Cholinesterases and postnatal development of the negative chronotropic effects of acetylcholine in albino rats

The negative chronotropic effects of acetylcholine were studied in the isolated atria of the hearts of albino rats aged 2, 15, 29 and 47 days and adult. In untreated preparations, i.e. with full cholinesterase activity, the strongest effects were observed in newborn animals; with advancing age the reaction grew weaker. If cholinesterase activity was inhibited with physostigmine, the differences between the various age groups were obliterated. It is thus evident that the actual acetylcholine sensitivity of the sinoatrial node tissue does alter during postnatal life, but that growing cholinesterase activity reduces the amount of acetylcholine diffusing from the medium into the acetylcholine receptor zone. The change which takes place in cholinesterase activity in the myocardial tissue during postnatal life is so great that is must be taken into account when considering the development of cholinergic control of cardiac function.     

Postnatal development of chronotropic responses to chemical and electrical stimulation of adrenergic nerve endings in the sinoatrial node of the heart of the albino rat

The positive chronotropic response to stimulation of adrenergic nerve endings in the sinoatrial node was studied in isolated atria from the hearts of rats of different ages. Dimethylphenylpiperazinium (DMPP) was used for chemical stimulation and transmural stimulation of the sinoatrial node region as electrical stimulation; in both cases noradrenaline is released from the nerve endings. With both stimulation methods, postnatal development was recorded in two phases. In the first phase, positive chronotropic responses are markedly increased and attained the maximum at the age of 14 days on using DMPP and of 24 days on using electrical stimulation. In the second phase, positive chronotropic responses diminish and at the age of about 45 days, with both stimulation methods, they become reduced to adult level. The first developmental phase can be attributed to an increase in the noradrenaline content of the nerve endings and the release of a larger amount of the transmitter during stimulation, together with an increase in the noradrenaline sensitivity of the cells of the sinoatrial node. It is not clear why positive chronotropic responses decrease in the second phase, when the noradrenaline content of the myocardial tissue continues to rise and pacemaker sensitivity to noradrenaline is not reduced.     

Postnatal development of electrophysiological properties of pacemaker cells in the rabbit

Using glass microelectrodes, the authors measured basic electrophysiological parameters of the true pacemaker cells of the rabbit in an attempt to elucidate the postnatal drop in the frequency of action potentials produced by the sinoatrial node. The average rate of slow diastolic depolarization falls markedly between birth and adulthood, while the duration of the action potentials of the pacemaker cells become prolonged. These changes explain age-determined chronotropic development: the lower rate of slow diastolic depolarization in adult animals causes later attainment of the threshold; the longer action potential also contributes to prolongation of the cycle of membrane voltage changes in the pacemaker cells. The direct role of the postnatal increase in the maximum diastolic potential value is small, owing to a concomitant increase in the threshold potential value.     

Postnatal changes of the tonic influence of the vagus nerves on the heart rate, and of the activity of choline acetyltransferase in the heart atria of rats

Postnatal changes in the resting heart rate and in its parasympathetic tonic inhibition have been measured in awake rats and compared with changes in the activity of choline acetyltransferase (ChAT) in the heart atria. The heart rate at rest increased from 372.min-1 on the 1st to 456 and 442.min-1 on the 15th and 24th day of life and then again decreased to 358 and 356.min-1 in 60-day-old and adult rats. Until the 15th day of postnatal life, the administration of atropine did not bring about an increase in the heart rate; the cardio-acceleratory effect of atropine (indicating the presence of tonic vagal inhibition of the heart) appeared only on the 18th day and increased steeply up to the 40th day of postnatal life. The activity of ChAT in the heart atria was measured as the difference between the synthesis of acetylcholine in atrial homogenates incubated in the absence and in the presence of bromoacetylcholine (BrACh), a specific inhibitor of ChAT; this procedure eliminated the contribution of carnitine acetyltransferase to the synthesis of acetylcholine. The activity of ChAT was found to increase steeply from the 1st to the 25th days of postnatal life; the steepest increase in the activity of the enzyme occurred between the 4th and the 15th days. Temporal correlation between the changes in the activity of ChAT, in the content of acetylcholine in the heart atria (Kuntscherová and Vlk 1979) and in the efficiency of transmural stimulation of sinoatrial region on the heart rate (Vlk 1979) indicate that the functional maturation of intracardiac cholinergic neurones, proceeding in rats during the first three weeks of their postnatal life, plays an important role in the onset and temporal development of the tonic parasympathetic inhibition of the heart rate.     

Postnatal changes in the activities of acetylcholinesterase and butyrylcholinesterase in rat heart atria

Postnatal development of the activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) in rat heart atria has been investigated with the use of 1.5-bis (allyldimethylammoniumphenyl) pentane-3-dibromide (BW 284 C51) as a selective inhibitor of AChE. Total cholinesterase activity (mumol acetylthiocholine hydrolysed X g-1 per hour) increased from 218 on the 1st day after birth to 426 on the 30th day and diminished to 340 in adult rats. The activity of AChE (mumol acetylthiocholine hydrolysed X g-1 per hour) underwent more dramatic changes, increasing more than 4-fold during the first month of life, from 13 on the 1st to 58 on the 30th day of life and then decreasing to 42 in adult rats. The proportion of AChE on total cholinesterase activity increased from 6% on the 1st day to 12-15% in animals aged 24 days and more. Since AChE is known to be specifically involved in the termination of the action of acetylcholine in the sinoatrial node, the observed postnatal changes in its activity are likely to play a role in the postnatal development of cardiac parasympathetic control.     

Immunocytochemical demonstration of the equilibrative nucleoside transporter rENT1 in rat sinoatrial node.

Adenosine exerts multiple receptor-mediated effects in the heart, including a negative chronotropic effect on the sinoatrial node. The aim of this study was to investigate the distribution of the equilibrative nucleoside transporter rENT1 in rat sinoatrial node and atrial muscle. Immunocytochemistry and/or immunoblotting revealed abundant expression of this protein in plasma membranes of sinoatrial node and in atrial and ventricular cells. Because rENT1-mediated transport is likely to regulate the local concentrations of adenosine in the sinoatrial node and other parts of the heart, it represents a potential pharmacological target that might be exploited to ameliorate ischemic damage during heart surgery.     

Gender differences in development of exploratory activity in rats

The dynamics of exploratory activity in male and female Wistar rats before weaning and over the course of puberty was investigated. Behavior of animals was examined on the 18th, 35th, and 50th postnatal days in a modified light-dark test and on the 24th postnatal day in an open-field test. It was found that exploratory activity of males considerably changed over the course of puberty period. From the 18th to 35th postnatal days, the number of animals exploring a light compartment increased. From the 35th to 50th postnatal days, the duration of the light compartment investigation increased. The only change in exploratory activity of female rats consisted in an increase in the number of transitions between the light and dark compartments from 35th to 50th postnatal days. The results suggest different mechanisms of exploratory behavior development in male and female rats.     

Effect of repeated exercise on postnatal development of the noradrenaline content of the albino rat heart, spleen and skeletal muscle

The authors studied the effect of repeated elevation of sympathetic activity on the postnatal development of the noradrenaline content of tissues of the albino rat. Between the ages of 15 and 29 days, young rats were forced to swim in water heated to 25 degrees C, 3 X 30 min on weekdays and 1 X 30 min on Saturdays and Sundays. At 30, 45 and 65 days, the noradrenaline content of the tissues was determined spectrofluorometricaLly by the trihydroxyindole method. The noradrenaline content of the heart of trained rats was higher than in the controls in all the given age groups and the size of the absolute difference rose with advancing age. The noradrenaline content of the spleen developed similarly. Repeated exercise did not lead to an increase in the noradrenaline content of skeletal muscle. The results show that the repeated elevation of the activity of sympathetic adrenergic neurones which occurs in young rats during exercise is a long-term factor stimulating the development of sympathetic innervation of the heart and spleen. The development of the neurones innervating skeletal muscle was not stimulated, probably because the activity of these neurones is not increased by stress.     

Functional-morphologic characteristics of the endocrine system of the progeny of adrenalectomized rats

The authors observed signs of premature maturation and activation of the hypophysio-adrenocortical system at the early postnatal period (the 70--90th hour) in the progeny of rats adrenalectomized 1 to 3 and 7 to 9 days before labour. A more pronounced in comparison with control (progeny of the sham-adrenalcetomized and intact rats) response to stress pointed to the early maturation of the suprahypophysial control mechanisms as well. Signs of functional exhaustion in the hypophysio-adrenocortical system and also in the thyroid gland were revealed in mature (6--10-month-old) rats born of adrenalectomized rats.     

Development of the caudate nucleus of rats in postnatal ontogenesis and formation of cholinergic mediation in it]

Morphological maturation of the rat's caudate nucleus and formation of cholinergic system in it were studied in postnatal ontogenesis. Under investigation were newborn rats, 7, 14, 21 day old rats and adult animals. The growth and maturation of the caudate nucleys were most intesne during the first two postnatal weeks, it was somewhat descreased by the beginning of the third week and continued in later terms as well. The structure of dendrites and axons became complicated during the first two weeks, the axo-dendritic contacts being also formed. The neuron structure in 14 days old rats was similar to that of adult animals. The activity of acetylcholinesterase in the caudate nucleus of 7 and 14 days rats was not great. It sharply increased by the 17th day and reached the level of adults with 3 weeks after birth. Possible correlations of the morpho-chemical maturation of the caudate nucleus and formation of motor activity in rats is discussed.     

Manifestation of active and passive defensive behavior in juvenile rats

The dynamics of freezing and flight reactions in juvenile rats was investigated. The rats were tested on the 20th, 25th, 35th, and 40th postnatal days. A sound of 6-sec duration (bell) was used as a threatening stimulus. The following parameters were recorded: number of rearing reactions and defecations within 5 min prior to stimulation, reactions to the bell, latent periods and durations of freezing reactions, freezing posture rigidity, and time of recovery of movements after freezing. It was shown that the intensity of freezing reduced in the period from the 20th to 35th postnatal day. The flight reactions were highest on the 25th and 40th days. Correlations between freezing indices and numbers of rearing and defecation reactions were different in rats of all age groups. The results suggest that the structure of defensive behavior changes with maturation of principal defensive reactions in rats within the first 40 postnatal days.     

Postnatal development of conditioned reflex behavior: comparison of the periods related to the onset of the different forms of hippocampal plasticity in rats

Fear conditioning, escape and active avoidance reactions in two-way avoidance paradigm were compared in rats of different ages. Fear conditioning, but not escape and active avoidance reactions could be acquired on the 16-17th postnatal days, and the acquisition was more effective than in adults. Escape behavior matured beginning from the 18th postnatal day reaching the adult level within the 3d-4th postnatal weeks. Maturation of the mechanisms of Pavlovian (fear reaction) and instrumental (escape reaction) conditioning did not facilitate the acquisition of two-way avoidance until the 4th postnatal week, young animals displayed low acquisition in this period. The maturation of these memory processes is proposed to be related to developmental stages of different mechanisms of hippocampal plasticity.     

Age related classification of neurons in the rat visual cortex: a Nissl study (author's transl)

Using Nissl preparations, 6, 8, 12, 16, 20, 30, 90, and 420 days old albino rats were investigated with regard to classification of several neuron types within layer IV and adjacent zones of layers III and V. Increasing maturation of the nervous tissue provides a greater classifiable variety of neuron types. About between the second and third postnatal week the degree of classification reach partially almost the same level as in the adult animals. Particular attention is paid to a large, frequently bipolar neuron type rich in cytoplasm.     

Proliferation of piriform neurons in the rat cerebellar cortex in the pre- and postnatal periods of development]

The method of histoautoradiography with the use of H3-thymidine was applied to the study; there were established the periods of appearance in the cerebellar anlage of albino rats of neuroblasts differentiated into the piriform neurones of the cerebellar cortex; dynamics of the proliferative activity of these cellular elements in the course of the pre- and postnatal periods of development of the experimental animals was investigated. On the basis of the material obtained a conclusion was drawn that the last cell divisions (the result of cell differentiation were Purkinje's cells of the cerebellar cortex) stopped by the 13th--15th day of the embryonic development. No incorporation of the labeled precursor into the DNA of the nuclei of the differentiating piriform neurons occurred later.     

Dependence of the cytotoxic effect of guanethidine on the degree of sympathetic activity

Young rats aged 15-29 days received a subcutaneous injection of guanethidine sulphate (5 mg/kg body weight) every day. Owing to damage to the postganglionic sympathetic neurones, on about the 60th day of life we observed a significant decrease in the noradrenaline concentration in these animals' hearts compared with the controls. If every guanethidine injection was followed immediately by intensive physical exercise, there was no drop in the heart noradrenaline concentration. Physical exercise of the same intensity performed a few hours before injecting guanethidine did not prevent the drop in the noradrenaline concentration in the heart. The results show that an exercise-induced increase in sympathetic activity, at a time when guanethidine is circulating in the blood and accumulating in the adrenergic neurones, inhibits the cytotoxic effect of guanethidine. Isolated physical exercise performed between the 15th and 29th day of life leads to an increase in the noradrenaline content of the heart of rats aged 60 days.     

Circahoral rhythms of quantitative protein change--a marker of the morphological maturity and functional activity of the retina

The development of hour changes in the protein content of the retinal ganglionic cells was studied in 1, 7, 14, 19, and 30 day old ground squirrels using cytophotometry. Such rhythms were also compared in sexually mature active and hibernating animals. In ground squirrels, as well as in the other immature born mammals, the recording of hour rhythms of the protein content of the ganglionic neurones becomes possible only after the morphological maturation of retina, from the 19th day of postnatal ontogenesis. During hibernation, no hour rhythms of the protein content of the retina ganglionic cells were recorded in ground squirrels. A conclusion has been drawn that the hour rhythms of protein metabolism can serve as an index of morphological maturity of the tissue and its functional activity.     

Radioautographic study of the hypothalamic serotoninergic structures in the perinatal period of the rat

The distribution of 3H-serotonin-binding structures in hypothalamus of 16 and 18 day old fetuses and of 9 day old rats was studied after intraventricular injection of 3H-serotonin. Rare 3H-serotonin-binding little differentiated cells were found predominantly in the intermediate zone of the 3rd ventricle in the retrochiasmatic area wall on the 16th and 18th days of prenatal development. In addition, an aggregate of heavily labeled neurones was observed in the suprachiasmatic area. At the same time 3H-serotonin-binding fibers first appeared, predominantly in the optic chiasma and perichiasmatic area. Radioactively labelled cells, which can be characterized by their morphology as differentiated neurones, were located in the dorsomedial nucleus on the 9th day of postnatal development. The number of serotonin-binding fibers markedly increased but the pattern of their distribution was, on the whole, similar to that in fetuses. The data obtained suggest that the main stages of structural organization of serotoninergic system of hypothalamus in rats are realized during perinatal period.     

Electrophysiology and pacemaker function of the developing sinoatrial node

The sinoatrial node performs its task as a cardiac impulse generator throughout the life of the organism, but this important function is not a constant. Rather, there are significant developmental changes in the expression and function of ion channels and other cellular elements, which lead to a postnatal slowing of heart rate and may be crucial to the reliable functioning of the node during maturation. In this review, we provide an overview of current knowledge regarding these changes, with the main focus placed on maturation of the ion channel expression profile. Studies on Na(+) and pacemaker currents have shown that their contribution to automaticity is greater in the newborn than in the adult, but this age-dependent decrease is at least partially opposed by an increased contribution of L-type Ca(2+) current. Whereas information regarding age-dependent changes in other transmembrane currents within the sinoatrial node are lacking, there are data on other relevant parameters. These include an increase in the nodal content of fibroblasts and in the area of nonexpression of connexin43, considered a molecular marker of nodal tissue. Although much remains to be done before a comprehensive view of the developmental biology of the node is available, important evidence in support of a molecular interpretation of developmental slowing of the intrinsic sinoatrial rate is beginning to emerge.     

Behavioral analysis of consequences of chronic blockade of NMDA-type glutamate receptors in the early postnatal period in rats

In view of the hypothesis that glutamatergic dysfunction of brain can underlie the negative symptoms of schizophrenia (including cognitive deficit), the aim of this study was to develop a model of cognitive impairment in Wistar male rats after administration of a noncompetitive NMDA-receptor antagonist in early postnatal period. Rat pups were daily subcutaneously injected with 0.05 mg/kg MK-801 on postnatal days 7-49. On the 27th and 28th days 24 h after the last previous injection, the MK-801-treated rats demonstrated lower spontaneous locomotor and exploratory activity in comparison with saline control, however, they retained the reaction of hyperlocomotion which developed immediately after the MK-801 administration. In these rats, the anxiety level in the elevated plus-maze (on the 40th postnatal day) was found to be decreased, and the spatial learning in food rewarded task was negatively affected (on the 50th-54th days). It is suggested that impairment of the input of sensory information and its correct assessment by the animals can be associated with the early neonatal blockade of NMDA glutamate receptors.     

Effects of a brief sensorimotor training on the development of behavioral inhibition in the mouse

In the young C57Bl6 mouse, the hyperexcitability phase (`pop-corn' stage) which normally occurs around the 16–18th postnatal day and lasts 3–6 days, was greatly shortened by an intensive sensorimotor training when the `pop-corn' stage appeared. It was prevented when the animals were trained for 4 days before it appeared. This might suggest, at least in part, that an early short duration sensorimotor training increased the rate of maturation of the inhibitory systems that sustain the development of the motor behavior.