The ontogeny of salt preference in rats

Abstract

Many mammals eat salt irrespective of need. This behavior, called salt preference or appetite, is studied primarily in adults. Little is known about its ontogeny. In these experiments, 3-18-day-old rat pups were offered saline, quinine, or ammonium chloride solutions by infusion through an anterior oral catheter, and intake was measured. At 6-18 days, pups showed the inverted U-shaped preference-aversion curve for NaCl that is characteristic of adult rats. Thus, rats express a preference for salt at a very early age. However, the curves were broader than the typical adult curve and were shifted along the concentration gradient in an age-related fashion. Consumption of quinine and ammonium chloride showed similar age-related changes. These changes may reflect the postnatal timing of the development of the rat gustatory system.

Salt appetite consequent on sodium depletion in the suckling rat pup

“…This early preference for salt has been shown previously (Bernstein & Courtney, 1987;Moe, 1986) so that it is the ability to respond to the deficit which matures at this age rather than the ability to detect salt by taste.…”

Ontogeny of urine preference and its relationship to NH4Cl preference and sodium hunger in suckling rat pups

“…Since urine preference in the adult is related to sodium need, the ontogeny of urine preference may be related to the ontogeny of the preference for sodium and sodium hunger (Bernstein & Courtney, 1987;Leshem, 1999;Leshem, Neufeld, & del Canho, 1994;Moe, 1986). Suckling rat pups also have a heightened preference for ammonium chloride that peaks at about 2 weeks of age and exceeds the preference for NaCl (Bernstein & Courtney, 1987;Leshem et al, 1994;Moe, 1986;Ogawa, Hasegawa, Ohgushi, & Murayama, 1994, Sollars & Bernstein, 1994, 1996Schafe & Bernstein, 1997).…”

“…Suckling rat pups also have a heightened preference for ammonium chloride that peaks at about 2 weeks of age and exceeds the preference for NaCl (Bernstein & Courtney, 1987;Leshem et al, 1994;Moe, 1986;Ogawa, Hasegawa, Ohgushi, & Murayama, 1994, Sollars & Bernstein, 1994, 1996Schafe & Bernstein, 1997). The heightened preference for NH 4 Cl is accompanied by a heightened neural response (Hill & Mistretta, 1990).…”

“…We further speculate that urine preference in the suckling also might be a means to recover sodium in the nest during the first 2 to 3 weeks of the pup's life because of its limited mobility and because sucklings up to 2 weeks of age are insensitive to sodium concentrations below 0.5 to 1.0 M (Bernstein & Courtney, 1987;Hill & Mistretta, 1990;Leshem & Epstein, 1989;Leshem et al, 1994;Moe, 1986) while the concentration of sodium in pup urine is only 0.02 M. Pup urine in the nest is a source of sodium-at least for the dam (Friedman et al, 1981;Gubernick & Alberts, 1983;Moore & Power, 1992). Hence, the preference for urine may guide the pup to its insensible concentration of sodium.…”

“…As weaning is approached, the spontaneous preference for sodium develops toward the adult sodium appetite (Bernstein & Courtney, 1987;Leshem et al, 1994;Moe, 1986) as does the response to sodium depletion (Leshem, 1999;Leshem et al, 1994); however, unlike 2-week-old pups, adult rats do not prefer NH 4 Cl to NaCl, and the salts are behaviorally dissociated because chorda tympani transection disrupts preference for NH 4 Cl, but not for NaCl (Breslin, Spector, & Grill, 1995;Sollars & Bernstein, 1996), amiloride blocks conditioned taste aversion to low concentrations of NaCl, but not to NH 4 Cl (Hill, Formaker, & White, 1990), and when rendered sodium hungry, adult rats do not increase NH 4 Cl intake (Breslin et al, 1995;Leshem et al, 1994;Markison, St. John, & Spector, 1995). Neurophysiological studies confirm the distinction between the two salts (Hill & Mistretta, 1990;Kitada, Mitoh, & Hill, 1998;Lundy, Pittman, & Contreras, 1997) and suggest that the parabrachial nucleus may mediate the neural and behavioral responses (Nishijo & Norgren, 1997).…”