Differential Morphological and Biochemical Recovery from Chemotherapy-Induced Peripheral Neuropathy Following Paclitaxel, Ixabepilone, or Eribulin Treatment in Mouse Sciatic Nerves

The reversibility of chemotherapy-induced peripheral neuropathy (CIPN), a disabling and potentially permanent side effect of microtubule-targeting agents (MTAs), is becoming an increasingly important issue as treatment outcomes improve. The molecular mechanisms regulating the variability in time to onset, severity, and time to recovery from CIPN between the common MTAs paclitaxel and eribulin are unknown. Previously (Benbow et al. in Neurotox Res 29:299-313, 2016), we found that after 2 weeks of a maximum tolerated dose (MTD) in mice, paclitaxel treatment resulted in severe reductions in axon area density, higher frequency of myelin abnormalities, and increased numbers of Schwann cell nuclei in sciatic nerves. Biochemically, eribulin induced greater microtubule-stabilizing effects than paclitaxel. Here, we extended these comparative MTD studies to assess the recovery from these short-term effects of paclitaxel, eribulin, and a third MTA, ixabepilone, over the course of 6 months. Paclitaxel induced a persistent reduction in axon area density over the entire 6-month recovery period, unlike ixabepilone- or eribulin-treated animals. The abundance of myelin abnormalities rapidly declined after cessation of all drugs but recovered most slowly after paclitaxel treatment. Paclitaxel- and ixabepilone- but not eribulin-treated animals exhibited increased Schwann cell numbers during the recovery period. Tubulin composition and biochemistry rapidly returned from MTD-induced levels of α-tubulin, acetylated α-tubulin, and end-binding protein 1 to control levels following cessation of drug treatment. Taken together, sciatic nerve axons recovered more rapidly from morphological effects in eribulin- and ixabepilone-treated animals than in paclitaxel-treated animals and drug-induced increases in protein expression levels following paclitaxel and eribulin treatment were relatively transient.

The role of glutamate signaling in pain processes and its regulation by GCP II inhibition

Glutamate is the predominant excitatory neurotransmitter used by primary afferent synapses and neurons in the spinal cord dorsal horn. Glutamate and glutamate receptors are also located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Not surprisingly, glutamate receptors have been an attractive target for new pain therapies. However, their widespread distribution and array of function has often resulted in drugs targeting these sites having undesirable side-effects. This chapter will review, in general terms, the current knowledge of glutamate and its effects at various glutamate receptors with regards to nociception. In addition, we will briefly review the glutamatergic drugs currently in use as treatments for pain, as well as known novel candidates in various stages of clinical trial. Lastly, we will summarize the data supporting a novel target for pain intervention by way of GCPII inhibition, which appears devoid of the side-effects associated with direct glutamate receptor antagonists and thus holds major promise for future therapy. GCPII (glutamate carboxypeptidase II) cleaves the prevalent neuropeptide NAAG into NAA and glutamate and there is widespread evidence and belief that targeting the glutamate derived from this enzymatic action may be a promising therapeutic route.

The preventive and therapeutic effects of GCPII (NAALADase) inhibition on painful and sensory diabetic neuropathy

Excitotoxic glutamate release occurs in several neurological disorders. One source is derived from the hydrolysis of the neuropeptide N-acetyl aspartyl glutamate (NAAG) by glutamate carboxypeptidase II (GCPII, also known as NAALADase). Drugs that attenuate glutamate transmission have been shown to relieve neuropathic pain, however side effects have limited their clinical use. It appears that GCPII is exclusively recruited to provide a glutamate source in hyperglutamatergic, excitotoxic conditions and therefore would be devoid of such side effects. Here we report on the therapeutic effects of an orally bio-available GCP II inhibitor on established painful and sensory neuropathy in the spontaneously diabetic BB/Wor rat. It significantly improved hyperalgesia, nerve conduction velocity and underlying myelinated fiber atrophy. The data suggest that GCP II inhibition may provide a meaningful and effective approach to the treatment of painful diabetic neuropathy.

GCPII (NAALADase) inhibition prevents long-term diabetic neuropathy in type 1 diabetic BB/Wor rats

AIMS/HYPOTHESIS

Hyperglutamatergic activity induced by ischemia is believed to underlie neuronal damage in a variety of neurological disorders, including neuropathic pain. Since ischemia is believed to be a prominent mechanism involved in diabetic polyneuropathy (DPN), we investigated the effect of the glutamate carboxypeptidase II (GCPII, EC #3.4-17.21; previously termed NAALADase), an enzyme responsible for the hydrolysis of the neuropeptide NAAG to NAA and glutamate, on the development of DPN in type 1 diabetic BB/Wor rats.

METHODS

Diabetic animals were treated with 10 mg/kg/day i.p. of the selective GCPII inhibitor GPI-5232 from onset of diabetes for 6 months. Hyperalgesia to thermal stimulation and nerve conduction velocity (NCV) were measured monthly. The effect on structural DPN was assessed by scoring of single, teased myelinated fibers, myelinated fiber morphometry and ultrastructural examination of C-fibers at 6 months.

RESULTS

GCPII inhibition showed significant but partial effects on hyperalgesia (p<0.001), nerve conduction slowing (p<0.01) axonal and nodal structural changes (p<0.001), small myelinated fiber atrophy, and degenerative changes of C-fibers.

CONCLUSIONS

GCPII inhibition has beneficial effects on hyperalgesia, nerve function, and structural degenerative changes in DPN, which are likely mediated by inhibition of ischemia-induced glutamate release.

Selective inhibition of NAALADase, which converts NAAG to glutamate, reduces ischemic brain injury

We describe here a new strategy for the treatment of stroke, through the inhibition of NAALADase (N-acetylated-alpha-linked-acidic dipeptidase), an enzyme responsible for the hydrolysis of the neuropeptide NAAG (N-acetyl-aspartyl-glutamate) to N-acetyl-aspartate and glutamate. We demonstrate that the newly described NAALADase inhibitor 2-PMPA (2-(phosphonomethyl)pentanedioic acid) robustly protects against ischemic injury in a neuronal culture model of stroke and in rats after transient middle cerebral artery occlusion. Consistent with inhibition of NAALADase, we show that 2-PMPA increases NAAG and attenuates the ischemia-induced rise in glutamate. Both effects could contribute to neuroprotection. These data indicate that NAALADase inhibition may have use in neurological disorders in which excessive excitatory amino acid transmission is pathogenic.

The role of striatal dopaminergic mechanisms in rotational behavior induced by phencyclidine and phencyclidine-like drugs

Phencyclidine (PCP) and phencyclidine-like drugs (TCP, dexoxadrol, MK-801, and SKF 10,047) were evaluated for their ability to induce rotational behavior in rats with unilateral 6-OHDA lesions of the medial forebrain bundle and for their ability to alter striatal dopamine (DA) overflow with microdialysis procedures. All of the compounds tested produced rotational behavior ipsilateral to the lesion, suggesting that they were enhancing extracellular dopamine in the intact striatum. The microdialysis studies, however, did not support this contention. There appeared to be a complete dissociation between the ability of the five compounds to produce ipsilateral rotations and their ability to enhance extracellular dopamine levels in the striatum. PCP was the only compound able to elicit significant increases in striatal dopamine overflow following i.p. injections and also produce dramatic rotational behavior. MK-801 was the most potent compound in enhancing rotational output while it had no effect at all on striatal dopamine overflow. Dexoxadrol also produced significant rotational output without having any effect on extracellular levels of dopamine following i.p. injections. TCP and SKF 10,047, at doses which produced significant rotational behavior, only elevated dopamine 16% and 12%, respectively, at peak effect. It is most parsimonious to conclude that the effects of PCP-like drugs on nigro-striatal function are mediated through their ability to act as indirect NMDA receptor antagonists and not through their ability to alter striatal dopamine activity.

Evidence that m-chlorophenylpiperazine-induced hyperthermia in rats is mediated by stimulation of 5-HT2C receptors

Intraperitoneal administration of m-chlorophenylpiperazine (m-CPP) to Wistar rats produced hyperthermia with a peak effect at 30 min. Pretreatment with low doses of metergoline (5-HT1/5-HT2 antagonist), mesulergine and mianserin (5-HT2C/5-HT2A antagonists) blocked m-CPP-induced hyperthermia. Pretreatment with propranolol (beta-adrenergic receptor antagonist that also has binding affinity for 5-HT1A, 5-HT1B and 5-HT2B sites), yohimbine (alpha 2-noradrenergic antagonist that also has binding affinity for 5-HT2B sites), MDL-72222 or ondansetron (5-HT3 antagonists) did not attenuate m-CPP-induced hyperthermia. Only high doses of ketanserin, LY-53857 and ritanserin (5-HT2A/5-HT2C antagonists) as well as spiperone (5-HT1A/5-HT2A/D2 antagonist) attenuated m-CPP-induced hyperthermia. Daily administration of m-CPP produced complete tolerance to its hyperthermic effect by day 5. However, there was no cross-tolerance to 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, a 5-HT2A agonist that also has high affinity for 5-HT2C receptors)-induced hyperthermia. m-CPP-induced increases in temperature were found to be significantly less in the Fawn-Hooded (FH) rat strain as compared to the Wistar rat strain; in prior studies, FH rats have been found to be subsensitive to other 5-HT2C-mediated pharmacologic responses. Altogether, these findings suggest that m-CPP-induced hyperthermia in rats is mediated by selective stimulation of 5-HT2C receptors.

Evidence that 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced hyperthermia in rats is mediated by stimulation of 5-HT2A receptors

The effects of various 5-HT receptor subtype-selective antagonists were studied on phenylisopropylamine hallucinogen 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced hyperthermia in Wistar rats, in an attempt to characterize the 5-HT receptor subtype mediating DOI-induced hyperthermia. Intraperitoneal administration of DOI to rats produced hyperthermia with a peak effect at 60 min. Pretreatment with propranolol (beta-adrenoceptor antagonist that also has binding affinity for 5-HT1A, 5-HT1B and 5-HT2C sites), MDL-72222 or ondansetron (5-HT3 antagonists) did not attenuate DOI-induced hyperthermia. In contrast, pretreatment with metergoline (5-HT1/5-HT2 antagonist), ketanserin, LY53857, mesulergine, mianserin and ritanserin (5-HT2C/5-HT2A antagonists), as well as spiperone (5-HT1A/5-HT2A/D2 antagonist), significantly attenuated DOI-induced hyperthermia. Furthermore, daily administration of DOI (2.5 mg/kg per day) for 17 days did not produce either tolerance to its hyperthermic effect or modify m-CPP-induced hyperthermia in rats. These findings suggest that DOI-induced hyperthermia in rats is mediated by stimulation of 5-HT2A receptors.

Functional and biochemical evidence for altered serotonergic function in the fawn-hooded rat strain

Administration of various doses of DOI (a 5-HT2A/5-HT2C agonist) produced hyperthermia that was significantly less in the FH rat strain relative to the Wistar rat strain. Similarly, administration of various doses of ipsapirone (a 5-HT1A agonist) produced hypothermia that was significantly less in the FH rat strain relative to the Wistar rat strain. Furthermore, m-CPP (a 5-HT agonist)-induced increases in growth hormone levels were also significantly less in the FH rat strain relative to the Wistar rat strain. There was no significant difference in the levels of either 5-HT or 5-HIAA between the two rat strains in the frontal cortex, hippocampus, hypothalamus, and striatum. In the brain stem, however, both 5-HT and 5-HIAA levels were significantly lower in the FH rat strain relative to the Wistar rat strain. On the other hand, 5-HT turnover rate was significantly higher in the hypothalamus and striatum and significantly lower in the hippocampus in the FH rat strain relative to the Wistar rat strain. These findings provide further evidence for altered serotonergic function in the FH rat strain and, in addition, suggest that the FH rat strain may prove to be a useful genetic model for some neuropsychiatric disorders with possible abnormalities in serotonergic function such as depression, obsessive-compulsive disorder, and the eating disorders.

Evidence that 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane-induced hypophagia and hyperthermia in rats is mediated by serotonin-2A receptors

The administration of various doses of the phenylisopropylamine hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) to rats produced dose-related decreases in 1-hr food intake in a food-restricted paradigm and in locomotor activity. DOM also produced dose-related increases in temperature. Pretreatment with propranolol [a beta adrenoceptor antagonist that also has high binding affinity for serotonin (5-HT) 5-HT1A, 5-HT1B and 5-HT2C sites], bemesetron or ondansetron (5-HT3 antagonists) did not attenuate either DOM-induced hypophagia or hyperthermia. In contrast, pretreatment with metergoline (a 5-HT1/5-HT2 antagonist) and ritanserin (a 5-HT2A/5-HT2C antagonist) significantly attenuated both DOM-induced hypophagia and hyperthermia. However, pretreatment with mesulergine (a 5-HT2C/5-HT2A antagonist) significantly attenuated DOM-induced hyperthermia but not hypophagia. On the other hand, spiperone (5-HT1A/5-HT2A/D2 antagonist) pretreatment significantly attenuated DOM-induced hyperthermia but accentuated DOM-induced hypophagia. Daily administration of DOM (1.0 mg kg-1 day-1) produced complete tolerance to its hypophagic effect by day 4 but did not produce cross-tolerance to m-chlorophenylpiperazine-induced hypophagia. In contrast, daily administration of DOM for 7 days did not produce either tolerance to its hyperthermic effect or modify m-chlorophenylpiperazine-induced hyperthermia in rats. These findings suggest that DOM-induced hypophagia and hyperthermia in rats are mediated by stimulation of 5-HT2a receptors.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

Differential effects of antidepressant treatments on fenfluramine-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of 5-HT releasing agent, fenfluramine, to rats produced increases in plasma prolactin and corticosterone concentrations. Short-term or long-term treatment with either clorgyline or imipramine did not affect baseline levels of prolactin or corticosterone. On the other hand, short-term but not long-term lithium treatment significantly increased baseline levels of corticosterone but not of prolactin. Short-term treatment with lithium but not clorgyline or imipramine potentiated fenfluramine-induced increases in plasma prolactin but not corticosterone. On the other hand, long-term treatment with clorgyline but not imipramine or lithium attenuated fenfluramine's effect on plasma prolactin but not on corticosterone. These findings demonstrate differential effects of antidepressant treatments on fenfluramine-induced increases in plasma prolactin and corticosterone in rats and are consistent with several other clinical and animal studies demonstrating dissimilar actions of different antidepressant treatments on two different 5-HT-mediated neuroendocrine functions.

Is NAN-190 an effective antagonist of the hypothermia and hyperglycemia induced by the 5-HT1A receptor agonist, 8-OH-DPAT?

The putative 5-HT1A receptor antagonist properties of 1-(2-methoxyphenyl)-4-[4-(2-phtalimmido)butyl] piperazine (NAN-190) were studied in mice. The responses studied were hypothermia- and hyperglycemia-induced by the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). NAN-190 (0.3-3 mg/kg) did not antagonize either response, but rather appeared to be additive with the effect produced by 8-OH-DPAT (0.25 mg/kg) alone, at least with respect to temperature. NAN-190, given alone in similar doses, caused hypothermia and hyperglycemia. These results suggest that NAN-190 has similar properties to 8-OH-DPAT with regard to temperature and glucose effects. Therefore, it does not appear to be a effective antagonist for all 5-HT1A-mediated responses.

Focal application of alcohols elevates extracellular dopamine in rat brain: a microdialysis study

Dopaminergic systems are thought to play a major role in the stimulant and reinforcing properties of drugs of abuse, including ethanol. The present study describes the effects of local perfusion with ethanol (and other alcohols) on extracellular dopamine in the striatum and nucleus accumbens. Following the establishment of basal dopamine levels (2-3 h), various concentrations of ethanol in artificial CSF (0.01-10% v/v) were slowly perfused through a microdialysis probe. Each dose of ethanol was found to increase dopamine concentrations in both the striatum and nucleus accumbens. This increase was dose-related in the striatum. The exclusion of calcium and inclusion of 12.5 mM magnesium in the perfusion medium prevented, or greatly attenuated the ethanol-induced dopamine (DA) release. Thus, the release of DA by ethanol is exocytotic in nature and involves calcium-dependent processes. The other alcohols tested, namely methanol and butanol, demonstrated a structure-activity relationship together with ethanol, in their ability to increase extracellular DA. The relative potency was butanol greater than ethanol greater than methanol. The diffusion of ethanol into the brain tissue was investigated following perfusion through the probe. Relatively low concentrations of ethanol were found in striatal tissue during perfusion and they declined rapidly with time, following the removal of ethanol from the perfusate. The concentrations of ethanol achieved in brain tissue following focal application through the microdialysis probe were relevant to human intoxication.

Modulation of the hypothermic and hyperglycaemic effects of 8-OH-DPAT by alpha 2-adrenoceptor antagonists

1. The effects of pretreatment with two novel and relatively specific alpha 2-adrenoceptor antagonists on the hypothermic and hyperglycaemic responses induced by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) were investigated in mice. The alpha 2-adrenoceptor antagonists used were, atipamezole, which occupies both central and peripheral receptors, and L 659,066, which poorly penetrates the blood brain barrier. 2. Atipamezole (1 and 3 mg kg-1) alone had no effect on body temperature but significantly attenuated the 8-OH-DPAT-induced hypothermic response. The hyperglycaemic effect of 8-OH-DPAT was also attenuated by pretreatment with atipamezole; however, 3 mg kg-1 atipamezole did cause some hypoglycaemia when administered alone. 3. Pretreatment with L 659,066 (3-30 mg kg-1) failed to alter the hypothermic effects of 8-OH-DPAT. All doses of L 659,066 tested attenuated 8-OH-DPAT-induced hyperglycaemia, but the highest dose (30 mg kg-1) produced hypoglycaemia when administered alone. 4. The results suggest that the attenuation of 8-OH-DPAT-induced hypothermia by alpha 2-adrenoceptor antagonist may be centrally mediated whereas the blockade of 8-OH-DPAT-induced hyperglycaemia may involve peripheral mechanisms.

Differential effect of lithium treatment on fenfluramine-induced decreases in food intake and locomotor activity in rats

Administration of fenfluramine to rats decreased 1-h food intake and locomotor activity. Short-term (2-6 days) but not long-term (21-25 days) lithium treatment potentiated fenfluramine-induced suppression of food intake. However, neither short-term nor long-term lithium treatment had any significant effect on fenfluramine-induced suppression of locomotor activity. These findings demonstrate a differential effect of lithium treatment on fenfluramine-induced suppression of food intake and locomotor activity. Furthermore, these findings are consistent with results from several other animal and clinical studies demonstrating a differential effect of lithium treatment on two different serotonin-mediated functions.

Hyperglycemic properties of serotonin receptor antagonists

Several serotonin (5-HT) receptor antagonists with varying specificities for the 5-HT receptor types, were studied with regard to their effects on blood glucose levels in mice. The non-selective antagonists, metergoline and methysergide, proved to be hyperglycemic at doses commonly used to antagonize 5-HT receptors. In contrast, ritanserin (a 5-HT2 and 5-HT1c antagonist) and MDL 72222 (a 5-HT3 antagonist) were effective only at doses which surpassed the dose range considered to be selective for their respective receptors. The results suggest that 5-HT systems play a role in maintaining glucose homeostasis and that 5-HT1 receptors may be particularly important in this function. Furthermore, the inherent hyperglycemic properties of non-selective serotonin antagonists described here, are pertinent to studies using these agents to investigate glucose metabolism.

Antagonism of 5-HT3 receptors attenuates the effects of ethanol on extracellular dopamine

The effect of a 5-HT3 antagonist, ICS 205-930, on ethanol-induced changes in extracellular dopamine, was investigated with in vivo microdialysis. Pretreatment of rats with ICS 205-930 effectively attenuated the subsequent increases in dopamine, in both the nucleus accumbens and corpus striatum. This suggests that 5-HT may be involved in the effects of ethanol on dopaminergic systems.

Behavior of streptozotocin-diabetic mice in tests of exploration, locomotion, anxiety, depression and aggression

The present study examined behavior of streptozotocin-diabetic mice in Porsolt's swim test, a putative animal model of depression, in the holeboard test of exploration and locomotor activity, in the plus maze test of anxiety, and in the resident-intruder paradigm of aggression. Two weeks after an IP injection of 200 mg/kg streptozotocin, which caused a 20% weight loss and increased fluid consumption and urination, male NIH Swiss mice were found to show lengthened duration of immobility in the swim test. One week of insulin treatment (0.1 IU/g/day) partially antagonized this change. The locomotor activity scores in the streptozotocin-treated mice were lower in the holeboard but higher in the plus maze than in the controls; therefore, the lengthened immobility was not likely to be due to a general motor impairment. No significant changes in the time spent in social interaction or aggressive behavior were found in the streptozotocin-treated mice. The results indicate that streptozotocin-treated mice show lengthened immobility in the swim test.

Carbamazepine and electroconvulsive shock attenuate beta-adrenoceptor and muscarinic cholinoceptor coupling to G proteins in rat cortex

We have recently found that lithium attenuates the function of G proteins, suggesting these proteins as the common site for both the antimanic and antidepressant therapeutic effects of lithium. Perturbation of G protein function may thus be a general characteristic of treatments effective in bipolar affective disorder. In the present study, we demonstrate that both chronic carbamazepine and repeated electroconvulsive shock treatment inhibit the coupling of both muscarinic cholinoceptors and beta-adrenoceptors to pertussis toxin-sensitive and cholera toxin-sensitive G proteins, respectively.

Time courses of behavioral and regional cerebral metabolic responses to different doses of meta-chlorophenylpiperazine in awake rats

The time course and relation to dose of regional cerebral metabolic rates for glucose (rCMRglc) and of motor behavior were measured in awake male adult Fischer-344 rats after administration of meta-chlorophenylpiperazine (MCPP), a serotonin-1B receptor agonist. rCMRglc was determined, using the quantitative autoradiographic [14C]deoxyglucose technique, in 71 brain regions at 5, 15, 30 and 60 min after administration of MCPP 2.5 mg/kg i.p., and at 15 min after MCPP 25 and 40 mg/kg. The time course of performance on a rotating rod was measured periodically for 60 min after MCPP 2.5 mg/kg, a dose which impaired locomotion and reduced rCMRglc maximally at 15-30 min after its administration. At 15 min, rCMRglc declined significantly in 28 (40%) of the areas studied (mean decline 16%). Most regions affected were telencephalic or diencephalic, corresponding to the projection areas of serotonergic fibers arising from the raphe nuclei. After higher doses of MCPP, a behavioral serotonin syndrome was observed with both rCMRglc increases and decreases (25 mg/kg) or only rCMRglc increases (40 mg/kg). Whereas behavioral and metabolic activation induced by high doses of MCPP may result from stimulation at postsynaptic serotonin receptors, rCMRglc reductions and hypomotility produced by MCPP 2.5 mg/kg resemble the effects of serotonin receptor antagonists and suggest that, at this low dose, MCPP acts at modulatory serotonin autoreceptors to reduce endogenous serotonin release.

Attenuation of hypothermic effects of ethanol by alpha 2-adrenoceptor blockers

Effects of selective alpha 2-adrenoceptor antagonists, atipamezole and idazoxan, on ethanol-induced hypothermia were investigated in mice. Ethanol significantly reduced (P less than 0.001) core temperature whilst both alpha 2-adrenoceptor antagonists were without effect when administered alone. However, both the 1 and 3 mg/kg doses of atipamezole significantly (P less than 0.05) attenuated the ethanol-induced reduction in body temperature 20 and 40 min after administration. The 3 mg/kg dose of idazoxan (but not the 1 mg/kg dose) also significantly (P less than 0.05) attenuated ethanol's hypothermic effect 20 min after administration but this effect was not statistically significant at 40 min. In a subsequent experiment using lower doses of atipamezole (0.03-1.0 mg/kg) the attenuation of ethanol-induced hypothermia caused by atipamezole was found to be dose-related. The effect of the benzodiazepine inverse agonist Ro 15-4513 on ethanol-induced hypothermia was also investigated. This compound possessed an intrinsic hypothermic action but neither attenuated nor enhanced the hypothermic effect of ethanol. These results suggest that alpha 2-adrenoceptor can, at least partially, modulate the hypothermic effects of ethanol.

Differential effect of clomipramine treatment on m-chlorophenylpiperazine-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of m-chlorophenylpiperazine (m-CPP, a serotonin agonist) to rats increased plasma prolactin and corticosterone concentrations. Long-term (21-day) and short-term (3-day) treatment with the tricyclic antidepressant, clomipramine, did not have any significant effect on baseline levels of either prolactin or corticosterone. Long-term but not short-term clomipramine treatment significantly potentiated m-CPP's effect on plasma prolactin. On the other hand, both long-term and short-term clomipramine treatment significantly attenuated m-CPP's effect on plasma corticosterone. These findings are consistent with other animal and clinical studies demonstrating a differential effect of antidepressant treatment on two different serotonin-mediated neuroendocrine functions.

Long-term lithium treatment in rats attenuates m-chlorophenylpiperazine-induced decreases in food intake but not locomotor activity

Administration of various doses of m-chlorophenylpiperazine (m-CPP, a 5-HT agonist) to rats produced dose-related decreases in food intake and locomotor activity. Long-term (21-25 days) but not short-term (3-7 days) lithium treatment attenuated m-CPP-induced decreases in food intake. However, neither short-term nor long-term lithium treatment had any significant effect on m-CPP-induced decreases in locomotor activity. These findings suggest development of functional subsensitivity of 5-HT1B receptors mediating decreases in food intake and provide further evidence that m-CPP's effects on food intake are mediated by different mechanisms from those regulating locomotor activity.

Long-term imipramine treatment potentiates m-chlorophenylpiperazine-induced changes in prolactin but not corticosterone or growth hormone levels in rats

Intravenous administration of m-chlorophenylpiperazine (m-CPP, a selective 5-HT agonist) to rats produced increases in plasma prolactin and corticosterone and a decrease in plasma growth hormone concentrations. Long-term but not short-term imipramine treatment potentiated m-CPP's effect on plasma prolactin, but not its effects on corticosterone or growth hormone. Short-term or long-term imipramine treatment did not produce significant changes in baseline levels of prolactin, corticosterone or growth hormone. These findings are compatible with development of functional supersensitivity of 5-HT receptors mediating prolactin release. Lack of potentiation of m-CPP's effects on corticosterone and growth hormone following long-term imipramine treatment suggests either differential regulation of these hormones by serotonergic and possibly other mechanisms, or different 5-HT receptor subtypes mediating the release of these hormones. Alternatively, adaptive changes in other aminergic neurotransmitter mechanisms such as the noradrenergic system may account for the differential effect of long-term imipramine treatment on m-CPP-induced neuroendocrine changes.

Hyperthermia induced by m-CPP in the rat and its modification by antidepressant treatments

Administration of the serotonin agonist m-chlorophenylpiperazine to rats produced a dose-related hyperthermia. Pretreatment with the serotonin receptor antagonist metergoline totally abolished this response, whereas similar treatment with haloperidol, phenoxybenzamine, naloxone, clonidine, pindolol, propranolol, methiotepin, and ritanserin was ineffective. In studies investigating the modification of the response by antidepressant treatments both acute (3 day) and chronic (22 day) administration of the MAO inhibitor clorgyline, as well as the tricyclics clomipramine and imipramine, attenuated the hyperthermic response to m-CPP. These findings are discussed with regard to the specificity of m-CPP-induced hyperthermia and its subsequent modification by antidepressant treatments, in order to evaluate this model's use as a probe for assessment of the serotonergic system.

Differential effects of long-term antidepressant treatments on 8-OHDPAT-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT) to rats produced increases in plasma prolactin and corticosterone concentrations. Long-term or short-term treatment with the MAO type A inhibiting antidepressant, clorgyline, or tricyclic antidepressants (imipramine and clomipramine), did not change baseline levels of either prolactin or corticosterone. Long-term but not short-term clorgyline treatment attenuated 8-OHDPAT's effect on plasma prolactin but not on corticosterone. On the other hand, long-term but not short-term treatment with clomipramine and to some extent imipramine also, accentuated 8-OHDPAT's effect on plasma prolactin but not on corticosterone. These findings demonstrate that long-term antidepressant treatment in rats produces a differential effect on 8-OHDPAT-induced increases in plasma prolactin and corticosterone, which is consistent with other clinical and animal studies demonstrating a differential effect of long-term antidepressant treatment on two different 5-HT-mediated neuroendocrine functions.

Differential neuroendocrine responses to the 5-HT agonist m-chlorophenylpiperazine in Fawn-Hooded rats relative to Wistar and Sprague-Dawley rats

The effect of various doses of the 5-HT agonist m-chlorophenylpiperazine (MCPP) on neuroendocrine function (prolactin and corticosterone responses) were compared in three different rat strains: Wistar, Sprague-Dawley (SD), and Fawn-Hooded (FH) rats. Administration of various doses of MCPP produced increases in plasma concentrations of prolactin and corticosterone in all three rat strains. The prolactin responses of FH rats to MCPP were significantly smaller than that of either Wistar or SD rats, while corticosterone responses were equivalent across all three strains. On the other hand, baseline concentrations of corticosterone, but not of prolactin, were significantly higher in FH animals relative to both Wistar and SD animals. There was no significant difference in either baseline hypothalamic concentrations of serotonin, 5-hydroxyindoleacetic acid, norepinephrine, or dopamine or brain concentrations of MCPP among these three rat strains. These findings support some other data indicating that FH rats, a strain with a peripheral platelet serotonin storage pool disorder, also possess alterations in some neuroendocrine functions which are modulated by serotonin.

Long-term imipramine treatment differentially affects fenfluramine-induced suppression of food intake and locomotor activity

Administration of fenfluramine to rats produced decreases in one-hour food intake and locomotor activity. Short-term (2-6 days) or long-term (21-25 days) treatment with the tricyclic antidepressant, imipramine, did not affect daily food intake, body weight gain or baseline locomotor activity when compared to saline treatment. However, long-term but not short-term imipramine treatment attenuated fenfluramine-induced decreases in one-hour food intake. On the other hand, neither short-term nor long-term imipramine treatment affected fenfluramine-induced decreases in locomotor activity. These findings demonstrate a differential effect of long-term imipramine treatment on fenfluramine-induced suppression of food intake and locomotor activity.

Clorgyline treatment differentially affects m-chlorophenylpiperazine-induced neuroendocrine changes

Intravenous administration of the 5-HT1B agonist, m-chlorophenylpiperazine (m-CPP) to rats produced increases in plasma prolactin (peak effect at 15 min), corticosterone (peak effect at 30 min) and a decrease in plasma growth hormone (peak effect at 15 min) concentrations. Short-term or long-term clorgyline treatment did not affect baseline levels of prolactin, corticosterone or growth hormone. Short-term clorgyline treatment attenuated m-CPP's effect on corticosterone but not on prolactin or growth hormone. On the other hand, long-term clorgyline treatment attenuated m-CPP's effect on prolactin but not on corticosterone or growth hormone. These findings are compatible with development of functional subsensitivity of 5-HT1B receptors mediating prolactin release following long-term clorgyline treatment. Attenuation of m-CPP's effect on corticosterone following short-term clorgyline treatment suggests either early adaptational changes in a 5-HT receptor subtype mediating corticosterone release, or clorgyline-induced increases in other neurotransmitters such as norepinephrine which may be responsible for attenuating m-CPP's effect on corticosterone.

Plasma and tissue pharmacokinetics of human interferon-alpha in the rat after its intravenous administration

The pharmacokinetic distribution of human lymphoblastoid interferon (IFN-alpha), (Wellferon, Burroughs Wellcome Company, Research Triangle Park, NC) in plasma and seven tissues was studied for up to 4 hr after intravascular administration as a bolus, 2 x 10(5) U/100 g, or as a 5-min infusion, 2 x 10(6) U/100 g, into anesthetized male Fischer 344 rats. IFN-alpha disappeared rapidly from plasma with elimination T 1/2 of 47 and 68 min, respectively, and was preferentially taken up by the kidney as compared with other organs. After i.v. injection, significant amounts of IFN-alpha, in order of descending concentration, were detected in kidney, lung, spleen, liver and lymph node, but not in brain or skeletal muscle. After a 5-min IFN-alpha infusion, significant amounts, in order of descending priority, were detected in kidney, lung, spleen, lymph node, liver, muscle and brain. The pharmacokinetic parameters of IFN-alpha are described for plasma and different tissues. After intravascular IFN-alpha administration, high blood levels are achieved immediately which encourages rapid distribution throughout body tissues, albeit at the cost of a high renal catabolic loss, in concentrations that can be predicted from our study.

Food intake, neuroendocrine and temperature effects of 8-OHDPAT in the rat

Administration of 8-hydroxy-2(di-n-propylamino)tetralin (8-OHDPAT) to rats produced dose-dependent decreases in food intake and hypothermia, increases in plasma prolactin and corticosterone, and a decrease in plasma growth hormone. 8-OHDPAT administration also induced the serotonin behavioral syndrome at all doses. Pretreatment with metergoline did not affect the 8-OHDPAT-induced behavioral syndrome or decrease in food intake but attenuated the prolactin increase and, furthermore, potentiated 8-OHDPAT-induced hypothermia. Pretreatment with ritanserin or naloxone did not modify 8-OHDPAT-induced changes in food intake, temperature or prolactin. Similarly, pretreatment with phenoxybenzamine, propranolol, clonidine, haloperidol and methiothepin also did not attenuate 8-OHDPAT-induced decreases in food intake. Administration of pindolol alone produced hyperthermia, decreased food intake and enhanced prolactin secretion. Pindolol thus appears to act as a partial 5-HT agonist in addition to being an antagonist at central 5-HT receptors.

Fenfluramine-induced suppression of food intake and locomotor activity is differentially altered by the selective type A monoamine oxidase inhibitor clorgyline

Administration of fenfluramine to rats produced decreases in 1-h food intake and locomotor activity. Short-term (2-6 days) or long-term (21-25 days) treatment with the monoamine oxidase (MAO) type A inhibiting antidepressant clorgyline potentiated fenfluramine-induced suppression of food intake but did not affect fenfluramine-induced suppression of locomotor activity. Although daily (4 h) food intake was not significantly less in clorgyline-treated animals relative to saline-treated controls, body weight gain was significantly less in clorgyline-treated animals relative to controls. These findings demonstrate a differential effect of clorgyline treatment on fenfluramine-induced suppression of food intake and locomotor activity.

Interactions between relatively selective monoamine oxidase inhibitors and an inhibitor of 5-hydroxytryptamine re-uptake, clomipramine

Features of interactions with combined antidepressants in man were evoked by clomipramine in rats pretreated with both the relatively selective monoamine oxidase (MAO) inhibitors clorgyline and deprenyl, but not when clomipramine was given to rats pretreated with deprenyl or clorgyline alone, i.e. inhibition of both MAO A and B was a likely prerequisite for clomipramine to elicit the syndrome (with the larger dose of clorgyline and deprenyl, MAO A and B inhibition exceeded 95%). The features evoked were myoclonic--forelimb flexor-extensor movements, wet dog shakes and head and body twitches; hyperthermia and ECG anomalies also developed, and locomotor activity was augmented. Myoclonic phenomena were prevented when the above pretreatment also included p-chlorophenyl-alanine, but were unaffected or even intensified when pretreatment instead included alpha-methyl-p-tyrosine; these phenomena were attenuated or abolished by pirenperone, a 5HT2 antagonist. Relevance of these findings to safer combinations of antidepressants is discussed.

Interactions of a non-selective monoamine oxidase inhibitor, phenelzine, with inhibitors of 5-hydroxytryptamine, dopamine or noradrenaline re-uptake

Interactions of combined antidepressants which occur in man were reproduced in rats pretreated with phenelzine, features elicited including myoclonic phenomena, an augmented lower limb flexor reflex, muscle fasiculation and fatalities, particularly with combinations incorporating 5-hydroxytryptamine (5-HT) re-uptake inhibitors. Combinations of antidepressants included phenelzine with 5-HT re-uptake inhibitors (paroxetine, fluoxetine, clomipramine); with "mixed" re-uptake inhibitors affecting 5-HT and noradrenaline (imipramine, amitriptyline); with noradrenaline re-uptake inhibitors (desipramine, maprotiline, nisoxetine) and with dopamine re-uptake inhibitors (benztropine, nomifensine). Myoclonic phenomena such as forelimb flexor-extensor movements, head and body twitches, occurred in phenelzine pretreated rats after paroxetine, fluoxetine, clomipramine, imipramine, amitriptyline and desipramine. Wet dog shakes, the most intense phenomenon, were obtained only after paroxetine, fluoxetine, clomipramine and imipramine. Myoclonic features were prevented when pretreatment included p-chlorophenylalanine but were unaffected when this incorporated alpha-methyl-p-tyyrosine; there were attenuated by methysergide, cyproheptadine, clozapine or pimozide. The myoclonic phenomena were reproduced by combination of 5-hydroxytryptophan but not L-3,4-dihydroxyphenylalanine with clomipramine. Electrocortical changes observed included 2-4 Hz, 5-8 Hz, large amplitude potentials unrelated to the myoclonic incidents and unaffected by sensory stimulation. Following phenelzine, brain monoamine oxidase (MAO) A inhibition was 99% and that of MAO B, 88%; 5-HT concentration was significantly elevated in the cortex and hypothalamus, as was hypothalamic noradrenaline. Peak and basal tensions of a lower-limb flexor reflex were elevated in phenelzine pretreated spinal rats by fluoxetine, paroxetine, clomipramine and imipramine, effects attenuated by cyproheptadine. Forelimb flexor-extensor movements and body twitches were elicited by fluoxetine and paroxetine in phenelzine pretreated spinal rats in the presence of electrical stimulation of the central stump of a divided posterior tibial nerve. Pressor responses were observed in phenelzine pretreated spinal rats given 5-HT re-uptake inhibitors, "mixed" re-uptake inhibitors and those affecting noradrenaline re-uptake; ECG anomalies occurred in such rats given clomipramine.

Interactions of non-selective monoamine oxidase inhibitors, tranylcypromine and nialamide, with inhibitors of 5-hydroxytryptamine, dopamine or noradrenaline re-uptake

Rats pretreated with tranylcypromine and given clomipramine, developed head and body twitches, forelimb flexor-extensor movements and wet dog shakes, phenomena which failed to develop when pretreatment incorporated p-chlorophenylalanine (PCPA) but were unabated when this included alpha-methyl-p-tyrosine (AMPT). Locomotor activity, itself enhanced by tranylcypromine, was further and significantly elevated compared to saline, by clomipramine or imipramine in grouped rats (n = 3) but not in single or paired rats; desipramine lacked such action. This effect of clomipramine was prevented when PCPA was incorporated into the pretreatment and that of imipramine by including PCPA or AMPT. Brain monoamine oxidase (MAO) A inhibition was 92% and that of MAO B, 80%. Cortical hydroxytryptamine (5-HT) and noradrenaline concentrations as well as hypothalamic 5-HT, were significantly elevated by tranylcypromine, as was dopamine in the striatum, nucleus accumbens and tuberculum olfactorium. Hyperthermia developed in tranylcypromine pretreated rats given paroxetine or fluoxetine. Myoclonic phenomena were elicited by paroxetine, fluoxetine, clomipramine or imipramine in nialamide pretreated rats but these were less intense than in rats pretreated with phenelzine or tranylcypromine. Fatalities were fewer than in rats pretreated with tranylcypromine or phenelzine. Brain MAO A inhibition was 92% and that of MAO B, 69%.

Clinical and experimental aspects of interactions between amine oxidase inhibitors and amine re-uptake inhibitors

Dangerous and even fatal interactions can occur in man following combinations of antidepressants which include non-selective MAO inhibitors. To ascertain the causation, interactions reproducing the clinical phenomena have been elicited in animals with these combinations, and the mechanisms involved have been explored by various pharmacological strategies; 5-HT re-uptake inhibitors proved especially hazardous in combination. Interactions could, however, be avoided even with the 5-HT re-uptake inhibitors, by combination with relatively selective MAO A or B inhibitors, an approach with potential clinical value.