Intracerebroventricular administration of atrial natriuretic peptide prevents increase of plasma ADH, aldosterone and corticosterone levels in restrained conscious dehydrated rabbits.

In order to investigate the effects of centrally administered ANP on plasma ADH, aldosterone and corticosterone levels as well as on blood pressure and on heart rate, 20 male New Zealand White (NZW) rabbits were used. Measurements were made on restrained conscious animals one week after the implantation of an indwelling intracerebroventricular (icv) cannula and two indwelling intravascular catheters (intracarotid and intrajugular). Animals were classified into two main groups, those with water available ad libitum ("euhydrated" group) and those who were dehydrated for 24 h ("dehydrated" group) before blood pressure and heart rate recordings and blood sampling for hormonal determination. Each group's individuals were divided into two subgroups of five animals each. Blood samples were collected at 0 min (control) and 30, 60, 90, 120 min following icv administration of 25 microl of either artificial cerebrospinal fluid (aCSF) (subgroups "aCSF") or human (h) ANP (1 microg) in aCSF (25 microl) (subgroups "hANP"). Blood pressure and heart rate were also recorded at the same times. Plasma ADH, aldosterone and corticosterone concentrations were determined using RIA. The results were analysed by analysis of variance (ANOVA). Blood pressure and heart rate values were unaffected by water deprivation or by ANP administration. Mean plasma corticosterone levels at all times (30-120 min) were significantly higher (p<0.001) than those at 0 min time. Plasma corticosterone levels in the "dehydrated+aCSF" group were significantly higher (p<0.05) than in each of the other groups ("dehydrated+hANP", "euhydrated+aCSF", "euhydrated+hANP"). Plasma corticosterone levels in each of those other groups did not differ significantly from one another. Dehydration resulted in a tendency to increase in aldosterone levels (p<0.07), and icv administration of hANP tended (p<0.08) to prevent in the "dehydrated+hANP" experimental group the increase in aldosterone levels observed in the control "dehydrated+aCSF" group from 30 to 120 min. Dehydration resulted in an increase in ADH levels (p<0.0001), and icv administration of hANP prevented (p<0.05) in "dehydrated+hANP" experimental group the increase in ADH levels observed in the control "dehydrated+aCSF" group from 90 to 120 min. The increase of corticosterone and ADH and the tendency towards increase in aldosterone in the control dehydrated groups could possibly be due to the combined stress stimulus of dehydration and restriction in the restrain box. These results indicate that centrally administered ANP, at the concentration achieved in the present study, neither affects blood pressure and heart rate in conscious restrained euhydrated and 24 h-dehydrated NZW rabbits nor decreases the ADH, aldosterone and corticosterone response to dehydration, but does apparently modulate ADH, aldosterone and corticosterone responses to other stimuli in the dehydrated state. In conclusion, the results of this study confirm that brain ANP may have an inhibitory effect on stimulated ADH, aldosterone and corticosterone release.

Effects of intracerebroventricular administration of atrial natriuretic peptide (ANP) on blood pressure, heart rate and plasma ADH and corticosterone levels in normal and dehydrated rabbits.

In order to investigate the effects of centrally administered Atrial Natriuretic Peptide (ANP) on plasma ADH and corticosterone levels as well as on blood pressure and on heart rate, 20 male New Zealand White (NZW) rabbits were used. Measurements were made on restrained conscious animals one week after the implantation of an indwelling intracerebroventricular (i.c.v.) cannula and two indwelling intravascular catheters (intracarotid and intrajugular). Animals were classified into two main groups, those with water available ad libitum ("euhydrated" group) and those who were dehydrated for 24h ("dehydrated" group) before blood pressure and heart rate recordings and blood sampling for hormonal determination. Each group's individuals were divided into two subgroups of five animals each. Blood samples were collected at 0 min (control) and 30; 60, 90, 120 min following i.c.v. administration of 25 microliters of either artificial cerebrospinal fluid (aCSF) (subgroups "aCSF") or human (h) ANP (1 microgram) in aCSF (25 microliters) (subgroups "hANP"). Blood pressure and heart rate were also recorded at the same times. Plasma ADH and corticosterone concentrations were determined by RIA. The results were analysed by ANOVA. Blood pressure and heart rate values were unaffected by water deprivation or by ANP administration. Mean plasma corticosterone levels at all times (30-120 min) were significantly higher (p < 0.001) than at 0 min time. Plasma corticosterone levels in the "dehydrated + aCSF" group were significantly higher (p < 0.05) than in each of the other groups ("dehydrated + hANP", "euhydrated + aCSF", "euhydrated + hANP"). Plasma corticosterone levels in each of those other groups did not differ significantly from one another. Dehydration resulted in an increase in ADH levels (p < 0.0001) and i.c.v. administration of hANP prevented (p < 0.05) in "dehydrated + hANP" experimental group, the increase in ADH levels observed in the control "dehydrated + aCSF" group from 90 to 120 min. The increase of corticosterone and ADH in the control dehydrated groups could possibly be due to the combined stress stimulus of dehydration and restriction in the restrain box. These results indicate that centrally administered ANP, at the concentration achieved in the present study, neither affects blood pressure and heart rate in conscious restrained euhydrated and 24h-dehydrated NZW rabbits nor decreases the ADH and corticosterone response to dehydration, but does apparently modulate ADH and corticosterone responses to other stimuli in the dehydrated state. In conclusion, the results of this study confirm that brain ANP may have an inhibitory effect on stimulated ADH and corticosterone release.

Effects of intracerebroventricular administration of atrial natriuretic peptide on subcortical EEG activity in conscious rabbits.

Atrial natriuretic peptide (ANP) influences the activity of rat hypothalamic neurons, modifies the membrane excitability of the rat forebrain neurons, and induces changes in membrane potentials in cultured rat glioma cells. In order to explore whether these effects are reflected in the electrical activity of larger subcortical brain areas, we investigated the electroenceophalographic activity (EEG) recorded from 20 male albino (New Zealand White) rabbits. Recordings of EEG were made on restrained, conscious animals 1 week after the implantation of an indwelling intracerebroventricular (i.c.v.) cannula (lateral right ventricle) and two stainless steel electrodes, implanted in the paraventricular (PVN) and supraoptic (SON) nuclei. Animals were classified into two main groups: those with water available ad libitum (group A) and those which were dehydrated for 24 h before EEG recordings (group B). Each group was divided into two subgroups (1 and 2) of five animals each. EEG was recorded at 0 min (control) and 30, 60, and 90 min following the i.c.v. injection of either 25 microliters artificial cerebrospinal fluid (aCSF; subgroup 1) or 1 microgram alpha-human ANP in 25 microliters a CSF (subgroup 2). Each EEG record duration was 6 s. For each EEG record the power spectrum of the digitized waveform was estimated in the frequencies 0.5-48 Hz using the fast Fourier transform, and the energy of each waveform was subsequently calculated. The results were analyzed by repeated-measures ANOVA and by the t-test. The analysis revealed that (1) water deprivation does not affect mean EEG energy and value (2) ANP attenuates (P < 0.05; in comparison with zero time) the mean energy value of EEG recorded from SON at 30 min and 60 min in the frequencies 8-48 Hz, whilst it tends to decrease (P < 0.1) the mean energy of EEG recorded from PVN at 30 min in the frequencies 8-15 Hz. Mean EEG energy changes caused by ANP would reflect its various (mainly inhibitory) effects on the electrical activity recorded from PVN and SON neurons in in vitro and in vivo studies.

Ultrastructural alterations of the rabbit sciatic nerve, spinal cord and cerebellum, following methionine sulphoximine administration.

Methionine sulphoximine (MSO) is a centrally acting neurotoxin which inhibits the glutamate metabolism enzymes and has convulsive properties. Small doses of MSO were administered to rabbits, either intravenously (i.v.) or intracerebroventricularly (ICV), and electron microscopic examination of the cerebellum, the spinal cord and the sciatic nerve was performed on the first day of rabbit hind leg rigid paralysis (myopathy with histological findings resembling myositis), which set in by the 2nd to 4th day after MSO administration. In the cerebellum focal minor alterations were found in the astrocytes (swelling and lucidity, diminution of glycogen granules) and sparsely in the presynaptic terminals (lucidity and clumping), whereas most of the neuron presented a normal appearance. In the spinal cord and in the sciatic nerve a dissociation of the axon from the myelin sheath was evident in a small number of myelinated nerve fibres, along with the appearance of vacuolated spaces. Mitochondrial disorganisation in the axons, as well as glial cell alterations, were also seen. The ultrastructural alterations were non specific, and since they were induced 2 to 4 days after the administration of either minimum doses (i.v.) or of extremely low doses (ICV) of MSO, they may be attributed to the inordinate increase of metabolism during the period of convulsions.

Plasma TSH, T3, T4 and cortisol responses to swimming at varying water temperatures.

The acute effect of 30-min swimming at a moderate speed, at three water temperatures (20, 26 and 32 degrees C) on plasma thyroid stimulating hormone (TSH), free thyroxine (F.T4), triiodothyronine (T3) and cortisol concentrations was studied in 15 élite male swimmers. Blood was sampled before and immediately after the events. The heart rate, which was continuously monitored during exercise, had the highest response at 32 degrees C and the lowest at 20 degrees C. Blood lactate concentrations were found to be similar after the three tests. Plasma TSH and F.T4 were found to be significantly increased (by 90.4% and 45.7% respectively) after swimming at 20 degrees C, decreased at 32 degrees C (by 22.3% and 10.1% respectively) and unchanged at 26 degrees C. Exercise at these three water temperatures did not significantly affect T3. Finally, plasma cortisol was found to be increased after swimming at 32 degrees C (by 82.8%) and 26 degrees C (by 46.9%), but decreased at 20 degrees C (by 6.1%).

Long-term effects of the administration of the convulsive substance DL-methionine-DL-sulfoximine to the rabbit.

Methionine sulfoximine (MSO) is a centrally acting neurotoxin which inhibits the glutamate metabolism enzymes and has convulsive properties. Administration of a small dose of MSO to rabbits, either intravenously or intracerebroventricularly, except for the already known convulsive effects, may also be responsible for hind leg myopathy (rigid paralysis with histological findings resembling myositis) which sets in by the 4th day after MSO administration.