the fields of aging and/or Alzheimer's disease. Effects of cannabinoids Some studies have shown that the stimulation of the cannabinoids results in the growth of new nerve cells in the hippocampus from both embryonic and adult stem cells. In 2005 a clinical study of rats at the University of Saskatchewan showed regeneration of nerve cells in the hippocampus. Studies have shown that a synthetic drug resembling THC, the main psychoactive ingredient in marijuana, provides some protection against brain inflammation, which might result in better memory at an older age. This is due to receptors in the system that can also influence the production of new neurons. Nonetheless, a study directed at Rutgers University demonstrated how synchronization of action potentials in the hippocampus of rats was altered after THC administration. Lack of synchronization corresponded with impaired performance in a standard test of memory. Recent studies indicate that a natural cannabinoid of cannabis, cannabidiol (CBD), increases adult neurogenesis while having no effect on learning. THC however impaired learning and had no effect on neurogenesis. A greater CBD to THC ratio in hair analyses of cannabis users correlates with protection against gray matter reduction in the right hippocampus. CBD has also been observed to attenuate the deficits in prose recall and visuo-spatial associative memory of those currently under the influence of cannabis, implying neuroprotective effects against heavy THC exposure. Neurogenesis might play a role in its neuroprotective effects, but further research is required. A few studies have reported a positive association between THC and hippocampal neurogenesis. Some of them hypotethize a biphasic effect, some of them express that part of the negative effects could be attributable to neuroadaptation due to exposure at a specific period of life, and that it could be reversed. Regulation Many factors may affect the rate of hippocampal neurogenesis. Exercise and an enriched environment have been shown to promote the survival of neurons and the successful integration of newborn cells into the existing hippocampus. Another factor is central nervous system injury since neurogenesis occurs after cerebral ischemia, epileptic seizures, and bacterial meningitis. On the other hand, conditions such as chronic stress, viral infection and aging can result in a decreased neuronal proliferation. Circulating factors within the blood may reduce neurogenesis. In healthy aging humans, the plasma and cerebrospinal fluid levels of certain chemokines are elevated. In a mouse model, plasma levels of these chemokines correlate with reduced neurogenesis, suggesting