Spinal analgesic actions of kappa receptor agonists, U-50488H and spiradoline (U-62066).

Administered i.p. to mice, the selective kappa receptor agonists U-50488H and spiradoline (U-62066) were more potent on the tail-flick than on the hot-plate analgesic assay. Both were more potent after i.s. rather than i.c. administration, a result consistent with earlier demonstrations that tail-flick analgesia is generally dependent upon spinal mechanisms. Intraspinal U-50488H was not effective in elevating rat tail-flick latencies. Both drugs increased the thresholds for cat spinal cord nociceptive neurons to respond to a noxious heat stimulus. However, maximal responses of spinal cord neurons to nociceptive stimuli were not altered. It is concluded that although spinal cord sites may be critical to kappa receptor analgesic mechanisms, the effects are quite distinct from spinal cord effects observed previously with classical narcotic analgesics.

Animal behavioral and neurochemical effects of the CNS toxic amino acid antitumor agent, acivicin.

The investigational amino acid antitumor agent, acivicin, has been reported to cause dose-related and reversible CNS toxicity in humans characterized by sedation, ataxia, hallucinations, personality changes, and other symptoms. In a series of studies aimed at characterizing this toxicity, we investigated several species as potential animal models, determined the effects of acivicin on neuronal action potentials, and measured drug effects on the brain content of several putative amino acid neurotransmitters. In mice, we were unable to demonstrate any effects of acivicin in a battery of tests used in identifying and classifying CNS-active agents of potential therapeutic utility. In rats, unlike phencyclidine and certain other psychotomimetic drugs, acivicin produced no impairment of shock avoidance or brightness discrimination in animals trained on an automated Y-maze. In contrast to the rodent species, acivicin effects were perceived as resembling those of cyclazocine by rhesus monkeys trained to discriminate between psychoactive drugs and saline by food reinforcement. Cats treated with acivicin exhibited dose-related symptoms of sedation, somnolence, and ataxia. Iontophoretically applied acivicin was shown to have no effect on the spontaneous firing rate of dorsal horn interneurones in spinal cats. At the time of peak CNS symptoms in cats treated with 100 mg/kg acivicin, content of gamma-aminobutyric acid (GABA; nmoles/mg protein) was elevated from 57-140% in cerebellum, diencephalon, midbrain, and corpus callosum compared to control animals. Brain contents of glutamate, glutamine, and aspartate were not altered in cats experiencing neurotoxicity. These studies have shown that some symptoms of acivicin CNS toxicity are shared by humans and higher non-human species such as the cat and the monkey but not by rodents. Acivicin itself is apparently not a CNS excitant or depressant, but metabolites of the drug could be. Acivicin may also cause increases in the GABA content of localized regions of brain.

U-50488H, a pure kappa receptor agonist with spinal analgesic loci in the mouse.

U-50,488H is a chemically novel analgesic that is a potent opioid-like agent on the mouse tail flick and electrically stimulated guinea pig ileum tests. U-50,488H is a very weak competitor for naloxone binding sites in brain and ileum. However, the drug has high affinity for kappa receptor binding sites revealed by competition for EKC sites in the presence of dihydromorphine. Morphine has both supraspinal and spinal sites of action since it was a potent analgesic after both intracranial and intraspinal injections. However, U-50,488H works predominantly at the spinal level. Dynorphin may be an endogenous ligand at this site. Studies on cat dorsal horn neurons suggest that U-50,488H analgesia may be due to an increase in threshold for neuron excitation.

Use of substance P fragments to differentiate substance P receptors of different tissues.

The C- and N-terminal fragments of substance P were compared to the parent molecule with respect to their ability to: (a) contract the isolated guinea pig ileum, (b) induce salivation in the rat, (c) excite single cat dorsal horn neurones, and (d) induce scratching by intracranial injections in mice. C-terminal fragments as small as the heptapeptide were potent SP agonists on all assay systems. C-terminal fragments containing five amino acids or less were, at most, only weakly active. The C-terminal hexapeptide was a potent SP receptor stimulant on the isolated guinea pig ileum and, when directly applied by microiontophoresis, on cat dorsal horn neurons. However, the same compound was only 2-5% as potent as substance P in eliciting salivation and scratching in vivo, an indication that this fragment may be especially labile to enzymatic degradation. N-terminal fragments were totally inactive on the isolated guinea pig ileum. On the rat salivation and central nervous system assays, however, N-terminal fragments were capable of weak SP-like activity. It is concluded that SP receptors exist in multiple forms which we have labelled SP1 and SP2 receptors for those insensitive or sensitive to N-terminal fragments, respectively.

Morphine depresses dorsal horn neuron responses to controlled noxious and non-noxious cutaneous stimulation.

Dorsal horn neurons of unanesthetized, decerebrated, low spinal cats were excited by controlled noxious and non-noxious natural stimulation as well as by intense transcutaneous electrical stimulation. Intravenous morphine (0.3--3.0 mg/kg) depressed the spontaneous activity, the electrically evoked discharge and the response to noxious cutaneous heat (greater than 45 degree C) of nociceptive dorsal horn neurons. In those nociceptive neurons receiving convergent non-noxious inputs, morphine also depressed responses to non-noxious cutaneous air-puff stimulation. The above morphine effects were all reserved by 0.3 mg/kg of naloxone i.v. In neurons, which were purely non-nociceptive, morphine had little or no effect on either spontaneous activity or evoked responses to non-noxious stimuli. These findings suggest that: 1) morphine has a spinal site of action in which noxious as well as non-noxious inputs are decreased; and 2) there is a group of purely non-nociceptive dorsal horn neurons which are not influenced by the spinal actions of morphine.

Morphine does not antagonize the substance P mediated excitation of dorsal horn neurons.

Multibarrelled microelectrodes were used to test the effects of iontophoretically released substance P (SP), morphine, glutamate, and naloxone on spinal cord dorsal horn neurons. Cells excited by SP were also excited by noxious stimuli, a finding consistent with the hypothesis that SP is the neurotransmitter released by primary nociceptor afferents to excite dorsal horn neurons. Iontophoretic morphine failed to depress the SP-induced discharges. Indeed, iontophoretic morphine frequently potentiated the SP responses. In addition to potentiating SP-induced discharges, iontophoretic morphine frequently increased both the spontaneous activity of dorsal horn neurons and the activity evoked in these cells by noxious cutaneous heat and iontophoretic glutamate. Naloxone did not antagonize these excitatory effects. Intravenous morphine only depressed spontaneous discharges. Nevertheless, iontophoretic morphine still produced excitatory effects in spinal animals pretreated with analgesic doses of intravenous morphine. It is concluded that such excitatory effects are toxic actions indicative of supratherapeutic morphine concentrations in the vicinity of the neuron being studied. Intravenously administered morphine depressed the spontaneous activity of dorsal horn neurons of spinal cats, but failed to depress their responses to SP. Morphine also failed to antagonize SP's biological effects in peripheral systems (contraction of isolated guinea pig ileum, rabbit hypotensive effect, rat sialogogic response). It is concluded that morphine is not a substance P receptor antagonist. The results are discussed with respect to the hypotheses that (1) the spinal analgesic effects of systemically administered morphine occur on presynaptic terminals of sensory neurons, and (2) an SP antagonist might be a unique analgesic agent.