Recent data have indicated that the long-lasting increase in tyrosine hydroxylase (TH) protein could be differently expressed in the anterior and posterior locus coeruleus (LC) after a single intraperitoneal injection of RU24722, which has been proposed as a potent activator of catecholaminergic systems. In the present study, we have evaluated the dose and time course responses and the effect of a repeated treatment with RU24722 at 3-day intervals on TH protein level in the anterior and posterior rat LC. The results showed that RU24722 induces a long-lasting increase of TH protein level in the anterior and posterior LC that was maximal 3 days following a single injection of 30 mg/kg. The increase in TH protein was maintained at a constant level after repeated administrations of RU24722 at 3-day intervals. Furthermore, we have investigated whether the effect of the drug on TH protein could be modulated via several hormonal systems. The long-term increase of TH steady-state content after RU24722 was still observed 15 days after castration, adrenalectomy, hypophysectomy, and thyroidectomy. The initial steady-state TH protein level was significantly higher in the anterior LC of thyroid- or hypophysectomized and in the posterior LC of hypophysectomized rats. However, this increase was reversed when animals were housed at 28 degrees C.

Thyrotropin-releasing hormone (TRH) has an antinociceptive action in the rat. Antinociception was observed using a thermal stimulus (tail-flick test) after TRH administration into lateral ventricle, nucleus raphe magnus, nucleus reticularis paragigantocellularis and amygdaloid nuclei. This effect was short-lived since it was completely abolished 60 min after intracerebroventricular administration. TRH may interact with the opioid systems as its antinociceptive effect was blocked by pretreatment with naloxone.

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