The ability of RX 77368 - a stabilised analogue of TRH - to provoke the secretion of prolactin and TSH in vivo

RX 77368, a stabilised analogue of TRH, has been evaluated for its ability to provoke the release of TSH and prolactin in vivo after intravenous administration to rats and human volunteers. The analogue caused a dose-related release of TSH in both species. In rats the compound caused release of prolactin but the dose-response relationship was bi-phasic, so that large doses caused only a diminished response. The prolactin response in human volunteers was equivocal and further experiments will be necessary to define the effect. Despite its greater biological stability the potency, efficacy and duration of effect with RX 77368 was similar to that of TRH in both species. The manner in which further endocrine investigations may aid the clinical development of RX 77368 is discussed.

Neuropharmacological evaluation of RX 77368--a stabilised analogue of thyrotropin-releasing hormone (TRH)

L-Pyroglutamyl-L-histidyl-L-2,3-dimethylprolineamide (Pyr-His-Dmp . NH2; RX 77368) a stabilised analogue of thyrotropin-releasing hormone (TRH) has been examined for neuropharmacological effects in animal tests. The compound was more potent than either TRH or clinically established drugs in four animal tests of antidepressant potential (reserpine reversal, clonidine antagonism, tremorine reversal and learned immobility). RX 77368 also antagonised barbiturate sleeping time. Given by itself to rats the peptide produced arousal as characterised by EEG and EMG measurements and delayed the onset of sleep. The arousal induced was not accompanied by increases in locomotor activity. The profile of pharmacological activity for RX 77368 did not correspond to the profiles of tricyclic antidepressants, psychic-stimulants or analeptic drugs. The possible clinical uses for such a molecule are discussed.

Thyrotropin-releasing hormone (TRH) analogues show enhanced CNS selectivity because of increased biological stability

TRH exerts both endocrinological and neuropharmacological actions. Two analogues of TRH, Pyr-His-Mep . NH2 (L-trans-3-methylprolineamide) and Pyr-His-Dmp . NH2 (L-3,3-dimethylprolineamide) have been examined for their neuropharmacological and endocrinological effects. Comparisons of their ability to provoke hyperthermia in rabbits demonstrated that both analogues were more potent than TRH, but like the parent peptide had only a limited ability to cross the blood brain barrier. This conclusion was confirmed by whole body autoradiographical studies. In contrast both analogues had a similar potency to TRH with respect to the ability to provoke TSH release. It is concluded that the increased potency in neuropharmacological tests results from enhanced bioavailability to CNS sites and that a similar rationale can be used to explain the CNS selectively claimed in the literature for other analogues of TRH.

Is thyrotropin releasing hormone an endogenous ergotropic substance in the brain?

The neuropharmacological profile for thyrotropin releasing hormone (TRH) has been compared with various other types of central nervous system stimulants, and the results of the comparison, considered together with the results obtained by other workers, suggest that TRH may function as an endogenous ergotropic substance. Its ergotropic properties suggest that TRH analogues with better biological stability than the parent compound may be useful agents for treating some psychiatric disorders; for hastening the recovery of consciousness and respiration after anaesthesia; for treating narcotic overdosage or dependence, narcolepsy, minimal brain damage, and various neuromuscular disorders; and for aiding the diagnosis of "brain death".

Modification of the proline residue of TRH enhances biological activity and inhibits degradation

An analogue of TRH with a modified proline residue (Pyr-His-MepNH2) has been synthesised and evaluated. In CNS, but not endocrine tests the analogue had improved activity compared to TRH and was more resistant to enzymatic degradation. A metabolite of TRH, His-Pro, was without biological activity. It is concluded that the biological actions attributed to TRH are due to the parent tripeptide rather than His-Pro and that these actions can be enhanced by improving the biological stability of TRH.

Evaluation of RX 72601 as an anti-curare agent

RX 72601, a new and potent anticholinesterase, has been evaluated for its ability to reverse the neuromuscular blockade induced by non-depolarizing muscle relaxants. In rats RX 72601 10 mug/kg i.v. proved effective in reversing the effects of alcuronium gallamine, pancuronium or tubocurarine and the drug exhibited a wide margin of safety. RX 72601 proved equally effective in reversing the effects of tubocurarine in both cats and baboons. In dogs and cats, effective anti-curare doses had little action on the cardiovascular or respiratory systems. Overall the results obtained indicate that RX 72601 may be a safe antagonist of non-depolarizing muscle relaxants and that it will be possible to use RX 72601 without prior administration of atropine.