This paper describes a hypothesis that attempts to account for how changes in noradrenergic systems in the brain can affect depression-related behaviors and symptoms. It is hypothesized that increased activity of the locus coeruleus (LC) neurons, the principal norepinephrine (NE)-containing cells in the brain, causes release of galanin (GAL) in the ventral tegmentum (VTA) from LC axon terminals in which GAL is colocalized with NE. It is proposed that GAL release in VTA inhibits the activity of dopaminergic cell bodies in this region whose axons project to forebrain, thereby resulting in two of the principal symptoms seen in depression, decreased motor activation and decreased appreciation of pleasurable stimuli (anhedonia). The genesis of this hypothesis, which derives from studies using an animal model of depression, is described as well as recent data consistent with the hypothesis. The formulation proposed suggests that GAL antagonists may be of therapeutic benefit in the treatment of depression.

Previous studies suggest that all effective antidepressant (AD) drugs decrease activity of locus coeruleus (LC) neurons. However, little data exist regarding blood levels of drug in these studies, and what data do exist suggest blood levels might have been very high. To assess whether decreased LC activity is produced by drugs that selectively block reuptake for either norepinephrine or serotonin at therapeutically relevant blood levels, effects of chronic administration of desipramine, paroxetine, and escitalopram on LC activity were measured across a range of doses and blood levels of drug. Further, effects of a range of doses of mirtazapine were examined; in that mirtazapine blocks α2 adrenergic receptors, it might be anticipated to increase rather than decrease LC activity. Finally, to begin to assess whether the response of LC to ADs was specific to these drugs, effects of four non-AD drugs (single dose) were measured. Drugs were administered via osmotic minipump for 14 d. Electrophysiological recording of LC activity (assessment of both spontaneous firing rate and sensory-evoked ‘burst’ firing) then took place under isoflurane anaesthesia on the last day of drug treatment. The blood level of drugs present at the end of the recording session was also measured. All AD drugs tested decreased LC spontaneous and sensory-evoked ‘burst’ firing, and this was observed across a wide range of blood levels for the drugs. Non-AD drugs did not decrease LC activity. The findings of this investigation continue to support the possibility that all effective AD drugs decrease LC activity.

No abstract

No abstract

No abstract