Biomedical subjects

M P Kristensen

Publications and source records attributed to M P Kristensen.

7 recordsLinked to original sources

PubMed1998

Relationships between hippocampal activity and breathing patterns.

Single cell discharge, EEG activity, and optical changes accompanying alterations in breathing patterns, as well as the knowledge that respiratory musculature is heavily involved in movement and other behavioral acts, implicate hippocampal regions in some aspects of breathing control. The control is unlikely to reside in oscillatory breathing movements, because such patterns emerge in preparations retaining only the medulla (and perhaps only the spinal cord). However, momentary changes in breathing patterns induced by affect, startle, whole-body movement changes, or compensatory ventilatory changes mediated by rostral brain regions likely depend on hippocampal action in aspects of control. Hippocampal activity was enhanced prior to sighs, and this enhancement was accompanied by increased slow theta activity. Theta frequency increased during apnea, prior to return of breathing. Consideration of hippocampal contributions to breathing control should be viewed in the context that significant interactions exist between blood pressure changes and ventilation, and that modest breathing challenges, such as exposure to hypercapnia or to increased resistive loads, bring into action a vast array of brain regions involving nearly every level of the neuraxis.

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PubMed1997

Activity changes of the cat paraventricular hypothalamus during phasic respiratory events.

We monitored the spatiotemporal organization of cellular activity in the medial paraventricular hypothalamus during spontaneously-occurring periods of increased inspiratory effort followed by prolonged respiratory pauses (sigh/apnea) in the freely-behaving cat. Paraventricular hypothalamic activity was assayed by video images of light captured with a stereotaxically-placed fibre optic probe. Respiratory activity was measured through electromyographic wire electrodes placed in the diaphragm. Sigh/apnea events appeared in all behavioural states, and especially during quiet sleep. Overall paraventricular hypothalamic activity declined transiently, with the onset of decline coinciding with the beginning of the sigh inspiratory effort, reached a nadir at apnea onset 4.4+0.5 s from the beginning of the sigh, increased during the course of the apnea, and subsequently rebounded above baseline to peak at 10.9+2.5 s after sigh onset. Scattered, small areas of the imaged region were activated or depressed independently of the overall image values. The data suggest that paraventricular hypothalamic activity changes dynamically during phasic respiratory events, and may contribute to the progression of the sigh/apnea. We speculate that the medial paraventricular hypothalamus influences breathing patterns through projections to parabrachial respiratory phase-shift regions, and that longer-latency influences may also be exerted indirectly through blood pressure effects from paraventricular hypothalamic projections to medullary cardiovascular nuclei. Additionally, the paraventricular hypothalamus may convey respiratory influences from other rostral structures, such as the hippocampus.

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PubMed1997

Light scattering changes follow evoked potentials from hippocampal Schaeffer collateral stimulation.

We assessed relationships of evoked electrical and light scattering changes from cat dorsal hippocampus following Schaeffer collateral stimulation. Under anesthesia, eight stimulating electrodes were placed in the left hippocampal CA field and an optic probe, coupled to a photodiode or a charge-coupled device camera to detect scattered light changes, was lowered to the contralateral dorsal hippocampal surface. Light at 660 +/- 10 (SE) nm illuminated the tissue through optic fibers surrounding the optic probe. An attached bipolar electrode recorded evoked right hippocampal commissural potentials. Electrode recordings and photodiode output were simultaneously acquired at 2.4 kHz during single biphasic pulse stimuli 0.5 ms in duration with 0.1-Hz intervals. Camera images were digitized at 100 Hz. An average of 150 responses was calculated for each of six stimulating current levels. Stimuli elicited a complex population synaptic potential that lasted 100-200 ms depending on stimulus intensity and electrode position. Light scattering changes peaked 20 ms after stimuli and occurred simultaneously with population spikes. A long-lasting light scattering component peaked 100-500 ms after the stimulus, concurrently with larger population postsynaptic potentials. Optical signals occurred over a time course similar to that for electrical signals and increased with larger stimulation amplitude to a maximum, then decreased with further increases in stimulation current. Camera images revealed a topographic response pattern that paralleled the photodiode measurements and depended on stimulation electrode position. Light scattering changes accompanied fast electrical responses, occurred too rapidly for perfusion, and showed a stimulus intensity relationship not consistent with glial changes.

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PubMed1996

State-dependent cellular activity patterns of the cat paraventricular hypothalamus measured by reflectance imaging.

Activity within the cat paraventricular hypothalamus (PVH) during sleep and waking states was measured by quantifying intrinsic tissue reflectivity. A fiber optic probe consisting of a 1.0 mm coherent image conduit, surrounded by plastic fibers which conducted 660 nm source light, was attached to a charge-coupled device camera, and positioned over the PVH in five cats. Electrodes for assessing state variables, including electroencephalographic activity, eye movement, and somatic muscle tone were also placed. After surgical recovery, reflected light intensity was measured continuously at 2.5 Hz during spontaneously varying sleep/waking states. Sequential state transitions from active waking to quiet waking, quiet sleep and active sleep were accompanied by progressively increased levels of PVH activity. Overall activity was highest during active sleep, and decreased markedly upon awakening. Moment-to-moment activity oscillated in the 0-0.1 Hz range, especially during active sleep and active waking; this oscillation diminished during quiet sleep. Distinct sub-regions of enhanced or diminished activity emerged within the imaged area in a state-dependent manner. We conclude that PVH activity changes with behavioral state in a regionally specific manner, and that overall activity increases during quiet sleep, and is even more enhanced in active sleep. PVH activation could be expected to stimulate pituitary release of adrenocorticotropic hormone (ACTH) and affect input to autonomic regulatory sites. Since ACTH and corticotropin releasing factor elicit arousal, and since the PVH projects to other brain areas which modulate state, we speculate that the PVH plays a role in shaping characteristics of sleep/waking states.

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PubMed1996

Concurrent reflectance imaging and microdialysis in the freely behaving cat.

We present a method to perform simultaneous microdialysis with light reflectance imaging of neural activity in a discrete brain region of the freely behaving animal. We applied this method to the dorsal hippocampus of freely behaving cats to (1) measure extracellular glutamate and reflectance variations across a sleep-waking cycle, (2) assess spatially coherent neural activity changes accompanying local perfusion of cocaine and (3) measure local changes in cell volume induced by infusion of hyper- and hypo-osmotic solutions. Higher extracellular glutamate concentrations corresponded to higher imaged neural activity. Sequential images showed that cocaine perfusion elicited a propagating reflectance change as cocaine reached the tissue. Microperfusion of hypo-osmotic solution ( - 100 mOsm), which increases cell volume, decreased reflectance. Microperfusion of hyperosmotic sucrose solutions, which reduce cell volume, increased reflectance in a dose-dependent manner. The data indicate that reflectance imaging can measure changes in cell volume, and could, thus, measure neural activity through activity/cell volume corollaries. Combining microdialysis and optical imaging enables investigation of the neurochemical bases of spontaneous neural activity patterns within discrete brain nuclei.

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PubMed1996

Hippocampal activity during transient respiratory events in the freely behaving cat.

We measured dorsal hippocampal activity accompanying sighs and apnea using reflectance imaging and electrophysiologic measures in freely behaving cats. Reflected 660-nm light from a 1-mm2 area of CA1 was captured during sighs and apnea at 25 Hz through a coherent image conduit coupled to a charge coupled device camera. Sighs and apnea frequently coincided with state transitions. Thus, state transitions without apnea or sighs were separately assessed to control for state-related activity changes. All dorsal hippocampal sites showed discrete regions of activation and inactivation during transient respiratory events. Imaged hippocampal activity increased 1-3 s before the enhanced inspiratory effort associated with sighs, and before resumption of breathing after apnea. State transitions lacking sighs and apnea did not elicit analogous optical activity patterns. The suprasylvian cortex, a control for site, showed no significant overall reflectance changes during phasic respiratory events, and no discrete regions of activation or inactivation. Spectral estimates of hippocampal electroencephalographic activity from 0-12 Hz showed significantly increased power at 3-4 Hz rhythmical slow activity before sighs and apnea, and increased 5-6 Hz rhythmical slow activity power during apnea, before resumption of breathing. Imaged activity and broadband hippocampal electroencephalogram power decreased during sighs. We propose that increased hippocampal activity before sigh onset and apnea termination indicates a role for the hippocampus in initiating inspiratory effort during transient respiratory events.

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PubMed1995

Imaging the dorsal hippocampus: light reflectance relationships to electroencephalographic patterns during sleep.

We assessed the correspondence of 660 nm light reflectance changes from the dorsal hippocampus with slow wave electroencephalographic (EEG) activity during quiet sleep (QS) and rapid eye movement (REM) sleep in four cats. An optic probe, attached to a charge-coupled-device (CCD) video camera, was placed on the dorsal hippocampal surface to collect reflectance images simultaneously with EEG, which was measured by macroelectrodes placed around the probe circumference. Spectral estimates of EEG and light reflectance amplitude indicated that reflectance changes occurred in a similar frequency range as EEG changes. Dividing the image into 10 subregions revealed that reflectance changes at the rhythmical slow wave activity band (RSA, 4-6 Hz) persisted in localized regions during QS and REM sleep, but regional changes showed considerable wave-by-wave independence between areas and from slow wave electrical activity. Peak frequencies for reflectance changes corresponded to fast RSA frequencies observed in the EEG. Optical changes most likely derive from fast-acting physical phenomena, rather than from alterations in blood perfusion, and provide increased spatial resolution over that offered by electrical measurements.

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