Cannabis could help prevent memory loss in the elderly, a new study has found.

Researchers in Germany, who are set to begin human trials later this year, say the drug may even help delay the onset of Alzheimer's disease by slowing the brain's natural ageing process.

The mental power of older mice improved dramatically after they were given THC, the psychoactive part of marijuana that makes users 'high'.

In fact, their brain connections in the hippocampus - which controls learning, memory and emotions - were firing as well as those of young adults.

It's hoped the same may apply to humans, with trials expected to begin this year.

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Old mice regained their speed of memory after consuming pot, German scientists found (file image)

Psychologist Professor Andreas Zimmer of Bonn University in Germany said the findings are essential in the movement to understand cannabis as a form of medicine.

More than half of the highly influential United States has voted in favor of legalization for medical use, and Germany followed suit just two months ago.

'Chronic, low-dose treatment with THC or cannabis extracts could be a potential strategy to slow down or even reverse cognitive decline in the elderly,' he said.

'Together, these results reveal a profound, long-lasting improvement of cognitive performance resulting from a low dose of THC treatment in mature and old animals.

'THC treatment for 28 days restored the learning and memory performance of mature and old animals in the water maze, novel object location recognition and social recognition tests to the levels observed in young mice.'

His team whose findings are published in Nature Medicine say it's too early to say if their remarkable results could be achieved in patients.

But they offer hope that drugs based on cannabis or THC could be given to middle or old aged people.

Co-author Dr Andras Bilkei-Gorzo said: 'Cannabis abuse is age-dependent. It's a problem for younger rather than older people.

'In safe doses the compound could improve their brains. It's possible it could delay the onset of Alzheimer's disease, but that's pure speculation.

'We hope to start clinical trials by the end of the year with a hundred - or even hundreds - of people.'

Previous studies have suggested chemicals found in marijuana are effective against Alzheimer's disease.

In the latest experiments three groups of mice that were 18 (old), 12 (mature) and two months (young) were regularly injected with low doses of THC for 28 days.

The equivalent ages in human years would be 64, 58 and 20, respectively.

Both before and after the treatment they were challenged with a series of tasks that tested their learning and memory skills.

These included negotiating their way around a water maze, locating objects and recognising other mice.

In young mice THC impaired their performance. But the same therapy actually improved learning and memory in both of the older sets of animals.

Prior to receiving THC they did poorly. But afterwards they were as efficient as the young mice had been before being given the compound that, interestingly, reduced their mental skills.

Prof Zimmer, a psychiatrist at Bonn University, said THC boosted genes that control neurons in the hippocampus - restoring them to the same patterns observed in young animals.

Psychoactive compounds found in marijuana - such as THC - exert their actions on the nervous system by interacting with the endocannabinoid system which deals with pain.

This is the brain's own internal version of cannabis chemicals which becomes desensitized as people get older. It has even been linked with beginning the process that leads to Alzheimer's.

Prof Zimmer said the endocannabinoid system declines during ageing but a direct link with symptoms has not been shown.

When someone smokes cannabis THC binds to the endocannabinoid receptor called CB1. It is the interaction between the drug and this receptor that makes people feel high.

Prof Zimmer said: 'Here we show a low dose of THC reversed the age-related decline in cogntive performance of mice aged 12 and 18 months.

'THC treatment restored hippocampal gene transcription patterns such that the expression profiles of THC-treated mice aged 12 months closely resembled those of THC-free animals aged 2 months.'

The study was published in Nature Medicine.