Transport of iodothyronines from bloodstream to brain: contributions by blood:brain and choroid plexus:cerebrospinal fluid barriers.

Thyroid hormone entering the brain from the cerebral circulation must first cross barriers at the the blood:brain and choroid plexus:cerebrospinal fluid interfaces. The route taken after entry through those barriers might bring about selective delivery of hormone to different regions of the brain and those differences might be crucial for the ultimate functional effects of the hormone. To determine whether and how distribution of hormone in the brain might vary according to the route of entry, film autoradiograms of serially sectioned brains were prepared after delivery of a pulse of 125I-labeled thyroid hormone into either the right lateral cerebral ventricle or the femoral vein. The results after intrathecal injection, reflecting the penetration of hormone into brain after crossing the choroid plexus:cerebrospinal fluid barrier, revealed a markedly limited, essentially periventricular distribution of radioactivity at both 3 and 48 h after hormone administration. Results after i.v. administration, which allows hormone access across both barriers, revealed an initial distribution pattern (at 3 h) generally similar to that seen after administration of markers of cerebral blood flow; at 48 h there was strong resolution in selected brain regions never noted to be labeled after intrathecal hormone injection. The functional implications of the differences in results produced by the two different routes of hormone entry are not known. However, ready access to circumventricular organs would appear to be favored by hormone crossing the choroid plexus:cerebrospinal fluid barrier whereas access to the panoply of nuclear triiodothyronine receptors would be favored by hormone crossing the blood:brain barrier. Therefore both routes of barrier transport should be taken into account in assessing the kinetics and actions of thyroid hormones in the central nervous system.

Thyroxine, triiodothyronine, and reverse triiodothyronine processing in the cerebellum: autoradiographic studies in adult rats.

Well confirmed evidence has demonstrated that the cerebellum is an important target of thyroid hormone action during development. Moreover, the presence of nuclear receptors and strong 5'-deiodinase activity in cerebella of adult rats have suggested that this region may continue to respond to thyroid hormones during maturity. Recent autoradiographic observations have focused attention on the cerebellar granular layer, in that [125I]T3 administered iv to adult rats was found to be selectively and saturably concentrated there. To determine the specificity of iodothyronine localization in the granular layer, we have now compared film autoradiographic observations made after iv [125I]T4 and iv [125I]rT3 with those found after iv [125I]T3. The results demonstrated that, as in the case of the latter hormone, labeling within the cerebellar cortex after iv [125I]T4 was both selective and saturable. Moreover, except for a lag in time to resolution and a longer retention time, the distribution of cerebellar radioactivity after iv labeled T4 was qualitatively similar to that seen after iv [125I]T3. However, the ability of T4 to become differentially concentrated in the granular layer of cerebellum was absolutely dependent on its ability to be converted intracerebrally to T3. Thus, pretreatment with ipodate, which blocks brain 5'-deiodinase activity and, therefore, the intracerebral formation of T3 from T4, completely prevented cerebellar granular layer labeling after iv [125I]T4 even though it did not interfere with differential labeling of this region by iv delivered [125I]T3. In the same experiments, propylthiouracil, a potent peripheral, but not central, 5'-deiodinase inhibitor, had no qualitative effect on the distribution of either T4 or T3 in cerebellum. By contrast with the results obtained after administering labeled T3 or T4, brain labeling after iv delivered [125I]rT3 was found to be no different from that produced by markers of cerebral blood flow, which rapidly enter and leave the brain without becoming incorporated into brain cells. This was so even during treatment with propylthiouracil and ipodate, both of which markedly prolonged the normally brief residence time of this iodothyronine in serum and brain. Overall, the autoradiographic results served to highlight the importance of the morphological approach for investigating thyroid hormone action and metabolism in brain. They demonstrated that only T3, whether entering as such from the circulation or formed in situ from T4 (but neither T4 itself nor iv administered rT3) was strongly, selectively, and saturably concentrated in the cerebellar granular layer of adult rats.(ABSTRACT TRUNCATED AT 400 WORDS)

[125I] triiodothyronine in the rat brain: evidence for neural localization and axonal transport derived from thaw-mount film autoradiography.

Previous thaw-mount light microscopic autoradiographic studies have shown that intravenously administered [125I] triiodothyronine is saturably concentrated and retained for at least 10 hours in discrete neural systems in the rat brain. To survey the brain more completely and to gain information about the time course of labeling, serial thaw-mount film autoradiograms were prepared from rat brains obtained at intervals through 48 hours after intravenous injection of high specific activity [125I] triiodothyronine. Parallel biochemical studies of whole brain homogenate extracts revealed that, at all time intervals, the label in the brain was mainly due to triiodothyronine itself (80%), or other organic iodocompounds (15%), but probably not due to free [125I] iodide (3%), which is rapidly transported out of the brain. The highly reproducible, well-defined labeling patterns seen on film indicated a widespread but selective localization of the hormone. At early times after intravenous injection of [125I] triiodothyronine, label was nonuniformly and prominently concentrated in selected regions of gray matter; evidence for saturability of hormone processing was obtained in competition studies with unlabeled triiodothyronine. Discrete labeling of fiber tracts (usually after 10 hours) left some regions of white matter conspicuously unlabeled. At 48 hours, many originally labeled gray regions showed markedly diminished or virtually complete loss of radioactivity, whereas others became newly or more prominently labeled. At that time, certain fiber tracts were also conspicuously labeled. The observed changing profiles of regional labeling over time are best explained by movement of the hormone from original sites of saturable incorporation in specific nuclei, to terminal fields, through the mechanism of axonal transport.

Intrathecal triiodothyronine administration causes greater heart rate stimulation in hypothyroid rats than intravenously delivered hormone. Evidence for a central nervous system site of thyroid hormone action.

To determine whether intracerebrally localized iodothyronines produce thyroid hormone-related functional effects, heart rate responses were compared in conscious hypothyroid rats given triiodothyronine (T3) by either the intrathecal or the intravenous route. A significant increase in heart rate occurred within 18 h after 1.5 nmol T3/100 g body wt was delivered intrathecally through a cannula previously placed in the lateral cerebral ventricle. Injection of the same T3 dose intravenously through an indwelling jugular catheter or injection of vehicle only by either route produced no significant increase in heart rate during the 48-h postinjection period of observation. These differences were observed even though integrated serum T3 concentrations were significantly lower after intrathecal than after intravenous T3 injection. The results indicate that thyroid hormone effects on heart rate are exerted within the brain as well as within the heart.

Effect of hypothyroidism and hyperthyroidism on triiodothyronine production in perfused rat liver.

This study was undertaken to evaluate the effect of altered thyroid states on hepatic T3 production in a functioning intact organ system, the isolated perfused liver. Thyroidectomized rats were treated for 3-4 weeks with vehicle, T4, 1.5 micrograms/100 g-1 day-1, or T4, 20 micrograms/100 g-1 day-1, to produce hypothyroidism, euthyroidism, or hyperthyroidism. Livers were perfused for 1 h with medium containing T4, 10 micrograms/dl, and T3 production was estimated by RIA. T3 production in the hypothyroid, euthyroid, and hyperthyroid groups, respectively, was 1.61 +/- (SE) 0.50, 5.18 +/- 0.55, and 15.62 +/- 1.61 ng/g-1 liver h-1. These differences in T3 production resulted entirely from changes in percent conversion of T4 to T3 which were 0.87 +/- 0.25%, 3.21 +/- 0.38%, and 12.02 +/- 1.82% in the hypothyroid, euthyroid, and hyperthyroid groups, respectively. The measured hepatic uptake of T4 decreased slightly with T4 administration from 188 +/- 13 to 162 +/- 7 and 144 +/- 10 ng/g liver in these same groups. The changes in T3 production were not accounted for by differences in biliary excretion or deiodination of T3. These studies demonstrate a stimulatory effect of T4 on the conversion of T4 to T3 which is important to altering net hepatic T3 production.

Iodothyronine homeostasis in rat brain during hypo- and hyperthyroidism.

Thyroid hormones are concentrated, retained, and metabolized in discrete neural systems in rat brain. To determine how iodothyronine requirements of brain compare with those of other thyroid hormone-dependent tissues, we measured effects of chronic thyroid hormone deficiency or excess on brain iodothyronine economy and particularly on the intracerebral rate of triiodothyronine formation from thyroxine. The results demonstrate that despite extremes of thyroxine availability, brain thyroxine and triiodothyronine concentrations and brain triiodothyronine production and turnover rates are kept within narrow limits. Adjustments in the activity of both brain and liver help to maintain these relatively stable conditions. Following thyroidectomy, fractional rates of triiodothyronine formation from thyroxine decrease to low levels in liver, whereas they increase markedly in brain; exactly the opposite direction of change occurs in brain and liver during hyperthyroidism. These responses suggest that brain iodothyronine homeostasis is important for the function of the whole organism. Because signs of nervous system dysfunction develop in hypothyroid and hyperthyroid individuals, it is possible that even relatively small deviations of brain iodocompound economy can produce significant changes in behavior and autonomic nervous system function.

Early ontogeny of iodocompound-processing neural systems in rat brain.

The distribution and localization of iodocompounds reaching the brain during early development were measured in rat pups nurtured on [125I]-containing milk from dams receiving daily [125I]-iodide injections. The regimen produced no measurable changes in growth and development of the offspring during the nursing period. Pup brains accumulated labeled iodocompounds at a faster rate than they grew and accumulated protein. The ratio of [125I]-iodocompounds in cerebrum relative to skeletal muscle increased progressively from day 11 through day 19. Significant differences in distribution of radioactivity in different brain regions were evident on day 1; developmental progress was associated with significantly different rates of regional accumulation of the isotope. On day 1 only 10% of the radioactivity in the postnuclear supernatant phase of brain homogenates was particle-bound; at the time of weaning, radioactivity in brain particles accounted for more than 50%. Growing nerve cell processes and myelin, known to be major targets of early thyroid hormone deficiency or excess, were also the major subcellular sites of [125I]-iodocompound localization in the developing rat brain. Overall, the ontogeny reflected progressive elaboration of iodocompound-processing neural systems resembling those recently recognized in adult brain.

Nervous system role of iodocompounds in blood pressure regulation.

Approximately one-third of hypertensive patients with thyroid dysfunction become normotensive on restoration to the euthyroid state. Considerable evidence suggests that the nervous system is involved in these responses, but the mechanism has been obscure. New information demonstrates that iodothyronines are localized and processed in discrete neural systems in rat brain. Moreover, following a latent period, heart rate is significantly increased by intrathecal administration of triiodothyronine in a dose which sustains but does not increase heart rate when given intravenously. These observations add to existing biochemical and autoradiographic evidence that autonomic effects of iodocompounds may be mediated through direct neuroregulatory (neuromodulator and neurotransmitter) functions within the nervous system.

Iodine-125-labeled triiodothyronine in rat brain: evidence for localization in discrete neural systems.

Autoradiograms prepared from adult rat brains demonstrate that nerve cells and neuropil in different brain regions selectively concentrate and retain intravenously administered triiodothyronine, by mechanisms susceptible to saturation with excess triiodothyronine. A neuroregulatory role for thyroid hormones, strongly supported by the observations, may account for their marked effects on behavior and the activity of the autonomic nervous system.

Growth and development of the neonatal rat: particular vulnerability of males to disadvantageous conditions during rearing.

Rat pups were nutured on radioactive iodine-containing milk by dams fed low-iodine (Remington) diets plus sodium 125I-iodide (Na 125I) supplements from 3 days before parturition to the time of weaning. Effects of individual features of this experimental regimen on developmental progress of the offspring were examined. Nutritional deficiencies inherent in the maternal Remington diet did not alter growth rate, if the pups were only minimally disturbed in the course of rearing. However, when combined with chronic handling stress (itself having no detectable effect on the weight gain of pups normally fed mothers), growth retardation was noted, without evidence of hypothyroidism. Addition of Na 125I to the maternal Remington diet reduced the growth rate of non-handled animals only. The strategy of measuring post-weaning catch-up growth served to magnify latent signs of pre-weaning growth retardation and revealed that males were more susceptible than females to early dietary deficiencies, radiation and handling stresses.

Synaptosomal [125I]triiodothyronine after intravenous [125I]thyroxine.

We administered [125I]thyroxine intravenously to adult male rats and measured uptake and subcellular distribution of the hormone and its metabolites in brain. Fractional brain uptake decreased after a large dose of iodothyronine, providing evidence for saturability of the uptake mechanism. Well-defined patterns of regional and subcellular labeling were noted within 1 h after [125I]thyroxine injection. Radioactivity in synaptosomes was always greater than in any other particle separated per gram of brain, increasing linearly relative to radioactivity in brain cytosol during the 1st h. Although [125I]triiodothyronine derived from [125I]thyroxine was not identified in serum at any time interval, it was measurable in synaptosomes within 20 min and in brain cytosol within 1 h after labeled hormone administration. Concentrations of the radioactive metabolite were twofold greater and ratios of [125I]triiodothyronine to [125I]thyroxine concentration were threefold greater in synaptosomes than in cytosol. Therefore, thyroxine may be converted to triiodothyronine within nerve terminals. Synaptosomal localization of iodothyronines and their metabolites may be relevant to the marked central and peripheral adrenergic nervous system effects of these aromatic amino acid hormones.

Triiodothyronine in rat submaxillary salivary gland: effects of altered adrenergic function.

To determine whether changes in adrenergic nerve terminal activity may influence tissue metabolism of iodothyronines, sympathetic nervous function of the rat submaxillary salivary gland was altered, and effects on salivary gland triiodothyronine (T3) uptake and retention measured. Following unilateral superior cervical ganglionectomy, denervated salivary gland contained 24% less (p less than 0.02) radioimmunoassayable T3/mg and took up 20% less (p less than 0.001) intravenously administered 125I-T3/mg than the contralateral innervated gland. Effects were similar at 7, 14 and 56 days following ganglionectomy and could not be accounted for by post-denervation changes in the delivery of the isotope, in total salivary gland water, in vascular volume or in extracellular or intracellular fluid spaces. When reserpine was administered to unilaterally ganglionectomized animals, uptake of 125I-T3/mg in the innvervated gland was reduced by 10% (p less than 0.005), relative to the denervated gland. The results suggest that loss or diminution of peripheral adrenergic nerve terminal activity reduces tissue uptake and retention of T3.

Localization of triiodothyronine in nerve ending fractions of rat brain.

Radioactive triiodothyronine reaching the rat brain after intravenous administration is rapidly and selectively taken up in the nerve ending fraction. A concentration gradient of radioactivity from brain cytosol to synaptosomes is observed at 5 min, increases linearly over the first hour, and is maintained for at least 10 hr. Radioactivity in the synaptosomes is due to triiodothyronine (90%) plus a single unidentified metabolite (10%). Approximately 85% of the synaptosomal radioactivity is released by osmotic disruption of the particles. The process of selective uptake, concentration, and retention of triiodothyronine in nerve terminals of the rat brain may be related to the sympathomimetic and behavior-altering effects of the thyroid hormones.