Inhibition of adenylate cyclase in rat brain synaptosomal membranes by GTP and phenylisopropyladenosine is enhanced in hypothyroidism

Thyroid hormones, serotonin and mood: of synergy and significance in the adult brain

The use of thyroid hormones as an effective adjunct treatment for affective disorders has been studied over the past three decades and has been confirmed repeatedly. Interaction of the thyroid and monoamine neurotransmitter systems has been suggested as a potential underlying mechanism of action. While catecholamine and thyroid interrelationships have been reviewed in detail, the serotonin system has been relatively neglected. Thus, the goal of this article is to review the literature on the relationships between thyroid hormones and the brain serotonin (5-HT) system, limited to studies in adult humans and adult animals. In humans, neuroendocrine challenge studies in hypothyroid patients have shown a reduced 5-HT responsiveness that is reversible with thyroid replacement therapy. In adult animals with experimentally-induced hypothyroid states, increased 5-HT turnover in the brainstem is consistently reported while decreased cortical 5-HT concentrations and 5-HT2A receptor density are less frequently observed. In the majority of studies, the effects of thyroid hormone administration in animals with experimentally-induced hypothyroid states include an increase in cortical 5-HT concentrations and a desensitization of autoinhibitory 5-HT1A receptors in the raphe area, resulting in disinhibition of cortical and hippocampal 5-HT release. Furthermore, there is some indication that thyroid hormones may increase cortical 5-HT2 receptor sensitivity. In conclusion, there is robust evidence, particularly from animal studies, that the thyroid economy has a modulating impact on the brain serotonin system. Thus it is postulated that one mechanism, among others, through which exogenous thyroid hormones may exert their modulatory effects in affective illness is via an increase in serotonergic neurotransmission, specifically by reducing the sensitivity of 5-HT1A autoreceptors in the raphe area, and by increasing 5-HT2 receptor sensitivity.

Influence of thyroid hormone on 5-HT(1A) and 5-HT(2A) receptor-mediated regulation of hippocampal BDNF mRNA expression

The aim of the present study was to determine the influence of thyroid hormone, T3, on the regulation of hippocampal BDNF expression by 5-HT receptor agonists. Chronic T3 administration prior to treatment with the 5-HT(1A) agonist, 8-OH-DPAT, significantly decreased BDNF mRNA in the dentate gyrus region of the hippocampus. Administration of 8-OH-DPAT did not alter hippocampal BDNF mRNA expression in naive, euthyroid rats. Pretreatment with the 5-HT(1A) antagonist, WAY 100635, completely blocked the 8-OH-DPAT-induced down-regulation of BDNF mRNA in chronic T3-treated rats. Acute T3 administration prior to 8-OH-DPAT treatment led to a small, but significant, decrease in hippocampal dentate gyrus BDNF mRNA. Acute or chronic administration of T3 did not alter the decrease in hippocampal BDNF mRNA induced by the 5-HT(2A/2C) receptor agonist, DOI. The influence of 8-OH-DPAT and DOI on hippocampal BDNF mRNA was also unaltered in rats rendered hypothyroid by propylthiouracil administration. Chronic T3 treatment or hypothyroidism did not influence the basal expression of hippocampal BDNF mRNA. The affinity and density of 5-HT(1A) receptors, and the hippocampal expression of 5-HT(1A) mRNA were also not influenced by chronic T3 treatment. The results of this study clearly demonstrate a powerful interaction between thyroid hormone and the 5-HT(1A) receptor in the regulation of hippocampal BDNF expression. Crosstalk between signal transduction cascades influenced by T3 and 5-HT(1A) receptors may mediate the synergistic effects of these systems on hippocampal BDNF expression.

Thyroid hormones and the treatment of depression: an examination of basic hormonal actions in the mature mammalian brain

Numerous clinical reports indicate that thyroid hormones can influence mood, and a change in thyroid status is an important correlate of depression. Moreover, thyroid hormones have been shown to be effective as adjuncts for traditional antidepressant medications in treatment-resistant patients. In spite of a large clinical literature, little is known about the mechanism by which thyroid hormones elevate mood. The lack of mechanistic insight reflects, in large part, a longstanding bias that the mature mammalian central nervous system is not an important target site for thyroid hormones. Biochemical, physiological, and behavioral evidence is reviewed that provides a clear picture of their importance for neuronal function. This paper offers the hypothesis that the thyroid hormones influence affective state via postreceptor mechanisms that facilitate signal transduction pathways in the adult mammalian brain. This influence is generalizable to widely recognized targets of antidepressant therapies such as noradrenergic and serotonergic neurotransmission.

Hypothyroidism alters the effect of GTP on adenylyl cyclase in forebrain and hindbrain synaptosomal membranes from 15-day-old rats

The effect of GTP concentration of forskolin-stimulated adenylyl cyclase activity was examined in synaptosomal membranes from 15-day-old rats that were hypothyroid owing to administration of propylthiouracil and a low-iodine diet to the mothers during pregnancy and suckling. In membranes from the forebrain hypothyroidism abolished the overall stimulatory effect of GTP, which was seen in the euthyroid case. In membranes from the hindbrain hypothyroidism had the opposite effect in that there was an enhancement of an overall stimulatory effect of GTP. It is suggested that these findings reflect changes during early development of the brain in the expression of various G-proteins and/or the expression of different isoforms of adenylyl cyclase.

Treatment with triiodothyronine decreases the abundance of the alpha-subunits of Gi1 and Gi2 in the cerebral cortex

Treatment of rats for 3 days with T3 halved the abundance of the alpha-subunits of Gi1 and Gi2 in synaptosomal membranes isolated from the cerebral cortex. It is suggested that these changes could contribute to behavioural abnormalities in hyperthyroidism. Similar T3 treatment did not alter abundance of Gi1 alpha or Gi2 alpha in the medulla oblongata nor did it alter abundance of G(o) alpha-subunits in three tested brain regions.

Adenosine-dependent regulation of cyclic AMP accumulation in primary cultures of rat astrocytes and neurons

The regulation of intracellular cyclic AMP (cAMP) formation by adenosine (Ado) and its analogues has been examined in primary cultures of rat-brain astrocytes and neurons. In the presence of the phosphodiesterase inhibitor, Ro 20-1724, basal levels of cAMP ranged from 40-120 pmol/mg protein in both cell types. Levels were not altered by treating the cells with Ado deaminase, which suggested that they did not produce appreciable amounts of endogenous Ado under standard culture conditions. In the astrocytes, microM quantities of agonists increased cAMP up to 30-fold higher than basal values; the relative potencies were typical of an A2 Ado receptor (NECA greater than Ado greater than R-PIA). Neuron-enriched cultures exhibited a maximum fourfold increase in cAMP in response to NECA; this was decreased a further eightfold when the cultures had prolonged exposure to the antimitotic agent, c-Ara, to eliminate greater than 98% of the nonneuronal cells. Low (nM) amounts of the Ado agonists inhibited cAMP formation in both cell types. In the astrocytes, the order of potency of inhibition of isoproterenol-stimulated cAMP formation was typical of an A1 receptor (R-PIA greater than Ado greater than NECA); maximum inhibition was 55-65%. Isoproterenol did not increase cAMP in the neuronal cultures. However, forskolin-stimulated formation was effectively (approximately 50%) inhibited by A1 Ado agonists; inhibition was not affected by prolonged treatment with c-Ara. From this study we tentatively concluded that rat astrocytes and neurons both contain inhibitory A1 Ado receptors, but that the stimulatory "A2" subtype is localized mainly on astrocytes.

Abundance of the alpha-subunits of Gi1, Gi2 and Go in synaptosomal membranes from several regions of the rat brain is increased in hypothyroidism

1. Rats (4 weeks old) were made hypothyroid by treatment with propylthiouracil together with a low-iodine diet for a further period of 4 weeks. Synaptosomal membranes were obtained from six anatomical regions of the brain. 2. The abundances in these membranes of the G-protein alpha-subunits Gi1 alpha, Gi2 alpha and Go alpha were measured by quantitative immunoblotting. 3. Hypothyroidism significantly increased the abundances of all three G-protein subunits in membranes from the cerebral cortex and the striatum. In the medulla oblongata and the hippocampus the abundances of Gi2 alpha and Go alpha were increased significantly. By contrast, in the cerebellum only Go alpha was increased, and in the hypothalamus only Gi2 alpha was increased. 4. It is suggested that this up-regulation of G-protein abundances may modify signalling pathways and may contribute to the functional changes that are observed in the central nervous system in hypothyroidism.

Diabetes decreases sensitivity of adipocyte lipolysis to inhibition by Gi-linked receptor agonists

(1) Streptozotocin-diabetes decreased the responsiveness of noradrenaline- or forskolin-stimulated lipolysis to inhibition by phenylisopropyladenosine (PIA), prostaglandin E1 (PGE1) and nicotinate in rat adipocytes. (2) Diabetes had no effect on high affinity binding of [3H]PIA to adipocyte plasma membranes. (3) Plasma membranes from diabetic animals had increased abundance of alpha-subunits of Gi1 and Gi2. The effect of pertussis toxin in overcoming inhibition of lipolysis by PIA was delayed in adipocytes from diabetic rats. (4) Diabetes decreased the GTP-dependent right-wards shift in the dose-curve for displacement of the antagonist [3H]DPCPX by PIA in adipocyte plasma membranes. (5) It is concluded that, despite increased abundance of Gi in diabetic adipocytes, less of this is functional. This may contribute to reduced sensitivity to PIA, PGE1 and nicotinate and explains some of the loss of control of lipolysis in insulin-dependent diabetes.