Monitoring on-line of extracellular gamma-amino-4-butyric acid using microdialysis coupled to immunosensor analysis

Stress induces CRF release in the paraventricular nucleus, and both CRF and GABA release in the amygdala

In the hypothalamus, corticotropin-releasing factor (CRF) initiates the hypothalamic-pituitary-adrenal (HPA) axis response to stress, resulting in the release of glucocorticoids, including cortisol. Extrahypothalamic CRF, particularly in the limbic system, also appears to play a role in the stress response. To further define brain CRF response to stress, immunosensor-based microdialysis probes were used to measure the extracellular levels of CRF in the paraventricular nucleus of the hypothalamus (PVN) and in the amygdala of sheep during a predator (dog) exposure stress. In addition, gamma amino butyric acid (GABA) was measured in the amygdala and cortisol was measured in venous blood. Exposure to the predator stress increased CRF in the PVN and both CRF and GABA in the amygdala. These were followed in time by a rise in venous cortisol. Application of a CRF antagonist to the amygdala, immediately prior to stress, had a small effect on the subsequent observed stress responses. This treatment, however, significantly reduced the responses to a repeat stress administered 2 days later, compared to nontreated animals. Application of a GABA antagonist to the amygdala prior to stress had no effect on the subsequent observed stress response but increased the response to the stress repeated 2 days later. Perfusion with 4-aminopyridine, a neuronal depolarising agent, into the PVN induced a release of CRF accompanied shortly thereafter by a small increase in CRF in the amygdala, and 5-10 min later by an increase in venous cortisol. Perfusion into the amygdala increased the levels of both CRF and GABA but had no effect on either PVN CRF or venous cortisol. These data support roles for both the PVN and amygdala in stress responsiveness. It suggests further that actions at the amygdala can strongly influence subsequent responsiveness to a further stress, mediated in part by both CRF and GABA actions.

Glucocorticoid feedback increases the sensitivity of the limbic system to stress

In the hypothalamus, corticotropin-releasing hormone (CRH) has a well-described role initiating the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Cortisol, released from the adrenal gland, exerts negative feedback on this axis. The role of extrahypothalamic CRH in stress responses is less well known. The purpose of this study was to measure the response of CRH in the amygdala to acute and repeated stress and to examine if cortisol had any effect on this response. Immunosensor-based microdialysis probes were used to measure CRH and cortisol in the amygdala and cortisol systemically in sheep exposed to a predator stress (a dog). Upon presentation of a dog, CRH increased in the amygdala of the sheep and then fell off. Cortisol levels rose both systemically and in the amygdala, and as they peaked, a second CRH response was observed. Repeated stress changed this response, with the magnitude of the first CRH peak decreasing while the second peak increased. Repeated stress also produced an exaggeration in both of the CRH peaks to presentation of a subsequent novel stress (a forelimb electric shock). Animals that had an escape route from the repeated dog stress did not show this exaggeration when faced subsequently with the novel stress. Administration of mifepristone, a glucocorticoid receptor antagonist, prior to the delivery of the repeat stress prevented subsequent changes in the CRH response. The data suggest that the amygdala shows a CRH response to presentation of a stressor acutely and repeatedly and that repeated stress can alter subsequent amygdala responsiveness to the same or a different stressor. This alteration appears dependent on circulatory glucocorticoids.

Rapid noninvasive measurement of hormones in transdermal exudate and saliva

Experiments were conducted in both sheep and humans to evaluate techniques for rapid sampling and measurement of testosterone, insulin, 17-beta estradiol, cortisol and glucose collected in saliva or transdermal exudate. Ultrasound and an electric current facilitated the latter collection. All but insulin were successfully measured in saliva, under resting conditions, and the measured hormones correlated best with blood levels 20-40 min prior to the saliva collection. With imposition of, and recovery from, an exercise stress, this correlation was weakened irrespective of considering the time lag between blood measures during this period and subsequent changes in saliva values. Provided an initial transdermal flux was established, all the hormones and glucose were successfully measured in the transdermal exudate at levels correlating with blood measures at the time of collection, and this held across stressor application and recovery. The transdermal exudate sampling and measurement apparatus is relatively portable, enabling noninvasive collection and analyte measurement, rapidly, at the site where the experiment is being conducted with minimal interference to subjects. This potentially offers a tool of considerable value to endocrine studies.

Immunosensors--principles and applications to clinical chemistry

INTRODUCTION

Immunosensors are affinity ligand-based biosensor solid-state devices in which the immunochemical reaction is coupled to a transducer. The fundamental basis of all immunosensors is the specificity of the molecular recognition of antigens by antibodies to form a stable complex. This is similar to the immunoassay methodology. Immunosensors can be categorized based on the detection principle applied. The main developments are electrochemical, optical, and microgravimetric immunosensors. In contrast to immunoassay, modern transducer technology enables the label-free detection and quantification of the immune complex.

METHODS

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors in these applications is most appropriate. Similarly, a series of clinical problems may be solved by continuous monitoring of certain analytes.

CONCLUSIONS

Clinical chemists should take advantage of immunosensors in clinical diagnostics. There are many recent developments in the immunosensor field which have potential impacts. The future role of this technique in intralaboratory, as well as bedside testing, will become even more important as the clinical laboratory is faced with increasing pressure to contain costs.

Measuring of extracellular cortisol and corticotropin-releasing hormone in the amygdala using immunosensor coupled microdialysis

A method is described for the measurement of cortisol or corticotropin-releasing hormone (CRH) from the brain, within, or in the dialysate of, a microdialysis probe using an antibody-linked assay. Polyclonal antibodies for either cortisol or CRH provide a specificity of measurement. These antibodies are affixed on a platinum electrode within the probe. Determination of bound cortisol or CRH is performed via an indirect assessment of competitive ligand also bound, and conjugated to HRP. This is activated and measured as current change. These probes were used for extracellular measurements in the amygdala, a limbic brain region, of sheep. Direct measurement in vivo compared favorably to the use of either the immunosensor or standard RIA techniques on dialysate emerging from a classical microdialysis probe, suggesting that the method could be used directly in-vivo. This is the first report in literature of rapid on-line measurement of CRH and cortisol from a discrete brain area. The probes have fast response times (>90% of maximum response within 30 s of start of analysis), high sensitivity (<0.1 pg/ml for CRH) and can acquire data every 2 min. They are stable in-vivo (>72 h) and regenerable, offering on-line measurement, with rapid time resolution, of neurohumoral substances.