A Google check of many sites found astrologers placing the chemicals under a sign, whereas such is usually confined to a body area and not its function. The latter comes under rulership of a planet.
The best site I found was that of Sharon Ward who provided a detailed account of research into Fatigue and the possible use of Dopamine in relieving it.
As dopamine is a 'buck me up feel good' chemical, she suggested Mars with Jupiter.
Aries is strongly associated with rulership of the head and the brain, so Mars energy as functioning for it is not surprising. Jupiter is the planet of 'good spirits'.
Neptune has general rulership over chemicals.
In cases of lack of positive energy, a harsh aspect between Mars-Neptune might be in the chart. With Neptune's transit through its own sign, it could act (negatively) stronger in creating low spirits
When I come down with a 'physical' cold, I don't even bother to check my chart. Either transiting Mars or Neptune is invariably influencing the other. I go very Neptune, feel sorry and full of self pity for myself and take myself off to a warm(Mars) bed(Neptune) to get over both.
Other forms of 'cold' might have a similar effect. Also look at aspects between Saturn to Neptune. Cold ice.
Moon would add 'the mood'.