Opioid modulation of ingestive behaviors in the spiny mouse (Acomys cahirinus)

The effect of naloxone on operant behavior for food reinforcers in DBA/2 mice

Mice are powerful models to investigate the genetic basis of food reward because many spontaneous obesity mutants exist and the murine genome is accessible to selectively targeted manipulations. Experiments in rats have shown that opioid receptor blockade reduces operant responding to food reinforcers. The present study investigated whether DBA/2J mice would display similar behavior in response to an opioid antagonist. Twelve male DBA/2J mice were trained to lever press for food reinforcers and subsequently randomized in a within subjects design for no injection, saline injection, or 10 mg/kg naloxone injection intraperitoneal (i.p.) 20 min before each daily trial under ad lib or food-deprived conditions. A significant main effect of injection occurred to reduce lever pressing by the mice. However, a greater pharmacological effect of naloxone occurred compared with saline on the operant responding only under the food-deprived conditions. Interestingly, the percentage of dispensed food pellets actually consumed was significantly reduced after naloxone injection compared with saline injection for either chow-based or sucrose pellets under ad lib or deprived feeding conditions. These data suggest that opioids specifically influence consumatory behavior in mice, but our findings on instrumental behavior were confounded by an independent inhibitory effect of an i.p. saline injection.

Drinking behavior in the spiny mouse (Acomys cahirinus) following putative dipsogenic challenges

Male spiny mice (Acomys cahirinus) were challenged with several putative dipsogenic stimulus conditions: hypertonic sodium chloride (NaCl), 24-h water deprivation, d,l-isoproterenol HCl, angiotensin II (AII) and polyethylene glycol (PEG), or control conditions, in within-subjects designs. Water intake and drinking pattern were monitored electronically in the home cage over a 2--6-h test period without food present, during the light portion of the L/D cycle. In addition, hematocrits were measured following several treatments and mean arterial blood pressure was monitored in response to several doses of AII. As expected, both water deprivation and hypertonic NaCl led to robust drinking with short latencies. PEG was also an effective dipsogen; while quite variable, latencies were often shorter than are typically reported for the rat. Isoproterenol induced a modest, but significant, dose-related drinking. Interference by AII's prominent pressor action might account, at least in part, for its relative ineffectiveness as a dipsogen. Comparisons are made with other rodent species similarly challenged.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Effect of insulin and 2-deoxy-D-glucose on feeding and plasma glucose levels in the spiny mouse

Adult male spiny mice (Acomys cahirinus) were challenged with 2-deoxy-D-glucose (2-DG) or regular insulin, and food intake or plasma glucose concentration was measured. Mice did not increase their food intake over baseline levels following treatment with 2-DG (62.5-1000 mg/kg). In contrast, regular insulin injections (1-50 U/kg) stimulated a modest, but significant increase in feeding, which was apparent within 2 hr at a low dose of 1 U/kg. However, a marked hyper- and hypoglycemia (compared to saline controls), respectively, were induced within 30 min by 2-DG (250 and 500 mg/kg) and regular insulin (1 and 3 U/kg). Reduced glucose levels may not account for the insulin-induced hyperphagia.