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Biomedical subjects

A C Bobbert

Publications and source records attributed to A C Bobbert.

14 recordsLinked to original sources
PubMed1998

Absence of endogenous circadian rhythmicity in blood pressure?

Currently available evidence reveals a predominant role of exogenous (so-called "masking") factors in the 24 h variation of blood pressure in humans. The existence of a (minor) endogenous circadian factor cannot be excluded, however. This possibility was tested by applying the rigorous unmasking conditions of the constant-routine protocol, that is, strict bed rest in a separate bedroom, total sleep deprivation, constant ambient temperature and illumination, and hourly equicaloric food and liquid intake. Twenty-five normotensive young individuals were subjected to a 26 h constant-routine procedure while hourly measurements were made of their blood pressure and heart rate. Repeated-measures analysis of variance failed to show a significant 24 h variation of blood pressure. The power of this test appeared satisfactorily high (>0.95). Heart rate, however, exhibited a significant circadian pattern, with a range of 6.7 beats/min (10% of the 24 h mean value). Moreover, the timing of the 24 h heart rate curves differed significantly between so-called morning (n = 10) and evening (n = 9) individuals. Mean peak values for the morning-types occurred at 11 AM, for the evening types nearly 6 h later. In conclusion, no evidence was found for the involvement of a circadian oscillator in the regulation of blood pressure.

Adult↗
PubMed1992

Pinealectomy decelerates the circadian food intake rhythm of cervically sympathectomized rabbits.

Long-term records of the free-running food intake rhythms were obtained from 26 young adult blinded rabbits that were subjected in the course of recording to bilateral superior cervical ganglionectomy (sympathectomy). This always resulted in acceleration of the rhythm (mean delta tau = -0.35 h). Some 4 months afterwards, 12 rabbits were pinealectomized and after another 4 months sham pinealectomized also. In the other 14 animals, these operations were performed in the reverse order. It appeared that suctioning away part of the ventricular walls during sham pinealectomy was followed by a slight reduction in tau (mean = -0.07 h) that could be largely attributed to the spontaneous gradual reduction of tau that occurred throughout all experiments. On the other hand, total pinealectomy in these already sympathectomized blinded rabbits always resulted in a substantial deceleration of the rhythms (mean delta tau = +0.23 h). These observations on blinded rabbits suggest that a sympathetically denervated pineal gland releases appreciable amounts of melatonin or of another hormone with a melatonin-like accelerating effect on the circadian pacemaker (SCN).

Animals↗
PubMed1991

Feedback in the rabbit's central circadian system, revealed by the changes in its free-running food intake pattern induced by blinding, cervical sympathectomy, pinealectomy, and melatonin administration.

Long-term records of the number of food approaches per 30 min were obtained from 16 rabbits in constant light and after blinding. Optic nerve sectioning usually resulted in a shortening of the free-running period (tau; mean reduction = -0.26 hr) of the oscillator governing the rabbit's food intake pattern. Subsequent resection of both superior cervical ganglia always reduced this tau (mean = -0.41 hr), and it could be further shortened (mean = -0.24 hr) by chronic administration of melatonin. These observations led to experiments in which the food intake of 15 blinded rabbits was recorded before and after pinealectomy. This always resulted in a lengthening of tau (mean = +0.27 hr), which was never seen after subtotal or sham pinealectomy in 10 other blinded animals (mean = -0.05 hr). Sympathectomy also reduced tau (mean = -0.50 hr) in the pinealectomized rabbits, whereas it induced a smaller reduction (mean = -0.29 hr) after sham pinealectomy. These results, together with previous observations on the effect of sympathectomy on the retinal "dark" discharge and on the influence of the latter upon the food intake pattern, indicate that the cervical sympathetic nerves may form part of three feedback loops for the rabbit's central circadian system. These have separate postganglionic pathways; there are also differences in the circadian phasing of their activities and in their effects on tau.

Animals↗
PubMed1987

Effects of illumination on the rabbit's EEG during the two phases of its circadian rhythm.

After exposure to a LD 12:12 regimen for several weeks, rabbits exhibit a programmed circadian rhythm in the RMS values of their occipital and frontal EEG's during 54-hr recordings in constant darkness. Illumination at levels of 80, 160 and 230 Lx raises these RMS values in both phases of the rhythm. The induced rise is large in the phase with low RMS values and small in the other, whereas light-induced changes in spectral composition of the EEG's are slight in either phase. Bilateral optic nerve sectioning results in similar changes in properties of the EEG and in the amplitude of its rhythm. The results are discussed with reference to the influence exerted by steady illumination on the overall-level of retinal maintained "dark" discharge.

Animals↗
PubMed1986

Properties of the two circadian oscillators influencing the rabbit's food intake pattern.

Continuous recordings of the pattern in food approaches during ad libitum-feeding were obtained from 15 rabbits which were exposed, for periods of 3-7 months, to continuous illumination (200-250 Lux), total darkness, light-dark alternations of L1:D23 h and L12:D12h and were blinded afterwards. It appears that the circadian pattern in food intake differs markedly in these circumstances and that it is governed by 2 loosely coupled circadian oscillators. These widely diverge in their effect on the eating pattern, in the response of their free-running periods to changes in steady state condition of the retina, and in their entrainment to either the onset or the ending of the illumination phases of 24-h LD regimens. The results indicate that the hypothalamic centres controlling the rabbit's food intake pattern must be strongly influenced by the retinal maintained 'dark' discharge.

Animals↗
PubMed1983

Circadian changes in the response of the rabbits retina to flashes.

It was previously shown that in rabbits the electroretinogram (ERG) and the visually evoked potential show a circadian rhythm consisting of a stable phase during which the animals respond to flashes with day time potentials and another phase with night time potentials (NTPs) and that the occurrence of the sharp phase transitions is programmed by the time course of the preceding light-dark schedule. From the present investigation it follows that photic sensitivity to flashes is markedly higher during the NTP-phase than in the other phase, whereas dark adaptation runs the same time course in both phases. With respect to the ERG it appears that, over a wide range of flash intensities its b wave only is affected by phase of the rhythm. The differences in photic responses between the 2 phases are discussed in the light of possibly synchronous changes in the influence of sympathetic nerve fibres on eye structures.

Animals↗
PubMed1981

Evidence for the existence of a retino-hypothalamo-retinal loop in rabbits.

Rabbits which have previously been exposed to 24-h light/dark alternations exhibit in their photic responses at all levels of the visual pathway, and in the electroretinogram, simultaneous circadian changes with a time course that is programmed by the preceding light/dark schedule. The occurrence of simultaneous changes in the response of the visual cortex to electrical stimulation at subcortical levels could not be established. After bilateral sectioning of the optic nerves, there were still changes in the electroretinogram which were now no longer programmed, but reflected the existence of a free-running rhythm. Bilateral sectioning of the cervical sympathetic nerves resulted in a complete absence of circadian changes in photic responses. The results suggest the possible existence of a retino-hypothalamo-retinal loop by means of which the responses to photic stimuli on a given day are modulated according to the time course of the light/dark alternations on the immediately preceding day. The possible role of this feedback loop in the origin of animal photoperiodism is discussed.

Animals↗
PubMed1978

Diurnal changes in the rabbit's visual evoked potential.

Rabbits that have been exposed to the natural cycles of day and night exhibit marked diurnal changes in the shape of their Visual Evoked Potential in constant environmental conditions. The results of exposure to artificial 24 hr light-dark cycles strongly suggest that it is the regular alternation of daylight and darkness which acts as the synchronizing "Zeitgeber" for the V.E.P. rhythm which exists after exposure to the natural cycles of day and night. It would seem further that the V.E.P. changes reflect a square-wave like rhythm in the sensitivity of the visual system to photic stimuli, in which the sensitivity is much higher at night than in the day-time. The probable importance of the diurnal V.E.P. rhythm for the occurrence of daily fluctuations in behaviour is discussed.

Animals↗
PubMed1978

Photoperiodic programming of diurnal changes in rabbit visual evoked potentials.

Rabbits which have been exposed for several weeks to the natural cycles of daylight and darkness, or to various fixed 24 hr light-dark alternations, exhibit in constant environmental conditions large diurnal changes in the shape of the Visual Evoked Potential. These changes run a square-wave like time course and seem to reflect a similar diurnal rhythm in the sensitivity of the visual system to photic stimuli. The relation between the durations of the two phases of the rhythm can, within wide limits, be changed by variations in length of the artificial or natural photoperiod. It further appears that after exposure to continuous illumination or darkness rabbits have either a free-running V.E.P. rhythm or no such rhythm at all. The probable importance of the diurnal changes in the visual system for the occurrence of seasonal fluctuations in behaviour is discussed.

Animals↗
PubMed1978

Seasonal fluctuations of the circadian changes in rabbit visual evoked potentials.

Two rabbits were exposed for more than a year to the natural cycles of daylight and darkness. At intervals of 2 months and less records were taken of the time course of the 24 hr rhythm appearing in constant conditions in their cortical flash-evoked responses. It appeared that the rhythm can be programmed as well by the natural light-dark alternations as by artificial cycles. In temperate zones this "natural programming" causes large seasonal fluctuations in the time course of the rhythm which reflects, over the major part of the year and with remarkable precision, the times of sunrise and sunset and, in consequence, the length of the day. The possible significance of fixing daylength temporarily in the nervous system is discussed in connection with Bünning's hypothesis on the causation of seasonal photoperiodism.

Animals↗