Lea Eriksson

Vasopressin release by histamine in the conscious goat

The vasopressin releasing activity of histamine was studied in conscious goats. Histamine was infused into the brain ventricles or intravenously and serial blood samples were taken from the jugular vein. Plasma vasopressin was assayed by a radioimmunoassay for arginine vasopressin (AVP). After i.v. infusions of 300 micrograms and 1 mg of histamine, a pronounced increase in plasma AVP up to greater than 100 pg/ml occurred. This was considered secondary to hypotension, since it only occurred with doses that markedly decreased arterial blood pressure. When given i.c.v. even smaller doses of histamine increased plasma AVP without any concomitant decrease in blood pressure. There was a simultaneous decrease in renal free water clearance in hydrated animals. Up to the dose of 300 microgramshistamine, the increase in AVP was modest. Histamine 1 mg i.c.v., given to either hydrated or non-hydrated animals, increased plasma AVP about tenfold, up to 38.3 +/- 23.3 pg/ml and 56.7 +/- 19.0 pg/ml, respectively. In individual experiments the correlation between the AVP level and the degree of antidiuresis was less apparent, since the kidney seemed to be very sensitive even to small changes in plasma AVP. It is concluded that histamine releases vasopressin through a central mechanism and it is reasonable to suppose that histamine may act as a neurotransmitter in the vasopressin releasing system.

Antidiuresis induced by infusions of histamine into the brain ventricles of conscious hydrated goats.

Histamine was infused into the third or lateral ventricle of conscious hydrated goats, and urine samples were analyzed for volume, osmolality and electrolytes. Doses of 10--1000 microgram of histamine induced dose-dependent antidiuretic responses both as to the maximum osmolality and the duration of the osmolality increase. Urine osmolality began to rise within a few minutes, reached its maximum within 0.5--2 h and was elevated for 1.5--4 h, depending on the dose. Thereafter a second increase in osmolality often occurred, which lengthened the effect of histamine dose-dependently up to about 10 h with the largest dose of histamine. Histamine (50--300 microgram) and the control solution given into the lateral ventricle increased the excretion of Na+ into the urine. After the largest dose of histamine (1000 microgram), however, the excretion of Na+ was significantly lower than in the control experiments. After the larger doses of histamine, effects on motor or autonomic functions were seen. These included decreased spontaneous motor activity, increased respiratory rate, defecation and miosis. It is suggested that the site of action of histamine is central, and that the release of vasopressin through the activation of the neurosecretory system is probably involved. In addition the changes in electrolytes may suggest an involvement of the release of other factors such as prolactin.

Plasma vasopressin levels after I.C.V. infusion of histamine agonists in the conscious goat

Histamine H1-agonists 2-pyridylethylamine (2-PEA) and 2-methylhistamine and H2-agonists 4-methylhistamine, dimaprit and impromidine were given i.c.v. to conscious goats and the release of arginine vasopressin (AVP) was measured. 2-PEA at very low doses (9 and 27 μmoles/animal, equivalent to H1-activity of about 0.5 and 1.5 μmoles histamine, resp.)_significantly increased plasma AVP. The H2-agonists did not cause consistent changes in AVP even if their relative doses were higher. It is concluded that the vasopressin releasing effect of histamine is due to H1-receptor activation.

Taurine in the osmoregulation of the Brattleboro rat

The function of taurine in mammalian osmoregulation was studied in the Brattleboro rat with hereditary hypothalamic diabetes insipidus (DI). DI rats are chronically dehydrated because of their inability to synthesize vasopressin. One day of water deprivation did not affect the water balance in rats with normal vasopressin synthesis, whereas DI rats were markedly dehydrated and lost considerably body weight. Taurine content and 3H-taurine accumulation by platelets were significantly higher in DI rats, with a further increase after one day of water deprivation. In DI rats, water deprivation also evoked a clear taurine increase in skeletal muscle and in the brain. These findings indicate that taurine has an osmoregulatory function in mammals.

Serum Cortisol Levels in Goats Exhibit Seasonal But Not Daily Rhythmicity

The aim of the study was to find out whether there is a daily rhythm in goat serum cortisol concentrations, whether the concentration profiles differ between normal light:dark and constant dark conditions, and whether any seasonal variations might be detected in daily cortisol secretion patterns. Seven Finnish landrace goats were kept at indoor temperature (18–23°C) under artficial lighting that approximately simulated the annual changes of daylength at 60°N. Blood samples were collected for cortisol measurements by radioimmunoassay at 2h intervals during six times of the year: winter (light:dark 6:18h), early spring (10:14h), late spring (14:10h), summer (18:6h), early fall (14:10h), and late fall (10:14h). Cortisol profiles were determined for two consecutive days, first in light:dark (LD) conditions and then in continuous darkness (DD). There was no significant daily rhythm in serum cortisol levels in any time of the year, nor did the profiles in LD and DD conditions show any differences. A significant seasonal variation was, however, detected among the overall cortisol levels. In winter, the concentrations were higher than in any other season, and from early spring to summer they were at their lowest. Under equal photoperiods, the cortisol levels were higher in fall than spring. The difference between winter and summer was confirmed in the following year in LD conditions. There was no correlation between the serum cortisol and progesterone levels. The results suggest that the possible circadian variation of cortisol secretion in goats is completely masked by external factors, and the lighting conditions do not have immediate effects on the daily secretion patterns. The seasonal variation in the overall cortisol levels is most probably related to the changes in photoperiod, because other conditions were relatively constant during the experiment.

Seasonal variation in endogenous serum melatonin profiles in goats: a difference between spring and fall?

The pineal hormone melatonin serves as a signal of day length in the regulation of annual rhythms of physiological functions and behavior. The duration of high melatonin levels in body fluids is proportional to the duration of the dark period of the day. Due to the direct suppression of melatonin by light, the overt melatonin rhythm may differ from the endogenous rhythm driven by the hypothalamic circadian clock. The aim of this study was to find out possible differences between the overt and endogenous melatonin rhythms in goats during the course of a year. Seven Finnish landrace goats (nonlactating females) were kept under artificial lighting that approximately simulated the annual changes of day length at 60°N. Blood samples for melatonin measurements by radioimmunoassay were collected at 2-h intervals during six seasons: winter (light:dark 6:18 h), early spring (10:14), late spring (14:10), summer (18:6), early fall (14:10), and late fall (10:14). Melatonin profiles were determined for 2 consecutive days, first in light-dark (LD) conditions and then in continuous darkness (DD). In LD conditions, the profiles matched the dark period with one exception: in winter, the mean peak duration was significantly shorter than the scotoperiod. In DD conditions, two types of endogenous melatonin patterns were found: a “winter pattern” (peak duration 13–15 h) in winter, early spring, early fall, and late fall, and a “summer pattern” (duration about 11 h) in late spring and summer. Thus, in equal habitual LD conditions in late spring and early fall (LD 14:10), the endogenous melatonin rhythms were not quite similar: the pattern in late spring resembled that in summer, and the pattern in early fall that in winter. These results suggest that, in addition to the light-adjusted overt melatonin rhythm, the endogenous rhythm of melatonin secretion varies during the course of a year.

Cardiovascular and behavioral changes after ICV infusion of TRH in the conscious goat

Thyrotropin releasing hormone was infused during 5 min into the lateral brain ventricle (ICV) of conscious goats in doses ranging from 125 to 4000 ng. Changes in blood pressure, heart rate and behavior were studied. TRH in doses of 500--4000 ng raised the mean blood pressure and both the magnitude and the duration of the response was related to the dose. The onset of the rise occurred in average 8 min after the start of the infusion. The heart rate fell somewhat with all doses. No consistent changes were observed in the respiration rate. TRH caused several behavioral changes beginning about 6 min after the start of infusion. The goats often decreased their locomotor activity. They were frequently bleating except after the lowest dose. These results support the view that TRH affects several vital functions in the central nervous system.

Cardiovascular and behavioural changes after i.c.v. infusions of histamine and agonists in conscious goat

Histamine (3 and 10 μmoles) raised the blood pressure in the conscious goat when given i.c.v. but lowered it when given intravenously. The results with 2-PEA support the view that central H1-stimulation raises the blood pressure. However, the results with dimaprit do not exclude the participation of central H2-receptors in the effect of histamine on the blood pressure.It appears that in the goat the stimulation of central H1- and H2-receptors mediates opposite actions on behaviour. H1-agonist increased and H2-agonist decreased the general activity of the animal.

The daily rhythms of melatonin and free fatty acids in goats under varying photoperiods and constant darkness

The purpose of the study was to explore parallel and divergent features of the daily rhythms of melatonin and plasma free fatty acids (FFA) in goats exposed to different lighting conditions. From these features, we attempted to analyze whether the endogenous melatonin rhythm plays any role in the maintenance of the FFA rhythm. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60°N (longest photoperiod, 18 h; shortest, 6 h). The ambient temperature and feeding regimen were kept constant. Blood samples were collected 6 times a year at 2 h intervals for 2 d, first in the prevailing light‐dark (LD) conditions and then after 3 d in constant darkness (DD). In LD conditions, the melatonin levels always increased immediately after lights‐off and declined around lights‐on, except in winter (18 h darkness), when the low daytime levels were restored clearly before lights‐on. The FFA levels also displayed a consistent rhythmicity, with low levels at night and a transient peak around lights‐on. In DD conditions, the melatonin profiles were very similar to those found in the habitual LD conditions, but the rhythm tended to advance. The FFA rhythm persisted also in DD, and the morning peak tended to advance. There was an overall parallelism between the two rhythms, with one significant exception. In winter in LD conditions, the morning rise in FFA levels coincided with lights‐on and not with the declining phase of melatonin, whereas in DD conditions, the FFA peak advanced several hours and coincided with the declining phase of melatonin. From this finding and comparisons of the calculated rhythm characteristics, i.e., phase‐shifts, phase differences, and correlations, we conclude that the daily rhythm of FFA levels is most probably generated by an endogenous oscillator, primarily adjusted by dawn, whereas the melatonin rhythm in this species is regulated by an oscillator primarily adjusted by dusk. The results did not exclude a modulatory effect of melatonin on the daily FFA profiles, but melatonin secretion, alone, does not explain the patterns sufficiently.

Histamine and histamine-N-methyltransferase in the goat brain

Histamine concentrations and histamine-N-methyltransferase (HMT) (EC 2.1.1.8) activities were assayed in the goat brain, since this animal has been used for the purpose of assessing the physiological role of histamine in the central nervous system. The highest histamine concentrations were found in the hypothalamus (616±88 ng/g), ‘thalamus’ (244±41), preoptic area (242±95) and ‘area postrema’ (170±52), and are three to tenfold higher than the average concentration in the whole brain (76±9). The pineal body and choroid plexus contained very high concentrations of histamine (5800 and 5400 ng/g, respectively) and the pituitary gland had about the same as the hypothalamus (564±248 ng/g). There was little variation in the HMT activity in various parts of the brain and in the glands. In the cerebrospinal fluid both histamine concentration and HMT activity were very low, some increase occurredpost mortem, but not within 20 minutes of death. The results indicate that the distribution of histamine and that of HMT in the goat brain are fairly similar to those in other mammalian brains, supporting the assumption that histamine might have a physiological function in the brain, especially in the hypothalamus.