This study shows that wheel running and sucrose consumption cause significant adaptations of cannabinoid CB1 receptor-mediated control of GABA transmission in the striatum and provide an emotional reserve against the effects of stress. A similar synaptic effect was achieved in animals chronically exposed to cocaine (Centonze et al, 2007a
), suggesting that changes in the sensitivity of striatal cannabinoid receptors represent a common synaptic correlate of the activation of the central reward pathway.
The synaptic responses to the selective cannabinoid CB1 receptor agonist HU210 were normal after a single day of exposure to running wheel or to sucrose, but remarkably potentiated after 7 days of treatments, indicating that cannabinoid receptors progressively adapt during these environmental manipulations. Furthermore, the synaptic adaptations caused by both exercise and sucrose exposure were slowly reversible after the discontinuation of the treatments, indicating that they were not irreversible.
Running wheel or sucrose selectively altered the sensitivity of cannabinoid receptors controlling GABA transmission in the striatum, whereas the sensitivity of glutamate synapses to CB1 receptor stimulation was unaltered, indicating the existence of differential regulation mechanisms of distinct cannabinoid receptors. The effects of cannabinoid CB1 receptor stimulation on sensitized striatal GABA synapses were mediated by a presynaptic mechanism in mice exposed to running wheel or to sucrose, as HU210 selectively reduced sIPSC and mIPSC frequency without altering their amplitudes, and depressed eIPSCs by enhancing PPR. These findings are compatible with the preferential location of CB1 receptors on GABAergic nerve terminals described in the striatum (Narushima et al, 2007
; Uchigashima et al, 2007
). Our data also show that the tested environmental manipulations do not result in a widespread dysregulation of the presynaptic control of striatal GABA synapses, as they did not alter the sensitivity of sIPSCs to the stimulation of other G-alpha-i/o-coupled receptors, such as GABAB receptors.
We have also shown that running wheel or sucrose altered the synaptic effects not only of the exogenous cannabinoid HU210 but also of endocannabinoids mobilized in the striatum in response to mGlu 5 receptor stimulation. This finding suggests that these alterations of cannabinoid transmission may have relevant synaptic consequences during the physiological activity of the striatum, which is mainly driven by glutamate inputs originating from the cortex and the thalamus (Wilson and Kawaguchi, 1996
; Stern et al, 1998
). It should be noted that the normal sensitivity of glutamate synapses to the stimulation of cannabinoid CB1 receptors supports the concept that glutamatergic drive to the striatum is intact after running wheel or sucrose drinking, and that the resulting mGlu 5 receptor activation may selectively impact GABA transmission because of the sensitized CB1 receptors controlling transmitter release.
The source of GABA inputs to striatal neurons is dual. Accordingly, striatal GABAergic principal neurons, besides inhibiting basal ganglia output nuclei, form functional synapses through their recurrent axon collaterals, establishing a feedback control over striatal neuron activity (Tunstall et al, 2002
; Guzman et al, 2003
; Koos et al, 2004
; Gustafson et al, 2006
). Inputs from GABAergic interneurons are another important source of synaptic inhibition of projection neurons, giving rise to a feed-forward inhibitory pathway that is independent of striatal output (Plenz 2003
; Tepper et al, 2004
; Gustafson et al, 2006
). Both the feedback and the feed-forward intrastriatal GABAergic pathways are likely modulated by cannabinoid CB1 receptors and undergo adaptations following wheel running or sucrose consumption. In line with this conclusion, CB1 receptors are expressed at very high concentrations on both axon collaterals of striatal projection neurons and on GABA interneurons (Herkenham et al, 1991
; Hohmann and Herkenham, 2000
Our previous report indicated that hypersensitivity of striatal GABA synapses to HU210 tightly correlated with the rewarding properties of cocaine, as this synaptic alteration appeared only when a CPP was induced. A single cocaine exposure that failed to induced CPP, conversely, also failed to sensitize striatal cannabinoid receptors (Centonze et al, 2007a
). It can be noted that, the CPP procedure selectively examines the positive reinforcing properties of addictive compounds (Acquas and Di Chiara, 1994
; Tzschentke, 1998
; Le Foll and Goldberg, 2005
), and previous behavioral studies indicated that the (endo)cannabinoid system, rather than contributing to the hedonic or psychomotor effects of cocaine consumption, is involved in the generation and maintenance of reward-based addictive behavior (De Vries and Schoffelmeer, 2005
; Le Foll and Goldberg, 2005
; Parolaro et al, 2005
). Here, we have observed that voluntary wheel running and sucrose consumption share with chronic cocaine the common neurobiological background of increasing the sensitivity of cannabinoid receptors modulating GABA transmission in the striatum (Centonze et al, 2007a
). It is conceivable, therefore, that this alteration might represent a synaptic correlate of reward-based behavior. Evidence of the rewarding and potentially addictive properties of running wheel is currently accumulating. Rodents, in fact, are highly motivated to gain access to running wheels and display CPP to an environment associated with wheel running (Iversen, 1993
; Belke, 1997
; Lett et al, 2000
; de Visser et al, 2007
). Furthermore, animals that run long distances daily show withdrawal signs when access to the running wheels is denied (Hoffmann et al, 1987
), and display increased vulnerability for cocaine self-administration and reinstatement after abstinence (Larson and Carroll, 2005
). On the other hand, intense rewarding properties of sucrose have also been reported (Lenoir et al, 2007
), and similar activation of the central reward pathway has been found after sweet tasting (Mark et al, 1991
; Hajnal et al, 2004
) and drugs of abuse (Di Chiara and Imperato, 1988
; Pontieri et al, 1996
). Furthermore, some behavioral and neurochemical signs of opiate withdrawal can be precipitated by naloxone in rats with sugar overconsumption (Colantuoni et al, 2004
Wheel running, however, also has a strong motor activating effect in mice, and the relationship between this experimental procedure and the mechanisms of reward is unclear, especially when animals are housed in groups, as in the case of this study. Similarly, at least in part, the effects of sucrose on cannabinoid CB1 receptors might also be unrelated to its rewarding properties, and reflect, for example, increased caloric assumption. However, we found striking similarities between the neurophysiological and behavioral effects of running wheel and of sucrose, which has reinforcing properties but not motor activating effects. These findings suggest, therefore, that the alteration of cannabinoid CB1 receptor function here described likely reflect the emotional effects of the two experimental paradigms, as also indicated by the evidence that cannabinoid CB1 receptor blockade abolishes the protective action of both running wheel and of sucrose on stress-induced behavior.
The DA system might have an important role in the sensitization of cannabinoid CB1 receptors after exercise or sucrose consumption, as both manipulations activate the DA system in the striatum (Mark et al, 1991
; Hajnal et al, 2004
; El Rawas et al, 2009
), and stimulation of D2-like receptors increase endocannabinoid levels (Giuffrida et al, 1999
; Beltramo et al, 2000
; Centonze et al, 2004
), and upregulates the expression of CB1 receptors in the striatum (Centonze et al, 2004
). Furthermore, endocannabinoids act as downstream effectors of D2-like receptor signaling in the inhibition of GABA transmission (Centonze et al, 2004
), and in the generation of corticostriatal long-term depression, a form of synaptic plasticity believed to underlie specific aspects of reward-based learning and psychostimulant addiction (Gerdeman et al, 2002
The commonalities among running wheel, sucrose consumption, and cocaine exposure might suggest that natural and drug-induced rewards trigger a common synaptic adaptation involving cannabinoid CB1-mediated transmission in the striatum. The importance of this adaptation in the behavioral consequences of the activation of the central reward pathway is evident when considering that exercise activates the endocannabinoid system also in humans (Sparling et al, 2003
), and that both the motivation for sweet food (Simiand et al, 1998
; Cota et al, 2003
; Ward and Dykstra, 2005
; Mahler et al, 2007
) and the rewarding properties of cocaine (Chaperon et al, 1998
) are significantly attenuated after the blockade of cannabinoid CB1 receptors.
Finally, our present results also show that voluntary exercise and sucrose consumption exert protective effects against the motor behavior induced by stress, and that the sensitization of cannabinoid CB1-mediated synaptic responses in the striatum are likely involved in these effects. Consistently, previous study proposed an association between stress-induced emotional alterations and the downregulation of striatal cannabinoid CB1 receptor-mediated transmission (Rossi et al, 2008
), and pharmacological blockade of CB1 receptors abolished the protective effects of both running wheel and sucrose drinking against stress (this study).