Integration of oral habituation and gastric signals in decerebrate rat pups.

During a series of oral infusions of a sweet solution, the ingestive responses of young rat pups habituate; pups stop responding to the infusions even when their stomachs are empty and the infused diet is nonnutritive. The rate of this oral habituation is enhanced by the addition of gastric fill signals, even in decerebrate pups. In intact but not in decerebrate pups, prior deprivation gates out the influence of gastric fill on habituation. This oral habituation system, responsive to multiple ingestion-related signals, may serve as the elemental process that integrates physiological state with ongoing behavior to control ingestion.

Control of ingestion by oral habituation in rat pups.

The role of habituation of mouthing activity in the control of ingestion was investigated in 6-, 12-, and 18-day-old rat pups. In pups at all ages, oral habituation to a flavored diet inhibited ingestion of a continuous oral infusion of that same diet. Twelve-day-old pups that had orally habituated to a diet continued to consume less of a continuous oral infusion of that diet both 30 min and 3 hr later, and the duration of suppressed ingestion was shown to be dependent on the rate of stimulus presentation during habituation experience. These data suggest that oral habituation may be a diet-specific influence on both intra- and intermeal patterning.

Oral habituation in rat pups is in the brainstem.

We have previously demonstrated a decremental, oral, experienced-based control of ingestion in rat pups that is potent, diet-specific, and long lasting. This control of ingestion is revealed in the decreases in mouthing responses of rat pups to repeated oral stimulation, a phenomenon that is well described as oral habituation. The present study examined the neural basis for oral habituation by examining the responses of decerebrate 10-day-old rat pups to repeated, brief infusions of a sucrose solution. Like neurologically intact pups, decerebrate pups showed distinct decreases in mouthing responses following a series of oral infusions. Thus, oral habituation is present in the hindbrain. Although intact pups demonstrated sensitization by showing increased responding to the first few stimulus presentations, decerebrates failed to show a sensitization to the stimulation. These results suggest that while the brainstem alone is sufficient for the expression of oral habituation, the forebrain also influences oral responsivity in intact animals.

Habituation of oromotor responding to oral infusions in rat pups.

The influence of oromotor experience on the pattern of ingestion in rat pups and the relation of this influence to age and pups' physiological state were investigated using a procedure designed to mimic the sham-feeding preparation in adult rats. Six-, 12-, and 18-day-old pups received brief intra-oral infusions of sucrose solutions once every minute. Small infusion volumes minimized postoral effects. Pups' oromotor responsiveness was assessed by recording the pattern of mouthing behavior continuously during the test. Pups were tested after 24, 6 or 0 h deprivation. During testing, the mouthing behavior of all pups except 24-hr deprived 6-day olds showed a marked decline. The specificity of this decrement was demonstrated in a second experiment in which the decremented response was restored by a switch in solution flavor. Finally, the influence of postoral signals on the decline in responsiveness was evaluated by comparing the oral responsiveness of 18-day-old pups following intragastric, oral, or no infusions. Oral infusions suppressed subsequent oral responding, but intragastric infusions did not. These results provide evidence for a habituation-like role of oromotor experience in determining patterns of ingestive behavior within a feeding test. Here, major determinants of the pattern of decline were pups' physiological state and developmental age.

Analysis of 2-DG autoradiograms using image-averaging and image-differencing procedures for systems-level description of neurobehavioral function.

Computer assisted 2-deoxyglucose (2-DG) autoradiography has been used to provide functional maps of areas of altered neural activity related to changes in an animal's behavior or state. The standard procedure for comparison of autoradiograms between different treatment groups has been to take measurement samples from predefined neuroanatomical regions and to average these across brains to attain statistical sensitivity for detecting treatment effects. Unfortunately, when sampling is restricted to predefined areas, important topographic information is lost along with the ability to reveal an unexpected change in neural activity. To preserve the rich topographical detail of metabolic information and to enhance the capacity to uncover novel areas of altered metabolic activity, we have developed a system for averaging entire images from 2-DG autoradiograms and for comparing the average images from two experimental groups by creating an image of differences. This procedure does not rely on sampling only preselected regions, but still allows statistical comparisons between experimental groups. The procedures we describe can be readily and inexpensively adapted for use in individual laboratories and are based on modifications of preexisting image analysis software. We show that, when average and difference images are created using standardized protocols for sectioning brain tissue and editing section images, they are impressively resolved and realistic and can serve as effective topographic descriptions of group differences in neural activity of functional and behavioral relevance.