the blood-brain barrier (BBB) has evolved to support the safety of your brain and body. It acts as a fence between the brain's blood vessels and components that make up brain tissue. The BBB defends the brain from absorbing disease-causing pathogens or toxins that may exist in the blood. Most neurotransmitters are unable to cross the blood-brain barrier and instead must be metabolised in the brain but there is the odd exception to the rule!
serotonin is your feel-good chemical messenger. It plays a crucial role in regulating mood, anxiety, appetite, sleep  and has been shown to promote patience . If you're looking to increase your serotonin levels, then orally ingesting serotonin would be in vain as it is unable to cross the BBB . Similarly, serotonin produced in your gut is unable to increase brain serotonin levels which are vital for maintaining mood.
the good news is you can still obtain serotonin through nutritional strategies. The amino acid tryptophan, which is found predominantly in animal proteins, is a building block to serotonin. To enter the brain, tryptophan must compete with other amino acids in your diet for transporter molecules to cross the blood-brain barrier. Unfortunately, it competes poorly compared to other preferential amino acids resulting in relatively small amounts being converted into serotonin.
if you don't obtain sufficient dietary tryptophan an alternative option is a more effective amino acid called 5-htp. Tryptophan is converted into 5-htp before becoming serotonin (5-ht) but it can also be taken as a food supplement. The isolated form can be extracted from the Griffonia seed, and its primary benefit is its ability to freely enter the brain without changing shape or structure.
dopamine is your reward and pleasure chemical messenger. According to the Natural Medicines database, dopamine is responsible for modulating emotional states, sex drive (libido) and body movements  and the capacity to handle your everyday stress . Dopamine is unable to cross the blood-brain barrier however sufficient consumption of the nutrient precursor to dopamine called tyrosine will increase dopamine in the brain . Tyrosine is converted to L-DOPA via a process called tyrosine hydroxylase before being converted into dopamine.
rich sources of tyrosine can be found in animal-based protein beef, fish and eggs; top plant-based sources of tyrosine are seaweed, pumpkin leaves and cooked mustard greens . If you're interested in a vegan-friendly tyrosine supplement, brain feed developed the world's 1st 750mg tyrosine supplement made from fermented corn.
acetylcholine is released when conducting cognitive tasks and is responsible for memory retention and learning  but it is also unable to cross the BBB. Its precursor is a nutrient called choline and the richest sources of choline are animal proteins such as beef, liver, eggs and chicken  however, it's also found in soybeans, mushrooms and tofu. Food-derived choline and other forms of choline, such as choline bitartrate, require a complex mechanism of action to enter the brain, which involves binding to transporter molecules  but there are some forms of choline that can avoid this challenge. Alpha Glyceryl-Phosphoryl-Choline (Alpha GPC) and Citicoline (CDP) can freely cross the BBB to be converted into acetylcholine. Alpha GPC is 40% choline by weight which is over double the amount contained in Citicoline (18.5%). Alpha GPC has been shown to increase acetylcholine levels 1-3 hours post-ingestion 
GABA is unable to cross the BBB but it can be broken in the muscles and it may offer a form of physical relaxation. Studies suggest that it is possible to increase the relative concentrations of GABA in the brain with the amino acid, theanine which can cross the BBB . It reduces the impact of excitatory pathways and in doing, so increases the relative concentration of GABA.
theanine is the predominant amino acid in green tea and it constitutes 1-2% of the dry weight before you boil the kettle, it might be helpful to know that you would need to consume 15-20 cups of tea to obtain the amounts ground in clinical study data . Most L-theanine supplements use synthetic Suntheanine but brain feed has 250mg 99% pure theanine capsules extracted green tea from the Hunan Province in South Central China. To keep it as pure as possible, it contains zero additives such as lemon balm, whose mechanism of action, ability to enter the brain, or synergistic benefit is unknown.
5. melatonin...it can be orally ingested!
melatonin is a chemical messenger that reduces the time taken to fall asleep. According to a 2013 review of melatonin studies, participants who took melatonin on average fell asleep seven minutes earlier, slept for eight minutes longer, and had an improved quality of sleep .
melatonin release is triggered by photosensitive receptors in the eyes that detect a reduction in UV light, and this evolutionary adaptation converts the brains store of serotonin into melatonin . Unlike other chemical messengers, melatonin can be orally ingested and can freely cross the BBB. Unfortunately, melatonin can only be obtained through prescription but you might be able to increase your levels of melatonin through other nutritional strategies such as increasing the availability of brain-derived serotonin.