this operant chamber that is equipped with an automated 45 mg pellet dispenser; it delivers a pellet when a pellet is removed from a feeding well, and is connected to a computer to keep precise records of the temporal distribution of these pellets. (Rats only remove these pellets when they eat a meal. A 45-mg pellet is what one could call “bite size” for a rat, so this record of dispensed pellets essentially portrays the feeding of a rat “bite by bite.”) The operant chamber is also equipped with a drinkometer. Right in front of the spout of a water bottle is a photocell beam that the rat breaks when it drinks. The computer keeps records of this beam breakage in front of the water spout, and essentially portrays the drinking of a rat “lick by lick.” Once the computer has recorded these patterns of feeding and drinking, it can generate a figure that shows the patterns over time for both feeding and drinking. In normal rats, we know that each rat shows a relatively stable pattern of eating (with two-thirds of its intake of anywhere from eight to fifteen discrete meals during the dark, and about 70% of its drinking during these meals) (Ervin et al., 1997; Le Magnen and Devos; 1980). We used two eatometers/drinkometers with computers on the 12th floor of the Kimball Tower with the permission of Dr. Gregory Ervin.
Male Wistars rats from Charles River Co. served as subjects in the experiments. All rats were singly-housed in hanging, stainless steel mesh cages in temperature (23EC) controlled rat colony rooms located in the 12th floor of the Kimball Tower. The rat colony rooms were maintained on a 12-hr light/dark cycle (lights on from 0630 to 1830 hr). Rats had unlimited access to water and food. Rats were implanted with a third ventricular (ICV) guide cannula (tube). The guide cannula was fitted with an obturator (cap), to prevent infection or dirty substances from entering. During testing, drugs or the vehicle alone (3 or 5 FL 0.9% NaCl) were injected through a stainless steel injection cannula that entered the third ventricle of the brain. Two days prior to the test, rats were placed in the “eatometer/drinkometer” to acclimate them and obtain records of their patterns of ad libitum feeding and drinking. Following compound injections, the patterns of feeding and drinking were graphed for periods of both 4 hr and 24 hr.
Fifteen male CD rats were used in the experiment. Out of the fifteen rats, ten learned how to remove the pellet from the dispenser of the eatometer/drinkometer, four of them did not learn how to remove it and one died for natural reasons. From the ten rats that learned how to use the dispenser, seven were injected with 1 nmol of MCH while after the injection the computer kept records of their drinking and feeding patterns. According to the records that the computer kept we observed that two rats ate a full meal in the next thirty minutes from the injection of MCH, while other two ate and drank water simultaneously (Figure 1). Also, three rats out of the seven injected did not eat or drink during the next half hour of having been injected (Figure 2).
According to the results we could observed that MCH acted more as an orexigenic compound rather than an anorexic compound. It was also observed that MCH could trigger drinking behavior in conjunction with feeding behavior, which is very peculiar of this drug previous findings. MCH was stated as a feeding and drinking suppressant, contrary to the conclusion of the present experiment. However, a previous chronic experiment with MCH performed by Dr. Gregory Ervin, showed that it behaved as an orexigenic compound.
References
- Ervin, G. N. , Dunn, A. P. , Talbot, C. A. , Brammer, S. A. , Schofield, A. , Branchek, T. A. , Noble, S. A. , & Heydorn, W. E. (1997) Unique features of feeding induced by intraventriuclar galanin 1-29 (GAL) in the rat, Annual Meeting of the Society for Neuroscience. New Orleans, LA.
- Le Magnen, J. , and Devos, M. (1980). Parameters of the meal pattern in rats. Neuroscience Biobehavioral Rev. 4(1), 1-11.
- Corwin, R. L. , Robinson, J. K. , & Crawley, J. N. (1993). Galanin antagonists block galanininduced in the hypothalamus and amygdala of the rat. European Journal of Neuroscience 5, 528-1533.
