Opiates, such as morphine, heroin, hydroxycodone, etc.... work on opiate receptors which are present in brain nuclei that are involved in reward, memory, pain perception, as well as a slew of other processes. Opiate receptors are natually responsive to endorphins (i.e. enkephalins) which are endogenous (naturally produced in the body) opiates.
THC is a cannabinoid derivative, which is a class of drug that comes from the cannabis sativa plant. This drug does not work by the same biochemical mechanism that opiates do. cannabinoids have their own subset of receptors which are also prevelent in many parts of the brain that are associated with appetite, memory, anxiety, etc.... Your body has endogenous cannabinoids (anadamide, 2-arachodonyl glycerol) which are ligands for 1 of 2 known cannabinoid receptors. Their is speculation on a 3rd receptor but that is very new and not well understood research going on right now.
the pharmacological properties of both Opiates and Cannabinoids are very different from each other. They are not really involved an any of the same processes however if administered in conjuctin with each other may have additive effects. This may be due to downstream regulation of different regions of the brain and/or processes along with the CNS depressent affect that both of these drugs have which may lead to synergistic like effects.
PhD. student Pharmacology