Lessons from nonmammalian species

Conditioned place preferences induced by hearing song outside the breeding season relate to neural dopamine D1 and cannabinoid CB1 receptor gene expression in female European starlings (Sturnus vulgaris)

The affective state induced by sensory stimuli changes to adaptively modify behaviors that are critical for survival and reproduction. In European starlings, during the spring breeding season, male courtship song is rewarding to females, but only to those that possess resources that are necessary for reproduction (i.e., nesting sites). In fall, starling song is non-sexual and proposed to maintain flocks. This suggests that in fall it may be adaptive for females to be rewarded by fall rather than spring, courtship song. We used a conditioned place preference (CPP) test to evaluate song-induced affective state in fall condition females and quantitative real-time PCR to measure expression of genes that modulate affective state (CB₁ endocannabinoid and D₁ dopamine receptors) in brain regions that were previously implicated in song-induced reward (i.e., the medial preoptic nucleus (mPOA) and ventromedial hypothalamus (VMH)). Fall condition females developed an aversion to a place that had been paired with playback of both male fall and courtship song, indicating that in general male song induces a negative affective state outside the breeding season. Song-induced aversion was stronger in birds conditioned towards an initial place preference. For mPOA, CB₁ receptor expression correlated positively with fall and spring song-induced CPP. D₁ receptor expression correlated negatively with fall (but not spring) song-induced CPP, and the ratio of CB₁ to D₁ receptor expression correlated positively with fall (but not spring) song-induced CPP. These correlations suggest that interactions between D₁ and CB₁ receptors in mPOA may play a role in modifying affective responses to song.

Cannabinoids Modulate Neuronal Activity and Cancer by CB1 and CB2 Receptor-Independent Mechanisms

Cannabinoids include the active constituents of Cannabis or are molecules that mimic the structure and/or function of these Cannabis-derived molecules. Cannabinoids produce many of their cellular and organ system effects by interacting with the well-characterized CB1 and CB2 receptors. However, it has become clear that not all effects of cannabinoid drugs are attributable to their interaction with CB1 and CB2 receptors. Evidence now demonstrates that cannabinoid agents produce effects by modulating activity of the entire array of cellular macromolecules targeted by other drug classes, including: other receptor types; ion channels; transporters; enzymes, and protein- and non-protein cellular structures. This review summarizes evidence for these interactions in the CNS and in cancer, and is organized according to the cellular targets involved. The CNS represents a well-studied area and cancer is emerging in terms of understanding mechanisms by which cannabinoids modulate their activity. Considering the CNS and cancer together allow identification of non-cannabinoid receptor targets that are shared and divergent in both systems. This comparative approach allows the identified targets to be compared and contrasted, suggesting potential new areas of investigation. It also provides insight into the diverse sources of efficacy employed by this interesting class of drugs. Obtaining a comprehensive understanding of the diverse mechanisms of cannabinoid action may lead to the design and development of therapeutic agents with greater efficacy and specificity for their cellular targets.

Neural endocannabinoid CB1 receptor expression, social status, and behavior in male European starlings

Many species modify behavior in response to changes in resource availability or social status; however, the neural mechanisms underlying these modifications are not well understood. Prior work in male starlings demonstrates that status-appropriate changes in behavior involve brain regions that regulate social behavior and vocal production. Endocannabinoids are ubiquitously distributed neuromodulators that are proposed to play a role in adjusting behavior to match social status. As an initial step to provide insight into this hypothesis we observed flocks of male starlings in outdoor aviaries during the breeding season. We used quantitative real-time PCR to measure expression of endocannabinoid CB1 receptors in brain regions involved in social behavior and motivation (lateral septum [LS], ventral tegmental area [VTA], medial preoptic nucleus [POM]) and vocal behavior (Area X and robust nucleus of the arcopallium; RA). Males with nesting sites sang to females and displaced other males more than males without nesting sites. They also had higher levels of CB1 receptor expression in LS and RA. CB1 expression in LS correlated positively with agonistic behaviors. CB1 expression in RA correlated positively with singing behavior. CB1 in VTA also correlated positively with singing when only singing birds were considered. These correlations nicely map onto the well-established role of LS in agonistic behavior and the known role of RA in song production and VTA in motivation and song production. Studies are now needed to precisely characterize the role of CB1 receptors in these regions in the production of status-appropriate social behaviors.

Similar anxiolytic effects of agonists targeting serotonin 5-HT1A or cannabinoid CB receptors on zebrafish behavior in novel environments

The discovery that selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine are present and bioaccumulate in aquatic ecosystems have spurred studies of fish serotonin transporters (SERTs) and changes in SSRI-sensitive behaviors as adverse outcomes relevant for risk assessment. Many SSRIs also act at serotonin 5-HT1A receptors. Since capitalizing on this action may improve treatments of clinical depression and other psychiatric disorders, novel multimodal drugs that agonize 5-HT1A and block SERT were introduced. In mammals both 5-HT1A and CB agonists, such as buspirone and WIN55,212-2, reduce anxious behaviors. Immunological and behavioral evidence suggests that 5-HT1A-like receptors may function similarly in zebrafish (Danio rerio), yet their pharmacological properties are not well characterized. Herein we compared the density of [(3)H] 8-hydroxy-2-di-n-propylamino tetralin (8-OH-DPAT) binding to 5-HT1A-like sites in the zebrafish brain, to that of similarly Gαi/o-coupled cannabinoid receptors. [(3)H] 8-OH-DPAT specific binding was 176±8, 275±32, and 230±36fmol/mg protein in the hypothalamus, optic tectum, and telencephalon. [(3)H] WIN55,212-2 binding density was higher in those same brain regions at 6±0.3, 5.5±0.4 and 7.3±0.3pm/mg protein. The aquatic light-dark plus maze was used to examine behavioral effects of 5-HT1A and CB receptor agonists on zebrafish novelty-based anxiety. With acute exposure to the 5-HT1A partial-agonist buspirone (50mg/L), or dietary exposure to WIN55,212-2 (7μg/week) zebrafish spent more time in and/or entered white arms more often than controls (p<0.05). Acute exposure to WIN55,212-2 at 0.5-50mg/L reduced mobility. These behavioral findings suggest that azipirones, like cannabinoid agonists, have anxiolytic and/or sedative properties on fish in novel environments. These observations highlight the need to consider potential ecological risks of azapirones and multimodal antidepressants in the future.