Lesions of the pedunculopontine tegmental nucleus increase sucrose consumption but do not affect discrimination or contrast effects.

Abstract

Pedunculopontine tegmental nucleus (PPTg) lesions block place preferences to drugs or food only when animals are nondeprived. PPTg lesions also disrupt operant responding, but lesioned rats cannot discriminate active from inactive levers. It is not clear, therefore, whether PPTg lesions block reward or disrupt the ability to differentiate changes in reward magnitude. These hypotheses were tested by measuring sucrose consumption, choice, and contrast effects after PPTg lesions. Both sham and lesioned rats consumed greater amounts of a sucrose solution as the concentration and level of deprivation were increased. Given a choice between 2 solutions, all rats consumed more of the higher concentration. Both groups exhibited contrast effects when the concentration was shifted from 32% to 4% within a session. Somewhat surprisingly, lesions increased sucrose intake when rats were food-restricted. These results suggest that PPTg lesions do not disrupt primary motivation or the ability to evaluate and respond to changes in reward strength.

Role of the pedunculopontine tegmental nucleus in sensorimotor gating and reward-related behavior in rats

“…It is unlikely that the reduced break point seen after muscimol injection in the PPTg can be attributed to an impaired perception of comparative reward values or a simple motivational deficit because in the food preference test there was no overall treatment effect on food intake and the animals still preferred the casein pellets over crumbled lab chow after muscimol treatment. These findings are consistent with the results from Olmstead and co-workers showing no difference in the consumption of sucrose solutions between PPTg-lesioned rats and shams (Olmstead et al 1999). However, since food preference under free choice conditions without any effort provides only an approximate measure of reward efficacy, we cannot completely rule out a role of the PPTg in incentive motivation under PR conditions.…”

Enhanced consumption of salient solutions following pedunculopontine tegmental lesions

“…After 10 days maintenance on food control (10–12g/rat/day – bodyweight did not fall to below 85% free feeding weight) ibotenic PPTg lesioned rats consumed significantly more 20% sucrose solution than sham controls (figure 9) (F (1,17) = 23.46, p = < 0.001). This deprivation-dependent effect of sucrose overconsumption in ibotenic PPTg lesioned rats has been previously noted (Olmstead et al , 1999). The consumption of 20% sucrose was not significantly different between sham rats maintained on free food and on food control (p > 0.05).…”

“…Previous interpretations of sucrose overconsumption by PPTg lesioned rats are in terms of disrupted behavioral control such as response-perseveration or loss of behavioral organization in conditions of high excitement (Olmstead et al , 1999; Alderson et al , 2001; Keating et al , 2002; Ainge et al , 2006; Winn, 2006). These theories have anatomical and behavioral support.…”

“…However, while causing no reduction in baseline spontaneous locomotion, PPTg lesions alter the sensitized locomotor response to drugs of abuse including amphetamine (Alderson et al , 2003) and nicotine (Alderson et al , 2008), again consistent with altered basal ganglia function. Rats bearing bilateral lesions of all neuronal types within PPTg also consume significantly more of high concentration sucrose solutions (>12% sucrose) than sham controls (Olmstead et al , 1999; Alderson et al , 2001; Ainge et al , 2006). This overconsumption effect is not observed for low concentration (4%) sucrose solution, drinking water or lab chow (Keating et al , 2002).…”

How best to consider the structure and function of the pedunculopontine tegmental nucleus: Evidence from animal studies

“…PPTg lesions also change the intravenous self-administration (IVSA) of drugs of abuse such as nicotine [187,188], heroin [189] and damphetamine [190,191]. The failure to respond for natural rewards in operant tests does not appear to be a failure of basic motivational process-eating and drinking in the home cage are normal [168] and taste perception is unaltered [174]. Rather, the problem appears to be to do with the production of appropriate behavior-a problem of action selection [170] rather than wanting or liking [182].…”