Short read
Petr Šrámek and colleagues at Charles University in Prague kept young men in water up to the neck for an hour per condition, at 32, 20 and 14 degrees, published in the European Journal of Applied Physiology in 2000. A control condition in room air without immersion served as the comparison. Testing the same men at three temperatures separated the effect of cold from that of water pressure. The modality was cold water immersion, not a sauna. The abstract does not state how many men took part.
After one hour at 14 degrees, plasma noradrenaline had risen by 530 % and plasma dopamine by 250 %, in a design that used room air without immersion as the control condition; adrenaline did not change. At the same temperature, metabolic rate rose by 350 %, heart rate by 5 %, systolic and diastolic blood pressure by 7 and 8 %, and urine output by 163 %. At 20 degrees, metabolic rate rose by 93 %. At 32 degrees, heart rate, blood pressure, renin activity, cortisol and aldosterone fell. The authors state that none of the water temperatures raised cortisol and attribute the cold responses mainly to increased sympathetic nervous activity.
The dopamine figure comes from blood plasma and says nothing direct about the brain, and a rise in plasma does not show a rise in the central nervous system. The exposure was one hour at 14 degrees up to the neck, not an ice bath and not two to three minutes. The study covers men only, measures acute responses and says nothing about adaptation.
The figure of 250 % is the most quoted number in cold exposure and belongs with its exposure and its site of measurement. The larger effect in the same experiment concerned noradrenaline, not dopamine.