Classification of opioids on the basis of their ability to antagonize bombesin-induced grooming in rats

Aversive properties of bombesin in rats

Aversive properties of bombesin were determined in the conditioned place-preference paradigm in rats and compared with the effects on spontaneous behavior. Bombesin induced excessive grooming and/or scratching behavior at doses of 80 ng, 400 ng, and 2.0 micrograms ICV. In the conditioned place-preference paradigm, doses of 400 ng and 2.0 micrograms ICV induced a profound aversion to the environment in which the animals had received peptide treatment. Eighty ng were partially effective, and 16 ng did not induce a significant change in preference. The grooming/scratching behavior was attenuated by pretreatment with 4 mg/kg morphine-sulfate. These results show that bombesin is strongly aversive at doses that induce grooming/scratching behavior. Although the relationship between these different effects is not known, the similarity in their dose-response relationship suggests that they may be mediated by a common mechanism.

Solubilization of the receptor for the neuropeptide gastrin-releasing peptide (bombesin) with functional ligand binding properties

The receptor for the neuropeptide gastrin-releasing peptide, the mammalian homologue of bombesin, was solubilized from rat brain and Swiss 3T3 cells by using the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS) and the cholesteryl hemisuccinate ester (CHS). Only the combination of the detergent CHAPS and the cholesteryl ester CHS in a glycerol-containing buffer satisfactorily preserved the binding activity upon solubilization. Specific binding activity was only solubilized from cell lines and tissue preparations known to express the GRP receptor. The dissociation constant (Kd) for the receptor solubilized from rat brain and Swiss 3T3 cells was 0.6 nM, similar to the value of 0.8 nM calculated for the membrane-bound receptor. Binding was saturable and reached equilibrium after approximately 2 h at 4 degrees C. The identity of the solubilized receptor with the membrane-bound one was further confirmed by the concordance of the relative binding affinities of various established bombesin analogues.

Localization of receptors for bombesin-like peptides in the rat brain

BN-like peptides and receptors are present in discrete areas of the mammalian brain. By radioimmunoassay, endogenous BN/GRP, neuromedin B, and ranatensin-like peptides are present in the rat brain. High-to-moderate concentrations of BN/GRP are present in the rat hypothalamus and thalamus, whereas moderate-to-high densities of neuromedin B and ranatensin-like peptides are present in the olfactory bulb and hippocampus, as well as in the hypothalamus and thalamus. While the distribution of neuromedin B and ranatensin-like peptides appears similar, it is distinct from that of BN/GRP. When released from CNS neurons, these peptides may interact with receptors for BN-like peptides. BN, GRP, ranatensin, and neuromedin B inhibit specific [125I-Tyr4]BN binding with high affinity. By use of in vitro autoradiographic techniques to detect binding of [125I-Tyr4]BN to receptors for BN-like peptides, high grain densities were found in the olfactory bulb and tubercle, the nucleus accumbens, the suprachiasmatic and paraventricular nucleus of the hypothalamus, the central medial and paraventricular thalamic nuclei, the hippocampus, the dentate gyrus, and the amygdala of the rat brain. Some of these receptors may be biologically active and mediate the biological effects of BN-like peptides. For example, when BN is directly injected into the nucleus accumbens, pronounced grooming results and the effects caused by BN are reversed by spantide and [D-Phe12]BN. Thus, the putative BN receptor antagonists may serve as useful agents to investigate the biological significance of BN-like peptides in the CNS.

Bombesin-like peptides stimulate phosphatidylinositol turnover in rat brain slices

The ability of bombesin (BN)-like peptides to stimulate phosphatidylinositol turnover in rat brain slices was investigated. BN (1 microM) significantly stimulated inositol-1-phosphate (IP1) but not inositol-4,5-biphosphate (IP2) or inositol-1,4,5-trisphosphate (IP3) production using frontal cortex slices in the presence of LiCl (7.5 mM); BN had no effect on cAMP or cGMP levels. BN and the structurally-related gastrin releasing peptide (GRP) elevated IP1 levels in a dose-dependent manner. Similarly, nanomolar concentrations of the GRP fragment (Ac-GRP20-27) significantly elevated IP1 levels, whereas micromolar concentrations of the inactive GRP1-16 did not. BN significantly elevated IP1 levels in those brain regions enriched in BN receptors such as the olfactory bulb, hippocampus, striatum, thalamus and frontal cortex, whereas IP1 levels were not significantly increased in areas which have a low density of BN receptors such as the cerebellum, medulla/pons and midbrain. These data suggest that CNS BN receptors may utilize phosphatidylinositol as a second messenger.

(D-Phe12) bombesin and substance P analogues function as central bombesin receptor antagonists

The potency of synthetic bombesin (BN) analogues with D-Phe12 substitutions and substance P analogues was investigated in the rat CNS. (D-Phe12,Leu14)BN, (D-Phe12)BN and (Tyr4,D-Phe12)BN inhibited binding to rat brain slices with IC50 values of approximately 2 microM. Similarly, spantide inhibited binding to rat brain slices with an IC50 value of 1.5 microM. Spantide inhibited specific (125I-Tyr4)BN binding as a result of decreased rate of association, whereas the rate of dissociation was unaffected. Neither the (D-Phe12)BN analogues nor the substance P analogues inhibited specific binding of 125I-VIP to rat brain slices. Central administration of BN (0.5 micrograms) induced grooming and suppressed feeding and resting. (Tyr4, D-Phe12)BN (5 micrograms) antagonized the behavioral effects of BN. Although spantide (2 micrograms) also antagonized many of the BN effects, it had intrinsic effects and hence the behavioral antagonism was not specific. These data suggest that although both (D-Phe12)BN and substance P analogues may function as central BN receptor antagonists, the (D-Phe12)BN analogues may be functionally the more useful class of antagonists.

Peptide transmitters of primary sensory neurons: similar actions of tachykinins and bombesin-like peptides

Previous studies have demonstrated that two peptides, substance P (SP) and substance K (SK), are contained in a common prohormone--beta-preprotachykinin. Both peptides are cleaved from the prohormone and appear to coexist throughout the brain. This study evaluated the behavioral activity of SK and compared it to the activities of SP, bombesin (BN), and structurally related peptides. After intraspinal injection, all of the peptides induced "bite/scratch" behaviors, which differed in durations of action. The specific rank order of these durations of action were: BN greater than gastrin releasing peptide (GRP) = ranatensin (RT) greater than neuromedin B (NMB) greater than kassinin (KASS) = SK = SP and ranged from dose-dependent maxima of approximately 2 min (SP) to approximately 100 min (BN). To examine the possibility that differences in durations of action are due to differences in rates of proteolytic degradation, each peptide was incubated in spinal cord homogenates at 37 degrees C, and the degradation rates were monitored by radioimmunoassay (RIA) and by bioassay. The lengths of incubation time required to produce approximately 90% degradation of peptide immunoreactivity varied across peptides from less than 5 min (SP) to more than 60 min (BN and RT). Degradation of bioactivity generally paralleled degradation of immunoreactivity. The results of this study suggest that durations of nociceptive effects produced by the peptides tested are due, in part, to their resistance to proteolytic degradation.

Neuromedin B-like peptides in rat brain: biochemical characterization, mechanism of release and localization in synaptosomes

Neuromedin B-like peptides were characterized in the rat brain. A rabbit antisera was utilized which recognized neuromedin B but not bombesin or GRP. Using gel filtration and HPLC techniques, a major and minor peak of immunoreactivity was present in rat brain extracts. In both cases the main peak of immunoreactivity coeluted with synthetic neuromedin B. The density of neuromedin B-like peptides ranged 50-fold being greatest in the olfactory bulb and hypothalamus, intermediate in the hippocampus, spinal cord, medulla/pons, pituitary, midbrain, thalamus, striatum and cortex and lowest in the cerebellum. Release studies indicated that neuromedin B-like peptides were secreted from hypothalamic, olfactory bulb and thalamic slices in a Ca++-dependent manner when KCl (75 mM) was present. Also, the neuromedin B-like peptides in the rat brain were localized to synaptosomes. These data indicate that neuromedin B-like peptides may function as regulatory peptides in the CNS distinct from bombesin/GRP.

Intrathecal FMRFamide (Phe-Met-Arg-Phe-NH2) induces excessive grooming behavior in mice

The molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) was administered intrathecally (i.t.) to mice and their behavior was monitored for 30 min. FMRFamide induced a dramatic and dose-related (5-12 micrograms) increase in grooming-related activities compared to saline-treated controls. The grooming behavior produced by 8 micrograms FMRFamide was not blocked by simultaneous i.t. administration of 10 micrograms of the following antagonists: atropine, phentolamine, methysergide, naloxone or spantide; peripheral administration of naloxone (3.5 mg/kg, s.c.) also failed to antagonize FMRFamide grooming. These data constitute the first report that FMRFamide produces behavioral changes in mammals.

Receptors for GRP/bombesin-like peptides in the rat forebrain

Binding sites in the rat forebrain were characterized using (125I-Tyr4)bombesin as a receptor probe. Pharmacology experiments indicate that gastrin releasing peptide (GRP) and the GRP fragments GRP as well as Ac-GRP inhibited radiolabeled (Tyr4)bombesin binding with high affinity. Biochemistry experiments indicated that heat, N-ethyl maleimide or trypsin greatly reduced radiolabeled (Tyr4)bombesin binding. Also, autoradiographic studies indicated that highest grain densities were present in the stria terminalis, periventricular and suprachiasmatic nucleus of the hypothalamus, dorsomedial and rhomboid thalamus, dentate gyrus, hippocampus and medial amygdaloid nucleus. The data suggest that CNS protein receptors, which are discretely distributed in the rat forebrain, may mediate the action of endogenous GRP/bombesin-like peptides.

The antagonism of bombesin in the CNS by substance P analogues

The ability of substance P analogues to inhibit the action of bombesin in the CNS was investigated using receptor binding and biological assays. The putative substance P antagonists inhibited binding to central receptors for both substance P and bombesin-like peptides. Spantide, which was the most potent analogue tested, reversed the bombesin induced hypothermia and grooming. Therefore the putative substance P antagonists may also antagonize the actions of bombesin in the CNS.

Studies on bombesin-induced grooming in rats

The gross behavior induced by centrally administered bombesin in rats was compared to that elicited by ACTH-(1-24) and the somatostatin analog, des AA1,2,4,5,12,13[D-Trp8]-somatostatin (ODT8-SS). Bombesin (0.001-1 microgram, ICV) caused dose-related excessive scratching which was qualitatively different from that associated with the other two groom-inducing agents. Bombesin-induced grooming was not markedly affected by behaviorally nondepressant doses of haloperidol, morphine, naloxone or neurotensin. Bombesin was active in genetically hypotrichotic (essentially furless) rats; and, again in such animals, even after numbing the area caudal to the shoulders with lidocaine. Tolerance and cross-tolerance studies with bombesin and ODT8-SS indicated that they produce scratching through different mechanisms. Bombesin caused scratching when injected directly into the periaqueductal gray, but not when administered intravenously. Neither hypophysectomy nor adrenalectomy markedly affected bombesin-induced grooming. This behavior appears to be initiated in the central nervous system and is produced independently of the pituitary-adrenal axis.

Intrathecal bombesin in rats: effects on behaviour and gastrointestinal transit

When bombesin is given intracerebroventricularly to rats, it is known to cause excessive scratching and inhibit gastrointestinal transit. We have administered bombesin via a permanent indwelling cannula into the subarachnoid space of the lumbar spinal cord of rats. By this route, bombesin elicited immediate excessive scratching and rapidly inhibited passage of a charcoal meal along the gastrointestinal tract. The A50 values for these effects were 0.004 (0.001-0.018) micrograms/rat and 0.34 (0.22-0.55) micrograms/rat, respectively. Bombesin-induced scratching and inhibition of transit are therefore mediated at spinal, as well as supraspinal, levels.

An animal model for preclinical screening of systemic antipruritic agents

Reliable antipruritic agents that can be given systemically are not available at present. This may be due to the lack of animal models for screening such compounds. Bombesin, a tetradecapeptide originally isolated from frog skin, induces dose-related excessive scratching when administered intracerebroventricularly (i.c.v.) to rats. With the help of a microcomputer, we monitored the scratching elicited by a standard, submaximal dose of bombesin (0.10 microgram, i.c.v.). This system provides 1) a sensitive and novel way of assessing drug-induced behavioral depression, and 2) a means of quantifying interactions between bombesin and possible antagonists. Thus, bombesin-induced grooming is antagonized by behaviorally nondepressant doses of methdilazine, trimeprazine, and chlorpromazine but not by morphine, haloperidol, diphenhydramine, hydroxyzine, mepyramine, cimetidine, or cyproheptadine. Methdilazine and trimeprazine are used clinically as antipruritic agents. The model therefore offers a means of evaluating new, systemic antipruritic agents, particularly those which may be active in treating histamine-independent pruritus.

ACTH-(1-24) and RX 336-M induce excessive grooming in rats through different mechanisms

ACTH-(1-24) (0.03-6 micrograms i.c.v.) and RX 336-M (7,8-dihydro-5',6'-dimethylcyclohex-5'-eno-1',2',8',14 codeinone) (1.5-6 mg/kg i.p.) induce dose-related excessive grooming and 'wet-dog' shaking in rats. In the present study, the grooming associated with these compounds was compared and analyzed pharmacologically. Grooming caused by RX 336-M and by ACTH-(1-24) was antagonized when rats were pretreated with comparable doses of morphine (0.5-4 mg/kg s.c.), however, only ACTH-(1-24)-induced grooming was attenuated by naloxone (1 and 10 mg/kg s.c.). ICI 154,129 (N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH) (30 mg/kg s.c.), a selective delta-opiate receptor antagonist, was ineffective against both ACTH-(1-24) and RX 336-M. Although haloperidol is known to antagonize grooming elicited by ACTH-(1-24) (e.g., Wiegant et al., 1977, European J. Pharmacol. 41, 343), even a high dose of this neuroleptic agent (5 mg/kg s.c.) only partially attenuated grooming caused by RX 336-M. Tolerance developed to the grooming elicited by RX 336-M, and by ACTH-(1-24), but there was no cross-tolerance. Both agents were active in genetically hypotrichotic rats; and, again in such animals, even after numbing the area caudal to the shoulders with lidocaine. Given the divergent results with naloxone, and, possibly, with haloperidol, and the lack of cross-tolerance, we conclude that the excessive grooming induced in rats by ACTH-(1-24) and by RX 336-M is mediated by different mechanisms.