The ontogeny of drinking evoked by activation of brain angiotensin in the rat pup

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

We chart the postnatal ontogeny of urine preference in the suckling rat. Twelve-day-old sucklings, when offered urine, NH4Cl, or NaCl, ingest more urine and NH4Cl than NaCl. When rendered sodium hungry by ivc renin or by sodium depletion, these sucklings prefer urine and NH4Cl to NaCl, dilute urine, or an NaCl and KCl mineral mix equimolar to urine; however, by 18 days of age, urine and NH4Cl are no longer preferred to NaCl. Hence, urine preference in the suckling may be specific and preparatory for the variety of purposes urine preference serves in the adult rat, and it might guide the pup to urinary sodium in the nest. Since preference for urine and NH4Cl covary during postnatal development, the high preference for NH4Cl in midterm sucklings might be because its ammonium flavor is similar to urine.

The ontogeny of salt hunger in the rat

Salt hunger is the behaviour of an animal suffering sodium deficiency. It is characterised by an increased motivation to seek and ingest sodium, and the ability to distinguish between sodium and other salts. Here I review the development of salt hunger in the rat. Salt hunger develops rapidly between birth and weaning. It can first be demonstrated 72 h postnatally when an intracerebroventricular injection of renin elicits greater swallowing of NaCl solution than water and greater mouthing of solid fragments of NaCl than of an artificial sweetener. However, sodium deficit per se cannot arouse the hunger at this age, and first elicits increased intake of NaCl only at 12 days-of-age. The next landmark is at 17 days-of-age when the hormonal synergy of aldosterone and central angiotensin II first elicits salt hunger, as it does in the adult. The specificity of the hunger for the sodium ion also develops postnatally: the 72 h-old sodium-hungry neonate does not distinguish between NaCl and other mono- and di-valent chloride salts but, increasingly during development, the sodium hungry pup distinguishes salts and by weaning age NaCl is clearly preferred to other salts almost as it is in adults. Early development may also be a sensitive period for determining lifelong preferences, and indeed, acute perinatal sodium depletion induces a lifelong enhancement of salt intake. Taken together, these findings demonstrate how a behaviour develops precociously and how, when the behaviour becomes important at weaning, the rat pup is competent to meet its sodium requirements, and may be adapted to anticipate sodium deficit.

Effects of physiological state on oral habituation in developing rats: cellular and extracellular dehydration

Hydrational state has been demonstrated to influence intake of various solutions in young rat pups. For instance, both cellular and extracellular dehydration produce an enhancement of intake in pups tested at 6 days of age. However, the behavioral mechanisms that result in increased intake following manipulations of hydrational state have been less extensively studied. The impact of hydrational state on behavioral responsiveness in young rat pups was examined by assessing the pattern of responding to a series of repeated oral infusions of diet. Pups were tested at 6, 12, or 18 days of age following either acute cellular dehydration produced by injection of 1 M NaCl or acute extracellular dehydration produced by injection of 10% polyethylene glycol (PEG). Oral responsiveness to a series of 30 brief infusions of one of four taste solutions (water, 10% sucrose, 0.135 M NaCl, or 1 M NaCl) was measured. Each infusion lasted 3 s and there was 1 min between infusions. The pattern of oral responding to solutions was affected by the developmental age of the pup, the hydrational state of the pup, and the solution offered, with the largest effects of dehydration observed in the youngest animals. In all conditions except one, pups habituated to repeated infusions. The exception was the failure of extracellularly dehydrated 6-day-old pups to display habituation to oral infusions of sucrose. These results suggest that, although intake is enhanced by both cellular and extracellular dehydration in very young pups, the behavioral changes responsible for the enhancement of intake after cellular dehydration are different from the behavioral changes resulting from extracellular dehydration. This dissociation of behavioral effects of dehydration in young pups demonstrates that intake measures alone may obscure subtle differences in behavior and argues for the utility of dissection of behavioral components in understanding the neural and physiological control of behavior.

Intracerebroventricular injection of renin in the neonatal rat reveals a precocious sodium appetite that is dissociated from renin-aroused thirst

Previous research on the ontogeny of sodium appetite in the rat has shown that sodium deficit first engenders sodium intake at 12 days of age, whereas direct stimulation of the brain renin--angiotensin system by intracranial injection of renin increases intake of NaCl solution as early as 3 days postnatally. Similar activation of brain angiotensin also increases thirst, so that the specificity of the precocious sodium intake remains undetermined. In this article we report experiments that dissociate neonatal renin-evoked sodium appetite and thirst, and establish the specificity of the appetite. Our findings confirm that sodium appetite can first be discerned at 3 days of age, and show that it rapidly develops until 12 days of age. During this developmental window, renin-evoked sodium appetite is dissociated from thirst because (a) NaCl is preferred to water, (b) the appetite develops faster than thirst, and (c) 3-day-old renin-stimulated pups will avidly lick dry NaCl. These results show that activation of brain angiotensin in the 3-day-old rat pup evokes a precocious and specific sodium appetite.

Salt appetite consequent on sodium depletion in the suckling rat pup

The ontogeny of the behavioral ability to compensate for sodium deficit was studied in the rat. The experiments showed that: 1) Before weaning age, sodium-depleted pups will increase their avidity for 3% NaCl solution; 2) the ability to select and drink a salt solution in response to a sodium deficit continues to evolve between 17-24 days of age, and that pups at these ages will modify their intake of salt and water as do adult rats when rectifying plasma osmolality; 3) The increased appetite for sodium is evident even when depleted preweanlings are dehydrated and provided with solid NaCl tablets to lick, showing that sodium appetite and hydrational status are already dissociated at this age; and finally, 4) sodium depletion first induces an increase in intake of orally infused 3% NaCl solution in 12-day-old pups. The picture of the development of salt appetite in the suckling rat that these findings present is of a precocious competence to meet a challenge to sodium homeostasis. In this respect salt appetite emerges in parallel to the other ingestive behaviors.

Functional roles of brain AT1 and AT2 receptors in the central angiotensin II pressor response in conscious young spontaneously hypertensive rats

Areas of adult rat brain that mediate the cardiovascular effects of central angiotensin II (ANG II) predominantly express AT1 ANG II receptors. In contrast, AT2 receptor expression in young rats is transiently increased, reaching a maximum during the first few weeks of life. This study was designed to determine the roles of brain AT1 and AT2 receptors in mediating the central pressor effects of ANG II in young (4-week-old) conscious spontaneously hypertensive rats (SHR). Mean arterial pressure responses to intracerebroventricular (i.c.v.) ANG II (100 ng in 5 microliters) were determined 10 minutes after i.c.v. injection of either the AT1 receptor antagonist Losartan (1.0, 2.5, 5.0, and 10.0 micrograms), the AT2 receptor ligand PD 123319 (3.5 x [10(-6), 10(-4), 10(-2), 10(0)] micrograms), or both. In control rats, i.c.v. Losartan prevented the pressor response to i.c.v. ANG II in a dose-dependent manner (P < 0.05), while i.c.v. PD 123319 alone was without effect. In other animals, pressor responses caused by i.c.v. ANG II-induced vasopressin secretion (VP-component) and sympathetic nervous system activation (SNS-component) were studied individually, with similar result; Losartan prevented the SNS-component, but reduced the VP-component by only 45%, indicating that both pressor components involve AT1 receptor activation. However, doses of Losartan were more effective when combined with 3.5 micrograms of PD 123319 than when given alone (P < 0.05); nearly eliminating the VP-component. These results suggest that i.c.v. ANG-II-induced pressor effects may involve activation of multiple receptor subtypes.

Ontogeny of angiotensinogen mRNA and angiotensin II receptors in rat brain and liver

The renin-angiotensin-system (RAS) is active in fetal and neonatal life. This study was undertaken to examine the ontogenic regulation of angiotensinogen (AT) gene expression and angiotensin II (A II) receptors in liver and brain. AT gene expression was studied in fetal, neonatal, adult and aged rats, using slot blot hybridization to quantify AT mRNA levels. During fetal life (gestational days 15-20), AT mRNA was more abundant in brain than in liver. Soon after birth, brain AT mRNA levels increased to a concentration 3 fold above fetal levels. In contrast, liver AT mRNA abundance increased 30-fold within 12 h of birth. Aging (3-20 months) resulted in a gradual decrease in AT mRNA in both the brain and liver. Liver A II receptors in the neonate were 2-fold higher than in the fetus, but returned to fetal levels by 8 weeks of age. In the brain, A II receptor abundance increased to a level 75% above fetal levels in 7 days old animals, but returned to fetal levels by 14 days of age. These studies suggest than in the fetus, the liver is not the primary source of AT but that unknown factors at parturition result in a dramatic increase in liver AT mRNA. In contrast, the more modest increases in brain AT mRNA parallel the gradual maturation of the CNS. In both tissues, further aging resulted in a gradual decrease in AT mRNA, reflecting either increased sensitivity to feedback downregulation by A II or age related increases in other extrahepatic sites of AT synthesis. Age related changes were also found in the A II receptor in both the liver and brain.

The ontogeny of salt appetite aroused by depletion in the rat

The emergence of the rat's ability to respond behaviorally to a bodily sodium deficit was examined. Sucklings were depleted of bodily sodium by furosemide injections and their ability to replenish sodium by imbibing 3% NaCl solution was measured at different ages. The results suggest that the appetite for salt, as a response to sodium deficit, matures at weaning age.

Ontogeny of renin-induced salt appetite in the rat pup

Intracerebroventricular injections of renin in suckling rat pups increased intake of NaCl solutions when they were orally infused 5 hr after injection. The appetite for saline solution was evident in pups as young as 3 days, was greater in females, and was specific insofar as intake of milk, either by suckling or by oral infusion, was not affected. Three-day-old pups increased intake only to 12% NaCl, the acceptable concentration of NaCl becoming lower in older pups. These results suggest, first, that, as is true for feeding and drinking, the brain mechanism for salt appetite is competent for expression of the behavior in the very young rat pup, and second, that its angiotensinergic neural substrate is distinct from that which mediates the dipsogenic effect of the hormone.

The development in infant rats of kassinin's potent and selective control of cell-dehydration thirst

In infant rats, kassinin exerts its antidipsogenic effect in the very early stages of neonatal life (2nd-3rd day). The inhibition of cell-dehydration drinking appears in rats of 2 days, and attains adult levels in pups of 9 days. Instead, the thirsts induced by suckling deprivation or by intracerebroventricular angiotensin II are inhibited by kassinin precociously (3rd day), but are unaffected by it in rats of 12-15 days. Kassinin also inhibits milk intake very early (3rd day) and this effect also disappears at 12 days of age. The pattern of ontogenetic results described here may be that of a brain kassinin-like tachykinin that, in the course of the development of the neural structures on which it acts, gains potent and selective control of cell-dehydration thirst.

Histamine-elicited drinking in weanling and adult rats

A total of 260 male and female adult (60-70 days of age) and weanling (22-25 days of age) Sprague-Dawley derived rats were used in these experiments. Subcutaneous administration of histamine (HA) elicited drinking in a dose-dependent manner for both ages tested, although the threshold dose varied with age. A dose of 5.0 mg/kg HA elicited significant increases in water intake for adults, whereas for weanlings a dose of 20 mg/kg HA was necessary. Adult rats exhibited decreased latency to drink after all doses of HA tested, whereas for weanlings, decreased latency was evident only after doses of HA sufficient to elicit increases in water intake. Combined antagonism of H1 and H2 receptors for HA, using dexbrompheniramine and cimetidine, respectively, inhibited HA-elicited drinking in adults and weanlings. Further investigation of the ontogeny of histamine- and food-related drinking may provide a useful approach to examine the physiological mechanisms underlying fluid consumption in adult animals and as they are gradually elaborated during ontogeny.

Thirst-induced anorexias and the ontogeny of thirst in the rat

In previous work (Leshem, M., Boggan, B., and Epstein, A.N. (1988). The ontogeny of drinking evoked by activation of brain angiotensin in the rat pup. Dev. Psychobiol. Vol.21, pp. 63-75) we showed that thirst elicited by activation of the brain's renin-angiotensin system in the suckling becomes specific to water after 16 days of age. However, in the suckling, we did not find the anorexia that reportedly accompanies angiotensin-induced thirst in the adult. This suggests the existence of a further stage in the ontogeny of thirst. Therefore, the present study pursued the ontogeny of thirst and its effects on milk intake through prepubescence into adulthood. Experiment 1 revealed that intracranial renin does not cause an anorexia to milk in prepubescent or adult rats. Experiment 2 showed that the absence of anorexia is true of thirst induced by renin but not by cellular dehydration, although both dipsogens suppressed milk intake when rats also had water available. Experiment 3 confirmed that the preweanling shows anorexia to solid food, as does the adult. Together with other work, these findings suggest that the ontogeny of the thirsts aroused by renin or intracellular dehydration is complete before weaning.