Cannabinoid receptor Type 1 densities reflect social organization in Microtus

Abstract

Across many species, endocannabinoids play an important role in regulating social play, reward, and anxiety. These processes are mediated through at least two distinct cannabinoid receptors (CB), CB1 and CB2. CB1 expression is found in appreciable densities across regions of the brain that integrate memory with socio-spatial information; many of these regions have been directly linked to the neurobiology of pair bonding in monogamous species. Using receptor autoradiography, we provide the first distributional map of CB1 within the brains of closely related monogamous prairie voles and promiscuous meadow voles, and compare receptor densities across sexes and species in limbic regions. We observe CB1-specific signal using [3H] CP-55,940 and [3H] SR141716A, though the latter exhibited a lower signal to noise ratio. We confirmed the presence of CB2 in prairie vole spleen tissue using [3H] CP-55,940. However, we found no evidence of CB2 in the brain using either [3H] CP-55,940 or [3H] A-836,339. The overall distribution of putative CB1 in the brain was similar across vole species and followed the pattern of CB1 expression observed in other species-high intensity binding within the telencephalon, moderate binding within the diencephalon, and mild binding within the mesencephalon and metencephalon (aside from the cerebellar cortex). However, we found profound differences in CB1 densities across species, with prairie voles having higher CB1 binding in regions implicated in social attachment and spatial memory (e.g., periaqueductal gray, hippocampus). These findings suggest that CB1 densities, but not distribution, correlate with the social systems of vole species.

Keywords: CB1; CB2; endocannabinoids; monogamy; pair bonding; social behavioral neural network.

Figure 1 –

Validation of [3H] CP-55,940 for autoradiography in prairie vole brains. Three adjacent sections of brain tissue were incubated with [3H] CP-55,940 with or without an unlabeled receptor competitor. Co-incubation of the radioligand with the CB2 blocker (HU308) had no discernible impact on binding in the HPC but increased binding in the PFC relative to control. Co-incubation of the radioligand with the CB1 blocker (AM251) significantly decreased signal in both regions. Representative autoradiographs depict each condition at the level of the HPC. * p < 0.05.

Figure 2 –

Validation of [3H] SR141716A and [3H] A-836,339 for autoradiography in prairie vole brains. Four adjacent sections of brain tissue were incubated with radioligand that was specific for CB1 ([3H] SR141716A) or CB2 ([3H] A-836,339) with complementary blockers for the same receptor. Co-incubation of [3H] SR141716A with AM251 decreased signal in the HPC but not in the PFC. Neither the [3H] A-836,339 condition with or without the competitor (HU308) produced quantifiable levels of signal. Representative autoradiographs depict each condition at the level of the HPC. * p < 0.05.

Figure 3 –

Validation of [3H] CP-55,940, [3H] SR141716A and [3H] A-836,339 for autoradiography in prairie vole spleens. Seven adjacent sections of spleen tissue were incubated with radioligand that was co-incubated with or without unlabeled blockers. [3H] CP-55,940 signal was decreased when co-incubated with HU308 (CB2 blocker) but not with AM251 (CB1 blocker). [3H] SR141716A signal was not significantly impacted by AM251 competition. We found no evidence of [3H] A-836,339 signal in spleen assay with or without a CB2 competitor (SR144528). * p < 0.05.

Figure 4 -

Representative autoradiograms of CB1 receptors, mapped with [3H] CP-55,940, throughout prairie vole brain. A1, primary auditory cortex; ACC, anterior cingulate cortex; AH, anterior hypothalamus; AOB, accessory olfactory bulb; AON, anterior olfactory nucleus; BLA, basolateral amygdala; BMA, basomedial amygdala; BST, bed nucleus of the stria terminalis; CBX, cerebellar cortex; CEA, central amygdala; CP, caudate-putamen; DG, dentate gyrus; ENT, entorhinal cortex; GPe, globus pallidus external; GPi, globus pallidum internal; IC, inferior colliculus; IPN, interpeduncular nucleus; LDTh, laterodorsal thalamus; LH, lateral hypothalamus; LS, lateral septum; MBO, mammillary bodies; MDTh, mediodorsal thalamus; MEA, medial amygdala; M1, primary motor area; MOB, main olfactory bulb; MPA, medial preoptic area; MRF, reticular formation; MRN, median raphe nucleus; NAc, nucleus accumbens; ORB, orbitofrontal cortex; PAG, periaqueductal gray; PCC, posterior cingulate cortex; PFC, prefrontal cortex; PG, pontine gray; PIR, piriform cortex; RSP, retrosplenial cortex; S1, primary somatosensory cortex; SC, superior colliculus; SN, substantia nigra; SUB, subiculum; V1, primary visual cortex; VP, ventral pallidum; VPTh, ventroposterior thalamus; VTA, ventral tegmental area.

Figure 5 –

Comparisons of CB1 density in prairie and meadow voles. Prairie voles had higher CB1 density in CA2 and CA3 (A) , DG and PAG (B), LDTh, MDTh, and VPTh (C), and in MBO (D). * p < 0.05.