We utilized variations in caloric availability and ambient temperature (T(a)) to examine interrelationships between energy expenditure and cardiovascular function in mice. Male C57BL/6J mice (n = 6) were implanted with telemetry devices and housed in metabolic chambers for measurement of mean arterial pressure (MAP), heart rate (HR), O(2) consumption (VO(2)), and locomotor activity. Fasting (T(a) = 23 degrees C), initiated at the onset of the dark phase, resulted in large and transient depressions in MAP, HR, VO(2), and locomotor activity that occurred during hours 6-17, which suggests torporlike episodes. Food restriction (14 days, 60% of baseline intake) at T(a) = 23 degrees C resulted in progressive reductions in MAP and HR across days that were coupled with an increasing occurrence of episodic torporlike reductions in HR (<300 beats/min) and VO(2) (<1.0 ml/min). Exposure to thermoneutrality (T(a) = 30 degrees C, n = 6) reduced baseline light-period MAP (-14 +/- 2 mmHg) and HR (-184 +/- 12 beats/min). Caloric restriction at thermoneutrality produced further reductions in MAP and HR, but indications of torporlike episodes were absent. The results reveal that mice exhibit robust cardiovascular responses to both acute and chronic negative energy balance. Furthermore, we conclude that T(a) is a very important consideration when assessing cardiovascular function in mice.

Pigeons learned to peck a keylight (82) when it was paired with a stimulus (81l that already evoked keypecking. Control procedures showed that 82 acquired control over responding because it was paired with 81 and because 81 had a conditioning history , thereby supporting the claim that 82 was a second-order conditioned stimulus. Second-order conditioning occurred as rapidly when 81 was a keylight as when it was a tone. Test procedures showed that after second-order conditioning, responding to 82 was markedly debilitated by the extinction of responding to 81, indicating that the ability of 82 to evoke a response importantly depends upon the continued ability of 81 to do so. Our demonstration that directed motor action in the pigeon is susceptible to second-order conditioning suggests a new interpretation of conditioned reinforcement in instrumental learning. Our dernonstration that the effectiveness of 82 depends upon the continued effectiveness of 81 indicates that 8-8 associations are formed in this version of the second-order conditioning experiment.In conditioning the alimentary and defensive reflexes of dogs, Pavlov noted that distinctive secretory and motor components of each reflex were evoked by the conditioned stimulus (Pavlov, 1927, Lecture Il). He emphasized the secretory component because it was more susceptible to accurate measurement and less susceptible to anthropomorphic interpretation. In recent years, the discovery of autoshaping (Brown & Jenkins, 1968) has focused attention on directed motor responses evoked by classically conditioned stimuli, and automated measurement has attenuated the difficulties Pavlov enumerated in studying motor behavior. In a typical autoshaping experiment, hungry pigeons approach and contact a localized stimulus such as a Iighted response key which signals presentations of food. There seems little doubt that this directed motor behavior provides a behavioral index of the associative status of the localized stimulus (Hearst & Jenkins, 1974) and, consequently, autoshaping provides a new experimental methodology for examining some old questions about associative learning.Autoshaping has been demonstrated with the method of first-order classical conditioning in which a neutral stimulus (SI) acquires control over 25 approach and contact responses because it signals a biologically significant event such as food or water. The first experiment in the present paper dernonstrates that the pigeon's keypeck can be autoshaped with the method of second-order conditioning in which a neutral stimulus (S2) acquires control over keypecking solely because it signals presentations of SI, a first-order conditioned stimulus (cf. Pavlov, 1927, Lecture III). This demonstration is important because it extends the empirical base of secondorder conditioning to include directed motor action, it implicates classical conditioning in the control of directed motor behavior in a wider range of situations than has been acknowledged (Hearst & Jenkins, 1974;Moore, 1973), and because it offers a new o...

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The primary purpose of the study was to test the hypothesis that reduced leptin signaling is necessary to elicit the cardiovascular and metabolic responses to fasting. Lean (Fa/?; normal leptin receptor; n = 7) and obese (fa/fa; mutated leptin receptor; n = 8) Zucker rats were instrumented with telemetry transmitters and housed in metabolic chambers at 23 degrees C (12:12-h light-dark cycle) for continuous (24 h) measurement of metabolic and cardiovascular variables. Before fasting, mean arterial pressure (MAP) was higher (MAP: obese = 103 +/- 3; lean = 94 +/- 1 mmHg), whereas oxygen consumption (VO(2): obese = 16.5 +/- 0.3; lean = 18.6 +/- 0.2 ml. min(-1). kg(-0.75)) was lower in obese Zucker rats compared with their lean controls. Two days of fasting had no effect on MAP in either lean or obese Zucker rats, whereas VO(2) (obese = -3.1 +/- 0.3; lean = -2.9 +/- 0.1 ml. min(-1). kg(-0.75)) and heart rate (HR: obese = -56 +/- 4; lean = -42 +/- 4 beats/min) were decreased markedly in both groups. Fasting increased HR variability both in lean (+1.8 +/- 0.4 ms) and obese (+2.6 +/- 0.3 ms) Zucker rats. After a 6-day period of ad libitum refeeding, when all parameters had returned to near baseline levels, the cardiovascular and metabolic responses to 2 days of thermoneutrality (ambient temperature 29 degrees C) were determined. Thermoneutrality reduced VO(2) (obese = -2.4 +/- 0.2; lean = -3.3 +/- 0.2 ml. min(-1). kg(-0.75)), HR (obese = -46 +/- 5; lean = -55 +/- 4 beats/min), and MAP (obese = -13 +/- 6; lean = -10 +/- 1 mmHg) similarly in lean and obese Zucker rats. The results indicate that the cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in Zucker rats and suggest that intact leptin signaling may not be requisite for the metabolic and cardiovascular responses to reduced energy intake.

Fasting produces multiple cardiovascular, metabolic, and behavioral responses. To examine the interrelationship between these responses, male spontaneously hypertensive rats (SHR; n = 8) implanted with cardiovascular telemetry devices were housed in metabolic chambers at 23 degrees C for 22-h daily measurements of physiological variables. The experimental apparatus was designed so that ingestive behavior was detected by photobeams and locomotion was detected by a load sensor. Cardiovascular and metabolic status were determined as both a function of the circadian cycle (12-h dark and 10-h light), as well as during periods of inactivity (no ingestion and minimal locomotion) within the dark and light phases. Data were obtained during baseline, 48-h of caloric deprivation, and 6 days of refeeding. Fasting produced significant reductions in mean arterial pressure (dark: -9.2+/-1.3 from 143.7+/-3.7 mm Hg; light: -8.6+/-1.8 from 140.1+/-3.7 mm Hg), heart rate (dark: -43.4+/-5.2 from 330.0+/-5.2 beats/min; light: -27.4+/-5.2 from 294.0+/-5.2 beats/min), and oxygen consumption (dark: -5.0+/-0.6 from 20.6+/-0.3 ml x min(-1) x kg (0.75); light: -2.7+/-0.2 from 14.9 +/-0.2 ml x min(-1) x kg(0.75)). Analysis of inactive periods during both light and dark phases revealed that these reductions were not dependent on behavioral effects. We conclude that fasting produces concurrent and interrelated reductions in cardiovascular and metabolic function in the SHR. The merging of cardiovascular telemetry, indirect calorimetry, and behavioral monitoring provides a powerful approach for investigation of the integrative physiological responses to energetic challenges.