Age-dependent cannabinoid CB1 receptor plasticity and search for histamine H4 receptors in the brain

Abstract

The aim of this thesis was to examine whether adaptive changes of cannabinoid CB₁ receptors occur following their direct and indirect activation through pre-treatment with an exogenous and endogenous cannabinoid depending on age and whether histamine H₄ receptor mRNA and functional H₄ receptors occur in the brain.<br /> This thesis focuses on the ³⁵S-GTPγS binding assay combined with other experimental methods. The motility of the mice after Δ⁹-THC treatment was studied in the Open Field Test. The concentration of the endogenous cannabinoid 2-arachidonoyl glycerol (2-AG) after JZL 184 treatment in the hippocampus was determined by liquid chromatography tandem mass spectrometry. Histamine H₄ receptor in the hippocampus of the mouse and guinea pig was detected using the reverse transcriptase polymerase chain reaction.<br /> Adolescent and aged mice were administered i.p. with Δ⁹-THC acute (single injection as a challenge dose) or chronically (6 injections over 3 days and a challenge dose), subsequently the motility of the animals was measured and CB₁ receptor G-Protein activation was quantified using ³⁵S-GTPγS binding method. A single dose of Δ⁹-THC decreased the motility in adolescent and old animals. Pre-treatment of mice with Δ⁹-THC attenuated the effect of a challenge dose of Δ⁹-THC by about 50 % in the rearing and resting time paradigm, independent of the age; with respect to the distance travelled, the attenuation of the effect tended to be less pronounced in old when compared to adolescent mice. Basal ³⁵S-GTPγS binding was lower in hippocampal membranes from vehicle-treated aged when compared to adolescent mice whereas Δ⁹-THC pre-treatment decreased basal binding significantly in adolescent but had no effect in membranes from old mice. The increase in CP 55,940-induced ³⁵S-GTPγS binding was less marked in vehicle-treated old than in adolescent mice. After pre-treatment with Δ⁹-THC, the CP 55,940-related increase in ³⁵S-GTPγS binding was not affected in old but was significantly reduced in adolescent mice. The results show that aging affects basal and CP 55,940-induced ³⁵S-GTPγS binding and the tolerance development of both parameters to Δ⁹-THC pre-treatment; in the behavioural paradigm, aging does not influence tolerance development.<br /> In the second part the concentration of 2-AG was increased by blockade of its degrading enzyme by JZL 184 and the effect of 2-AG on basal and CP 55,940-induced ³⁵S-GTPγS binding was measured. Adolescent mice were pre-treated with JZL 184 at 4, 10 or 40 mg/kg over a period of 14 days. JZL 184 at 4 mg/kg and 10 mg/kg did not affect measured parameters and increased hippocampal 2-AG without affecting CP 55,940-induced ³⁵S-GTPγS binding, respectively. JZL 184 at 40 mg/kg increased 2-AG but also tended to decrease the CP 55,940-induced ³⁵S-GTPγS binding. The data suggest that 10 mg/kg of JZL 184 when administered over a longer time period leads to a sustained increase in 2-AG and does not cancel out the activation of CB₁ receptors.<br /> In the third part the H₄ receptor mRNA is was detected in the cerebral cortex of mice and guinea pigs. The search for a functional correlate was, however, negative; thus, a H₄ receptor-related facilitation of ³⁵S-GTPγS binding in cortical membranes of either species could not be shown.