The pituitary-adrenal response to “minimized” schedule-induced drinking

Previous research has indicated that hungry rats engaging in polydipsic drinking while on an intermittent food-delivery schedule exhibit a significant drop in plasma corticosterone levels during a 30-min experimental session; rats that do not have water available during sessions maintain their presession corticosterone levels, however. This effect of schedule-induced drinking has been interpreted as reflecting primarily psychological factors rather than peripheral physiological factors. This interpretation was supported in the present experiments. In Experiment 1, corticosterone levels in polydipsic rats were determined at 0, 10, 20 and 30 min during an intermittent-feeding session in order to assess the time course of the effect. In Experiment 2, fluid consumption by polydipsic rats during a 30-min session was reduced by presenting a saccharine solution previously paired with illness in a conditioned taste aversion procedure. The results of both experiments indicate that significant reductions in corticosterone from presession levels are found even when the amount of fluid consumed during a session is relatively small.     

Ethanol intake during schedule-induced polydipsia

Schedule-induced polydipsia was established by maintaining 4 male Sprague-Dawley rats on concurrent variable interval 1 min food reinforcement and continuous water reinforcement. Five ethanol concentrations (2, 4, 8, 16, and 32% W/V) were substituted for water. The number of liquid reinforcements and volume of liquid consumed varied inversely with ethanol concentration while the quantity of ethanol intake varied directly with the concentration. Ethanol reinforcements were above water control levels during the first 10 min ethanol was present. For the same interval increases occurred in the number of ethanol, but not water reinforcements when each concentration was presented a second time. Decreases occurred in the food baseline as a function of concentration with intermediate concentrations having the greatest effect. These decreases were greater the second time the ethanol concentrations were presented.     

Human thermoregulation and the cardiovascular system

A key but little understood function of the cardiovascular system is to exchange heat between the internal body tissues, organs and the skin to maintain internal temperature within a narrow range in a variety of conditions that produce vast changes in external (exogenous) and/or internal (endogenous) thermal loads. Heat transfer via the flowing blood (i.e. vascular convective heat transfer) is the most important heat-exchange pathway inside the body. This pathway is particularly important when metabolic heat production increases many-fold during exercise. During exercise typical of many recreational and Olympic events, heat is transferred from the heat-producing contracting muscles to the skin surrounding the exercising limbs and to the normally less mobile body trunk and head via the circulating blood. Strikingly, a significant amount of heat produced by the contracting muscles is liberated from the skin of the exercising limbs. The local and central mechanisms regulating tissue temperature in the exercising limbs, body trunk and head are essential to avoid the deleterious consequences on human performance of either hyperthermia or hypothermia. This brief review focuses on recent literature addressing the following topics: (i) the dynamics of heat production in contracting skeletal muscle; (ii) the influence of exercise and environmental heat and cold stress on limb and systemic haemodynamics; and (iii) the impact of changes in muscle blood flow on heat exchange in human limbs. The paper highlights the need to investigate the responses and mechanisms of vascular convective heat exchange in exercising limbs to advance our understanding of local tissue temperature regulation during exercise and environmental stress.     

Human cardiovascular dose-response to supplemental oxygen

Aim: The aim of the study was to examine the central and peripheral cardiovascular adaptation and its coupling during increasing levels of hyperoxaemia. We hypothesized a dose‐related effect of hyperoxaemia on left ventricular performance and the vascular properties of the arterial tree. Methods: Oscillometrically calibrated arterial subclavian pulse trace data were combined with echocardiographic recordings to obtain non‐invasive estimates of left ventricular volumes, aortic root pressure and flow data. For complementary vascular parameters and control purposes whole‐body impedance cardiography was applied. In nine (seven males) supine, resting healthy volunteers, aged 23–48 years, data was collected after 15 min of air breathing and at increasing transcutaneous oxygen tensions (20, 40 and 60 kPa), accomplished by a two group, random order and blinded hyperoxemic protocol. Results: Left ventricular stroke volume [86 ± 13 to 75 ± 9 mL (mean ± SD)] and end‐diastolic area (19.3 ± 4.4 to 16.8 ± 4.3 cm²) declined (P < 0.05), and showed a linear, negative dose–response relationship to increasing arterial oxygen levels in a regression model. Peripheral resistance and characteristic impedance increased in a similar manner. Heart rate, left ventricular fractional area change, end‐systolic area, mean arterial pressure, arterial compliance or carbon dioxide levels did not change. Conclusion: There is a linear dose–response relationship between arterial oxygen and cardiovascular parameters when the systemic oxygen tension increases above normal. A direct effect of supplemental oxygen on the vessels may therefore not be excluded. Proximal aortic and peripheral resistance increases from hyperoxaemia, but a decrease of venous return implies extra cardiac blood‐pooling and compensatory relaxation of the capacitance vessels.     

Human cardiovascular adjustments to acute hypoxaemia

Traditionally, cardiovascular adjustments to hypoxaemia are viewed as resultants of competing local vasodilation and vasoconstriction via arterial chemoreflexes with net effects of increased cerebral and coronary blood flows (local) and reduced flow to visceral organs and muscle (reflex). Although true in asphyxia, breathing activates lung mechanoreceptors which reduce vagal outflow and apparently, in humans, abolishes sympathetic vasomotor activity (SNA). During rest, moderate to severe hypoxaemia (PaO2 = 35 to 27 mmHg) caused no splanchnic, cutaneous or muscle vasoconstriction. Local vasodilator effects of hypoxaemia were not sufficient to overwhelm vasoconstriction; splanchnic arterioles responded normally to infused noradrenalin (NA) during hypoxaemia. Possibly, central effects of hypoxaemia blunt SNA or peripheral, prejunctional effects impair neuronal release of NA. Persistent orthostatic tolerance with normal skeletal muscle vasoconstriction and retained spinal venomotor reflexes during hypoxaemia argue against prejunctional inhibition of NA release. Results so far suggest that beyond a certain threshold, hypoxaemia centrally inhibits SNA. In contrast to rest, even moderate hypoxaemia during exercise markedly increases plasma NA concentration (and SNA), but the usual relationship among splanchnic blood flow, plasma NA and heart rate was not observed--NA and heart rate rose together, whereas the predicted splanchnic vasoconstriction was not observed. In moderate hypoxaemia, muscle blood flow and cardiac output are greater than in normoxia at a given submaximal oxygen uptake; but at maximal oxygen uptake, blood pressure, total vascular conductance and maximal cardiac output are unaffected. Given the fixed upper limit to cardiac output and the greater capacity of active muscle to vasodilate and exceed cardiac pumping capacity during hypoxaemia, we conclude that blood pressure is maintained by baroreflex- (not chemoreflex-) mediated vasoconstriction in the active muscle which must be the primary target of increased SNA and the source of NA.     

Adrenal demedullation suppresses schedule-induced polydipsia in rats

The phenomenon of schedule-induced polydipsia (SIP) has been classified as nonregulatory, thus attempts to identify behavioral controls have taken precedence over physiological explanations. The present investigation initially focused upon SIP as a directional behavior with underlying physiological correlates. Rats were bilaterally adrenalectomized and given free access to demineralized water and 2% saline solution during SIP. Each animal redirected its consumption pattern to compensate for the natriuresis, and an overall decline in total fluid volume consumed was measured. A second experiment demonstrated suppressed SIP levels with adrenal demedullation suggesting that catecholamines may play a role in this phenomenon. These results indicate that the adrenal medulla is important in the maintenance of schedule-induced polydipsia, but the precise relationship between central and circulating levels of catecholamines, and SIP remains to be determined.     

Aggression and cardiovascular response in children

OBJECTIVE: To examine cardiovascular response as a function of children's aggression after controlling for the effects of known risk factors. METHOD: Participants were 140 children in second, fourth, and sixth grades. Teachers completed the Matthews Youth Test for Health, a measure that includes questions pertaining to children's aggression. Measures of blood pressure and heart rate were obtained during baseline, academic quiz, and recovery. RESULTS: Increasing age and body mass index were associated with increased cardiovascular responses. Aggressive children exhibited higher heart rates at baseline and lower heart rate reactivity. Aggressive children with a positive parent history of hypertension exhibited the greatest cardiovascular response. CONCLUSIONS: These results provide further support for the identification of behavioral factors that increase cardiovascular risk in children.     

Injected testosterone fails to influence schedule-induced attack

Three pigeons attacked a rear-projected conspecific target during exposure to a response-independent Fixed-Time 120 sec (FT 120 sec) and food delivery schedule. After baseline attack rates were established, subjects experienced two conditions in which they received injections (IM) of testosterone (10 mg and 20 mg; 100 mg/ml, in suspension) twenty minutes before session start. No concomitant change in attack rate or temporal location of attack was observed. Findings support observations suggesting that social context is an important determiner of the effects of testosterone on aggressive behavior.     

Unilateral substantia nigra lesions and schedule-induced polydipsia

The present experiment investigated the effects of unilateral 6-hydroxydopamine lesions of the substantia nigra (SN) on schedule-induced polydipsia (SIP). Lesions were made in either the "dominant" or "non-dominant" hemisphere as defined by an amphetamine rotation test. It was found that unilateral lesions of either the "dominant" or "non-dominant" SN significantly reduced SIP and also significantly impaired somatosensory responsiveness as indicated by the "tactile extinction test." Somatosensory neglect was significantly greater following a lesion in the "dominant" hemisphere than "non-dominant" hemisphere.     

Effects of limited-target availability on schedule-induced attack

Pigeons exposed to a 180-sec fixed-time food schedule could attack a rear-projected conspecific target that was available either (a) continuously throughout the interfood interval, (b) randomly during one 30-sec portion of each interfood interval, or (c) during the final 90 sec of each interval. During continuous-target availability, attack was maximal shortly after food ingestion and progressively decreased thereafter. During random-target availability, five of seven pigeons attacked less per target-access period the later that period occurred within the interfood interval, whereas two subjects exhibited relatively high local attack rates even when access periods occurred within the final third of the interval. When the target was available only during the second half of the interfood interval, attack occurred as soon as the target was presented and progressively decreased throughout the remainder of the target-access period. In general, these results show that schedule-induced attack can be increased by limiting the availability of that target and also indicate that such attack can reliably occur at times other than shortly after food delivery.     

Mental task demands and cardiovascular response patterns

Though it is well known that solving mental tasks elicits tonic increases in cardiovascular activity, a good theory explaining the specificity of this effect is lacking. It is also unclear why different kinds of mental tasks elicit different response patterns. The aim of the experiment was to compare cardiovascular response patterns during three tasks matched for their duration (8 min) and probability of success (.5): a simple RT task and two numeral tasks. One of them (a RUN task) involved program running (performing basic arithmetical operations), the other (an EDIT task) required searching for a problem solution. Seven cardiovascular variables, measured by a Portapres monitor, were analyzed. The analysis showed, among other findings, that the tonic increase in blood pressure and heart rate was greater during the RUN task than during the RT task. On the other hand, the EDIT task and the RT task produced almost identical response patterns.     

Modulation of schedule-induced polydipsia by Pavlovian conditioned states

It has previously been porposed that Random-Interval schedules fail to induce polydipsic drinking in rats because such schedules fail to provide gradients of reinforcement probability. In the present study, food-deprived rats received single pellet meals delivered on either a Fixed-Time or a Random-Time 120-sec schedule, with half of each group receiving signalled, and half unsignalled, pellet deliveries. Acquisition of polydipsia was retarded in animals exposed to the Random-Time Unsignalled condition. However, the imposition of a CS+ for pellet delivery on the identical Random-Time schedule resulted in levels of polydipsia comparable to those obtained in both Fixed-Time conditions. These results suggest that the establishment of a Pavlovian conditioned inhibitory signal, as well as local periods of low reinforcement probability, may be sufficient for the development of schedule-induced behaviors such as polydipsia.     

Measurement and development of schedule-induced drinking in pigeons

Three food-deprived pigeons were exposed to fixed-interval (FI) schedules of grain presentation ranging in value from 15-sec to 4-min. A photocell arrangement located above the water cup recorded frequency, duration, conditional probability, and temporal location of drinking. Drinking developed at inter-food intervals of 2-min or 4-min, gradually increased in magnitude, and stabilized after a large number of experimental sessions. Two of the three pigeons showed substantial levels of water intake, and all birds developed typical patterns of adjunctive drinking in which bouts occurred either shortly after reinforcement, or within the first half of the inter-food interval. These data suggest that previous difficulties in extending the generality of schedule-induced drinking to pigeons may reflect failures to properly arrange an effective combination of procedural variables.     

The development of water-intake regulation during schedule-induced polydipsia

Water intake by rats exhibiting asymptotic levels of schedule-induced polydipsia during a fixed-interval (FI) 1-min food schedule was decreased by reducing the diameter of the drinking spout orifice from 2.5 mm (LT-condition) to 1.0 mm (ST-condition). During initial exposure to the ST condition, rate of licking was immediately enhanced but session intake was not regulated. Lick rate continued to increase across subsequent ST sessions and session intake similar to that during LT conditions was achieved after several ST sessions. However, volume regulation failed to occur during the first session following each transition from LT to ST conditions. Restrictions in the rate of consumption increased the duration but not the frequency of post-pellet licking bouts, and resulted in decreased rates of bar-pressing with enhanced pause time. Reinforcement frequency across conditions was unaffected. Increased licking rates during ST sessions also resulted in enhanced licking rates during sugsequent LT sessions. These results suggest that volume regulation during schedule-induced polydipsia is achieved through reductions in competing responses, and changes in both the discriminative control of drinking and topography of licking.     

Acquisition of schedule-induced polydipsia in the mongolian gerbil

Three food-deprived Mongolian gerbils reliably displayed schedule-induced polydipsia when fixed-time food schedules were increased to 3-min interpellet intervals. No polydipsia was evident with shorter interpellet intervals. These data further extend the species generality of schedule-induced polydipsia, and indicate the importance of systematic variation of relevant variables before concluding that a given species does not display schedule-induced polydipsia.