Thyroid hormone action: nongenomic modulation of neuronal excitability in the hippocampus.

Years of effort have failed to establish a generally-accepted mechanism of thyroid hormone (TH) action in the mature brain. Recently, both morphological and pharmacological evidence have supported a direct neuroactive role for the hormone and its triiodinated metabolites. However, no direct physiological validation has been available. We now describe electrophysiological studies in vivo in which we observed that local thyroxine (T4) administration promptly inhibited field excitatory postsynaptic potentials recorded in the dentate gyrus (DG) with stimulation of the medial perforant pathway, a result that was found to be especially pronounced in hypothyroid rats. In separate in vitro experiments, we observed more subtle but statistically significant responses of hippocampal slices to treatment with the hormone. The results demonstrate that baseline firing rates of CA1 pyramidal cells were modestly reduced by pulse-perfusion with T4. By contrast, administration of triiodothyronine (T3) was often noted to have modest enhancing effects on CA1 cell firing rates in hippocampal slices from euthyroid animals. Moreover, and more reliably, robust firing rate increases induced by norepinephrine were amplified when preceded by treatment with T3, whereas they were diminished by pretreatment with T4. These studies provide the first direct evidence for functional, nongenomic actions of TH leading to rapid changes in neuronal excitability in adult rat DG studied in vivo and highlight the opposing effects of T4 and T3 on norepinephrine-induced responses of CA1 cells studied in vitro.

Thyroid hormone distribution in the mouse brain: the role of transthyretin.

Transthyretin is the major thyroxine-binding protein in the plasma of rodents, and the main thyroxine-binding protein in the cerebrospinal fluid of both rodents and humans. The choroid plexus synthesizes transthyretin and secretes it to the cerebrospinal fluid. Although it was suggested that transthyretin might play an important role in mediating thyroxine transfer from the blood into the brain across the choroid plexus-cerebrospinal fluid barrier, newer findings question this hypothesis. Because thyroid hormone passage across brain barriers is a precondition for its action in the CNS, and because brain is an important target of thyroid hormone action, we investigated the role of transthyretin in mediating thyroid hormone access to and distribution within the brain in a transthyretin-null mouse model system. In this report we describe the results derived from use of film autoradiography, a technique that yields definitive morphological results. Film autoradiograms were prepared at 3 and 19 h after intravenous injection of either high specific activity [(125)I]thyroxine or [(125)I]triiodothyronine. Image analyses were designed to demonstrate regional changes in hormone distribution, and to highlight alterations in iodothyronine delivery from ventricles to brain parenchyma. We find no qualitative or quantitative differences in these parameters between the transthyretin-null and the wild-type mouse brain after either [(125)I]thyroxine or [(125)I]triiodothyronine administration. The data presented here now provide definitive evidence that, under standard laboratory conditions, transthyretin is not required for thyroid hormone access to or distribution within the mouse brain. This study also provides the first map of iodothyronine distribution in the brain of the mouse.

Evidence that 3,3',5-triiodothyronine is concentrated in and delivered from the locus coeruleus to its noradrenergic targets via anterograde axonal transport.

Recent immunohistochemical studies of rat brain triiodothyronine reveal heaviest localization in locus coeruleus perikarya. The cellular distribution is similar to that observed in concomitant studies of tyrosine hydroxylase immunohistochemistry: heavy clumps of immunoreactive triiodothyronine are distributed within locus coeruleus cytosol and in cell processes, leaving cell nuclei unstained. At the same time, in locus coeruleus targets, cell nuclei as well as surrounding neuropil are prominently triiodothyronine labeled. These observations, combined with diverse evidence linking thyroid hormone with norepinephrine at many levels of physiological and pathophysiological function, led to the hypothesis that the locus coeruleus binds and accumulates triiodothyronine and delivers the hormone via anterograde axonal transport to postsynaptic locus coeruleus targets, where nuclear triiodothyronine receptors are densely concentrated. Furthermore, the hypothesis predicts that destruction of locus coeruleus nerve terminals would interrupt this neural route of triiodothyronine delivery and prevent or reduce triiodothyronine labeling of nuclear receptors in noradrenergic target cells. To test this formulation, we gave the specific locus coeruleus lesioning agent, N-(2-chloroethyl)-N-2-bromobenzylamine hydrochloride (DSP-4), to adult male rats and examined their brains three, five and seven days thereafter by triiodothyronine and, in alternate sections, tyrosine hydroxylase immunohistochemistry. Treatment with DSP-4 resulted in specific and selective reduction in tyrosine hydroxylase and triiodothyronine immunohistochemical labeling in cell nuclei and in nerve cell processes within the neuropil of the hippocampus and cerebral cortex at all time periods examined. The results demonstrate that full occupancy of locus coeruleus target cells by triiodothyronine requires the presence of intact locus coeruleus projections and supports the proposal that, like norepinephrine, triiodothyronine delivery to noradrenergic targets occurs through delivery by locus coeruleus terminals. These findings provide strong support for earlier proposals that triiodothyronine functions as a co-transmitter with norepinephrine in addition to or as part of its genomic role in the cells receiving noradrenergic innervation.

Thyroid hormones as neurotransmitters.

During brain development, before the apparatus of neurotransmission has been set into place, many neurotransmitters act as growth regulators. In adult brain, their role in neurotransmission comes to the fore but neuronal plasticity and other growth-related processes are their continuing responsibility. This has been clearly demonstrated for catecholamines. Previous as well as recent evidence now indicates that thyroid hormones may participate in the developing and adult brain through similar mechanisms. Immunohistochemical mapping of brain triiodothyronine (antibody specificity established by numerous appropriate tests) demonstrated that the hormone was concentrated in both noradrenergic centers and noradrenergic projection sites. In the centers (locus coeruleus and lateral tegmental system) triiodothyronine staining, like that of tyrosine hydroxylase, was heavily concentrated in cytosol and cell processes. By contrast, in noradrenergic targets, label was most prominent in cell nuclei. Combined biochemical and morphologic data allows a construct of thyroid hormone circuitry to unfold: The locus coeruleus is conveniently located just beneath the ependyma of the 4th ventricle. Thyroxine, entering the brain via the choroid plexus, is preferentially delivered to subependymal brain structures. High concentrations of locus coeruleus norepinephrine promote active conversion of thyroxine to triiodothyronine, leading to the preeminence of the locus coeruleus as a site of triiodothyronine concentration. Results of treatment with the locus coeruleus neurotoxin DSP-4 established that axonal transport accounts for delivery of both triiodothyronine and norepinephrine from locus coeruleus to noradrenergic terminal fields. The apparatus for transduction of thyronergic and noradrenergic signals at both membrane and nuclear sites resides in the postsynaptic target cells. Upon internalization of hormone in post-synaptic target cells, genomic effects of triiodothyronine, norepinephrine, and/or their second messengers are possible and expected. The evidence establishes a direct morphologic connection between central thyronergic and noradrenergic systems, supporting earlier proposals that triiodothyronine or its proximate metabolites may serve as cotransmitters with norepinephrine in the adrenergic nervous system.

Immunohistochemical mapping of brain triiodothyronine reveals prominent localization in central noradrenergic systems.

Many lines of evidence support a close association between thyroid hormones and noradrenergic systems in peripheral tissues. However, there is little certainty regarding interactions of the two systems in brain. We now report that triiodothyronine is concentrated in both nuclei and projection sites of central noradrenergic systems. Immunohistochemical mapping of the hormone revealed the following: (1) Locus coeruleus and all other noradrenergic cell groups identified were the most prominently labeled neural centers in the brain. (2) The hormone was also concentrated in the widely dispersed targets of noradrenergic projections. (3) Triiodothyronine labeling in noradrenergic target cells was most prominent over the cell nuclei, indicating that the hormone was bound to its receptors. Therefore, targets of noradrenergic innervation should be responsive to triiodothyronine. (4) Unlike that in noradrenergic target cells, triiodothyronine staining was decidedly perikaryal in locus coeruleus (A-6) and the other A-1 to A-7 cell groups; the staining pattern in locus coeruleus cytosol and processes was heavy, clumped and similar to that seen in contiguous sections immunostained for tyrosine hydroxylase. Results of radio-immunoassay, immunoabsorption and pharmacological tests demonstrated the specificity of the antibody for triiodothyronine and ruled against cross-reactivity with norepinephrine or its metabolites as the basis for the staining reactions. Although other possibilities consistent with these new observations are given consideration, it appears that the structure and activity of central noradrenergic systems may be major determinants of triiodothyronine distribution patterns and actions in brain. If the noradrenergic system processes both triiodothyronine and norepinephrine and conducts them both to nerve cell groups receiving its terminal arborizations, specific postsynaptic receptors would be available for transduction of both sets of messages. The evidence provides a morphological basis for earlier proposals that triiodothyronine may play a neuromodulatory or neurotransmitter role in the adrenergic nervous system.

Film autoradiography identifies unique features of [125I]3,3'5'-(reverse) triiodothyronine transport from blood to brain.

1. Steady-state iodothyronine profiles in plasma are composed of thyroid gland-synthesized hormones (mainly thyroxine) and tissue iodothyronine metabolites (mainly triiodothyronine and reverse triiodothyronine) that have entered the bloodstream. The hormones circulate in noncovalently bound complexes with a panoply of carrier proteins. Transthyretin (TTR), the major high-affinity thyroid hormone binding protein in rat plasma, is formed in the liver. It is also actively and independently synthesized in choroid plexus, where its function as a chaperone of thyroid hormones from bloodstream to cerebrospinal fluid (CSF) is undergoing close scrutiny by several groups of investigators. Because TTR has high-affinity binding sites for both thyroxine and retinol binding protein, its potential role as a mediator of combined thyroid hormone and retinoic acid availability in brain is of further interest. 2. While they are in the free state relative to their binding proteins, iodothyronines in the cerebral circulation are putatively subject to transport across both the blood-brain barrier (BBB) and choroid plexus CSF barrier (CSFB) before entering the brain. Previous autoradiographic studies had already indicated that after intravenous administration the transport mechanisms governing thyroxine and triiodothyronine entry into brain were probably similar, whereas those for reverse triiodothyronine were very different, although the basis for the difference was not established at that time. Intense labeling seen over brain ventricles after intravenous administration of all three iodothyronines suggested that all were subject to transport across the CSFB. 3. To evaluate the role of the BBB and CSFB in determining iodothyronine access to brain parenchyma, autoradiograms prepared after intravenous administration of [125I]-labeled hormones (revealing results of transport across both barriers) were compared with those prepared after intrathecal (icv) hormone injection (reflecting only their capacity to penetrate into the brain after successfully navigating the CSFB). 4. Those studies revealed that thyroxine and triiodothyronine were mainly transported across the BBB. They shared with reverse triiodothyronine a generally similar, limited pattern of penetration from CSF into the brain, with circumventricular organs likely to be the main recipients of iodothyronines (with or without retinol) transported across the CSFB. 5. Analysis of all of the images obtained after intravenous and icv hormone administration clarified the basis for the unique distribution of intravenously injected reverse triiodothyronine. The hormone is excluded by the BBB but may be subject to limited penetration into brain parenchyma via the CSF. 6. Overall the observations single out reverse triiodothyronine as the iodothyronine showing the most distinctive as well as the most limited pattern of transport from blood to brain.(ABSTRACT TRUNCATED AT 400 WORDS)

An acute dose of desmethylimipramine inhibits brain uptake of [125I]3,3',5-triiodothyronine (T3) in thyroxine-induced but not T3-induced hyperthyroid rats: implications for tricyclic antidepressant therapy.

The tricyclic antidepressant, desmethylimipramine (DMI), a highly selective inhibitor of presynaptic uptake of norepinephrine (NE), has also been shown to reduce [125I]3,3',5-triiodothyronine (T3) uptake in rat brain synaptosomes. Using DMI as a probe to examine 1) possible noradrenergic influences on thyroid hormone (TH) actions in brain and 2) TH:affective disorder relationships, we found that a single dose of DMI produces a small (7.4-25%) but significant (P < or = .05) decrease in brain uptake of both labeled T3 (T3) and labeled thyroxine (T4) across the spectrum of thyroid states from hypothyroid (HYPO) to euthyroid to T4-induced hyperthyroid. Therefore, it was noted with considerable interest that DMI appeared not to interfere with brain T3 uptake in T3-induced hyperthyroid (T3-HYPER) rats. To confirm this finding, thyroidectomized male rats were made T3-HYPER through administration of T3 (20 micrograms/kg) for 3 weeks or maintained without TH supplement for 6 weeks, becoming HYPO. Rats were given i.v. T3 and 5 min later i.p. DMI or saline. They were decapitated at 3 hr and brains retrieved for radiochemical analysis. Each experiment was run in three separate trials, with three to four rats in each treatment category (DMI or saline). Evaluation by analysis of variance showed that T3 concentrations (percentage of dose) were significantly lower in DMI than in saline-treated rat brain for HYPO (-15%; P = .0034) but not T3-HYPER rats (-2%; P = .6595). These results suggest that, as it does in the case of NE, DMI tends to block TH uptake sites in rat brain. The data also demonstrate a differential affinity for those sites in which T3 > DMI > T4 and suggest that T3 might augment tricyclic antidepressant therapy more effectively than T4.

Desmethylimipramine, a potent inhibitor of synaptosomal norepinephrine uptake, has diverse effects on thyroid hormone processing in rat brain. II. Effect on in vivo 5'-deiodination of [125I]thyroxine.

We have studied the effects of desmethylimipramine (DMI), a tricyclic antidepressant, on thyroid hormone (TH) handling in rat brain in an effort to discover a pharmacological basis for reported interactions between TH, affective disorders and psychotropic drugs. An acute dose of DMI has been used in order to determine the primary effects of the drug in brain without perturbations from secondary effects. Recently we have reported that a single dose of DMI significantly decreases brain uptake of both [125I]thyroxine (T4) and [125I]3,3',5-triiodothyronine (T3) across the spectrum of thyroid states from hypothyroid (HYPO) to euthyroid (EU) to T4-induced hyperthyroid (HYPER). To investigate further the effects of DMI on brain processing of TH, we have measured effects of the drug on in vivo rates of T4 to T3 conversion in a series of experiments in which DMI (25 mg/kg) was given to HYPO, EU and HYPER male rats in conjunction with i.v. [125I]T4. Decreased in vivo conversion ratios (T3/T4 ratios) suggest that acute DMI treatment causes a significant decrease in 5'-deiodinase activity in balance of brain (but not cerebellum) in all DMI treated rats as compared to their saline treated controls (ANOVA, P < 0.0001). For assurance that reduced T3/T4 in DMI treated rat brain is not the result of DMI enhancement of 5-deiodination of T3 or T4, the effect of DMI on concentrations of labeled I-, rT3, and T2 (3,3'- and 3',5'-) was also observed. In no case was there a significant increase in any metabolite in DMI treated rats for any tissue studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Desmethylimipramine, a potent inhibitor of synaptosomal norepinephrine uptake, has diverse effects on thyroid hormone processing in rat brain. I. Effects on in vivo uptake of 125I-labeled thyroid hormones in rat brain.

Several lines of evidence point to an interaction between amine uptake inhibitors (tricyclic antidepressants) and thyroid hormones. To examine this issue under conditions which would minimize secondary effects of drug treatment, desmethylimipramine (DMI), a highly specific norepinephrine uptake inhibitor, was given acutely as a single i.p. dose one hour before i.v. [125I]triiodothyronine (T3*) or [125I]thyroxine (T4*). Tissues were analysed after rat decapitation at 3, 5, 10, and 20 min intervals thereafter. DMI had a small but significant inhibitory effect on the brain uptake of both T3* (7.4%) and T4* (19%) over their respective 20-min time courses as indicated by two-way ANOVA. To examine the drug response further and to determine the effect of thyroid status on the response, hypothyroid (HYPO) and T4-induced hyperthyroid (HYPER) rats, were given i.v. T3* and, 5 min later, i.p. DMI or saline. They were killed 3 h later and tissue analysed. Because DMI effects on T4* uptake could not be evaluated over a 3 h period without blocking T4* to T3* conversion, sodium ipodate (60 mg/kg) was given in 2 doses before i.v. T4*. Under these conditions, DMI significantly reduced brain concentrations of the administered T3* and T4* in HYPO (15% and 19%) and in HYPER rats (13% and 25%). These results suggest that, as it does in the case of norepinephrine, DMI blocks the uptake site for T3 and T4 in rat brain. No information is available regarding the relationship, if any, between the thyroid hormone and norepinephrine uptake sites.

Effects of hypothyroidism on rat circadian activity and temperature rhythms and their response to light.

Male rats made hypothyroid by administration of propylthiouracil plus sodium ipodate in drinking water were compared to controls in terms of period of circadian activity and temperature rhythms, amount of gross motor activity, and mean temperature. Animals were studied under entrainment, constant darkness (DD), and constant dim light (LL). There was no difference in the period of the circadian activity rhythm between groups in DD. However, hypothyroid rats showed significant blunting of the period-lengthening response to increasing ambient illumination. As expected, the period of the circadian temperature rhythm increased in controls with increasing ambient illumination. In contrast, the period of the circadian temperature rhythm in hypothyroid animals actually shortened under LL compared to DD. This blunting of the period-lengthening response to increasing ambient illumination of both activity and temperature rhythms in hypothyroid animals could not be explained by differences in activity level or mean temperature between the groups.

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.

Thyroparathyroidectomy produces a progressive escape deficit in rats.

Abnormal thyroid status and affective disorders have been associated in the human clinical literature. It has recently been shown that pretreatment with thyroid hormone can prevent escape deficits produced by inescapable shock in an animal analogue of depression. In this report we provide evidence that hypothyroid status can produce an escape deficit in rats. While sham-operated rats improved their performance on a simple escape task over three days of testing, thyroparathyroidectomized rats showed a pronounced decrease in their responses. Markov transition analysis was used to obtain conditional probabilities of escaping given a prior escape or failure to escape for the two groups. This analysis shows that the structure of the data set may be similar for the two groups. These results suggest that if intact rats learn to escape, then hypothyroid rats may learn not to escape.

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)

Synthesis of side chain-modified iodothyronines.

New side chain-modified iodothyronines have been synthesized. They include: 1-[4-(4-hydroxyphenoxy)-3,5-diiodophenyl]-1,2-ethanediol (T2EG); alpha-hydroxy-4-(4-hydroxyphenoxy)-3,5-diiodobenzeneacetic acid (T2HAA) and their 4-methyl ether derivatives (MT2EG, MT2HAA); 1-[4-(4-hydroxyphenoxy)-3,5-diiodophenyl]-2-aminoethanol (T2EA); 1-[4-(4-hydroxy-3-iodophenoxy)-3,5-diiodophenyl]-1,2-ethaned iol (T3EG); 1-[4-(4-hydroxy-3-iodophenoxy)-3,5-diiodophenyl]-2-aminoetha nol (T3EA); and alpha-hydroxy-4-(3-iodo-4-hydroxyphenoxy)-3,5-diiodobenzeneacet ic acid (T3HAA). These model compounds are being used to study thyroid hormone metabolism and to determine structure-activity relationships of iododiphenylether derivatives.

[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.