The etiology of depressive illness has been linked with brain monoaminergic neuronal dysfunction, yet the development of sensitive markers of endogenous depression has proven difficult.
Using catheters placed in an internal jugular vein, we estimated the release of brain monoamine neurotransmitters in 19 healthy volunteers and in 9 patients with nonbipolar depressive illness refractory to medication at rest and following intravenous desipramine hydrochloride. Venoarterial plasma concentration gradients were used to quantify the amount of neurotransmitters stemming from the brain. Cerebral oxidative metabolism was assessed concurrently from measurements of oxygen and carbon dioxide gas exchange via the process of regional indirect calorimetry.
The brains of these patients exhibited reduced venoarterial norepinephrine (4.0 ± 2.7 nmol/L vs 0.7 ± 1.3 nmol/L) and homovanillic acid concentration gradients (8.3 ± 7.8 nmol/L vs 3.1 ± 1.9 nmol/L), and used an energy source other than glucose. Internal jugular 5-hydroxyindoleacetic acid concentration gradients were not reduced in the patients with depressive illness. While both the reduction in norepinephrine turnover and the defect in cerebral metabolism were normalized following pharmacological blockade of the norepinephrine transporter with desipramine, paradoxically it was the brain's turnover of dopamine that bore a significant relation to the patients' clinical status (rs = 0.79, P = .02). The positive nature of this relationship remains difficult to reconcile.
In accordance with the monoamine hypothesis, a deficit in brain norepinephrine and dopamine exists in patients with depressive illness. Moreover, the brains of these patients use an energy source other than glucose, a situation that is normalized following the acute pharmacological blockade of the norepinephrine transporter with the tricyclic antidepressant, desipramine.