Sodium Appetite Sensitization

Excerpt

Sodium is necessary for the survival of most, if not all, animals. It is the most prevalent of extracellular solutes, and because it is largely excluded from the intracellular fluid compartment by the cell plasma membrane, it in a sense comprises the “true grit” of the extracellular fluid. The maintenance of near-ideal concentrations of intracellular constituents is necessary for optimum cellular fitness and function. The concentration of impermeable solutes such as sodium in the extracellular fluid compartment is the major determinant of water movement across the plasma membrane. A high concentration of sodium in the extracellular space causes water to move out of cells and produces dehydration of the intracellular fluid compartment, whereas a low concentration of extracellular solute causes overhydration of intracellular space. In addition to its importance in establishing the distribution of fluid across semipermeable membranes, the relative concentration of extra- and intracellular sodium can also influence cellular functions. For example, the resting membrane potential of cells is in large part dependent on the relative concentrations of intracellular versus extracellular sodium, and the shape of action potentials generated by neurons is affected by the ratio of these two sodium concentrations.

Animals cannot synthesize sodium, so that when the ion is lost from the body it must be acquired from external sources and ingested in order to restore homeostasis. Many mammalian species, especially omnivores and herbivores, display motivated behaviors of seeking and ingesting sodium-containing substances when they are sodium deficient. Collectively, the set of behaviors and the motivational state (i.e., drive) that mobilizes and directs animals to finding and consuming substances containing sodium is most often referred to as sodium or salt appetite. A common operational definition of sodium appetite is simply a significant increase in the intake of unpalatable saline solutions (usually 1.5% to 3% NaCl w/v) over a specified period in response to a deficit. Such hypertonic saline solutions are normally consumed in minimal quantities, or not consumed at all, by sodium-replete animals.