Circuits regulating pleasure and happiness: evolution and role in mental disorders

Area of Science:

• Neuroscience
• Evolutionary Biology
• Psychiatry

Background:

• Emotional behaviors like reward-seeking and distress-avoiding are evolutionarily conserved, essential for survival and reproduction in animals.
• Early vertebrate ancestors possessed sophisticated systems for motion control and action-benefit evaluation, including the habenula.
• The human amygdalo-hippocampal complex evolved from early land animal endbrains, while the neocortex is a more recent mammalian development.

Purpose of the Study:

• To develop a biological framework for the subcortical regulation of human emotional behavior based on evolutionary principles.
• To explore the potential role of this framework in explaining the pathogenesis of key mental disorder symptoms.

Main Methods:

• The study adopts an evolutionary perspective, tracing the development of brain structures from early animals to humans.
• It focuses on the conserved habenular system and its role in regulating reward-seeking and distress-avoiding behaviors.
• The proposed framework integrates the amygdalo-hippocampal complex, habenula, mesencephalon, and ventral striatum.

Main Results:

• The habenular system has been conserved throughout vertebrate evolution, consistently regulating the intensity of pleasure-related and happiness-related behaviors.
• The framework suggests that human emotional regulation relies on a conserved amygdalo/hippocampal-habenular-mesencephalic-ventral striatal system.
• Maladaptations within this system are proposed as a basis for the primary symptoms observed in human mental disorders.

Conclusions:

• The evolutionary development of the forebrain provides a biological basis for understanding human emotional behavior regulation.
• Dysfunction in the ancient subcortical circuits, particularly the amygdalo/hippocampal-habenular-mesencephalic-ventral striatal system, may underlie mental disorders.
• This framework offers a novel perspective on the pathogenesis of psychiatric conditions.