Dopamine-containing substantia nigra units are unresponsive to changes in plasma glucose levels induced by dietary factors, glucose infusions or insulin administration in freely moving cats

Dopamine-containing neurons in the pars compacta of the substantia nigra showed no significant change in activity during 48 hours of food deprivation in cats that were maintained on either a high carbohydrate diet or a low carbohydrate-high protein diet. Plasma glucose levels declined significantly during this time period in the high carbohydrate diet group, and increased slightly in the low carbohydrate-high protein diet group. In addition, there was no significant change in the activity of dopaminergic neurons in food deprived cats during feeding behavior, during which glucose levels were restored to normal. Intravenous infusion of glucose in freely moving cats, which elevated plasma glucose levels from 82 to 719 mg/100 ml and midbrain glucose from 4.3 to 12.2 mumoles/g, was also without effect on the activity of dopaminergic neurons. Insulin administration to cats maintained on a diet of standard cat chow and fasted for 18 hours decreased plasma and brain glucose to 32.8 mg/100 ml and 2.1 mumoles/g, respectively, but, again, there was no significant change in nigral unit activity. These data demonstrate that central dopaminergic neurons are unresponsive to fluctuations in brain and plasma glucose, and argue against a role for central dopamine systems in the regulation of feeding behavior and energy metabolism.