Our findings provide evidence that behavioral effects of cocaine are affected by dietary energy intake. ADF produced changes in baseline locomotion indicative of enhanced mesolimbic dopamine function. Although the acute rewarding effects of cocaine were not altered by ADF in the CPP paradigm, there was an enhancement of locomotion in the second conditioning session, indicative of enhanced context-dependent sensitization. The reason that we did not see behavioral sensitization to repeated cocaine injection is likely due to the occurrence of stereotypy. Repeated high doses of cocaine injection will induce locomotor sensitization initially, followed by stereotypy sensitization and reduced locomotor activity. At the same time, other consequences of cocaine, which might be considered more deleterious, were mitigated by ADF, and associated with changes in dopamine metabolism in the OB. In a previous study, rats were maintained for 10 days on a limited daily feeding caloric restriction diet, and their locomotor response to acute cocaine administration increased compared to rats on a non-restricted diet (Stamp et al. 2008
). Taken together with our data, these findings suggest that different patterns and levels of dietary energy restriction can differentially affect behavioral responses to cocaine The olfactory system plays a pivotal role in the rewarding effects of foods (Ferriday and Brunstrom 2008; Martin et al. 2009
) and increasing evidence suggests important roles of the OB, as a component of the mesolimbic dopaminergic system, in cocaine addiction (Sellings et al. 2006
; Ikemoto 2007
; Volkow et al. 2008
). For example, it was reported that dopamine signaling in the mesolimbic system is essential for hyperphagia in mouse models of obesity and type 2 diabetes (Szczypka et al. 2000
). Our findings therefore suggest that dietary energy intake can influence the vulnerability of mesolimbic system components to the damaging effects of cocaine.
Although the specific mechanism by which ADF abrogates cellular and molecular effects of cocaine in the OB remains to be established, previous findings and data in the present study suggest that ADF increases the resistance of neurons to a range of insults including some adverse effects of drugs of abuse. We previously reported that ADF can protect neurons against a range of insults including excitotoxins (Bruce-Keller et al. 1999
), dopaminergic toxins (Duan and Mattson 1999
), ischemic stroke (Xu and Mattson 1999) and pathogenic mutations in genes that cause inherited forms of Alzheimer’s and Huntington’s diseases (Halagappa et al. 2007; Duan et al. 2003
). One of the general mechanisms responsible for the neuroprotective effects of ADF involves increased levels (or prevention of depletion) of neurotrophic factors that include BDNF and bFGF and of stress resistance proteins that include HSP-70, GRP-78 and HO-1 (Wan et al. 2003
; Arumugam et al. 2009). ADF also suppresses the production of inflammatory cytokines that include TNF-α, IL-6 and IL-1β (Arumugam et al. 2009). Cocaine treatment reduces BDNF protein levels in the prefrontal cortex (Fumagalli et al. 2007
) and causes a biphasic effect on stress resistance proteins in the hippocampus, with an early (5 h) increase followed by a delayed (24 h) suppression (Hayase et al. 2003
). Cocaine has also been reported to induce inflammatory responses in neurons (Dey and Snow 2007
) and neural progenitor cells (Crawford et al. 2006
). Conceivably, each of these mechanisms might be involved in the influences of dietary restriction on stimulant toxicities.
Interestingly, we found that neither acute or chronic cocaine treatment affected levels of dopamine or its metabolites in the OB. Previous studies have shown that cocaine alters dopamine metabolism in some brain regions, such as the nucleus accumbens and amygdala, but with little or no effects on dopamine metabolism in other brain regions (Kalivas et al., 1988
; Hadfield and Milio, 1992
; Alburges and Wamsley, 1993
). However, because we did not directly measure dopamine release or reuptake, we cannot rule out an effect of cocaine on dopaminergic neurotransmission in the OB. It would therefore be informative to determine the effects of dopamine receptor antagonists and agonists on OB neurogenesis and gene expression.
Throughout adult life, OB interneurons die and are replaced, although the turnover rate of individual OB interneurons appears to be quite variable and can be influenced by sensory experience (Mouret et al., 2009
). Newly generated neurons are produced by stem cells located in the subventricular zone of the forebrain and then migrate through the rostral migratory stream to reach the OB (Gheusi et al. 2000
; Havrda et al. 2008
). We labeled forebrain stem cells by administering BrdU to the mice and then waited 48 h, a time period sufficient to allow migration of newly generated cells into the OB. We found that acute and chronic cocaine treatment reduced numbers of BrdU-labeled cells in the OB. Chronic cocaine had a greater effect than acute cocaine. ADF largely prevented the inhibitory effect of cocaine on the production of new OB cells. Previous work has shown that hippocampal neurogenesis is reduced in cocaine-treated animals (Yamaguchi et al. 2004
; Andersen et al. 2007
). This reduced hippocampal neurogenesis can lead to increased cocaine self-administration (Noonan et al., 2010
). In the present study, we show that cocaine suppresses the production of new OB cells and that ADF blocks this adverse effect of cocaine on OB plasticity.
Impaired OB neurogenesis may contribute to the impaired olfactory perception that has been documented in cocaine addicts (Gordon et al. 1990
; Bauer and Mott 1996
). Because OB neurogenesis was apparently maintained in cocaine-treated mice that had been maintained on an ADF diet, replacement of OB interneurons may be an important mechanism to protect against the adverse effects of cocaine. It has been shown that neurogenesis is required for normal functioning of the olfactory neural circuits and for associated olfactory behaviors (Breton-Provencher et al., 2009
). The reduction in newly-generated OB cells caused by cocaine administration suggests a role for impaired neurogenesis in the olfactory deficits of human cocaine addicts (Gordon et al. 1990
; Bauer and Mott 1996
). The ability of ADF to prevent the cocaine-induced reduction in numbers of newly-generated cells in the OB suggests the possibility that dietary energy restriction might mitigate the adverse effects of cocaine on olfactory perception in cocaine addicts. In this regard, it will be important to determine whether dietary energy restriction begun after cocaine administration can restore OB structure and function. Although weight loss often occurs in cocaine addicts (Quach et al. 2008
), the roles of reduced energy intake in modifying the impaired production of new OB cells caused by cocaine are unclear. Interestingly, olfactory bulb lesions can result in altered levels of anxiety and aggression (Mucignat-Caretta et al. 2004
), suggesting a potential role for impaired olfactory plasticity in these behaviors which are common in cocaine addicts.
The molecular mechanisms by which cocaine impairs and ADF maintains OB neuroplasticity remain to be established. Some of the data summarized above indicate that ADF produces rather generalized improvements in cellular and/or tissue responses to stress, but this does not necessarily mean that this explanation is complete. In particular, the causal changes are likely to involve differential changes in different parts of the limbic circuitry underlying the behavioral effects of cocaine, just as trait activity, in terms of locomotor response in a novel environment, likely involves HPA axis feedback to the hippocampus, producing further changes in other parts of the limbic system (Lemaire et al., 1999
). A number of stress paradigms have been shown to affect hippocampal neurogenesis (Lee et al., 2006
; Bergstrom et al., 2007
; Mineur et al., 2007
). However, many consequences of stressful experiences vary based on the chronicity and predictability of stress. Predictable chronic mild stress, which may be most similar to ADF, increases hippocampal neurogenesis (Lee et al., 2002
; Parihar et al., 2009
). However, whether stressors affect olfactory neurogenesis in a manner similar to hippocampal neurogenesis remains to be determined. Our gene array data suggest involvement of several metabolic and signaling pathways. In mice fed ad libitum
, cocaine increases the expression of multiple genes involved in mitochondrial energy metabolism including NADH dehydrogenase, succinate dehydrogenase and ATP synthase. These changes suggest that cocaine induces an altered metabolic state. Consistent with this notion, previous brain imaging studies have shown that cocaine administration reduces glucose utilization in the striatum and mesolimbic systems in monkeys and human subjects (Baxter et al. 1988
; Lyons et al. 1996
). Genes involved in GABAergic neurotransmission (GABA A receptor and glutamic acid decarboxylase) and membrane hyperpolarization (potassium channels) were up-regulated by both acute and chronic cocaine treatment, consistent with increased inhibitory tone in the OB.
An additional alteration induced by both acute and chronic cocaine administration was the up-regulation of genes involved in membrane trafficking (clathrin, clathrin-interacting protein 1, and phosphatidylinositol clathrin binding assembly protein). The latter changes may be associated with altered trafficking of the dopamine transporter that can occur following treatment with addictive drugs (Saunders et al. 2000
). We found that numerous genes encoding olfactory receptors were down-regulated in response to cocaine treatment, suggesting an additional mechanism whereby cocaine impairs olfactory perception. However, because cocaine was the only drug of abuse examined in this study, we do not know whether the mechanisms underlying the effects of cocaine on the OB were specific for cocaine or were instead due to less specific cytotoxic effects of this drug. Nevertheless, a remarkable result of our gene array analysis was that ADF completely prevented the effects of cocaine on OB gene expression, suggesting that ADF interrupts the damaging effects of cocaine at a very early step in the process. These findings, particularly in combination with the reversal of the deleterious effects of cocaine on OB neurogenesis, suggest that dietary restriction may reduce or ameliorate some of the adverse consequences of cocaine abuse.