Dopamine is transmitted via three major pathways. In obsessive compulsive disorder (OCD) transmission of dopamine from the substantia nigra to the basal ganglia and caudate nucleus-putamen (neostriatum) is thought to be increased. These regions of the brain are concerned with sensory stimuli and movement. The pathways projecting to the forebrain, mesolimbic system, and the tubero-infundibular system remain unaffected in OCD.
The principal centres for noradrenergic neurones are the locus coeruleus and the caudal raphe nuclei. In obsessive compulsive disorder (OCD) the transmission of noradrenaline is increased from the locus coeruleus to the frontal cortex, thalamus, hypothalamus and limbic system compared with normal. Nerves projecting from the locus coeruleus to the cerebellum and midbrain exhibit normal noradrenergic activity in OCD. The transmission of noradrenaline from the caudal raphe nuclei is also unchanged.
The principal serotonin centres in the brain are the caudal and rostral raphe nuclei. Transmission of serotonin ascending from the rostral raphe nuclei to the basal ganglia, thalamus, limbic cortex and cerebral cortex is decreased in OCD compared with normal. The other major pathways for serotonin transmission which project to the cerebellum and descending the spinal cord, remain unchanged.