Cannabinoid receptor density is increased in a newborn piglet but not an adult rat model of traumatic brain injury

Objectives
Traumatic brain injury (TBI) is a leading cause of death and disability in the paediatric age group and also among adults with lifelong impairments and concomitant socioeconomic consequences. There is evidence from animal experiments and patient studies that cannabinoid signalling is involved in TBI either by modulating neuroinflammation or for neuroprotective pathways. However, almost no data exist on changes of cannabinoid receptors (CBR) after TBI. The present study was performed to investigate CBR in two different animal models of TBI. The identification of disease-related targets within the cannabinoid system is a precondition for potential molecular imaging of TBI patients with Positron-Emission-Tomography (PET), e.g. for monitoring of neuroprotective pharmacotherapy.
Methods
Thirteen female newborn piglets (post-TBI survival time: 6 h) underwent moderate fluid percussion (FP) injury (n = 7) or sham operation (n = 6) with an impact pressure of 3.8 ± 0.3 atmospheres. Furthermore, male Sprague-Dawley rats were randomized into four groups (post-TBI survival time: 6, 24, and 72 h), anaesthetized and subjected to sham injury/craniotomy (control, n = 3-5) or mild-to-moderate controlled cortical impact injury (CCI) (n = 5, 2 mm depth at 4 m/sec). From brains of both species, cryostat sections were cut (rat 12 µm, pig 20 µm) and density of CB1/2R ([3H]CP-55,940) and CB1R ([3H]SR141716A) was assessed with in vitro autoradiography. If appropriate, CB1- and CB2-specific ligands SR141716A and SR144528, respectively, were applied for receptor blockade.
Results
In the newborn piglet model, we found a statistically significant overall increase of [3H]CP-55,940 binding in the brains of injured animals (15 of 24 investigated brain regions, including cerebral cortex, hippocampus, thalamus hypothalamus and midbrain; max: +140%, mean: +47%). No significant increases in [3H]SR141716A binding were observed.
In contrast, in adult rats, no statistically significant changes in CBR density were detected, although in few brain regions increases as well as decreases of up to 30% of [3H]CP-55,940 binding were found.
Conclusions
In conclusion, the expression density of CBR is significantly altered after experimental TBI in newborn piglets. Because CB1R show no significant alterations after injury, it is very likely, that the increases are of CB2R origin, probably due to activated microglia.
In a mild-to-moderate adult rat model, no statistically significant changes in CBR density are found, which can either be attributed to species differences in e.g. brain morphology, or differences in the severity of the employed models.
Data from newborn pigs indicate involvement of a cannabinoid mechanism in paediatric TBI. The identification of the underlying mechanisms, supported for instance by molecular imaging with PET, could help to detect clinically relevant neuroreceptor changes after TBI and provide valuable insights which may prove helpful in the development of cannabinoid-based neuroprotectants.