Neural degeneration following chronic stimulant abuse reveals a weak link in brain, fasciculus retroflexus, implying the loss of forebrain control circuitry.

There is increasing evidence that the fasciculus retroflexus (FR) represents a 'weak link' following the continuous administration of drugs of abuse. A variety of drugs which predominantly potentiate dopamine, including D-amphetamine, methamphetamine, MDMA, cocaine, and cathinone, all induce degeneration in axons from lateral habenula, through the sheath of FR, to midbrain cells such as SN, VTA, and raphe. For some drugs, such as cocaine, this is virtually the only degeneration induced in brain. Continuous nicotine also selectively induces degeneration in FR, but in the other half of the tract, i.e. in axons from medial habenula through the core of the tract to interpeduncular nucleus. This phylogenetically primitive tract carries much of the negative feedback from forebrain back onto midbrain reward cells, and the finding that these descending control pathways are compromised following simulated drug binges has implications for theories of drug addiction but also psychosis in general.