Pharmacological and human studies with a highly potent opioid peptide, D-Met2, Pro5-enkephalinamide.

D-Met2, Pro5-enkephalinamide (DMPEA) is an opioid peptide having analgesic activity in animals more potent after intravenous administration than morphine. It is less toxic but in animals it showed a higher dependence capacity than morphine. Besides analgesia DMPEA produces in rodent behavioral symptoms similar to those evoked by morphine or beta-endorphin, resembling the actions of neuroleptica. In human trials DMPEA was found to produce unpleasant sensations, no euphoria, and sometimes even dysphoria. DMPEA increases the serum levels of prolactin, growth hormone and, to a less extent, of TSH. Those effect of DMPEA on pituitary hormones. Finally, the human studies indicated that DMPEA antagonized pain (measured with the submaximum effort tourniquet technique), but did not affect adversely and even improved attention and short-term memory; it had no effect on the long-term memory. As the subjective effects of DMPEA are not pleasant, and no patient desired to obtain another treatment, some optimism as to low habit-forming properties of DMPEA may be justified.

Comparison of tolerance development and dependence capacities of morphine, beta-endorphin, and [D-Met2, Pro5]-enkephalinamide.

The tolerance-development capacities of beta-endorphin, [D-Met2, Pro5]-enkephalinamide, and morphine were compared in rats, and the dependence capacity of morphine was compared with that of the enkephalin analogue in mice. Tolerance to the analgesic effect, as measured by the tail-flick test, developed somewhat more rapidly in the [D-Met2, Pro5]-enkephalinamide-treated group than in the others. A similar relationship was found for the dependence capacity. Considering that the enkephalin analogue displayed the strongest analgesic activity, the well-known correlation between antinociceptive and tolerance development/dependence capacities of opiates seems to be valid for opioid peptides as well.

In vivo antagonism by naloxone of morphine, beta-endorphin and a synthetic enkephalin analog.

The in vivo equivalent of pA2 values were determined in rat tail-flick and mouse hot-plate tests for naloxone against morphine, beta-endorphin and a synthetic enkephalin analog, (D-Met2,Pro5)-enkephalinamide, as analgesics. In mice the apparent pA2 value of naloxone against morphine (6.86) was similar to that found by previously and essentially the same values were obtained against the opioid peptides, indicating homogenous receptor population for the analgesics studied. In rats the pA2 of naloxone against morphine (7.17) was lower than against either beta-endorphin (7.55) or (D-Met2,Pro5)-enkephalinamide (7.51), warning that in rats such a homogeneity of the "analgesic" receptor population as was observed for mice may not exist.

Effect of apomorphine on the antinociceptive activity of morphine.

Apomorphine pretreatment potentiated the analgesic effect of morphine in a dose-dependent manner both in rats and in mice measured by five different tests (writhing, hot plate, inflamed foot, tail-pinch and tail-flick procedures). Furthermore, apomorphine augmented the antinociceptive activity of morphine in tolerant animals as well. In morphine dependent mice the nalorphine precipitated jumping--a withdrawal symptom--was found inhibited by apomorphine treatment. The results are discussed in the light of the numerous but contradictory data available in the literature.

The effect of anti-parkinsonian drugs on chlorpromazine-induced depression of operant behaviour.

Rats were conditioned in automatic Skinner boxes on a discrete trial avoidance-escape schedule. The chlorpromazine-induced conditioned reflex inhibition could be reversed by apomorphine and amantadine, but not by atropine, trihexyphenidyl and diethazine. These findings seem to provide an additional tool for differentiating the atropine-like and dopaminergic anti-parkinsonian drugs.