The effect of cholecystokinin octapeptide on rat striatal 3H spiperone binding.

In the rat striatum sulphated CCK8 has no significant effect on equilibrium binding of 3H-spiperone but has a considerable, although transient, effect under non-equilibrium conditions. Under non-equilibrium conditions (during the association phase of ligand binding) and at high ligand concentrations (1 nM), CCK8 displaces specific binding and at low ligand concentrations (0.1 nM) CCK8 enhances specific binding. CCK8 has no effect on 3H-spiperone dissociation kinetics.

In vivo evidence for the selectivity of ICI 154129 for the delta-opioid receptor.

The in vivo selectivity of the novel delta opioid-receptor antagonist N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH (ICI 154129) was examined in several opioid-selective models. Antagonism at the delta receptor was demonstrated in the striatal head-turn model in the rat. Intrapallidal injection of the relatively selective delta-receptor agonist D-Ala2,D-Leu5-enkephalin (0.5 micrograms) slowed the head-turn time and this effect was completely prevented by prior subcutaneous administration of ICI 154129 (30 mg/kg). The role of delta receptors in two classical test situations was studied using the mixed opioid agonist etorphine and the antagonists naloxone and ICI 154129. The drug ICI 154129 (30 mg/kg, s.c.) failed to prevent the antinociceptive effects and stimulation of locomotor activity produced by etorphine, whereas the relatively selective mu-opioid receptor antagonist, naloxone was effective in both test situations. The possible involvement of delta receptors in morphine-induced dependence was studied by monitoring the abstinence behaviour precipitated in rats given pellets of morphine by either ICI 154129 or naloxone. Naloxone (0.5 mg/kg, i.p.) precipitated a characteristic withdrawal syndrome in conscious rats and, at a much smaller dose (0.02 mg/kg, i.p.), induced shaking behaviour in pentobarbitone-anaesthetised rats. No withdrawal signs were observed in either model after injection of ICI 154129 (30 mg/kg, s.c.), suggesting that the delta receptors are not involved in dependence on morphine.

GABA receptor multiplicity. Visualization of different receptor types in the mammalian CNS.

Receptors for GABA in the mammalian brain are not homogeneous. A clear separation exists between receptors which recognize the antagonist bicuculline and a population which does not. These two classes have been designated GABAA and GABAB sites respectively. Within the GABAA category there may also be many subtypes which exhibit subtle pharmacological differences. Numerous centrally-active agents influence GABAA site function generally via an allosteric interaction. By comparison very few substances currently available interact with GABAB sites. The GABAA and GABAB sites show many contrasting characteristics not least of which is their distribution pattern within the rat brain. Autoradiographic analysis has indicated that although both receptors may be present within many regions, in some areas only one type is present. For example, GABAA sites only are present in the lamina molecularis of the olfactory bulb and granule cell layer of the cerebellum whereas GABAB sites are present in the interpeduncular nucleus without any evidence of GABAA sites. In the spinal cord GABAB sites are unevenly distributed with high densities in laminae I-IV. GABAA sites are more uniformly distributed throughout the dorsal and ventral horns. In conclusion, there is now good evidence for multiple GABA receptors and the way is open to determine the functional significance of the GABAB receptor in relation to the now classical GABAA site.

Are baclofen-sensitive GABAB receptors present on primary afferent terminals of the spinal cord?

The site of action of the antispastic drug baclofen has long been considered to reside in the spinal cord although supraspinal effects have also been reported. This beta-chlorophenyl derivative of the neurotransmitter gamma-aminobutyric acid (GABA) depresses both monosynaptic and polysynaptic transmission in the cord possibly through a decrease in transmitter release rather than by any antagonism at postsynaptic receptors. Recently, baclofen has been shown to be a selective ligand for a bicuculline-insensitive GABA receptor (GABAB) site that occurs widely in the mammalian central nervous system including the spinal cord. The apparent importance of the cord in the therapeutic effects of this drug prompted us to ask whether they involve GABAB site activation. As an initial step we have located these receptors by autoradiography, comparing them with classical GABAA sites. We report here that GABAB sites, unlike GABAA sites, are present in high concentrations in laminae I, II, III and IV of the dorsal horn and that after the neonatal administration of capsaicin this binding is reduced by 40-50%.

Actions of substance P, MIF, TRH and related peptides in the substantia nigra, caudate nucleus and nucleus accumbens.

Neurones in the substantia nigra were found to be sensitive to iontophoretically applied substance P, substance P 1-9 methyl ester and substance P 1-9 amide. Substance P 1-2, 4-9 and 5-9 methyl esters, thyrotropin releasing hormone (TRH), Pyroglutamyl-histidyl-2 methyl prolineamide (methyl TRH), Pyroglutamyl-histidyl-2 methyl prolineamide (methyl TRH), histidyl-proline-diketopiperazine (His-Pro) and MSH releasing inhibiting factor (MIF) were without effect on neurones in this area. Thyrotropin releasing hormone (TRH), methyl TRH, His-Pro and MIF were inactive on neurones in the caudate nucleus and nucleus accumbens. Bilateral injections of substance P and substance P 1-9 methyl ester into the ventral tegmental area (VTA) of conscious rats produced locomotor activity, while similar injections of substance P 4-9 and 5-9 methyl esters did not. The locomotor activity produced by amphetamine was prolonged by TRH, while MIF was devoid of such activity. The data suggest that substance P and substance P 1-9 have similar effects in the substantia nigra, although the mechanism of action is unclear. Thyrotropin releasing hormone and MIF probably do not have acute actions in the brain areas tested.

Bicuculline-insensitive GABA receptors on peripheral autonomic nerve terminals.

The action of gamma-aminobutyric acid (GABA) and related compounds on rat isolated atria and mouse and guinea pig isolated vas deferens has been studied. GABA depressed the evoked but not basal release of [3H]noradrenaline from atria (IC50 4 micro M) and reduced the twitch responses of the vas deferens (IC50 3 micro M) in a dose-dependent manner. These depressant effects were not prevented by recognized GABA antagonists such as bicuculline and picrotoxin. Numerous GABA analogues, in particular 3-aminopropanesulphonic acid, failed to mimic the action of GABA. However, beta-p-chlorophenyl GABA (baclofen) was stereospecifically active. Other related beta-substituted derivatives were also active but to a lesser degree than GABA. Pretreatment of the vas deferens with the neuronal GABA uptake inhibitors 2,4-diaminobutyric acid or cis-3-aminocyclohexanecarboxylic acid potentiated the action of GABA. These data suggest the presence of a bicuculline-insensitive GABA receptor on autonomic nerve terminals. Preliminary observations indicate a lack of chloride ion dependence in the action of GABA at this site.

Effect of pretreatment with bronchodilator drugs on in vitro responsiveness of guinea pig lung adenylate cyclase.

Pretreatment of guinea pigs with 5 microgram/kg isoprenaline or 10 microgram/kg salbutamol s.c. thrice daily for 7 days reduced the responsiveness of lung slice and tracheal ring adenylate cyclase to isoprenaline, but not to prostaglandin E1. Pretreatment of guinea pigs with isoprenaline also reduced the sensitivity of tracheal smooth muscle strip adenylate cyclase to isoprenaline. Cross-tolerance developed to noradrenaline in lung slices obtained from guinea pigs pretreated with isoprenaline. Propranolol blocked the response of lung slice adenylate cyclase of control and isoprenaline-pretreated animals to approximately the same degree. The presence of phentolamine in the incubation medium did not affect the reduced sensitivity to isoprenaline. Possible mechanisms of development of tolerance to sympathomimetic bronchodilator drugs are discussed.

In vitro profile of some opioid pentapeptide analogues.

The opiate agonist potency of thirteen synthetic enkephalin pentapeptides has been examined on the electrically stimulated guinea pig ileum and mouse vas deferens preparations in comparison with methionine and leucine enkephalins, beta-endorphin and normorphine. Their antagonism by naloxone (Ke) was also assessed on each preparation. Our findings are compatible with, and are discussed in the context of, the hypothesis that these preparations possess at least two populations of receptors.