TABLE 3.
Calculated Emax and EC50 values from agonist-stimulated GTPgS binding experiments in CB1 (+/+) and (+/−) membranes from cerebellum and spinal cord tissue
| Agonist | CB1 (+/+) |
CB1 (+/−) |
(+/−) Emax/(+/+) Emax | ||
|---|---|---|---|---|---|
| Emax | EC50 | Emax | EC50 | ||
| Cerebelluma | |||||
| A-834,735D | 211.1 ± 11.4 | 22.3 ± 5.6 | 164.9 ± 14.1*# | 29.4 ± 11.9 | 0.78 |
| WIN55,212-2 | 202.5 ± 7.6 | 39.4 ± 7.6 | 152.6 ± 4.9**# | 50.4 ± 8.0 | 0.75 |
| CP55,940 | 198.7 ± 5.2 | 4.1 ± 0.6 | 166.3 ± 7.6*# | 6.1 ± 1.4 | 0.84 |
| JWH-073 | 185.5 ± 1.7 | 25.6 ± 1.2 | 95.5 ± 3.9**** | 38.3 ± 7.1 | 0.51 |
| CP47,497 | 212 ± 8.6 | 102.1 ± 18.2 | 133.3 ± 7.6*** | 94.5 ± 22.8 | 0.63 |
| THC | 119.9 ± 4.6^ | 24.3 ± 4.7 | 73.8 ± 8.7**^ | 52.4 ± 29.0 | 0.62 |
| Spinal cordb | |||||
| A-834,735D | 57.8 ± 2.4 | 15.0 ± 3.0 | 35.0 ± 3.0** | 24.1 ± 9.6 | 0.61 |
| WIN55,212-2 | 56.3 ± 3.8 | 65.4 ± 22.0 | 36.9 ± 2.4** | 84.8 ± 21.6 | 0.66 |
| CP55,940 | 49.6 ± 5.9 | 8.61 ± 5.1 | 30.4 ± 4.3* | 9.5 ± 6.9 | 0.61 |
| JWH-073 | 41.3 ± 3.5 | 77.8 ± 29.9 | 20.6 ± 7.7# | 46.6 ± 91.0 | 0.49 |
| CP47,497 | 41.2 ± 3.9 | 93.2 ± 38.1 | 17.1 ± 2.2**#$ | 45.9 ± 24.2 | 0.42 |
| THC | 11.4 ± 2.3^ | 6.1 ± 11.2 | ND | ND | ND |
ND, not determined; ^indicates significant differences versus all other ligands within (+/+) Emax values. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001 for (+/+) versus (+/−) samples within tissue. ^P < 0.05 for THC versus other wild-type Emax values. #P < 0.05 for JWH-073 versus other (+/−) Emax values. $P < 0.05 for CP47,497 versus other (+/−) Emax values. No significant differences in EC50 were detected across brain region or genotype for each drug.
In cerebellum, all agonists elicited significantly lower [35S]GTPγS binding in (+/−) membranes than in (+/+) membranes. Comparison of CB1 (+/+) Emax values across drugs revealed that THC elicited the least stimulation of all ligands tested in (+/+) and (+/−) tissue.
Spinal cord membranes showed a similar pattern of results in which stimulation was significantly reduced in tissue from (+/−) mice compared with (+/+) mice, although the Emax value of JWH073 in CB1 (+/−) mice did not statistically differ from its Emax value in CB1 (+/+) mice (P = 0.052). The lack of detectable stimulation by THC in spinal cord tissue from CB1 (+/−) mice precluded the calculation of the Emax and EC50 values of this compound, although the moderate efficacy agonists CP47,497 and JWH-073 differed from WIN,55,212-2 in (+/−) spinal cord. In CB1 (+/+) spinal cord tissue, the Emax value of THC was significantly reduced compared with each of the other ligands. This loss of efficacy is expressed additionally as the CB1 (+/−) Emax/ CB1 (+/+) Emax to show relative differences between high receptor conditions of cerebellum and low receptor conditions of spinal cord.