Rats given dopamine-depleting brain lesions as neonates are subsensitive to dopaminergic antagonists as adults

Extensive damage to central dopamine (DA)-containing neurons are known to produce akinesia and sensory neglect when the lesions are made in adult rats. Similar behavioral impairments occur when dopaminergic function is disrupted temporarily by DA receptor blocking agents, and brain-damaged rats are particularly sensitive to the effects of those drugs. The present experiments offer a striking contrast to these well-accepted findings that central DA-containing neurons are critical to the initiation of voluntary movement. After near-total destruction of the dopaminergic neurons in 3-day-old rats, there were no conspicuous behavioral dysfunctions at any time during the subsequent 5-8 months, even when the animals were given large doses of DA receptor blocking agents. These findings suggest that some other neuronal system had replaced the absent dopaminergic neurons in the central control of movement.

Rats given dopamine-depleting brain lesions as neonates are subsensitive to dopaminergic antagonists as adults

Extensive damage to central dopamine (DA)-containing neurons are known to produce akinesia and sensory neglect when the lesions are made in adult rats. Similar behavioral impairments occur when dopaminergic function is disrupted temporarily by DA receptor blocking agents, and brain-damaged rats are particularly sensitive to the effects of those drugs. The present experiments offer a striking contrast to these well-accepted findings that central DA-containing neurons are critical to the initiation of voluntary movement. After near-total destruction of the dopaminergic neurons in 3-day-old rats, there were no conspicuous behavioral dysfunctions at any time during the subsequent 5-8 months, even when the animals were given large doses of DA receptor blocking agents. These findings suggest that some other neuronal system had replaced the absent dopaminergic neurons in the central control of movement.

Inhibitory controls of ingestion in 6-day-old rat pups

The ability of gastric preloads of milk, water, and isotonic saline to inhibit ingestion was studied in deprived 6-day-old rat pups being fed through oral cannulas. Preloads were delivered through gastric cannulas or by gavage, and both the volume of the load and the interval between the preload and testing were varied in order to assess the relative contributions of pre- and post-absorptive cues from the loads. Pups' willingness to initiate ingestion in short (10 min) tests (appetite) was unaffected by the degree of gastric filling produced by a load but was depressed by post-absorptive changes in hydrational state. On the other hand, pups terminated ingestion in long (30 min) tests (satiety) in response to gastric filling. Cessation of intake in long tests was only slightly influenced by post-absorptive factors. These results reveal two distinct phases of ingestion in infant rats and show that each is controlled in a different manner.

Effect of dopamine-depleting brain lesions on suckling and weaning in rats

Rats given large dopamine-depleting brain lesions as adults exhibit severe impairments in ingestive behavior and sensorimotor function. In contrast to these well-known effects, virtually complete destruction of central dopaminergic neurons produced no such dysfunctions when it occurred in neonates. Indeed, rats continued to suckle and grow, albeit somewhat more slowly, and they could be weaned readily when they were 27 days old. Although most brain-damaged animals did not survive weaning when they were 18 days old, whereas controls exhibited no difficulty, this failure appears to be the consequence of their reduced body weight and related inability to maintain body temperature in a relatively cool environment (22 degrees C). Such premature weaning occurred more successfully when growth was stimulated by rearing brain-damaged pups in small litters or when ambient temperatures were raised to 31 degrees C so as to minimize heat loss. These results demonstrate that the effects of near-total dopamine-depleting brain lesions are considerably less severe when they occur in infants than when they occur in adults, and, consequently, they reveal a capacity for neural plasticity during development that is no longer present at maturity.

Apparent sprouting of striatal serotonergic terminals after dopamine-depleting brain lesions in neonatal rats

Near-total dopamine-depleting brain lesions produced in 3-day-old rats by intracerebroventricular injection of the neurotoxin 6-hydroxydopamine led to pronounced increases in striatal serotonin (5-HT) and 5-hydroxyindoleacetic acid contents 1-8 months later. This effect was associated with an increase in in vitro high affinity 5-HT uptake, suggesting that proliferation of new serotonergic terminals had occurred within the striatum. No such effect was obtained when comparable brain lesions were produced in adult rats.

Determinants of suckling versus drinking in weanling albino rats: influence of hydrational state and maternal contact

Preferences of weanling albino rats (15, 20, and 25 days of age) for nonnutritive suckling or drinking were determined using a Y-maze. Age, hydrational state, and the opportunity to contact the mother all influenced choice behavior. When tested in a maze that allowed continuous maternal contact in the "drinking-goal box," dehydrated rats chose to drink, while nondehydrated rats chose to suckle. Testing dehydrated rats in a maze that precluded maternal contact in the "drinking-goal box" eliminated drinking preference at all ages. These observations identify maternal presence as an important condition for the expression of drinking behavior during the weaning period and confirm our previous demonstration of dehydration's inhibition of suckling behavior.

Olfactory contributions to dehydration-induced anorexia in weanling rats

By 20 days of age, dehydration produces a clear anorexia, even though weanling rats have had only limited feeding and drinking experience. Their lack of ingestive experience makes weanlings good subjects for studying the physiological mechanisms subserving anorexic phenomena because learned contributions are unlikely to add significant complications. Twenty-day-old rats dehydrated by hypertonic saline injection were anorexic when offered milk or solid food (rat chow), but not when offered sucrose solutions (Experiment I). However, when the scent of almond was associated with sucrose solutions, or with water, intake of these solutions was depressed by dehydration (Experiment II). Thus for dehydrated rats, olfactory stimulation may help produce dehydration-induced anorexia. Making rats anosmic by intransal lavage with ZnSO4 (Experiment III) eliminated the anorexia to almond-scented water and partially eliminated that to milk. For these fluids, an odor cue seems a requirement for the occurrence of dehydration-anorexia. In Experiment IV, we found that dehydration-anorexia did not occur when milk was infused directly into pups' mouths. This finding suggests that the inhibitory process mediating dehydration-anorexia influences the approach to food and not the actual consumatory response that occurs once food is in the mouth. The inhibition of feeding produced by dehydration, therefore, may depend largely on olfactory cues, and seems to operate at a distance, as rats locate and approach food.

Dehydration inhibits suckling behavior in weanling rats

The relation between dehydration and suckling behavior was determined in weanling rats 14, 20, and 23 days of age. After 15 days of age, intracellular and extracellular dehydration sharply reduced both the number of rats that suckled and the amount of milk consumed. Rehydration returned both behaviors to control levels. Thus, during the weaning period, the internal determinants of suckling are not homologous with those of drinking but are more homologous with those governing feeding.

Physiological and behavioral consequences in rats of water recycling during lactation

Radioactive water injected into 10-day-old rats were recovered the following day from their mothers' blood. This transfer of water was nearly eliminated by ligating the injected pups' urethras, a result indicating that mother rats consume their offspring's urine. Preventing urine consumption by ligating urethras of all pups in a litter doubled the plasma volume deficit in dams produced by 24 hr of water deprivation alone and led to increased isotonic saline, but not water, intakes following after water deprivation. Mother rats' daily water intakes increased after they were deprived of pup urine, and their pup-licking behavior increased following water deprivation. These findings indicate that urine consumption by mother rats has both physiological and behavioral consequences and suggest that pup licking is, in part, an ingestive behavior.

Sensory determinants of suckling behavior in weanling rats

Exteroceptive determinants of nipple attachment in albino rat pups were studied by altering maternal sensory features. Rats 15, 20, 25, and 30 days of age were permitted to locate and attach to the nipples of their anesthetized mother after she had received one of the following treatments: (a) thermalnipple and peripheral skin temperatures were lowered to either 31 or 28 degrees C, (b) tactile--the mother's ventrum was shaved clean; (c) thermotactile--the mother was shaved, and her peripheral temperature was lowered to 31 degrees C; and (d) olfactory--all nipples were thoroughly cleaned with methylene chloride and ethanol solvents. Only the nipple-wash treatment consistently affected nipple attachment. Washing the nipple disrupted suckling; returning a vacuum distillate of the wash extract or of pup saliva reinstated attachment to normal levels. Thus, in sighted weanling rats, as in younger rat pups, olfactory stimuli remain the major determinants of nipple attachment.