Peculiarities of regulation of high-frequency oscillations of cardiac rhythm in rat ontogenesis

Dynamics of high-frequency components of heart periodogram whose main part is respiratory arrhythmia (RA) as well as consequences of vagotomy, block of M-cholinoreceptors by atropine and of β-adrenoreceptors by propranolol was studied in Wistar white rats in a large age diapason from 2–4 days to adults. It was established that results of the actions in immature rats did not essentially differ from those observed in adult rats and described in animals of other species and in human. In rats of young age, predominant in genesis of RA is peripheral mechanism. Vagotomy produces an elevation of the RA amplitude due to a sharp increase of the inspiration time as well as to deafferentation. Sympathetic nervous system produces restricting action on RA. This role is preserved in animals of all age groups. Participation of parasympathetic innervation in the RA genesis is revealed from the third week and continues increasing to the mature age. However, in adult rats, the peripheral mechanism of the RA formation is preserved, as disturbance of parasympathetic innervation leads not to the disappearance of RA, but only to a decrease of its amplitude.     

Trypanosoma cruzi: acetylcholine content and cholinergic innervation of the heart in rats.

The acetylcholine content, as well as the density and distribution of cholinergic innervation were studied in the hearts of rats after inoculation with the Y strain of Trypanosoma cruzi. Cholinergic innervation was studied by histochemical techniques using acetylcholinesterase in the sinoatrial node, in fragments of the right atrium and auricular appendages. Acetylcholine was assayed on a strip of guinea pig ileum suspended in Tyrode's solution. Twenty days after inoculation, the cardiac content of acetylcholine dropped 40% and the cholinergic innervation was markedly reduced in 80 to 100% of the rats. However, at 70 and 97 days the cardiac content of acetylcholine was not different from that of the controls. The cholinergic innervation in the heart of the animals sacrificed 98 and 180 days after inoculation was normal in 60 to 66% of the rats. The results indicate that the cardiac parasympathetic innervation is damaged during the acute phase of experimental Chagas' disease, but returns to normal during the chronic phase in most animals.     

Effects of Extracellular Diadenosine Tetraphosphate on Action Potentials in the Atrial and Ventricular Myocardium of the Rat Heart during Early Postnatal Ontogenesis

Diadenosine tetraphosphate (AP4A) belongs to a wide group of naturally derived endogenous purine compounds that have recently been considered as new neurotransmitters in the autonomic nervous system. It has been shown that AP4A induces inhibitory effects and modulates adrenergic control in the heart of adult mammals. Nevertheless, the physiological significance of AP4A in early postnatal development, when sympathetic innervation remains yet immature, has not been investigated. The aim of the present study was to elucidate the effects of AP4A on the heart bioelectrical activity in early postnatal ontogenesis. Action potentials (AP) were recorded using the standard microelectrode technique in multicellular isolated right atrial (RA), left atrial (LA), and ventricle (RV) preparations from male Wistar rats at postnatal days 1, 14, and 21 and from 60-day animals that were considered as adults. The application of AP4A caused significant reduction of AP duration in atrial (RA and LA) preparations from rats of all ages. Also, AP4A caused significant AP shortening in RV preparations from rats of various ages; however, the effect was more pronounced in 21-day-old and adult rats. AP4A failed to alter automaticity of RA preparations from the rats at postnatal days 1, 14, and 21 and weakly decreased spontaneous rhythm in RA preparations from the adult rats. The effect of AP4A was partially abolished by P2-receptor blocker PPADS in LA preparations from both 21-dayold and adult rats, while it failed to suppress AP4A-caused AP shortening in preparations from 1- and 14-dayold animals. Thus, extracellular AP4A causes shortening of AP both in the atrial and ventricular myocardium in the rats of early postnatal ontogenesis and in adults. The effect of AP4A depends on age only for ventricular myocardium where it may be attributed with growing contribution of diadenosine polyphosphates to the control of myocardium inotropy.     

Age-related changes in the innervation of the prostate gland

The adult prostate gland grows and develops under hormonal control while its physiological functions are controlled by the autonomic nervous system. The prostate gland receives sympathetic input via the hypogastric nerve and parasympathetic input via the pelvic nerve. In addition, the hypogastric and pelvic nerves also provide sensory inputs to the gland. This review provides a summary of the innervation of the adult prostate gland and describes the changes which occur with age and disease. Growth and development of the prostate gland is age dependent as is the occurrence of both benign prostate disease and prostate cancer. In parallel, the activity and influence of both the sympathetic and parasympathetic nervous system changes with age. The influence of the sympathetic nervous system on benign prostatic hyperplasia is well documented and this review considers the possibility of a link between changes in autonomic innervation and prostate cancer progression.     

Early dietary sodium restriction disrupts the peripheral anatomical development of the gustatory system

Dietary sodium restriction has profound effects on the development of peripheral taste function and central taste system anatomy. This study examined whether early dietary sodium restriction also affects innervation of taste buds. The number of geniculate ganglion cells that innervate single fungiform taste buds were quantified for the midregion of the tongue in two groups of rats: those fed either a low-sodium diet and those fed a sodium replete diet (control rats) from early prenatal development through adulthood. The same mean number of ganglion cells in developmentally sodium-restricted and control adult rats innervated taste buds on the midregion of the tongue. However, the characteristic relationship of the larger the taste bud, the more neurons that innervate it did not develop in sodium-restricted rats. The failure to form such a relationship in experimental rats was likely due to a substantially smaller mean taste bud volume than controls and probably not to changes in innervation. Further experiments demonstrated that the altered association between number of innervating neurons and taste bud size in restricted rats was reversible. Feeding developmentally sodium-restricted rats a sodium replete diet at adulthood resulted in an increase in taste bud size. Accordingly, the high correlation between taste bud volume and innervation was established in sodium-replete rats. Findings from the current study reveal that early dietary manipulations influence neuron-target interactions; however, the effects of dietary sodium restriction on peripheral gustatory anatomy can be completely restored, even in adult animals.     

Whisker plucking alters responses of rat trigeminal ganglion neurons

Whisker plucking in developing and adult rats provides a convenient method of temporarily altering tactile input for the purposes of studying experience-dependent plasticity in the somatosensory cortex. Yet, a comprehensive examination of the effect of whisker plucking on the response properties of whisker follicle-innervating trigeminal ganglion (NVg) neurons is lacking. We used extracellular single unit recordings to examine responses of NVg neurons to controlled whisker stimuli in three groups of animals: (1) rats whose whiskers were plucked from birth for 21 days; (2) rats whose whiskers were plucked once at 21 days of age; and (3) control animals. After at least 3 weeks of whisker re-growth, NVg neurons in plucked rats displayed normal, single whisker receptive fields and could be characterized as slowly (SA) or rapidly adapting (RA). The proportion of SA and RA neurons was unaffected by whisker plucking. Both SA and RA NVg neurons in plucked rats displayed normal response latencies and angular tuning but abnormally large responses to whisker movement onsets and offsets. SA neurons were affected to a greater extent than RA neurons. The effect of whisker plucking was more pronounced in animals whose whiskers were plucked repeatedly during development than in rats whose whiskers were plucked once. Individual neurons in plucked animals displayed abnormal periods of prolonged rhythmic firing following deflection onsets and aberrant bursts of activity during the plateau phase of the stimulus. These results indicate that whisker plucking exerts a long-term effect on responses of trigeminal ganglion neurons to peripheral stimulation.     

Whisker plucking alters responses of rat trigeminal ganglion neurons

Whisker plucking in developing and adult rats provides a convenient method of temporarily altering tactile input for the purposes of studying experience-dependent plasticity in the somatosensory cortex. Yet, a comprehensive examination of the effect of whisker plucking on the response properties of whisker follicle-innervating trigeminal ganglion (NVg) neurons is lacking. We used extracellular single unit recordings to examine responses of NVg neurons to controlled whisker stimuli in three groups of animals: (1) rats whose whiskers were plucked from birth for 21 days; (2) rats whose whiskers were plucked once at 21 days of age; and (3) control animals. After at least 3 weeks of whisker re-growth, NVg neurons in plucked rats displayed normal, single whisker receptive fields and could be characterized as slowly (SA) or rapidly adapting (RA). The proportion of SA and RA neurons was unaffected by whisker plucking. Both SA and RA NVg neurons in plucked rats displayed normal response latencies and angular tuning but abnormally large responses to whisker movement onsets and offsets. SA neurons were affected to a greater extent than RA neurons. The effect of whisker plucking was more pronounced in animals whose whiskers were plucked repeatedly during development than in rats whose whiskers were plucked once. Individual neurons in plucked animals displayed abnormal periods of prolonged rhythmic firing following deflection onsets and aberrant bursts of activity during the plateau phase of the stimulus. These results indicate that whisker plucking exerts a long-term effect on responses of trigeminal ganglion neurons to peripheral stimulation.     

Effects of thyrotoxicosis and selective hepatic autonomic denervation on hepatic glucose metabolism in rats

Thyrotoxicosis is known to induce a broad range of changes in carbohydrate metabolism. Recent studies have identified the sympathetic and parasympathetic nervous system as major regulators of hepatic glucose metabolism. The present study aimed to investigate the pathogenesis of altered endogenous glucose production (EGP) in rats with mild thyrotoxicosis. Rats were treated with methimazole in drinking water and l-thyroxine (T(4)) from osmotic minipumps to either reinstate euthyroidism or induce thyrotoxicosis. Euthyroid and thyrotoxic rats underwent either a sham operation, a selective hepatic sympathetic denervation (Sx), or a parasympathetic denervation (Px). After 10 days of T(4) administration, all animals were submitted to a hyperinsulinemic euglycemic clamp combined with stable isotope dilution to measure EGP. Plasma triiodothyronine (T(3)) showed a fourfold increase in thyrotoxic compared with euthyroid animals. EGP was increased by 45% in thyrotoxic compared with euthyroid rats and correlated significantly with plasma T(3). In thyrotoxic rats, hepatic PEPCK mRNA expression was increased 3.5-fold. Relative suppression of EGP during hyperinsulinemia was 34% less in thyrotoxic than in euthyroid rats, indicating hepatic insulin resistance. During thyrotoxicosis, Sx attenuated the increase in EGP, whereas Px resulted in increased plasma insulin with unaltered EGP compared with intact animals, compatible with a further decrease in hepatic insulin sensitivity. We conclude that chronic, mild thyrotoxicosis in rats increases EGP, whereas it decreases hepatic insulin sensitivity. Sympathetic hepatic innervation contributes only to a limited extent to increased EGP during thyrotoxicosis, whereas parasympathetic hepatic innervation may function to restrain EGP in this condition.     

Effect of the early postnatal administration of cortisol on development of the nervous control of cardiac function in albino rats

When infant rats were treated with cortisol, in daily s.c. doses of 20 mg X kg-1, between the ages of 2 and 15 days, the noradrenaline content of their heart and spleen, between the ages of 23 and 65 days, was lower than in the controls. The decrease in the noradrenaline content did not diminish with advancing age; on the contrary, it was the most pronounced at 65 days. Cortisol treatment did not affect the noradrenaline content of skeletal muscles. The functional significance of the decrease in the noradrenaline content was studied in nervous control of the sinoatrial node of the heart. In agreement with the drop in noradrenaline concentration, transmural stimulation of the sinoatrial node region of isolated atria led to a mild, but statistically significant reduction of the function of sympathetic nerve endings, whereas parasympathetic innervation of the node showed no signs of impairment. This peripheral functional deficiency of sympathetic innervation of the node is not seen in the intact organism, where it is masked by central nervous mechanisms. Rats given cortisol postnatally had a significantly higher heart rate at 23, 33 and 44 days, because, in the presence of normal sympathetic influence, the tone of the parasympathetic nerves was reduced. The heart rate was highest at 23 days; with advancing age the difference diminished and at 65 days it was statistically nonsignificant.     

Role of sympathetic and parasympathetic mechanisms in formation of secondary cardiac rhythms in rats ontogenesis

In progeny of Wistar rats aged from birth to 3 week, there was studied participation of sympathetic and parasympathetic mechanisms in regulation of cardiac rhythm and its rhythmic oscillations (secondary cardiac rhythms), whose spectral composition was analyzed using rapid Fourier transformation. Consequences, which changed in the process of development, of blockade of -adrenoreceptors by propranolol, of -adrenoreceptors by phentolamine, and of muscarinic cholinoreceptors by atropine as well as of chronic desympathization by guanethidine (isobarine). It was found that due to heterochronia in establishment of functions of sympathetic and parasympathetic nervous systems, reactions to blockade of adreno- and cholinoreceptors for the first 3 weeks of postnatal ontogenesis changed not only quantitatively, but also qualitatively. Blockade of adrenoreceptors in newborn animals leads to an increase of power of the rhythm oscillations in all low-frequency diapasons. The baroreflex function of parasympathetic innervation is well expressed as early as in newborns. Tonic function with respect to frequency of heart rate and power of oscillations in the high-frequency diapason becomes evident only by the 3-week age.Translated from Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, Vol. 41, No. 1, 2005, pp. 69–75.Original Russian Text Copyright © 2005 by Bursian, Sizonov, Semenova, Kulaev, Timofeeva, Polyakova, Dmitrieva.     

Increased sympathetic innervation in the cerebral and mesenteric arteries of hypertensive rats

The density of catecholamine-containing nerve fibers was studied in the cerebral and mesenteric arteries from normotensive Wistar-Kyoto rats (WKY), spontaneously hypertensive rats (SHR), and stroke-prone SHR (SHRSP) in the growing (SHR, WKY) and adult (SHR, SHRSP, WKY) animals. Cerebral arteries from SHR showed an increased adrenergic innervation from day 1. The nerve plexuses reached an adult pattern earlier in SHR than in WKY. The arteries from adult SHR and SHRSP (22 weeks old) showed a markedly higher nerve density than WKY. There was a positive linear correlation between blood pressure and nerve density for four cerebral arteries. The mesenteric arteries were not innervated at birth. However, hyperinnervation of these arteries in the SHR was already present at 10 days of age as compared with WKY. Sympathectomy with anti-nerve growth factor and guanethidine caused a complete disappearance of fluorescent fibers in the mesenteric arteries from SHR and WKY, and in the cerebral arteries of WKY. The same procedure caused only partial denervation of the cerebral arteries from hypertensive animals. We postulate that the increase in nerve density in the cerebral arteries from the hypertensive rats may contribute to the development of arterial hypertrophy in chronic hypertension through the trophic effect of the sympathetic innervation on vascular structure.     

Innervation of the proximal urethra of ovariectomized and estrogen-treated female rats

The proximal urethra plays a central role in maintaining urinary continence, and sympathetic excitatory innervation to urethral smooth muscle is a major factor in promoting tonic contraction of this organ. Elevated estrogen levels are often associated with incontinence in humans. Because elevated estrogen levels result in degeneration of sympathetic nerves from the closely related uterine smooth muscle, we examined the effects of chronic estrogen administration on proximal urethral innervation. Ovariectomized virgin female rats received either vehicle or 17 beta-estradiol for 1 week, and smooth muscle size and parasympathetic, sensory and sympathetic nerve densities were assessed quantitatively throughout the first 3 mm of the proximal urethral smooth muscle. In vehicle-infused ovariectomized rats, parasympathetic nerves immunoreactive for vesicular acetylcholine transporter were most abundant, while calcitonin gene-related peptide-immunoreactive sensory nerves and tyrosine hydroxylase-immunoreactive sympathetic nerves were less numerous. The densities of parasympathetic and sensory nerves remained constant along the proximal urethra, while sympathetic nerves showed a significant increase along a proximal-distal gradient. Administration of 17beta-estradiol for 7 days via subcutaneous osmotic pump did not change smooth muscle area in sections, and neither densities nor total innervation of any nerve population was altered. These findings reveal a rich cholinergic innervation of the proximal urethra, and a pronounced gradient in sympathetic innervation. Unlike the embryologically similar uterine smooth muscle, estrogen does not influence muscle size or composition of innervation, indicating that estrogen's actions on innervation are highly target-specific. Thus, estrogen's effects on urinary continence apparently occur independently of any significant remodeling of smooth muscle or resident innervation.     

Transient and permanent effects of androgen during synapse elimination in the levator ani muscle of the rat.

Young male rats were castrated at 7 days of age, and treated with testosterone propionate daily from 7 to 34 days of age. At 13 months of age, motor axons and terminals innervating the levator ani (LA) muscle were stained with tetranitroblue tetrazolium (TNBT). The number of separate axons innervating individual muscle fibers was counted, and muscle fiber diameter was measured. Previous studies have shown that this androgen treatment increases muscle fiber diameter and delays synapse elimination, measured as (1) a greater percentage of muscle fibers innervated by multiple axons and (2) larger motor units. The present results indicate that the androgenic effect on synapse elimination is permanent, in that high levels of multiple innervation persisted for 12 months after the end of androgen treatment. In contrast, the effect on muscle fiber diameter was not maintained for this period. This dissociation of androgenic effects on the pattern of innervation from androgenic effects on muscle fiber diameter offers further evidence that the androgenic maintenance of multiple innervation is not dependent on muscle fiber size. In addition, circulating testosterone levels were measured at 50 and 60 days of age in animals similarly treated with androgen or oil from 7 to 34 days of age. By 60 days of age, testosterone levels in hormone-treated animals had dropped below detectability, comparable to levels in oil-treated controls. This provides additional evidence that androgen treatment during juvenile development can have permanent effects on the adult pattern of innervation in the LA muscle.     

Transient and permanent effects of androgen during synapse elimination in the levator ani muscle of the rat

Young male rats were castrated at 7 days of age, and treated with testosterone propionate daily from 7 to 34 days of age. At 13 months of age, motor axons and terminals innervating the levator ani (LA) muscle were stained with tetranitroblue tetrazolium (TNBT). The number of separate axons innervating individual muscle fibers was counted, and muscle fiber diameter was measured. Previous studies have shown that this androgen treatment increases muscle fiber diameter and delays synapse elimination, measured as (1) a greater percentage of muscle fibers innervated by multiple axons and (2) larger motor units. The present results indicate that the androgenic effect on synapse elimination is permanent, in that high levels of multiple innervation persisted for 12 months after the end of androgen treatment. In contrast, the effect on muscle fiber diameter was not maintained for this period. This dissociation of androgenic effects on the pattern of innervation from androgenic effects on muscle fiber diameter offers further evidence that the androgenic maintenance of multiple innervation is not dependent on muscle fiber size. In addition, circulating testosterone levels were measured at 50 and 60 days of age in animals similarly treated with androgen or oil from 7 to 34 days of age. By 60 days of age, testosterone levels in hormone-treated animals had dropped below detectability, comparable to levels in oil-treated controls. This provides additional evidence that androgen treatment during juvenile development can have permanent effects on the adult pattern of innervation in the LA muscle.     

Influence of surgical and chemical orchidectomy on weight and distribution of AChE-nerve fibres in thymuses of adult rats

The thymus is a crossroad between the immune and neuroendocrine systems. As such, it is innervated by acetylcholinesterase (AChE)-positive fibres of the vagus, the recurrent laryngeal and the phrenic nerves. It is well know, that the innervations density of the thymus increases with age. In our study, adult rats were orchidectomized (surgically and chemically by the application of a luteinizing hormone-releasing hormone). The density of AChE-positive nerve fibres in thymuses, as well as the weight of thymuses was examined. The authors found that both surgical and chemical orchidectomy result in macroscopic and microscopic regeneration of the atrophied thymuses. In regenerated rat's thymuses after orchidectomy the density of AChE-positive nerve fibres was markedly higher in comparison with the control animals. The distribution, as well as the density of AChE-positive nerve fibres in regenerated thymuses after orchidectomy evokes the images of its innervations like in young animals before age-related involution. The authors also found a markedly higher weight of thymuses of orchidectomized rats in comparison with the control groups. In recent study the authors proved that after 8 weeks surgical orchidectomy leads to the regeneration of thymic AChE-positive innervation and chemical orchidectomy by administration of luteinizing hormone-releasing hormone after 4 weeks of adult rats.     

Postnatal development of postganglionic parasympathetic neurones in the heart of the albino rat.

Electrical stimulation of the sinoatrial node region of isolated atria in medium containing physostigmine (0.1 micrograms/ml) produces a negative chronotropic effect whose intensity and duration depend mainly on the amount of acetylcholine released from postganglionic parasympathetic fibres endings. This technique was used to study functional maturation of the given neurones during postnatal development of albino rats. Preparations from animals of different ages were stimulated with 2-second bursts of rectangular pulses (frequency 50 Hz, pulse duration 0.02 ms, voltage 22.5--27.5 V) and frequency changes of the preparation were registered by recording extracellular action potentials. At 10 days the negative chronotropic effect is very weak and at 15 days it is only slightly stronger, but at 18 days it is almost the same as in adult animals. At 24 and 34 days the reaction is somewhat stronger than in adulthood. It can be concluded from these observations that functional maturation of the postganglionic parasympathetic neurones innervating the sinoatrial node in albino rats occurs between the 10th and 20th day of postnatal life.     

SCN efferents to peripheral tissues: implications for biological rhythms.

The suprachiasmatic nucleus (SCN) is the principal generator of circadian rhythms and is part of an entrainment system that synchronizes the animal with its environment. Here, we review the possible communication of timing information from the SCN to peripheral tissues involved in regulating fundamental physiological functions as revealed using a viral, transneuronal tract tracer, the pseudorabies virus (PRV). The sympathetic nervous system innervation of the pineal gland and the sympathetic outflow from brain to white adipose tissue were the first demonstrations of SCN-peripheral tissue connections. The inclusion of the SCN as part of these and other circuits was the result of lengthened postviral injection times compared with those used previously. Subsequently, the SCN has been found to be part of the sympathetic outflow from the brain to brown adipose tissue, thyroid gland, kidney, bladder, spleen, adrenal medulla, and perhaps the adrenal cortex. The SCN also is involved in the parasympathetic nervous system innervation of the thyroid, liver, pancreas, and submandibular gland. Individual SCN neurons appear connected to more than one autonomic circuit involving both sympathetic and parasympathetic innervation of a single tissue, or sympathetic innervation of two different peripheral tissues. Collectively, the results of these PRV studies require an expansion of the traditional roles of the SCN to include the autonomic innervation of peripheral tissues and perhaps the modulation of neuroendocrine systems traditionally thought to be controlled solely by hypothalamic stimulating/inhibiting factors.     

Respiratory Cardiac Arrhythmia in Postnatal Ontogenesis of Rats

Development of the respiratory cardiac arrhythmia and the role of parasympathetic nervous system in its origin have been studied in rats aged from 4–6 days to 6 months of life. In rat pups of the first week of life, small fluctuations of cardiac rhythm were observed with the frequency close to fluctuations of respiratory rhythm. However, at this age they had neither regular character nor clear connection with phases of the respiratory cycle. On the 2–3rd week the amplitude of fluctuations rose and their association with respiration was established; however, unlike the respiratory arrhythmia observed in other animals and human, in rat pups there was deceleration but not acceleration of heart beating. By to the 6-week age the respiratory arrhythmia reached the maximal values, then its amplitude began to decrease. Bilateral transection of the vagus nerves in rat pups did not cause reduction of the respiratory arrhythmia. Thus, in rats the central influences on the heart can be transduced by bypassing the system of vagus nerves.     

The origin and selective innervation of early muscle fiber types in the rat

The diversity of muscle fiber types present in adult animals is present also in the fetus. Fibers generated early and late in fetal development undergo a stereotyped sequence of myosin expressions in giving rise to these fiber types. The differentiation of these fetal fiber types does not require innervation. However, evidence obtained from experiments identifying the types of fibers innervated by single motors suggests that the nervous system comes to recognize this diversity, at least during early postnatal life. Reinnervation experiments suggest that this recognition can occur in the absence of the timing cues normally present in the genesis of fiber types. Thus, a selective innervation of muscle fiber types occurs during development. The role of rearrangement of initial synaptic connections in generating this selectivity is discussed.     

Retinoic acid is detected at relatively high levels in the CNS of adult rats

Retinoic acid (RA) is essential for cellular growth and differentiation in developing and adult animals. The central nervous system (CNS) suffers developmental defects if embryonic levels of RA are too high or too low. The production and function of RA in adult brain are unclear. We report that RA is present throughout the brain and spinal cord of adult, vitamin A-deficient (VAD) rats treated with a physiological amount of all-trans-retinol. The hippocampus/cortex contained the highest proportion of RA in the brain (27.2 +/- 2.9% of the organic phase radioactivity, and 23.5 +/- 0.8% of the organic phase radioactivity extracted from spinal cord was RA). RA comprises a higher proportion of the retinoid pool in the CNS compared with amounts reported in other target tissues (E Werner and HF DeLuca. Arch Biochem Biophys 393: 262-270, 2001). However, RA is not preferentially transported from the blood to the brain. There were 2.90 +/- 0.20 fmol RA/g tissue transported to the brain of VAD rats treated with 2.00 nmol [20-(3)H]all-trans-retinoic acid, but higher amounts of RA were delivered to the liver, testis, and spleen. Because RA is not transported preferentially to brain, this tissue likely synthesizes RA more efficiently than other target tissues.