TRH and TRH receptor system in the basolateral amygdala mediate stress-induced depression-like behaviors.

Chronic stress is a potent risk factor for depression, but the mechanism by which stress causes depression is not fully understood. To investigate the molecular mechanism underlying stress-induced depression, C57BL/6 inbred mice were treated with repeated restraint to induce lasting depressive behavioral changes. Behavioral states of individual animals were evaluated using the forced swim test, which measures psychomotor withdrawals, and the U-field test, which measures sociability. From these behavioral analyses, individual mice that showed depression-like behaviors in both psychomotor withdrawal and sociability tests, and individuals that showed a resiliency to stress-induced depression in both tests were selected. Among the neuropeptides expressed in the amygdala, thyrotropin-releasing hormone (TRH) was identified as being persistently up-regulated in the basolateral amygdala (BLA) in individuals exhibiting severe depressive behaviors in the two behavior tests, but not in individuals displaying a stress resiliency. Activation of TRH receptors by local injection of TRH in the BLA in normal mice produced depressive behaviors, mimicking chronic stress effects, whereas siRNA-mediated suppression of either TRH or TRHR1 in the BLA completely blocked stress-induced depressive symptoms. The TRHR1 agonist, taltirelin, injection in the BLA increased the level of p-ERK, which mimicked the increased p-ERK level in the BLA that was induced by treatment with repeated stress. Stereotaxic injection of U0126, a potent inhibitor of the ERK pathway, within the BLA blocked stress-induced behavioral depression. These results suggest that repeated stress produces lasting depression-like behaviors via the up-regulation of TRH and TRH receptors in the BLA.

Salsolinol and the peripheral sympathetic activity: the effects of hypophysectomy, adrenalectomy and adrenal medullectomy.

The endogenous isoquinoline salsolinol (SALS) is a recently identified prolactin (PRL) releasing factor, a selective and potent stimulator of PRL secretion both in vivo and in vitro. SALS decreased the peripheral tissue dopamine (DA) level dose dependently, consequently increased the NE/DA ratio, indicating reduced release of newly formed norepinephrine (NE) from sympathetic terminals. The aim of our study was to investigate the effect of adrenal medullectomy (MEDX), adrenalectomy (ADX) and hypophysectomy (HYPOX) on the action of SALS on the PRL secretion, and on the catecholamine concentration of the selected sympathetically innervated peripheral tissues (atrium, spleen, etc). The experiments were done in male rats of 200-300 g body weight kept in air conditioned room with regular lighting. We used high-pressure liquid chromatography with electrochemical detection (HPLC-EC) for measurement of NE and DA concentrations, and radioimmunoassay for prolactin measurement. In MEDX as well as in ADX rats, SALS (25 mg/kg i.p.) was able to reduce DA level and increase the NE/DA ratio. The changes of prolactin secretion (increase by SALS) were not affected either by ADX or MEDX. Therefore the presence of the adrenal gland is not required for the changes of prolactin secretion, nor for the reduction of peripheral sympathetic activity induced by SALS. Investigating the possible effect of pituitary hormones on the peripheral sympathetic system, the action of SALS has been tested in HYPOX rats. We have found that the effect of SALS on peripheral sympathetic terminals is not affected by HYPOX, consequently the role of pituitary hormones in the effect of SALS on the peripheral catecholamine metabolism may be excluded.

Stress- as well as suckling-induced prolactin release is blocked by a structural analogue of the putative hypophysiotrophic prolactin-releasing factor, salsolinol.

Prolactin is secreted from the anterior lobe of the pituitary gland in response both to suckling and to stress. We recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol), produced in the neurointermediate lobe of the pituitary gland, as well as in the medial basal hypothalamus, can selectively release prolactin from the anterior pituitary. Therefore, it has been proposed that salsolinol is a putative endogenous prolactin-releasing factor (PRF). Here, we report that one structural analogue of salsolinol, 1-methyl-3,4-dihydroisoquinoline (1MeDIQ), can block salsolinol-induced release of prolactin, but does not affect prolactin release in response to thyrotropin releasing hormone (TRH), alpha-methyl-p-tyrosine (alpha MpT) (an inhibitor of tyrosine hydroxylase), domperidone (a D(2) dopamine receptor antagonist), or 5-hydroxytryptophan (5-HTP), a precursor of serotonin). 1MeDIQ profoundly inhibited suckling-, immobilization-, as well as formalin-stress induced prolactin release without any influence on corticosterone secretion. The 1MeDIQ-induced reduction in prolactin response to immobilization stress was dose-dependent. These results suggest that salsolinol can play a pivotal role in the regulation of prolactin release induced by either physiological (suckling) or environmental (stress) stimuli.

[3-Me-His(2)]-TRH combined with dopamine withdrawal rapidly and transiently increases pyroglutamyl aminopeptidase II activity in primary cultures of adenohypophyseal cells.

TRH is hydrolyzed by pyroglutamyl aminopeptidase II (PP II), a highly specific ecto-enzyme which is localized on the surface of lactotrophs. To study whether PP II activity may be rapidly regulated during a burst of prolactin secretion, we used an in vitro model in which primary cultures of adenohypophyseal cells were incubated with 500 nM dopamine (DA) for 24 h prior to treatments. We observed a rapid increase of PP II activity when 100 nM [3-Me-His(2)]-TRH, a TRH agonist, was added at removal of DA. PPII activity was maximal after 20 min of treatment and reduced to time 0 activity at 30 min. Dopamine withdrawal alone, slightly and transiently, modified the enzyme activity: an initial activation at 15 min was followed by a transient inhibition at 20 min. The specific contribution of [3-Me-His(2)]-TRH in this paradigm was a transient enhancement of PP II activity. If DA was not removed, [3-Me-His(2)]-TRH was ineffective. These data demonstrate that during in vitro conditions that mimic a suckling episode, adenohypophyseal PP II activity is rapidly and reversibly adjusted.

Overexpression of the G protein G11alpha prevents desensitization of Ca2+ response to thyrotropin-releasing hormone.

Doubly transfected human embryonal kidney cells (clone E2M11 of the HEK 293 cell line) expressing both thyrotropin-releasing hormone (TRH) receptors and G11alpha protein in high amounts were used to analyze the desensitization phenomenon of the Ca2+-mobilizing pathway. Quite unexpectedly, we did not observe any significant desensitization of the [Ca2+]i response to TRH in these cells after repeated or prolonged incubation with the hormone (up to 5 h). Under the same conditions, the TRH-induced [Ca2+]i response was completely desensitized in the parent cell line (293-E2 cels) expressing TRH receptors alone. In both cell lines, inositol phosphate response was desensitized after TRH exposure, although basal levels of inositol phospates in TRH-pretreated cells were much higher than in "naive" TRH-unexposed cells. These data suggest a significant role of the G protein G11alpha in desensitization of the Ca2+-mobilizing pathway occuring after repeated or long-term exposure of target cells to TRH-receptor agonists.