5-HT3 receptor antagonists ameliorate emesis in the ferret evoked by neutron or proton radiation

BACKGROUND

Nausea and vomiting produced by sub-lethal doses of X- or gamma-rays can be ameliorated by serotonin subtype-three (5-hydroxytryptamine; 5-HT3) receptor antagonists. The effectiveness of these anti-emetics on blocking the emetic responses induced by fission neutron or proton radiation exposure was evaluated in the ferret animal model.

HYPOTHESIS

5-HT3 receptor antagonists or bilateral vagotomy will ameliorate that emesis evoked by fission neutrons or protons.

METHODS

Groups of ferrets were exposed to whole-body or head-shielded radiations of varying qualities: fission spectrum neutons, high-energy protons, or gamma-rays. Prior to that exposure, some groups were either vagotomized or received subcutaneous (s.c.) or oral (p.o.) treatment with various doses of the 5-HT3 receptor antagonist antiemetics eusatron and ondansetron.

RESULTS

We demonstrated that both eusatron and ondansetron effectively abolished the emesis normally induced by 2-Gy doses of either 60Co gamma or neutron:gamma, mixed-field irradiation, the latter with a neutron-to-total dose ratio (Dn/Dt) of 0.9+/-2% (%SD). Different routes of delivery of the anti-emetics yielded different degrees of inhibition of the emetic responses; p.o. treatment was less efficacious than s.c. treatment for the emesis to fission neutrons. Eusatron was significantly more effective than ondansetron on a mg x kg(-1) basis. Bilateral vagotomy also attenuated or abolished the emetic responses to the mixed-field neutron exposures. Furthermore, emesis induced by exposure to 2.5 Gy of 200-MeV protons was effectively abolished by ondansetron.

CONCLUSION

These results are consistent with the concept that similar physiological and pharmacological mechanisms underlie the emetic responses to different qualities of radiation.

Modulation of amphetamine-stimulated [3H]dopamine release from rat pheochromocytoma (PC12) cells by sigma type 2 receptors

An important regulatory mechanism of synaptic dopamine (DA) levels is activation of the dopamine transporter (DAT), which is a target for many drugs of abuse, including amphetamine (AMPH). sigma receptors are located in dopaminergic brain areas critical to reinforcement. We found previously that agonists at sigma2 receptors enhanced the AMPH-stimulated release of [3H]DA from slices of rat caudate-putamen. In the present study, we modeled this response in undifferentiated pheochromocytoma-12 (PC12) cells, which contain both the DAT and sigma2 receptors but not neural networks that can complicate investigation of individual neuronal mechanisms. We found that enhancement of AMPH-stimulated [3H]DA release by the sigma agonist (+)-pentazocine was blocked by sigma2 receptor antagonists. Additionally, the reduction in the effect of (+)-pentazocine by the inclusion of ethylene glycol bis(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid led us to hypothesize that sigma2 receptor activation initiated a Ca2+-dependent process that resulted in enhancing the outward flow of DA via the DAT. The source of Ca2+ required for the enhancement of reverse transport did not appear to be via N- or L-type voltage-dependent Ca2+ channels, because it was not affected by nitrendipine or omega-conotoxin. However, two inhibitors of Ca2+/calmodulin-dependent protein kinase II blocked enhancement in AMPH-stimulated release by (+)-pentazocine. Our findings suggest that sigma2 receptors are coupled to the DAT via a Ca2+/calmodulin-dependent protein kinase II transduction system in PC12 cells, and that sigma2 receptor antagonists might be useful in the treatment of drug abuse by blocking elevation of DA levels via reversal of the DAT.

Regulation of [3H]norepinephrine release from guinea pig hippocampus by sigma2 receptors

The binding profile of the sigma2 receptor ligand endo-N-(8-methyl-8-azabicyclo[3.2.1.]oct-3-yl)-2,3-dihydro-(1-methyl)eth yl-2-oxo-1H-benzimidazole-1-carboxamidehydrochloride (BIMU-8) had previously been determined, but its agonist/antagonist status at sigma2 receptors had not been identified. We therefore investigated the effects of BIMU-8 for its ability to regulate the stimulated release of [3H]norepinephrine from slices of guinea pig hippocampus. BIMU-8 alone, at a concentration chosen to occupy 50% of sigma2 receptors, had no significant effect on N-methyl-D-aspartate (NMDA)-stimulated release of [3H]norepinephrine. We have shown previously that the sigma receptor agonist (+)-pentazocine inhibits NMDA-stimulated release in a concentration-dependent manner, producing a biphasic inhibition curve. Similarly, the sigma receptor agonist 1S,2R-(-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl )cyclohexylamine (BD737) produced a broad inhibition curve. The inhibition by low concentrations of (+)-pentazocine or BD737 that selectively activated sigma1 receptors was reversed by the sigma1-selective receptor antagonist (1-(cyclopropylmethyl)-4-2'-oxoethyl)piperidine HBr (DuP 734). In the current study, when the sigma1 component of inhibition by (+)-pentazocine was blocked by DuP 734, the remaining component of inhibition mediated by sigma2 receptors was reversed by BIMU-8. Our results suggest that (1) BIMU-8 is an antagonist at sigma2 receptors and that (2) sigma2 receptors contribute to regulation of norepinephrine release in guinea pig hippocampus.

Neurotensin, N-acetyl-aspartylglutamate and beta-endorphin modulate [3H]dopamine release from guinea pig nucleus accumbens, prefrontal cortex and caudate-putamen

Dopaminergic hyperactivity in nucleus accumbens and dopaminergic hypoactivity in prefrontal cortex are thought to underlie positive and negative symptoms of schizophrenia, respectively. The caudate putamen is the neuroanatomical substrate for extrapyramidal side effects resulting from chronic antipsychotic treatment. We sought to identify potential endogenous regulators of dopamine release that might produce differential effects in these brain areas. We tested neurotensin, N-acetyl-aspartyl-glutamate and beta-endorphin for potential regulation of [3H]dopamine release in these regions of guinea pig brain. All three peptides stimulated dopamine release, above basal activity, at all concentrations tested in the three regions. Neurotensin significantly enhanced and N-acetyl-aspartyl-glutamate had no significant effect on N-methyl-D-aspartate-stimulated release from all three regions. In contrast, beta-endorphin significantly inhibited N-methyl-D-aspartate-stimulated release in nucleus accumbens and caudate putamen. These results suggest that these neuropeptides may regulate endogenous dopamine release and therefore may be potential therapeutic targets for antipsychotic drug development.

Regulation of [3H] dopamine release from mesolimbic and mesocortical areas of guinea pig brain by sigma receptors

The role of sigma (sigma) receptors in brain function is poorly defined. They are located in limbic areas, including nucleus accumbens (NAC) and prefrontal cortex (PFC), both of which are thought to be involved in schizophrenia. Many antipsychotics (APs), including haloperidol, bind with high affinity to sigma receptors. Dopaminergic hyperactivity in NAC is thought to underlie positive symptoms of schizophrenia, while dopaminergic hypoactivity in PFC is thought to underlie negative symptoms. Sigma receptors regulate N-methyl-D-aspartate (NMDA)-stimulated [3H] dopamine ([3H]DA) release in caudate-putamen (CP), the neuroanatomical substrate for extrapyramidal side effects resulting from chronic AP treatment. In the current study, we investigated whether sigma receptors could similarly regulate DA release in mesolimbic and mesocortical tissue, and the relative participation of different sigma receptor subtypes in this process. We found that, in NAC, regulation of DA release by the prototypical sigma agonist (+)pentazocine was mediated predominantly by the sigma 1 receptor, whereas in the PFC a portion of the (+)pentazocine effect was likely mediated by the sigma 2 receptor. We also observed, in both the NAC and PFC, that regulation of DA release by the sigma agonist BD737 was mediated primarily by the sigma 1 receptor. In addition, we determined that (+)pentazocine or BD737 effects on DA release were not mediated via opioid receptors, nor the phencyclidine (PCP) binding site within the NMDA receptor-operated cation channel, nor by sigma receptor effects upon [3H]DA accumulated by noradrenergic terminals in PFC.

5-Hydroxytryptamine 1a receptor agonists block prepulse inhibition of acoustic startle reflex

Presentation of a nonstartling stimulus (prepulse) 100 msec before a startle-eliciting auditory stimulus (pulse) reduces startle reflex amplitude in mammals. Prepulse inhibition of acoustic startle reflex is smaller in schizophrenics than in nonschizophrenics, a phenomenon that has been hypothesized to reflect sensorimotor gating deficits underlying schizophrenic psychosis. Five 5-hydroxytryptamine1a (5-HT1a, serotonin) receptor agonists: 8-hydroxy-2-(di-n-propylamino) tetraline (8-OHDPAT), 5-methoxydimethyltryptamine, buspirone, gepirone and ipsapirone, were tested for effects on prepulse inhibition and startle reflex amplitude in rats. All five agents reduced prepulse inhibition at doses that had no effect on startle reflex amplitude or motor activity. Reduction of prepulse inhibition by 8-OHDPAT was antagonized by (-)propranolol, a 5-HT1a receptor antagonist, and partially by haloperidol, a dopamine D2 receptor antagonist, but not by ketanserin or methysergide, 5-HT2 receptor antagonists. 8-OHDPAT did not reduce prepulse inhibition in subjects pretreated with reserpine or tetrabenazine to deplete neuronal amines, but interpretation of this result is complicated because reserpine and tetrabenazine given alone reduced prepulse inhibition. The results indicate that 5-HT1a receptor agonists block prepulse inhibition of acoustic startle reflex, possibly via dopaminergic mechanisms.