Nigrostriatal dopaminergic neurons exert a regulatory action on different effector neurons within the basal ganglia thus influencing the control of movement. Although these neurons do not contain CB 1 receptors, they are significantly affected by either the activation or the blockade of the endocannabinoid system, leading to important changes in the motor activity (Fernández‐Ruiz, 2009 ; Fernández‐Ruiz et al ., 2010 ). It is generally accepted that these effects are exerted through CB 1 receptors located in other neuronal subpopulations (i.e. GABAergic, glutamatergic and opioidergic neurons). These neurons are located in the close vicinity of, and connected with, dopaminergic neurons (see van der Stelt and Di Marzo, 2003 ). It is also important to note that these midbrain dopaminergic neurons, although lacking CB 1 receptors, may produce and release endocannabinoid ligands from their somas and dendrites (as shown for the midbrain dopaminergic neurons located in the ventral‐tegmental area; Melis et al ., 2004 ; Riegel and Lupica, 2004 ), thus facilitating the retrograde signalling function of these transmitters and CB 1 receptors in excitatory and inhibitory synapses (reviewed in Seutin, 2005 ). Lastly, even though most of the cannabinoid effects on dopaminergic transmission are indirect and exerted through GABA‐ and/or glutamate‐containing neurons, there are some recent studies that propose additional or alternative mechanisms that involve a closer relationship between the endocannabinoid and the dopaminergic systems (see below).

Dopamine has been also linked to the action of cannabinoids (see Fernández‐Ruiz et al ., 2010 ; El Khoury et al ., 2012 ). However, the different subpopulations of dopaminergic neurons within the CNS and, in particular, those neurons whose cell bodies are located in the substantia nigra and that project to the caudate‐putamen, the so‐called nigrostriatal dopaminergic neurons, do not appear to contain cannabinoid CB 1 receptors (see Fernández‐Ruiz, 2009 ; Fernández‐Ruiz et al ., 2010 ), the cannabinoid receptor type mostly involved in the control of synaptic activity. CB 1 receptors are also absent from other dopaminergic neuronal subpopulations (e.g. mesocorticolimbic neurons), although this does not exclude possible interactions between cannabinoids and dopamine in the control of those behaviours (e.g. brain reward, motivation, emotion) regulated by these neurons in physiological and physiopathological conditions (e.g. addiction). However, this has been the subject of a recent review (Fernández‐Ruiz et al ., 2010 ) and will not be addressed in the present one, which will concentrate exclusively in these interactions at the level of the basal ganglia.

It is well established that the endocannabinoid system, formed by different signalling lipids, the enzymes involved in their synthesis and degradation, and their target receptors, plays a modulatory function in important processes of the CNS. This includes the control of movement (see Fernández‐Ruiz, 2009 ), learning and memory (see Zanettini et al ., 2011 ), emotional behaviour (see McLaughlin and Gobbi, 2012 ), nociception (see Guindon and Hohmann, 2009 ), brain reward (see Solinas et al ., 2008 ), feeding behaviour (see Kirkham, 2009 ) and emesis (see Parker et al ., 2011 ), among others. This modulatory function is exerted through the ability of endocannabinoids and their receptors to participate in the retrograde signalling in different synapses located in those brain structures that regulate these processes (Castillo et al ., 2012 ). This is facilitated by the presynaptic location of cannabinoid CB 1 receptors, the key neuronal cannabinoid receptor type, that allow endocannabinoids to directly modulate the function of most of neurotransmitters including glutamate, GABA, opioid peptides, acetylcholine and 5‐HT (see Heifets and Castillo, 2009 ; Kano et al ., 2009 ). This function is particularly important in the case of glutamatergic and GABAergic synapses, in which, through well‐defined processes of short‐ and long‐lasting synaptic depression, it prevents an excess of excitation or inhibition, respectively, (Lovinger, 2008 ) that may lead to pathological conditions if prolonged and/or enhanced.

Cannabinoid–dopamine interactions at the basal ganglia

As mentioned above, there is solid anatomical, biochemical, physiological and pharmacological evidence that supports the idea that dopamine is the key regulatory transmitter in the control of movement exerted at the basal ganglia level (see Smith and Villalba, 2008). The activation of dopaminergic transmission in this circuitry produces hyperkinesia, whereas its inhibition results in a reduction of movement. By contrast, activation of the endocannabinoid system has been associated with motor inhibition and even catalepsy (see Fernández‐Ruiz, 2009), so that it has been widely speculated that the hypokinetic effect of cannabinoid agonists might be produced through a reduction in dopaminergic activity, given their ability to modify the action of several substances acting on the dopamine system. For example, cannabinoid agonists potentiated reserpine‐induced hypokinesia (Moss et al., 1981) and dopamine receptor antagonist‐induced catalepsy (Anderson et al., 1996), whereas they reduced quinpirole‐induced hyperlocomotion (Marcellino et al., 2008) and amphetamine‐induced hyperactivity (Gorriti et al., 1999) in laboratory rodents (for a complete summary of the behavioural data associated with the activation of CB 1 receptor‐mediated signals within the basal ganglia, see Fernández‐Ruiz and Gonzáles, 2005; Fernández‐Ruiz, 2009). Based on these data, several authors have proposed the idea of an inverse correlation between the two transmitter systems, with a reduced endocannabinoid tone accompanied by increased dopaminergic activity occurring in hyperkinetic conditions, and the opposite associated with a reduction in movement (see Fernández‐Ruiz, 2009). However, there are recent reports of a long‐lasting activation of striatal dopaminergic function, reflected in an enhanced tyrosine hydroxylase expression, by CB 1 receptor agonists (Bosier et al., 2012). The inverse correlation between both systems has been proposed for physiological conditions and also for pathological events, for example, Parkinson's disease, the most prevalent disorder affecting the basal ganglia (Obeso et al., 2008). The endocannabinoid system becomes hyperactivated in Parkinson's disease in parallel to the dopamine deficiency produced by the progressive degeneration of nigrostriatal dopaminergic neurons, resulting in the occurrence of motor symptoms such as bradykinesia, rigidity and tremor (see Fernández‐Ruiz, 2009). As will be discussed in the last section, this inverse correlation may serve for the development of cannabinoid‐based therapies for this disease.

As mentioned above, the most intriguing aspect of this pharmacological interaction between both systems is that it occurs in the absence of CB 1 receptors on the dopaminergic neurons (Herkenham et al., 1991a), which would imply that the mechanism enabling this interaction would be largely, if not exclusively, indirect and based on the necessary mediation of GABA‐ and/or glutamate‐containing neurons that do contain these receptors (see Fernández‐Ruiz, 2009; Fernández‐Ruiz et al., 2010). However, three recent experimental observations have challenged this classic idea. First, certain eicosanoid‐derived cannabinoids, including anandamide, N‐arachidonoyl‐dopamine (NADA) and AM404, have been found to bind and activate TRPV1 receptors (see Starowicz et al., 2007). Also, these receptors have been located in dopaminergic neurons within the basal ganglia (Mezey et al., 2000), allowing a direct action of these endocannabinoid/endovanilloid compounds on dopaminergic transmission. Second, there is also recent evidence that indicates that CB 1 receptors are able to form heteromers with other metabotropic receptors, including the dopamine D 1 and D 2 receptor types located, among others, in striatal projection neurons, enabling both systems to directly interact at postsynaptic level (see Ferré et al., 2009). These studies have provided interesting novel insights in terms of the function and therapeutic potential of the endocannabinoid signalling in the basal ganglia, as well as its interaction with dopaminergic transmission, from both basic and clinical perspectives. Lastly, CB 2 receptors have recently been identified in nigrostriatal dopaminergic neurons in the human brain (García et al., 2015), enabling endocannabinoids to act through the other major cannabinoid receptor type to directly modulate dopaminergic transmission, although the distribution of this receptor type in the brain is much more restricted than that of the CB 1 receptor and is frequently associated with pathological conditions (Fernández‐Ruiz et al., 2007). These mechanisms will be addressed in more detail below, after examining the classical indirect mechanism first proposed to explain the endocannabinoid–dopamine interactions (see Figure 1 for a representative diagram of these interactions).

Figure 1 Open in figure viewer PowerPoint Summary of the different neuronal mechanisms proposed to explain the interactions between the endocannabinoid signalling system and dopaminergic transmission at the level of the basal ganglia.

Effects of cannabinoids on dopaminergic transmission exerted through CB 1 receptors located in GABAergic and glutamatergic neurons As mentioned above, the abundant presence of endocannabinoid elements, that is, CB 1 receptors and their endogenous ligands, in the basal ganglia (Herkenham et al., 1991b; Mailleux and Vanderhaeghen, 1992; Tsou et al., 1998; Bisogno et al., 1999; Breivogel and Sim‐Selley, 2009), supports the idea that the endocannabinoid system plays an important modulatory role in the function of these brain structures (see Fernández‐Ruiz, 2009). It is generally accepted that those substances that enhance the endocannabinoid activity, preferentially the direct agonists of the CB 1 receptor, generate a dose‐dependent motor inhibition in laboratory animals that may even produce catalepsia with the highest doses (see Fernández‐Ruiz, 2009). This has been also observed in human smokers of cannabis and is associated with a detrimental effect on striatal dopaminergic functioning (Kowal et al., 2011). Similar results were obtained by administering the so‐called indirect cannabinoid agonists that are inhibitors of the endocannabinoid inactivation processes, for example, the enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase and the endocannabinoid transporter (Fernández‐Ruiz, 2009). These hypokinetic effects were generally reversed by the administration of rimonabant or other CB 1 receptor antagonists, supporting the idea that this receptor type is the key cannabinoid receptor involved in motor effects of cannabinoid compounds. In addition, rimonabant and other antagonists of CB 1 receptors produce by themselves a certain degree of hyperlocomotion, because many of them are inverse agonists (see Fernández‐Ruiz, 2009), whereas mice lacking CB 1 receptors exhibited several motor anomalies (see Valverde et al., 2005), supporting the key role played by these receptors (for a complete summary of the behavioural data associated with the activation/inhibition of CB 1 receptor‐mediated signals within the basal ganglia, see Fernández‐Ruiz and Gonzáles, 2005; Fernández‐Ruiz, 2009). A priori, the motor effects of cannabinoid agonists were explained as the normal consequence of their activity on those neuronal subpopulations that contain CB 1 receptors within the basal ganglia circuitry. Striatal projection GABAergic neurons and subthalamonigral glutamatergic neurons were the first CB 1 receptor‐containing neurons identified in relation with the motor effects of cannabinoids (Herkenham et al., 1991a; Mailleux and Vanderhaeghen, 1992; Tsou et al., 1998; Fusco et al., 2004). Further studies, conducted mostly with immunohistochemical procedures, demonstrated that CB 1 receptors were also located in corticostriatal glutamatergic afferences (Köfalvi et al., 2005; Uchigashima et al., 2007) and in some subpopulations of striatal GABA interneurons (Fusco et al., 2004; Uchigashima et al., 2007). In all cases, the neurons containing CB 1 receptors are GABAergic or glutamatergic neurons, thus supporting the idea that the first event associated with the activation of these receptors is an alteration in the activity of GABA and glutamate synapses but not the dopaminergic synapses. The changes in this neurotransmitter would occur secondarily to a primary effect on GABA or glutamate transmission, and they would be due to the connection of dopaminergic transmission with these neurons. However, as mentioned above, it is also possible that dopaminergic neurons located in the substantia nigra may be responsible for producing endocannabinoids for the activation of CB 1 receptors located in GABAergic or glutamatergic neurons, as found for dopaminergic neurons located in the ventral tegmental area (Melis et al., 2004; Riegel and Lupica, 2004). In addition, endocannabinoids may be also produced by striatal‐projecting neurons in order to target CB 1 receptors located in corticostriatal glutamatergic neurons and inhibit glutamate release, a response that appears to be regulated by the interaction of D 2 and adenosine A 2A receptors located in striatal cholinergic interneurons (Tozzi et al., 2011). All these findings are supported by the different anatomical studies mentioned above, but also by numerous pharmacological, electrophysiological and neurochemical studies that addressed the interaction of cannabinoid agonists with substances acting on the dopamine system, in relation to the motor effects in laboratory animals, studies that have been mentioned in the above section.

Effects of eicosanoid‐related cannabinoids exerted through TRPV1 receptors located in dopaminergic neurons As mentioned above, further investigations have, however, provided new elements to re‐evaluate the idea that the effects of endocannabinoids on dopaminergic transmission in the basal ganglia are necessarily indirect and mediated by CB 1 receptors located in GABA‐ or glutamate‐containing neurons. For example, it is now well known that anandamide and some of its analogues, for example, AM404, but not classic cannabinoids such as. Δ9‐tetrahydrocannabinol (Δ9‐THC), may behave as full agonists for the TRPV1 receptors (see Starowicz et al., 2007). These receptors have been identified in the basal ganglia located, among other markers, in nigrostriatal dopaminergic neurons (Mezey et al., 2000). The activation of these receptors with capsaicin or with other potential vanilloid ligands produced hypokinesia in rats (Di Marzo et al., 2001). Anandamide produced the same behavioural effect accompanied by a reduction in the activity of dopaminergic terminals in the striatum (de Lago et al., 2004), and this effect was reversed by capsazapine, thus supporting that it is exerted by the activation of TRPV1 receptors (de Lago et al., 2004). Further in vitro studies using perfused striatal fragments confirmed the activity of anandamide and the lack of effect of classic cannabinoids, such as Δ9‐THC, that do not bind to vanilloid‐like receptors, indicating that the TRPV1, rather than the CB 1 receptor, is the key target involved in these effects (de Lago et al., 2004). Other authors reported that the activation of TRPV1 receptors in the substantia nigra pars compacta, rather than producing an inhibition, stimulated dopamine release, although these effects seem to be mediated by TRPV1 receptors located in glutamatergic neurons, rather than by those located in dopaminergic terminals (Marinelli et al., 2003; 2007). Another interesting compound active at the TRPV1 receptor is NADA, an arachidonic acid derivative with properties of endocannabinoid and endovanilloid ligands (Starowicz et al., 2007). NADA seems to be synthesized through the conjugation of an arachidonic acid molecule directly with dopamine (Hu et al., 2009), excluding earlier suggestions that it would be synthesized through the hydroxylation of N‐arachidonoyl‐tyrosine followed by decarboxylation, by the same enzymes as those involved in dopamine synthesis. Its physiological significance is yet poorly understood, but some evidence suggests that it can serve as an antioxidant and neuroprotective compound (Bobrov et al., 2008). In addition, making the issue even more complex, a further study by Ferreira et al. (2009) revealed that N‐acyldopamines, such as NADA, were able to control the activity of dopaminergic terminals in the striatum via ion channels other than TRPV1 receptors, an effect that was not observed with anandamide or capsaicin. Importantly, NADA was likely to be synthesized in the substantia nigra in conditions of hyperactivity (Marinelli et al., 2007). Another recent observation that makes the issue even more complex suggests that anandamide may inhibit the dopamine transporter function by a receptor‐independent mechanism, an effect found in heterologous cells and synaptosomal preparations and mimicked by the anandamide analogue methanandamide, not by arachidonic acid (Oz et al., 2010). In addition, inhibition of FAAH or COX‐2 failed to alter the effect of anandamide, thus indicating that this effect is not related to the metabolism of this endocannabinoid (Oz et al., 2010). Authors also found that the effect was not attenuated by Pertussis toxin, excluding the involvement of CB 1 , CB 2 or GPR55 receptors, but not excluding that of TRPV1 receptors. Other authors also reported an inhibition of the dopamine transporter by different cannabinoid ligands in the rodent striatum (Price et al., 2007; Pandolfo et al., 2011). The inhibition was seen with the non‐selective cannabinoid agonists WIN55,212‐2 and O‐2545, and also with cannabidiol and NADA, but not with anandamide and 2‐arachidonoyl glycerol (Pandolfo et al., 2011). The effect was also seen with various CB 1 receptor antagonists/inverse agonists such as AM251 (Pandolfo et al., 2011). As expected, authors concluded that these effects were likely to be CB 1 receptor‐independent (Pandolfo et al., 2011).

Interaction of CB 1 and dopamine receptors at the postsynaptic level As mentioned above, CB 1 receptors do not appear to be located in dopaminergic neurons, with the only exception of a study that described direct interactions of the CB 1 receptor with the D 2 presynaptic receptor, which would be only possible if both receptors are located in the same neurons (O'Neill et al., 2009). However, most of the authors believe that CB 1 receptors are not located on dopaminergic neurons, but in striatal GABAergic projection neurons (striatonigral and striatopallidal pathways, respectively), in which they co‐localize with D 1 or D 2 receptors (Hermann et al., 2002; Martín et al., 2008). This may facilitate postsynaptic interactions between endocannabinoids and dopamine at the level of G‐protein/adenylyl cyclase signal transduction (Giuffrida et al., 1999; Meschler and Howlett, 2001; Nguyen et al., 2012). In addition, there is strong evidence supporting the formation of heteromers between CB 1 and D 2 receptors, and also adenosine A 2A receptors (see Ferré et al., 2009; Brugarolas et al., 2014). These CB 1 , D 2 and A 2A receptor heteromers were found in the dendritic spines of GABAergic neurons projecting to the globus pallidus, but their functional properties and their role in striatal function still need further investigation (see Ferré et al., 2009). This type of postsynaptic mechanism facilitates the direct interaction between cannabinoids and dopamine allowing, in this case, a bidirectional regulation, endocannabinoids to dopamine and vice versa. Thus, on one side, the motor effects of CB 1 receptor agonists have been associated with an activation of signalling via the neuronal phosphoprotein DARPP‐32, which has been linked to intracellular responses elicited by D 1 and D 2 receptors in the striatal projection neurons, whereas the genetic inactivation of DARPP‐32 resulted in an attenuation in the motor effects of cannabinoids (Andersson et al., 2005). On the other side, D 2 receptors controlled anandamide production in the striatum. This may serve as an inhibitory feedback mechanism counteracting dopamine‐induced facilitation of psychomotor activity (Giuffrida et al., 1999), as well as controlling G i/o protein availability for CB 1 receptors (González et al., 2009) and facilitating endocannabinoid‐mediated long‐term synaptic depression of GABAergic neurons (Kreitzer and Malenka, 2007), an effect also seen in the ventral tegmental area (Pan et al., 2008). A similar interaction of endocannabinoids with D 1 receptors has been recently proposed (Martín et al., 2008) and this proposal has been extended to glutamatergic synapses in which dopamine and its receptors also promote endocannabinoid‐mediated synaptic depression (see Lovinger and Mathur, 2012). In fact, the changes in corticostriatal glutamatergic synapses derived from the deficiency in dopamine occurring in Parkinson's disease have been proposed as a key factor in the pathogenesis of this disease (Lovinger and Mathur, 2012). Similarly, the formation of receptor heteromers (e.g. CB 1 , D 1 /D 2 , A 2A ) in striatal neurons may be of interest from a pharmacological point of view for the treatment of Parkinson's disease symptoms, in particular, levodopa‐induced dyskinesias. However, a recent study has demonstrated that levodopa disrupts the crosstalk between A 2A ‐CB 1 ‐D 2 receptors in experimental models of Parkinson's disease in rodents (Pinna et al., 2014) and primates (Bonaventura et al., 2014).