49: Homeostasis, Motivation, and Addictive States

Introduction

• Drinking Occurs Both in Response to and in Anticipation of Dehydration

  • Body Fluids in the Intracellular and Extracellular Compartments Are Regulated Differentially

  • The Intravascular Compartment Is Monitored by Parallel Endocrine and Neural Sensors

  • The Intracellular Compartment Is Monitored by Osmoreceptors

  • Motivational Systems Anticipate the Appearance and Disappearance of Error Signals

• Energy Stores Are Precisely Regulated

  • Leptin and Insulin Contribute to Long-Term Energy Balance

  • Long-Term and Short-Term Signals Interact to Control Feeding

• Motivational States Influence Goal-Directed Behavior

  • Both Internal and External Stimuli Contribute to Motivational States

  • Motivational States Serve Both Regulatory and Nonregulatory Needs

  • Brain Reward Circuitry May Provide a Common Logic for Goal Selection

• Drug Abuse and Addiction Are Goal-Directed Behaviors

  • Addictive Drugs Recruit the Brain's Reward Circuitry

  • Addictive Drugs Alter the Long-Term Functioning of the Nervous System

  • Dopamine May Act As a Learning Signal

• An Overall View

The temperature of the air at higher latitudes can fluctuate by 70°C (158°F) or more over the year, yet some birds and mammals live year round in such environments without hibernating or estivating. These animals keep their core temperatures within a narrow range, on the order of a few degrees, during both the fierce blizzards of winter and the sultry days of summer. This regulatory feat is just one of many that keep key physiological variables within limits favorable to vital processes of the body, such as cell division, energy metabolism, macromolecular synthesis, and cell signaling.

The active maintenance of a relatively constant internal environment is called homeostasis. Constancy of the internal environment is the basis of the freedom of action we and other animals enjoy because it partially decouples our physiology from immediate external conditions and greatly extends the range of available habitats. For example, salmon are able to live in both fresh and salt water because they can regulate the osmolality of their extracellular fluid. Hagfish, conversely, are confined to marine habitats because they cannot regulate the osmolality of their extracellular fluid, which reflects that of the external environment.

In climates with wide variation in temperature the budget-minded homeowner may set the thermostat to a lower value during the winter and a higher one during the summer; more sharply contrasting daytime and nighttime settings may be chosen when heating or air-conditioning costs are high. Similarly, in physiological systems the means and variances of regulated variables may be adjusted over the course of the day, the seasons, and the life cycle. For example, a dehydrated camel conserves water by letting its temperature increase above normal before beginning to sweat; at night it lets its temperature decrease below normal, thus starting the day cooler and delaying the onset of sweating. The zoologist Nicholas Mrosovsky has coined the term rheostasis to refer to the linkage of regulatory targets and ranges to chronobiological and life-cycle events.

Regulation is achieved through interlinked control systems with both physiological and behavioral outputs (Box 49–1...