Efferent acoustic neurons within the lateral superior olivary nucleus of the guinea pig

The relative potencies of the stereoisomers of ketamine, N-allylnormetazocine and cyclazocine were determined at mu opiate and sigma opiate/phencyclidine (PCP) receptors in vitro in rat brain homogenates, as well as in a discriminative stimulus behavioral paradigm in rats trained to discriminate PCP from saline. In all cases the in vivo and in vitro data were in agreement. The (+)-isomers of N-allylnormetazocine and cyclazocine are highly specific sigma opiate/PCP ligands.     

Specific Binding of [H] phencyclidine in rat central nervous tissue: further characterization and technical considerations

The interaction of phencyclidine (PCP) with its specific receptor sites in the central nervous system has been further characterized. Kinetic association and dissociation rate constants of 2.9 × 10⁶ M⁻¹ and 4.8 × 10⁻¹ were determined, yielding a kinetic K_D of 1.6 × 10⁻⁷ M, in agreement with the K_D previously determined at equilibrium. Permissible separation time of 13 s was calculated from the kinetic data, well above the actual separation time of less than 10 s in the rapid filtration assay. Presoaking of filters in 0.01% poly-l-lysine eliminated displacable [³H]PCP adsorption to filter material. Binding data obtained via centrifigation assays was identical to that obtained with the rapid filtration method. Stereospecificity of the PCP receptor was demonstrated by the finding that (+)-ketamine is four-fold more potent than (−)-ketamine in displacing specifically bound [³H]PCP. Several proteolytic enzymes including trypsin, papain and thermolysin potently inactivated PCP receptors. Detailed regional distribution studies showed highest density of PCP receptors in subicular cortex and hippocampus, intermediate levels in hypothalamus, striatum, frontal cortex and cerebellum, lower levels in brainstem and spinal cord, and negligible levels in corpus callosum, a white- matter control area. Benzomorphan opiates with PCP-like behavioral effects interact with the PCP receptor. These data support the pharmacological relevance of the PCP receptor site as demonstrated by the rapid filtration method.     

Blockade of long-term potentiation by phencyclidine and sigma opiates in the hippocampus in vivo and in vitro

Long-term potentiation (LTP) of synaptic transmission in the rat hippocampus in vivo and in vitro, was studied using field potentials. Pretreatment with phencyclidine (PCP) or 'sigma' opiates blocked LTP in vivo while mu and kappa opiates and the antagonist naloxone were ineffective. Scopolamine (20 mg/kg i.p.) neither prevented LTP nor antagonized the LTP-blocking effect of PCP. In vitro, PCP up to 100 microM did not alter synaptic activation of CA1 pyramidal cells by stratum radiatum stimulation but blocked LTP in a dose-dependent manner (ED50: 3 microM). The sigma opiate, cyclazocine, also prevented the induction of LTP in vitro while morphine and procaine were ineffective.     

-Receptor antagonist reverse ‘non-opioid’ stress-induced analgesia

We have evaluated the effects of antagonists to the μ, and opiate receptors on ‘opioid’ and ‘non-opioid mediated’ analgesia. Our findings indicate that μ receptors might mediate ‘opioid’ and receptors ‘non-opioid’ analgesia. It is suggested that endogenous opiates that act on μ or receptors could be responsible for the different types of stress-induced analgesia.     

An increase in opiate receptor-sites is associated with enhanced cardiovascular depressant, but not respiratory depressant action of morphine

Rats were treated for 4 weeks with a constant infusion of 2 mg/kg/h of the opiate antagonist naloxone. This treatment increased μ-, σ- and ξ-binding sites by 60–180% in several brain regions, suggesting effective blockade of the 3 types of opiate sites. The significance of changes in opiate binding sites for opiate receptor mediated physiological responses were examined using cardiovascular and respiratory responses to morphine (assessed after elimination of naloxone) as physiological parameters. Chronically naloxone-treated rats showed no alteration in respiratory responses to morphine, whereas there was a marked supersensitivity to depressor and bradycardic effects and a loss of pressor and tachycardic effects of morphine. These data are the first indication that cardiovascular effects of opiates vary with changes in central opiate receptor levels. Our observations, moreover, show that there are complex relationships between receptor number and receptor-mediated effects of opiates.     

Quantitative localization of [H]TCP binding in rat brain by light microscopy autoradiography

The anatomical localization of phencyclidine (PCP)/σ-opiate receptors in rat brain was determined by quantitative light microscopy autoradiography using the new ligand N-(1-[2-thienyl]cyclohexyl)[³H]TCP). TCP is a potent analog of PCP which possesses a higher affinity for PCP/σ-opiate receptor than those PCP itself. The higest of [³H]TCP binding was detected in the hippocampus. Intermediate levels were found in frontal cortex, striatum, amygdala and cerebellum. Specific [³H]TCP binding was undetectable in anterior commissure and corpus callosum. The distribution pattern of [³H]TCP binding sites is similar to the pattern obtained with [³H]PCP but more sharply defined. On the basis of its greater potency and specificity, [³H]TCP may prove superior to [³H]PCP as a molecular probe for the study of brain sigma opiate/phencyclidine receptors.     

Selective localization of different types of opiate receptors in hippocampus as revealed by in vitro autoradiography

The visualization of opiate binding sites within the hippocampus of the rat has been achieved by means of an in vitro autoradiography. In line with the concept of multiple opiate receptors, different opioid agonists revealed a particular distribution pattern. Whereas the selective δ-receptor agonist [³H]d-Ala², d-Leu⁵-enkephalin specifically labelled binding sites in the CA₂ area, [³H]etorphine grains displayed a uniform dense distribution throughout the pyramidal cell layers from CA₁ to CA₄.     

[H]-etorphine and [H]-diprenorphine receptor binding in vitro and in vivo: differential effect of Na and guanylyl imidodiphosphate

The in vivo and in vitro cerebral receptor binding kinetics of the opiate agonist etorphine and the antagonist diprenorphine were investigated in the rat. Although of similar receptor affinity in vitro in Tris buffer brain homogenates, etorphine exhibited considerably less affinity than diprenorphine in vivo. The hypothesis was tested whether the opiate receptor regulators, Na⁺ and GTP, are responsible for the low in vivo receptor affinity of the agonist. [³H]Etorphine and [³H]diprenorphine dissociation curves were similarly affected by Na⁺ and guanylyl imidodiphosphate (GPP(NH)P), a hydrolysis resistant GTP analog when added separately in vitro. However, the combination of Na⁺ and GPP(NH)P greatly accelerated only the [³H]etorphine off-rate over that with Na⁺ alone and reproduced the rapid dissociation half-life observed in vivo (t_1/2 < 1 min). In contrast, the receptor dissociation rate of the antagonist was not further accelerated by Na⁺ plus GPP(NH)P over that with Na⁺ alone. Moreover, Na⁺ plus GPP(NH)P decreased [³H]etorphine, but not [³H]diprenorphine, equilibrium binding in vitro. These results suggested that the lower in vivo affinity of etorphine than of diprenorphine is predominantly caused by the combined action of Na⁺ and GTP. Furthermore, the data are consistent with the hypothesis that both etorphine and diprenorphine bind to the opiate receptor in its high affinity form, but that only the agonist etorphine is capable of converting the high affinity form to one of low affinity in the presence of Na⁺ and guanine nucleotide. Assuming that production of the low affinity form reflects biological activation of the opiate receptor system, then this hypothesis is consistent with the high pharmacological potency of etorphine (agonistic ED₅₀ 1μg/kg) relative to its low apparent in vivo receptor affinity, as well as with the low fractional receptor occupancy of etorphine ( 2%) at its analgesic ED₅₀. Finally, in vivo [³H]etorphine and [³H]diprenorphine displacement curves obtained with unlabeled diprenorphine and etorphine showed that [³H]etorphine labels only a subpopulation of the total [³H]diprenorphine binding sites. It remains to be determined which subsites of the opiate receptor system mediate the agonistic actions of etorphine.     

Effect of phencyclidines on hippocampal pyramidal cells

Phencyclidine (PCP) and several behaviorally active or inactive structural analogs were administered i.v. to urethane-anesthetized rats in order to determine their effects on CA1 pyramidal cell discharges elicited by contralateral CA3 (cCA3) stimulation.PCP and the behaviorally active m-amino derivative (m-NH₂PCP) depressed, in a dose-dependent manner, the amplitude of the population spike evoked in CA1 by a cCA3 stimulation (ED 50s: 0.9 mg/kg for PCP, 0.5 mg/kg for m-NH₂ PCP). However, the behaviorally inactive derivatives m-nitro (m-NO₂ PCP) and PCP methyliodide (PCP CH₃I) were ineffective up to 10 mg/kg.PCP (0.1–0.3 mg/kg i.v.) also decreased the duration of inhibition of CA1 discharges in a paired-stimulus paradigm; this was in contrast to the effects of thiopental and diazepam.In midcollicular-transected, urethane-anesthetized rats, the inhibitory effect of PCP on cCA3-CA1 transmission was not observed but the drug was still as effective as in intact rats in the paired-stimulus paradigm.In animals subjected to 6-hydroxydopamine lesions of the hippocampal noradrenergic innervation (average 85%) decrease in NE content), the potency of PCP in inhibiting cCA3-CA1 transmission was the same as in a group of sham-operated controls.These results suggest the following conclusions: (i) PCP exerts at least 2 separate types of effects in CA1, both of which result from a central action of the drug; (ii) PCP decreases the monosynaptic excitation of CA1 pyramidal cells and this action requires the integrity of brainstem afferents; (iii) PCP may decrease recurrent inhibition or afterhyperpolarization in CA1 via a mechanism which is independent of these connections and, therefore, could result from a direct action of the drug at the level of the hippocampus; (iv) finally, no evidence was found to suggest that the noradrenergic innervation of the hippocampus is critically involved in the action of PCP on CA1 discharges.     

Effect of morphine on regional cerebral oxygen consumption and supply

The effects of acute opiate receptor stimulation on regional cerebral oxygen consumptilon and blood flow were examined in 9 regions of the artificially respired, α-chloralose-anesthetized cat. Regional cerebral arterial and venous oxygen saturation were examined microspectrophotometrically and regional cerebral blood flow was monitored using radioactively tagged microspheres (15 ± 3 μm in diameter). Oxygen consumption was calculated as the product of flow and oxygen extraction. In 8 cats, after control cerebral blood flow was obtained, and in 8 experimental cats after this same measurement was obtained before and 40 min after the administration of 1.5 mg/kg morphine sulfate; the cats' heads were simultaneously sawed in 3 places and quickly frozen in liquid nitrogen-cooled propane. Systolic and diastolic blood pressure were significantly decreased by treatment. The heterogeneity of venous oxygen saturation was significantly reduced by morphine. Average cerebral blood flow, oxygen extraction and consumption were not altered significantly by morphine. Regional cerebral blood flow in the hypothalamus, thalamic oxygen extraction, and hypothalamic and thalamic oxygen consumption were significantly decreased by treatment. This low dose of morphine may produce changes in cerebral neuronal and/or synthetic activity which lowers oxygen consumption in some regions rich in opiate receptors, while not affecting overall brain oxygen supply or consumption.