Cold exposure elevates thyrotropin-releasing hormone gene expression in medullary raphe nuclei: relationship with vagally mediated gastric erosions. the central mechanisms through which hypo- or hyperthyroidism influences autonomic activity, we hypothesized that the autonomic disorders observed in altered thyroid statuses may be related to the influence of thyroid hormones on TRH-synthesizing neurons EO 1428 in the medullary Rpa, Rob, and PPR. We have recently reported that thyroid hormones exert a feedback regulation on TRH mRNA levels in these nuclei (59). In the present study, the expression of Fos-like protein, as observed by immunohistochemistry, was used as a marker to evaluate activity-dependent alterations of the neuronal function in these nuclei in different thyroid statuses. Double immunostaining was used to observe whether Fos-like immunoreactivity (IR) was localized specifically in the TRH-synthesizing neurons. The correlations between the numbers of Fos-like IR positive neurons in these ventral medullary nuclei and serum T4 levels were studied in hypothyroid animals. MATERIALS AND METHODS Animals and treatments. Male Sprague-Dawley rats weighing 270C320 g (Harlan, CA) were maintained on rat Purina chow and tap water ad libitum and housed under conditions of controlled temperature (22 2C) and illumination (light on 0600C1800) for at least 7 days before any treatment. Rats were then divided into four groups and treated for 4 wk: values less than 0.05 were considered statistically significant. RESULTS Serum T4 levels in different thyroid statuses after 4 wk of treatment. Serum T4 levels in euthyroid rats were 3.1 0.4 g/dl. Rats drinking 0.1% PTU had 84% lower serum T4 levels (0.5 0.0 g/dl) compared with euthyroid rats. Daily injection of T4 (2 g100 g?1day?1) in PTU-treated rats prevented this decrease and brought serum T4 levels to 2.6-fold higher than the euthyroid controls (8.0 0.5 g/dl). Animals with hyperthyroidism induced by daily intraperitoneal injection of a high dose EO 1428 of T (10 g100 Rabbit Polyclonal to MOBKL2B g?1day?1) showed a sixfold higher level of serum T4 (19.2 1.7 g/dl) compared with the euthyroid rats. Numbers of Fos-like IR positive neurons in the Rpa, Rob, and PPR in different thyroid statuses. Rats injected intraperitoneally with vehicle for 4 wk had only a few Fos-like IR positive neurons in each of the observed regions in the Rpa, Rob, and PPR (Table 1; Figs. 2C4). Hypothyroidism selectively induced remarkable increases of Fos-like IR positive neurons in the Rpa, EO 1428 Rob, and PPR by 10- to 70-fold but not in the surrounding areas or in other nuclei within EO 1428 the caudal ventral medulla (Table 1; Figs 2C4). The most abundant numbers of Fos-like IR positive neurons were observed in the rRpa compared with other observed regions (Table 1; Fig. 2). Simultaneous T4 replacement (2 g100 g?1day?1) in PTU-treated rats significantly, although not completely, prevented the induction of Fos-like IR in the Rpa, Rob, and PPR by 62C95% (Table 1; Figs. 2C4). Daily high dose of T4 injection (10 g100 g?1day?1) did not significantly change the number of Fos-like IR positive neurons in the raphe nuclei (Table 1; Figs. 2 and ?and3)3) but significantly decreased Fos-like IR in the PPR compared with euthyroid controls (Table 1). Open in a separate window Fig. 2. Tendency of the number of Fos-like immunoreactivity (IR) positive neurons in Rpa and Rob in different thyroid statuses after 4 wk of treatment. Each column is EO 1428 mean of 20 sectionsregion?1rat?1 and 4C6 rats/group. Control, euthyroid; 6- 0.001 compared with euthyroid group. ? 0.001 compared with hypothyroid group. Negative correlations between serum T4 levels and the numbers of Fos-like IR positive neurons in the Rpa, Rob, and PPR. Correlations between serum T4 levels and the numbers of Fos-like IR positive neurons in the Rpa, Rob, and PPR were observed during the development of hypothyroidism. Serum T4 levels gradually decreased as the PTU treatment continued, and they reached a significantly low level of 16% compared with the control value after 2 wk of treatment. Thereafter, T4 levels remained low until the end of the 4-wk treatment period (Fig. 5A). During progression of the hypothyroidism, there were significant negative correlations between serum T4 levels and the numbers of Fos-like IR positive neurons in all regions examined (i.e., the r, m, and c regions of the Rpa, Rob, and PPR) (Table 2; Fig. 5). The negative correlation between T4 levels and the numbers of Fos-like IR in the rRpa are shown in Fig. 5 as an example in graph.