Evidence for early opioid modulation of licking responses to sucrose and Intralipid: a microstructural analysis in the rat

Abstract

The behavioural mechanisms underlying the effects of the opioid antagonist naloxone (0.3-3 mg/kg i.p.), and the opioid agonists morphine (0.3-3 mg/kg s.c.), and U-50, 488H (0.3-3 mg/kg s.c.) on ingestive behaviour were investigated using a microstructural analysis of licking patterns for sucrose solutions and Intralipid (fat emulsions) in a brief contact test. Naloxone dose-dependently decreased the total number of licks and the number of bouts for sucrose and Intralipid, but did not affect mean bout duration. Morphine dose-dependently increased the total number of licks and the number of bouts for both test fluids. For Intralipid but not for sucrose drinking, morphine actually decreased mean bout duration. U-50, 488H significantly affected total licks, although the dose-effect relationship showed an inverted U-shaped function. There was a dose-dependent increase in mean bout duration following administration of U-50, 488H and an increase in bout number, although only the lowest dose differed significantly from the control condition. The results show that microstructural analysis can distinguish between the effects of naloxone, morphine and U-50, 488H on licking behaviour and indicate that selective opioid receptor subtypes may be differentially involved in ingestive processes.

Involvement of Endogenous Enkephalins and β-Endorphin in Feeding and Diet-Induced Obesity

“…Given that the number of bouts of licking initiated are thought to reflect the incentive motivational properties of the stimulus, whereas the length of bouts reflects hedonic properties (see Supplementary Materials for further discussion, D'Aquila, 2010; Davis and Smith, 1988;Higgs and Cooper, 1998), our results suggest endogenous enkephalins contribute to the overall motivational properties of palatable stimuli, independent of their hedonic impact. This finding is interesting for several reasons.…”

“…More importantly, significant main effects of genotype were apparent for total licks (F (1, 26) = 12.48, po0.01, Figure 1a) and bouts of licking (F (1, 26) = 9.11, po0.01, Figure 1b), but not mean bout length (Figure 1c), such that PENK KOs exhibited fewer total licks and licking bouts across the two sucrose concentrations. This general decrease in bout number may reflect a role of endogenous enkephalins in supporting incentive motivational aspects of feeding behavior (see Supplementary Materials for further discussion, D'Aquila, 2010; Davis and Smith, 1988;Higgs and Cooper, 1998). Interestingly, although there was no main genotype effect on bout length, there was a significant interaction between sucrose concentration and genotype for this measure (F (1, 26) = 11.87, po0.01, Figure 1c) because of PENK KOs exhibiting shorter licking bout lengths at 2% sucrose, but longer bouts at 20% sucrose.…”

“…Microstructural analysis of licking behavior, commonly used to parse hedonic and motivational aspects of feeding (D'Aquila, 2010;Davis and Smith, 1988;Frisina and Sclafani, 2002;Higgs and Cooper, 1998; for review see Dwyer, 2012), was conducted in proenkephalin knockout (PENK KO) and β-endorphin-deficient (BEND KO) mice during consumption of sweet solutions, followed by measurement of body weight gain during long-term access to a 'cafeteria diet' (Sampey et al, 2011).…”

The Endomorphin System and Its Evolving Neurophysiological Role

“…They tend to increase food intake, namely that of sucrose (Higgs and Cooper, 1998), other carbohydrates , or fat (Higgs and Cooper, 1998; in rodents exposed to different environmental conditions (Giraudo et al, 1998a,b;Itoh et al, 1998;Leventhal et al, 1998a,b;. The -opioid receptor agonists, which also exhibit orexigenic activity (Morley et al, 1982;Woods and Leibowitz, 1985;Gosnell et al, 1986; THE ENDOMORPHIN SYSTEM AND ITS NEUROPHYSIOLOGICAL ROLE Glass et al, 1999), modulate feeding behavior and gustatory information through -opioid receptors located in the hypothalamus (Pfeiffer and Herz, 1984;Kuhn and Windh, 1989) and NTS (Matsuo et al, 1984;MoufidBellancourt and Velley, 1994).…”

Clozapine increases reward evaluation but not overall ingestive behaviour in rats licking for sucrose

“…the number of licks per bout), is sensitive to the nature and the concentration of the tastants, and might represent a measure of reward evaluation or "hedonic impact". In contrast, the number of bouts is sensitive to stimuli other than the direct contact with the reward, such as post-ingestive cues, and might represent a measure of behavioural activation or motivation (D'Aquila 2010; Higgs and Cooper 1998;Schneider et al 1990). In rats licking for a sucrose solution, dopamine D1-receptor antagonism reduces the bout number without affecting mean bout size (D'Aquila 2010; Liao and Ko 1995), while dopamine D2-like receptor antagonism produces on the within-session bout number time course, an effect resembling extinction in paradigms of instrumental responding for a reward (D'Aquila 2010), and reduces bout size (D'Aquila 2010; Genn et al 2003;Liao and Ko 1995;Schneider et al 1990), mimicking the effect of sucrose dilution (Schneider et al 1990).…”