Hj. Teschemacher

On the central sites for the antinociceptive action of morphine and fentanyl

The inhibition by morphine and fentanyl of a nociceptive response, the licking reaction elicited by electrical stimulation of the tooth-pulp, was studied in rabbits. Both drugs were injected into the entire ventricular system, and into its separate parts and morphine was applied to definite brain structures. Spread of the applied drug was examined autoradiographically, using 14C-labelled morphine. Intraventricular injection of morphine was 500–1000 times more effective than intravenous injection. The onset of action was much slower than after systemic application; this latency is interpreted as being caused by the slow permeation of morphine to the receptor areas in the periventricular structures. Injection into separate parts of the ventricular system did not inhibit the nociceptive reaction when the spread of substance was restricted to one or both lateral ventricles. Inclusion of the third ventricle in the area of distribution resulted in only a slight increase in effectiveness. Application of the drugs into the aqueduct and into the fourth ventricle induced a complete inhibition of the nociceptive reaction. When morphine spread into the surrounding of the cisterna cerebellomedullaris (with exclusion of ventricular system) no effect was observed. Uni- or bilateral microinjection of morphine into various diencephalic and mesencephalic periventricular structures induced a pronounced inhibition of the nociceptive reaction. In at least one part of these experiments, however, morphine may have acted indirectly, by spreading to caudal parts of the ventricular system. The most effective sites of the antinociceptive action of morphine-like substances were situated in the fossa rhomboides and structures near it. Autoradiographical studies showed the sites for the antinociceptive action of morphine to be located in the ventricular wall of these regions at a depth of about 1–2 mm. Diencephalic sites of action were of minor importance, at least when the licking reaction was used as test. The functional structures involved in the drug effects are discussed.

Sites of action of morphine involved in the development of physical dependence in rats

In rats made dependent on morphine by implantation of morphine pellets, withdrawal, as precipitated by intraventricular injection of morphine antagonists, was compared to withdrawal as precipitated by systemic antagonist application. The results, most clearly those obtained with a hydrophilic compound, diallyl-nor-morphinium-bromide, point to periventricularly located sites of action for the release of most withdrawal signs by antagonists. Jumping, reaching only low levels after i.ventr. injection of levallorphan and nalorphine, was very pronounced when the benzomorphane derivative SH 254, was used. In the case of writhing and diarrhea, the situation is more complicated. Possibly, central as well as peripheral mechanisms are involved in the expression of these signs.

Sites of action of morphine involved in the development of physical dependence in rats. II. Morphine withdrawal precipitated by application of morphine antagonists into restricted parts of the ventricular system and by microinjection into various brain areas.

Morphine withdrawal was precipitated by injection of various morphine antagonists into restricted parts of the ventricular system or by microinjection of levallorphan into specific brain areas of rats made dependent on morphine by repeated pellet implantation. When the antagonists could spread only within the lateral ventricles and the 3rd ventricle, a weak withdrawal syndrome was induced; by antagonist administration into the restricted 4th ventricle, however, strong withdrawal signs like jumping were elicited even at small dosages. In microinjection experiments, structures in the midbrain and the lower brain stem proved to be the most sensitive to antagonist action. Although microinjections into thalamic nuclei also had some effect, it could not be excluded that the effects were due to uncontrolled spreading of the drug. This became especially clear from experiments with tritium-labeled levallorphan.It is concluded that brain structures located in the anterior parts of the floor of the 4th ventricle and/or caudal parts of the periaqueductal gray matter are important sites of action for the development of physical dependence on morphine.

The importance of lipid-solubility for the central action of cholinolytic drugs

Abstract Cholinolytic drugs differ widely in their relative central activities. With the quaternary and some tertiary amines, central activity is very small, whereas the antiparkinsonian drugs have a high central activity. Because the so-called blood-brain barrier is lipid in character and allows highly lipid-soluble substances to penetrate faster than poorly lipid-soluble substances, the question was investigated whether central activity is influenced by the lipid-solubility of the drugs. Accordingly the partition coefficients heptane/water of the drugs were measured and compared with the relative central cholinolytic activities. The relative central activity was represented by the central to peripheral cholinolytic active doses. The two values were determined by different biologic methods: antiarecoline action, investigated by means of the analgesia test in the mouse; inhibition of the EEG arousal reaction in the rabbit; intensification of the morphine induced stupor in the rat; mydriatic action in mouse and rabbit; and inhibition of chromodacryorrhoea in the rat. The results show a correlation between the relative central activity and the partition coefficient heptane/water of the drugs. This means that the relative central activity increases as the lipid-solubility of the drug increases.

Permeation morphinartig wirksamer Substanzen an den Ort der antinociceptiven Wirkung im Gehirn in AbhÄngigkeit von ihrer Lipoidlöslichkeit nach intravenöser und nach intraventrikulÄrer Applikation

SummaryThe antinociceptive action of morphine and of a series of similar substances following intravenous and intraventricular administration was investigated by means of the tooth-pulp-test in rabbits; the relative effectiveness of the substances after the two methods of administration was compared with their lipid-solubility.1.Morphine was about 900 times as effective when administered intraventricularly than when injected intravenously; this difference was even more pronounced in the case of normorphine and (quaternary) N-methylmorphine, but was slightly less for dihydromorphine and hydromorphone. In the case of levorphanol, pethidine, etorphine, fentanyl and other synthetic analgesics, the difference in effectiveness between the two methods of administration was incomparably smaller (in the range of 1∶10).2.The quotient effectiveness intravenous administration/effectiveness intra-ventricular administration bore a close relation to the lipid solubility of the substances derived from the partition coefficient (Pc) heptane/water and dichlor-ethane/water at pH 7.4. A similar correlation between Rf-values from thin-layer chromatographie and this quotient was found. Morphine and its derivatives showed very low lipid-solubility (Pc heptane/water < 0.00001); that of the synthetic analgesics was higher, reaching Pc-values above 10. Thus it is concluded that the permeation of morphine and its hydrophilic derivatives into the CNS is impeded, whereas no important hindrance exists for permeation of the more lipophilic compounds having pcs above 0.01.3.Determination of the concentration of labelled substances in the brain (14C-morphine,3H-dihydromorphine,3H-fentanyl and3H-etorphine) at the time of a defined antinociceptive effect confirmed this interpretation. In the case of morphine and dihydromorphine, brain concentrations were only 1/20 of the plasma level, while fentanyl and etorphine reached brain concentrations which were up to 10 times that in the plasma. Furthermore, the studies of concentration in the brain showed the gradation of effectiveness of the substances after intraventricular administration to be approximately equal to the gradation of their “intrinsic activity”.4.There was a close correlation between the lipid solubility of the substances and the rate of onset of their effect following intraventicular administration. This relation was much less pronounced after intravenous injection.5.The results are discussed in view of differences in the kinetics of distribution of the substances after intravenous and intraventricular application.

A material with opioid activity in bovine milk and milk products.

SummaryChloroform-methanol extracts of lyophilized milk, of commercially available dried milk or baby food and of casein digests were tested for opioid activity on the guinea-pig ileum longitudinal muscle-myenteric plexus preparation. Compounds with opioid activity — which proved to be resistant to peptidases — were detected in certain batches of baby food, casein digest, and cow milk in considerably varying amounts.

Morphine abstinence syndrome in rabbits precipitated by injection of morphine antagonists into the ventricular system and restricted parts of it.

The abstinence syndrome as precipitated in morphine-dependent rabbits by systemic administration of nalorphine was compared with the syndrome induced by application of nalorphine to the entire cerebroventricular system, or separated parts of it. Systemic nalorphine administration precipitated a syndrome characterized by motor excitation and other more peculiar symptoms. Small, intraventricularly-applied nalorphine dosages induced a similar behaviour pattern although some abdominal symptoms, present after systemic withdrawal, were lacking. Preconvulsive and convulsive symptoms, which are only abortive upon systemic withdrawal, became more prominent with higher doses of nalorphine, administered intraventricularly. A nearly identical behaviour pattern was observed when nalorphine was injected into the 4th ventricle. In contrast, the symptomatology was weak after injection of nalorphine into the anterior parts of the ventricular system when the antagonist could not reach the caudal parts (plug in the aquaeductus mesencephali). The convulsive symptoms after intraventricular withdrawal were accompanied by a large increase in body temperature. Bradycardia and irregularities in heart rhythm were also pronounced only after intraventricular withdrawal. These results indicate that, structures easily reached by nalorphine from the 4th ventricle and probably located in medullary and pontine parts of the brain stem, are important sites of action of morphine for the development of physical dependence on this drug.

Autoradiographic studies concerning the supraspinal site of the antinociceptive action of morphine when inhibiting the hindleg flexor reflex in rabbits

Abstract Previous investigations in unrestrained, conscious rabbits showed that the site of action of morphine when it inhibits the hindleg flexor reflex is located in structures easily reached from the 4th ventricle. In the present investigation using 14C-morphine, the site of action of the drug was localized more precisely. The distribution pattern of morphine around the 4th ventricle after intraventricular and intracisternal injection was visualized by autoradiography and correlated with the pharmacological effect in the individual experiments. Thus, structures bordering the caudal parts of the 4th ventricle and the extraventricular cerebrospinal fluid space within this region could be excluded as sites of action. On the other hand, structures in the floor of the rostral part of the fossa rhomboidea are highly putative as sites of morphine action. The concentration of morphine along the permeation route from the ventricle to nearby structures was determined by combining the results of radioassay, autoradiography and optical scanning. The concentrations calculated for structures at different distances from the ventricular wall were compared with those concentrations obtained after systemic application of equieffective morphine dosages. It was concluded that the relevant receptors are located somewhat more than 1000 μm away from the ventricular wall. When a weak antinociceptive effect is observed, this region corresponds to the border zone of the autoradiographically-blackened area. The results are discussed with reference to the technical problems of this procedure as well as the anatomical structures and physiological mechanisms involved in supraspinal control of spinal nociceptive reflex activity.

Differentiation between spinal and supraspinal sites of action of morphine when inhibiting the hindleg flexor reflex in rabbits

The effect of morphine on the ipsilateral hindleg flexor reflex evoked by radiant heat or by electric stimulation of the paw was studied in rabbits. Morphine was injected systemically into the whole ventricular system and into restricted parts of it. Morphine, when administered intraventricularly at very low doses, induced reflex inhibition which was comparable to the effect obtained when the drug was applied systemically. By means of radioassay of 14C-labelled morphine, it was shown that a spinal site of action is not involved in this effect. Administration of morphine to restricted anterior parts of the ventricular system (consisting of the 3rd ventricle and the lateral ventricles) was ineffective whereas morphine injected into the 4th ventricle induced a strong reflex inhibition. Intracisternal injection was ineffective when the 4th ventricle was closed by a plug. Reflex inhibition induced by systemic morphine administration was reversed when levallorphan was injected intraventricularly or systemically. Experiments in which the concentration of 3H-levallorphan in the spinal cord was studied, showed that also after systemic morphine administration a spinal site of action is, if at all, of little importance. From these results it is concluded that a very strong morphine-induced reflex inhibition originates from brain structures easily reached by the drug from the fossa rhomboidea.

Hydrophobic interactions responsible for unspecific binding of morphine-like drugs.

SummaryThe unspecific binding of four narcotic analgesics 3H-dihydromorphine, 14C-morphine, 3H-etorphine and 3H-fentanyl to human albumin, human plasma, rabbit plasma and several tissue homogenates from rabbits was investigated using equilibrium dialysis and ultrafiltration. At a drug concentration of 10−7 M in human plasma, dihydromorphine is bound to an extent of 14%, morphine to 23%, etorphine to 88% and fentanyl to 70%. These differences in binding are due to different degrees of hydrophobic interaction between the drugs investigated and the plasma or tissue components. The hydrophobic interactions are due to the unionized form of the drugs. The ionized form is bound to a negligible extent with all four compounds, possibly in part by ionic mechanism. Binding increased with increasing ionic strength of the protein solution, with raising temperature between 0°C and 37°C and with increasing pH values of the protein solution, features which are characteristic of hydrophobic interactions. Scatchard plots of the binding data, from which the total binding constants nk were derived, indicated high concentrations of binding sites compared with drug concentrations found analgesically effective in vivo.