Early life experience alters behavioral responses to sweet food and accumbal dopamine metabolism

Abstract

Neonatal handling in rats persistently alters behavioral parameters and responses to stress. Such animals eat more sweet food in adult life, without alterations in lab chow ingestion. Here, we show that neonatally handled rats display greater incentive salience to a sweet reward in a runway test; however they are less prone to conditioned place preference and show less positive hedonic reactions to sweet food. When injected with methylphenidate (a dopamine mimetic agent), non-handled rats increase their sweet food ingestion in the fasted state, while neonatally handled rats do not respond. We did not observe any differences regarding baseline general ambulatory activity between the groups. A lower dopamine metabolism in the nucleus accumbens was observed in handled animals, without differences in norepinephrine content. We suggest that early handling leads to a particular response to positive reinforcers such as palatable food, in a very peculiar fashion of higher ingestion but lower hedonic impact, as well as higher incentive salience, but diminished dopaminergic metabolism in the nucleus accumbens.

Enhancing glutamatergic transmission during adolescence reverses early-life stress-induced deficits in the rewarding effects of cocaine in rats

“…The maternalseparation model is a well-validated early-life stress model which produces distinct phenotypes which are hallmarks of many stressrelated disorders (Ladd et al, 2000;Meaney, 2001;Sanchez et al, 2001). However, the effects of maternal separation on reward function is somewhat controversial with several reports detailing either increased (Flagel et al, 2003;Kosten et al, 2000;Matthews et al, 1999) or decreased reward seeking (Martini and Valverde, 2012;Matthews et al, 1999;Silveira et al, 2010). Here we show that our MS protocol produces a robust deficit in reward processes as indicated by a decrease in cocaine intake.…”

“…It should be noted that this involved extensive handling and repeated IP injections over a three week period during adolescence. It has been well documented that various manipulations to rodents during the early-life period can have profound effects on hedonic behaviour in adulthood (Flagel et al, 2003;Kosten et al, 2000Kosten et al, , 2005Kosten et al, , 2006Lynch et al, 2005;Martini and Valverde, 2012;Matthews et al, 1999;Moffett et al, 2006Moffett et al, , 2007Nylander and Roman, 2013;Silveira et al, 2010;Yajie et al, 2005), as such, we conducted a separate experiment where maternally-separated animals were injected with vehicle alone, while another group of maternallyseparated animals received no injections during the adolescent period. We found the maternal-separation induced reduction in cocaine-taking remained intact independent of whether the animals had received adolescent handling and injections or had not (presented as area under the curve for reasons of succinctness).…”

“…However, this highly-plastic neuronal environment makes the brain highly susceptible to pathological insult (Bondi et al, 2014;Enoch, 2011;Meyer-Lindenberg and Tost, 2012;O'Connor and Cryan, 2014) including psychological stress which has been well established as predisposing individuals to addictive disorders later in life (De Bellis, 2002;Enoch, 2011;Koob, 2008). Indeed, preclinical studies have demonstrated psychological stress induced early in life can affect reward-function in adulthood; both increasing rewardinduced behavioural-reinforcement (Flagel et al, 2003;Kosten et al, 2000;Matthews et al, 1999) and decreasing reward intake, possibly through anhedonia (Martini and Valverde, 2012;Matthews et al, 1999;Silveira et al, 2010). Thus, a need to clarify the mechanisms by which exposure to a psychological stress can alter drug-intake in adulthood remains.…”

Intrauterine growth restriction increases the preference for palatable foods and affects sensitivity to food rewards in male and female adult rats

“…For instance, found that animals exposed to postnatal overfeeding did not develop a CPP to sweet foods and had decreased levels of dopamine D2 receptors in the NAcc. Silveira et al (2010) also obtained the puzzling findings of increased palatable food intake accompanied by reduced pTH levels in the nucleus accumbens of female rats at baseline (n ¼ 5À 6) and after exposure to sweet food (n ¼ 5-6). Data expressed as percent of control levels (Adlib/Adlib group).…”

Early life handling decreases serotonin turnover in the nucleus accumbens and affects feeding behavior of adult rats

“…Norepinephrine also plays an important role in the regulation of hunger and satiety (Wellman, 2000), and it is described that handling during early postnatal life influences central noradrenergic transmission (Baamonde et al, 1999(Baamonde et al, , 2002. While associated with sucrose sham-feeding (Hajnal & Norgren, 2004) and palatable food consumption (Fallon et al, 2007), the total amount of norepinephrine in the nucleus accumbens was measured in another study from our group (Silveira et al, 2009) and it was not different between neonatally handled and nonhandled animals in adulthood. Besides that norepinephrine transporter knockout mice have a consumption of sucrose comparable to wild-type mice (Perona et al, 2008).…”