Effects of thyroid state on brain stem responses to iontophoretic noradrenaline

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.

Impaired response of experimental diabetic mice to tricyclics: a possible beta-adrenergic mechanism

Diabetes is reportedly associated with alterations in peripheral and central noradrenergic systems. The latter might be involved in the antidepressant effects of imipramine-like drugs in both humans and animals. Therefore, it is possible that diabetics show an impaired responsiveness to tricyclics. To test this possibility the effects of streptozotocin (STZ)-induced experimental diabetes in mice were assessed in two psychopharmacological tests: 1) the reversal of apomorphine- (16 mg/kg) induced hypothermia and 2) the hypoactivity induced by a direct beta-agonist (clenbuterol 0.06 mg/kg). At day 15 after STZ or vehicle treatment, imipramine (4 mg/kg) antagonized the apomorphine-induced hypothermia in diabetic (D) and nondiabetic (ND) mice and clenbuterol produced hypoactivity in both groups. At day 30 and 45, the ability of imipramine (1, 2, 4, 8, 16 mg/kg), clomipramine (8 mg/kg) and desipramine (2 mg/kg) to reverse apomorphine-induced hypothermia disappeared at the same time that clenbuterol lost its ability to induce hypomotility in D mice. These impaired responses on both tests were corrected by a short period of insulin therapy. These two tests may reflect central beta-adrenergic functions. Therefore, these data suggest that the impaired responsiveness of diabetic mice might be due at least in part to a noradrenergic dysfunction. Possibly, in diabetes, a beta-adrenoceptor desensitization identical to that observed at the peripheral level occurs in the central nervous system. The possibility that a thyroid hormone deficiency may be involved was also tested. Decreased T3 plasma levels were found in D mice concomitant with the impaired pharmacological responses and T3 supplementation turned these responses to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothyroidism elicits electrophysiological noradrenergic subsensitivity in rat cerebellum

discharge rats of Purkinje neurons were compared in control and hypothyroid adult rats. Purkinje neurons in hypothyroid rats fired significantly faster and were less sensitive to iontophoretically applied norepinephrine than those in control rats. The subsensitivity of the Purkinje neurons appeared to be primarily due to an alteration in the beta-receptor--adenylate cyclase complex, because the sensitivity of these cells to locally applied N6-monobutyryl adenosine 3'-5'-monophosphate (N6 cyclic AMP) did not change significantly. The sensitivity of the Purkinje neurons to norepinephrine could be restored in hypothyroid rats by administration of triiodothyronine.

Regulation of alpha 1-adrenoceptors in the cerebral cortex of the rat by thyroid hormones

The influence of thyroid hormones on the concentration and properties of alpha 1-adrenoceptors in a crude membrane fraction obtained from the rat cerebral cortex was investigated using the [3H]-WB 4101 binding assay. Animals were made hypothyroid by feeding 6-propyl-2-thiouracil for 8 weeks. Hyperthyroidism was induced by triiodothyronine injections (50 microgram/100 g body weight) for 9 days. 1. The binding of [3H]-WB 4101 was saturable and of high affinity in controls as well as in hyper- and hypothyroid animals. The maximal number of binding sites (Bmax), which amounted to 95 fmol/mg protein in control animals, was increased by 27% in cortical membranes from hyperthyroid rats and reduced by 23% in the hypothyroid group. 2. The reduction in [3H]-WB 4101 binding due to 6-propyl-2-thiouracil feeding was reversible by triiodothyronine treatment. 3. Dissociation constants (KD) calculated from saturation experiments (0.25 nM) or kinetic data (0.21 nM) remained unchanged in altered thyroid states. 4. Inhibition of [3H]-WB 4101 binding by adrenergic agonists and antagonists revealed no differences between euthyroid and hypothyroid animals. The higher affinity of prazosin to the binding sites compared with yohimbine indicated that [3H]-WB 4101 predominantly labeled alpha 1-adrenoceptors. It is concluded that thyroid hormones regulate the number of alpha 1-adrenoceptors in membranes of the rat cerebral cortex, leaving their affinities unchanged.

Effects of altered thyroid state on the inhibition produced by locus coeruleus in Purkinje cells in the rat

The number of cerebellar Purkinje cells inhibited after locus coeruleus stimulation was found to be greater in hyperthyroid rats than in control healthy animals; these in turn showed a higher percentage of inhibited cells than hypothyroid rats. It is concluded that thyroid hormone is capable of modulating synaptic activity in the LC-PC pathway.