Effect of melanostatin and thyroliberin on the biosynthesis and release of dopamine by rat brain striatal P2 fractions

Systemic administration of thyrotropin-releasing hormone enhances striatal dopamine release in vivo

We examined the effects of systemically administered thyrotropin-releasing hormone (TRH) on the release of dopamine (DA), as assessed by brain microdialysis within the corpus striatum of anesthetized rats. A single dose (10 micrograms i.v.) elevated DA levels in brain extracellular fluid (ECF) by 240% above baseline levels after 150 min. Systemic tyrosine ([TME] 20 mg/kg i.v.) also increased DA release (by 190% after 150 min), while combined treatment with both agents was associated with significant potentiation of the DA response (to 640% after 150 min). None of the treatments significantly altered striatal tissue levels of DA or its metabolites. A large dose of TRH (50 micrograms i.v.) significantly increased DA release (by 1150%) whether or not animals had received an active or denatured prolactin (PRL) antiserum prior to the experiment, suggesting that the TRH effect is not mediated by PRL. Although TRH is rapidly metabolized in plasma and penetrates the blood-brain barrier only poorly, our results suggest that even relatively small doses of the hormone can affect striatal dopaminergic neurotransmission.

In vitro study of frog (Rana ridibunda Pallas) neurointermediate lobe secretion by use of a simplified perifusion system. IV. Interaction between dopamine and thyrotropin-releasing hormone on alpha-melanocyte-stimulating hormone secretion

The interaction between dopamine and TRH on alpha-melanocyte-stimulating hormone (MSH) release from the intermediate lobe of amphibian pituitary has been studied in vitro using the perifusion model. Dopamine (10(-10) to 10(-6) M) was responsible for a dose-related inhibition of alpha-MSH secretion. The inhibitory effect of dopamine (10(-8) and 3.16 X 10(-8) M) was completely abolished in the presence of haloperidol (10(-5) and 10(-6) M, respectively). It has been previously established that, in amphibians, TRH stimulated alpha-MSH release in vitro and that the action of TRH was not mediated via an inhibition of the release of endogenous dopamine (M. C. Tonon, P. Leroux, M. E. Stoeckel, S. Jégou, G. Pelletier, and H. Vaudry, 1986, Endocrinology 112, 133-141). In the present study we demonstrate that TRH (10(-7) M) reverses the inhibitory effect of dopamine (for concentrations ranging from 3.16 X 10(-8) to 10(-6) M) on alpha-MSH secretion and that the effects of TRH and dopamine are additive. Thus, these results indicate that the intracellular events associated with TRH-induced stimulation and dopamine-induced inhibition of alpha-MSH release are not linked together.

Neurotensin potentiates the potassium-induced release of endogenous dopamine from rat striatal slices

The effects of neurotensin on the release of endogenous dopamine (DA) from rat striatal slices were investigated. DA was extracted by the aluminium adsorption method and assayed electrochemically by high-performance liquid chromatography. Neurotensin (2-100 microM) potentiated the potassium-induced release of striatal endogenous DA in a concentration-dependent manner. This neurotensin-induced release of DA was completely abolished in calcium-free medium containing EGTA 1 mM. The presence of tetrodotoxin 2 microM did not alter the effect of neurotensin 10 microM. On the other hand, neurotensin did not affect the uptake of [3H]DA into the striatal slices. In additional experiments, we confirmed that the KCl-induced release of DA in the nucleus accumbens was also potentiated b neurotensin. These results suggest that neurotensin has a direct effect on the central DA nerve terminals and releases DA in a Ca2+-dependent manner.

Regulatory peptides as a source of new drugs--the clinical prospects for analogues of TRH which are resistant to metabolic degradation

The biological properties of several analogues of TRH (Pyr-His-Pro X NH2) are reviewed. Analogues discussed include those with modifications to the Pyr moiety (e.g. DN-1417, CG 3509 and CG 3703), the Pro moiety (e.g. RX 77368) and MK-771 which has both terminal residues modified. The analogues have enhanced biological half-lives compared to TRH because of their resistance to enzymatic degradation. Neuropharmacological evaluation indicates the analogues to be active in antidepressant screening tests, to reverse the effects of diverse CNS depressants and to promote arousal when given alone. The analogues all exhibit enhanced potency compared to TRH in such tests. In contrast they appear equipotent to TRH in endocrine tests. An explanation is offered for this paradox in terms of metabolic stability and bioavailability to the requisite sites of action. The prospects for clinical utilization of the neuropharmacological properties common to TRH and its analogues are considered.