Ontogeny of the ionic specificity of sodium appetite in the rat pup

Abstract

Sodium-deficient adult rats prefer NaCl to other monochloride salts (e.g., Denton, 1991; Schulkin, 1991). However, it is not known when or how this specificity develops. Our experiments charted the development of the ionic specificity of sodium appetite aroused by sodium depletion or intracerebroventricular injection of renin. We compared intake of 3% NaCl to three other monochlorides, potassium (K), ammonium (NH4), and lithium (Li), and calcium chloride (CaCl2) at various ages between 72 hr postnatal and weaning. This revealed a biphasic developmental scheme: The adult pattern of discrimination between the salts emerges between 3 and 18 days of age. Subsequently, the preference for Na over the other salts increases into adulthood.

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.…”

“…Hence, the preference for urine may guide the pup to its insensible concentration of sodium. Although there is no direct evidence that pups ingest urine in the nest as do dams, thirsty, hungry, or sodiumdepleted pups will lick appropriate fluids (Leshem, 1999;Leshem, Boggan, & Epstein, 1988;Leshem, del Canho, & Epstein, 1993;, 1989Leshem, Langberg, & Epstein, 1993;Leshem et al, 1994;1999;Weller, Smith, & Gibbs, 1990;Wirth & Epstein, 1979).…”

“…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).…”