Intrathecal administration of thiorphan, bestatin, desipramine and fluoxetine differentially potentiate the antinociceptive effects induced by beta-endorphin and morphine, administered intracerebroventricularly

Uses of fluoxetine in nociceptive pain management: A literature overview

Fluoxetine is one of the top ten prescribed antidepressants. Other therapeutic applications were approved for fluoxetine including, anxiety disorders, bulimia nervosa, and premature ejaculation. However, the role of fluoxetine in nociceptive pain management is still unclear. In this review, we discuss an overview of five possible roles of fluoxetine in pain management: intrinsic antinociceptive effect, enhancement of acute opioid analgesia, attenuation of tolerance development to opioid analgesia, attenuation of dependence development and abstinence syndrome, and attenuation of opioid induced hyperalgesia. Conflicting data were reported about fluoxetine intrinsic anti-nociceptive effect in preclinical and clinical studies except for inflammatory pain. Similar controversy was described in preclinical and clinical studies which explored the possible enhancement of opioid analgesia by fluoxetine co-administration. However, fluoxetine was found to have a promising effect on opioid tolerance and dependence in animal and human studies. Regarding opioid induced hyperalgesia, no studies examined fluoxetine effects in this regard. Our literature review revealed that, the most likely beneficial use of fluoxetine in nociceptive pain management is for alleviation of inflammatory pain and attenuation of opioid tolerance and dependence. Non-steroidal anti-inflammatory and corticosteroids carry many adverse effects and toxicities. Effective alleviation of opioid tolerance and dependence represents a huge health burden and growing unmet medical need. Moreover, most agents used to attenuate these phenomena are either experimental or poorly tolerable drugs which limit their transitional value. Fluoxetine offers an effective, safe, and tolerable alternative for management of both inflammatory pain and opioid tolerance and dependence presently available to clinicians.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Prophylactic treatment with the tricyclic antidepressant desipramine prevents development of paclitaxel-induced neuropathic pain through activation of endogenous analgesic systems

Neuropathic pain impacts approximately 3-4.5% of the global population and remains an unresolved health problem. The management of neuropathic pain has two distinct goals-prevention of development and control of established neuropathic pain. We examined the impact of both prophylactic and therapeutic treatments with the tricyclic antidepressant desipramine on the development and maintenance of toxic neuropathic pain induced by the chemotherapeutic agent paclitaxel. We also investigated the involvement of endogenous analgesic (i.e., endogenous opioid and endocannabinoid) systems in the antinociceptive actions of desipramine in these two distinct phases of neuropathic pain. Chronic subcutaneous infusion of desipramine via osmotic pumps suppressed both the development and maintenance of paclitaxel-induced neuropathic pain. However, only prophylactic desipramine treatment blocked the development of neuropathic pain throughout the three month observation interval; neuropathic pain did not return. The opioid receptor antagonist naloxone blocked the antinociceptive effects of both prophylactic and therapeutic desipramine treatments throughout the entire timecourse of desipramine-induced antinociception. By contrast, cannabinoid CB₁ and CB₂ receptor antagonists partially attenuated the antinociceptive actions of desipramine in a manner that was restricted to the development phase of paclitaxel-induced neuropathic pain only. Paclitaxel decreased cell viability in TMD231 tumor cells in an MTT assay in vitro. Notably, desipramine (1nM-1μM) alone did not alter tumor cell viability and did not prevent the cytotoxic effects of paclitaxel under identical conditions. The highest concentration of desipramine (10μM) reduced tumor cell viability alone and enhanced the cytotoxic effects of paclitaxel. Our study identifies a previously unrecognized preemptive analgesic strategy that prevents development of paclitaxel-induced neuropathic pain, and also dissects receptor mechanisms underlying desipramine-induced antinociceptive effects. This information may be applied to improve current therapeutic strategies with the goal of preventing and managing neuropathic pain induced by chemotherapeutic treatment.

Effects of Morphine and Time of Day on Pain and Beta-Endorphin

Clients report more pain at some times of day than at others due, in part, to the temporal variation of the body's inhibitory pain response. The analgesic effectiveness of morphine varies with the time of day, perhaps due to the inhibiting or enhancing effects of the drug on plasma beta-endorphin (BE). This experiment was designed to examine the timed effects of morphine on the pain-induced BE response. Six groups of treatment mice (injected with morphine sulfate) and 6 groups of control mice (injected with saline) were exposed to an acute pain stimulus at 4-h intervals, and blood was collected. Plasma BE was analyzed using radioimmunoassay. Control mice showed a robust cir-cadian BE-response rhythm with a peak at 0000 and a nadir at 1200, whereas the BE response of mice that received morphine was arrhythmic. Animals that received morphine tolerated the noxious stimulus longer, but the analgesia varied with time of day. These results indicate that morphine abolishes the rhythmic BE response to pain and does not inhibit pain equally at all times of day. Morphine doses should be titrated to maximize the endogenous pain control system while achieving analgesia with decreased dosages.

Electroacupuncture at 2/100 hz activates antinociceptive spinal mechanisms different from those activated by electroacupuncture at 2 and 100 hz in responder rats

We examined the effects of intrathecal injection of desipramine and fluoxetine (selective inhibitors of norepinephrine and 5-HT uptake, resp.), thiorphan and neostigmine (inhibitors of enkephalinase and acetylcholinesterase, resp.), gabapentin (a GABA releaser), and vigabatrin (an inhibitor of GABA-transaminase) on the antinociception induced by 2 Hz, 100 Hz, or 2/100 Hz electroacupuncture (EA) applied bilaterally to the Zusanli (ST36) and Sanyinjiao (SP6) acupoints using the rat tail-flick test. We show that 2 Hz EA antinociception lasts longer after the administration of drugs that increase the spinal availability of norepinephrine, acetylcholine, or GABA; 100 Hz EA antinociception lasts longer after drug that increases the spinal availability of norepinephrine; 2/100 Hz EA antinociception lasts longer after drugs that increase the spinal availability of endogenous opioids or GABA. We conclude that the antinociceptive effect of 2/100 Hz EA is different from the synergistic effect of alternate stimulation at 2 and 100 Hz because the effect of the former is not changed by increasing the spinal availability of serotonin and lasts longer after the administration of vigabatrin. The combination of EA with drugs that increase the availability of spinal neurotransmitters involved in the modulation of nociceptive inputs may result in a synergistic antinociceptive effect in the rat tail-flick test.

Study of antinociceptive activity of SSRI (fluoxetine and escitalopram) and atypical antidepressants (venlafaxine and mirtazepine) and their interaction with morphine and naloxone in mice

Objective:

to study the probable site of antinociceptive action of SSRI (fluoxetine, escitalopram) and atypical antidepressants (mirtazapine, venlafaxine) and their interaction with morphine and naloxone.

Materials and Methods:

the study was conducted on albino mice (25-35 grams) of either sex. Different doses of morphine (0.5 and 1 mg/kg), fluoxetine (2, 5 and 10 mg/kg), venlafaxine (30, 40 and 50 mg/kg), mirtazapine (3, 5 and 7 mg/kg) and escitalopram (2.5, 5 and 10 mg/kg) were administered subcutaneously to obtain their subanalgesic doses using tail flick analgesiometer. Tail flick latencies were obtained at 15, 30, 60 and 120 min. after drug administration. Naloxone (1 mg/kg) was administered 10 minutes prior to test drug for testing antagonism.

Observations:

fluoxetine (5 and 10 mg/kg), mirtazapine (5 and 7 mg/kg) and venlafaxine (40 and 50 mg/kg) were found to have antinociceptive activity but not at lower doses. Escitalopram failed to show any antinociceptive activity at any of the doses used. The antinociceptive effect of all the drugs was antagonized by naloxone (1 mg/kg). Further, subanalgesic doses of fluoxetine, mirtazapine and venlafaxine showed analgesic activity with suboptimal dose of morphine (0.5 mg/kg).

Result and conclusion:

fluoxetine, mirtazapine and venlafaxine have antinociceptive activity whereas escitalopram doesn’t; their site of action seems to be the same as that of opioid analgesics (‘mue’ receptors). However, other pathways (cholinergic, histaminic, noradrenergic, GABAergic) may be involved in mediation of their analgesic activity, deserving further elucidation. Results apparently show that these drugs may be useful in the management of pain as monotherapy or in combination with other opioids.

The role of 5-HT(3) receptors in the additive anticonvulsant effects of citalopram and morphine on pentylenetetrazole-induced clonic seizures in mice

Citalopram, a selective serotonin reuptake inhibitor (SSRI), is frequently used in the treatment of major depressive disorders. In addition to its antidepressant features, citalopram shows some anticonvulsive properties at lower doses, whereas higher doses, ingested in cases of suicide, have been associated with seizures. Moreover, some reports support the enhancing effect of morphine on different responses of SSRIs such as analgesic and anticonvulsant properties. Although the exact mechanisms of these additive effects are not yet fully understood, 5-HT(3) receptor has recently been shown to play an important role in the central effects of SSRIs and morphine. In this regard, we used a model of clonic seizures induced by pentylenetetrazole (PTZ) in male NMRI mice to investigate whether morphine and citalopram exhibit additive anticonvulsant effects and, if so, whether this effect is mediated through modulation of 5-HT(3) receptors. In our study, citalopram at lower doses (0.5 and 1 mg/kg, ip) significantly increased the seizure threshold (P<0.01) and at a higher dose (50 mg/kg) had proconvulsive effects. Moreover, morphine at low and noneffective doses had additive effects on the anticonvulsive properties of citalopram. This additive effect was prevented by pretreatment with low and noneffective doses of tropisetron (a 5-HT(3) receptor antagonist) and augmented by 1-(m-chlorophenyl)-biguanide (mCPBG, a 5-HT(3) receptor agonist). Moreover, low doses of morphine (0.1 and 0.5 mg/kg) alone or in combination with potent doses of 5-HT(3) receptor agonist or antagonist could not alter the proconvulsive properties of citalopram at higher dose (50 mg/kg), ruling out the contribution of 5-HT(3) to this effect. In summary, our findings demonstrate that 5-HT(3) receptor mediates the additive anticonvulsant properties of morphine and low-dose citalopram. This could constitute a new approach to augmenting the efficacy and curtailing the adverse effects of citalopram.

The Analgesic Activity of Bestatin as a Potent APN Inhibitor

Bestatin, a small molecular weight dipeptide, is a potent inhibitor of various aminopeptidases as well as LTA4 hydrolase. Various physiological functions of Bestatin have been identified, viz.: (1) an immunomodifier for enhancing the proliferation of normal human bone marrow granulocyte-macrophage progenitor cells to form CFU-GM colonies; Bestatin exerts a direct stimulating effect on lymphocytes via its fixation on the cell surface and an indirect effect on monocytes via aminopeptidase B inhibition of tuftsin catabolism; (2) an immunorestorator and curative or preventive agent for spontaneous tumor; Bestatin alone or its combination with chemicals can prolongate the disease-free interval and survival period in adult acute or chronic leukemia, therefore, it was primarily marketed in 1987 in Japan as an anticancer drug and servers as the only marketed inhibitor of Aminopeptidase N (APN/CD13) to cure leukemia to date; (3) a pan-hematopoietic stimulator and restorator; Bestatin promotes granulocytopoiesis and thrombocytopoiesis in vitro and restores them in myelo-hypoplastic men; (4) an inhibitor of several natural opioid peptides. Based on the knowledge that APN can cleave several bioactive neuropeptides such as Met-enkaphalins, Leu-enkaphalins, beta-Endorphin, and so on, the anti-aminopeptidase action of Bestatin also allows it to protect endopeptides against their catabolism, exhibiting analgesic activity. Although many scientific studies and great accomplishments have been achieved in this field, a large amount of problems are unsolved. This article reviews the promising results obtained for future development of the analgesic activity of Bestatin that can be of vital interest in a number of severe and chronic pain syndromes.

Evidence for epsilon-opioid receptor-mediated beta-endorphin-induced analgesia

Among the opioid receptors, which have been pharmacologically classified as mu, delta, kappa and epsilon, the existence of the epsilon receptor has been controversial, and this receptor is generally not recognized as a member of the opioid peptide receptor family because it has not been precisely characterized. However, results from pharmacological, physiological and opioid receptor binding studies clearly indicate the presence of epsilon-opioid receptors, which are distinct from mu-, delta- or kappa-opioid receptors. This putative epsilon-opioid receptor is stimulated supraspinally by the endogenous opioid peptide beta-endorphin, which induces the release of Met-enkephalin, which, in turn, acts on spinal delta2-opioid receptors to produce antinociception. In this article, this beta-endorphin-sensitive epsilon-opioid receptor-mediated descending pain control system, which is distinct from that activated by the mu-opioid receptor agonist morphine, is described and the physiological role of the beta-endorphin-mediated system in pain control activated by cold-water swimming and intraplantar injection of formalin is discussed.

Peptidase inhibitor-induced antidiuresis mediated through angiotensin and opioid receptors in the rat hypothalamus

We examined the effects on urine outflow rate after microinjections of thiorphan (a carboxypeptidase inhibitor) and bestatin (an aminopeptidase inhibitor) into the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei of anesthetized hydrated rats to determine the possible role of neuropeptides in the regulation of urine production. After individual microinjection of the peptidase inhibitors into the nuclei, only thiorphan at 100 nmol administered into the PVN significantly decreased the urine outflow rate. Two consecutive microinjections of the peptidase inhibitors at 100 nmol each into the nuclei induced potent antidiuresis. These effects after microinjections of the peptidase inhibitors into the PVN and SON were diminished by pretreatment with [Sar1,Ile8]angiotensin (ANG) II (an ANG II receptor antagonist) and naloxone (an opioid receptor antagonist) in the PVN and with [Sar1,Ile8]ANG II in the SON, respectively. A vasopressin (AVP) receptor antagonist, d(CH2)5-D-Tyr(Et)VAVP (i.v.), completely blocked the antidiuresis by microinjections of the peptidase inhibitors into both the nuclei. Urinary osmotic pressure was significantly increased by consecutive microinjections of the peptidase inhibitors into the PVN and SON. These results suggest that endogenously-released ANG II and opioid peptides in the PVN and ANG II in the SON regulate urine production mediated through increased AVP release.

The reduction of antinociceptive effect of morphine administered intraventricularly is correlated with the decrease of serotonin release from the spinal cord in streptozotocin-induced diabetic rats

1. The antinociceptive effect of morphine (25 micrograms) administered into the 3rd ventricle was significantly attenuated in streptozotocin-induced diabetic rats as measured by the tail-flick assay. 2. The release of serotonin (5-HT; 5-hydroxytryptamine) from the spinal cord caused by intraventricular injection of morphine (25 micrograms) was significantly reduced in streptozotocin-induced diabetic rats. 3. No differences of 5-HT contents of the spinal cord (lumbar cord) between streptozotocin-induced diabetic- and vehicle-treated rats were found. 4. It is concluded that the reduction of antinociception produced by intraventricular injection of morphine in streptozotocin-induced diabetic rats might be, at least partly, due to the decrease of 5-HT release from the spinal cord.

Influence of antidepressant drugs administration on the morphine inhibitory effect in mice vasa deferentia

The effect of chronic and acute antidepressant drug administration on peripheral opioid mechanisms was investigated. For this purpose, the inhibitory effect of morphine on electrically-induced contractions of mouse vas deferens was measured. Acute antidepressant administration did not induce any change in morphine inhibition. Chronic fluvoxamine and chlorimipramine treatment induced strong and significant hypersensitivity to morphine-inhibition. Chronic desipramine, desmethylclomipramine and nortriptyline treatment induced the opposite effect. These results seem to show differential opioidergic regulation depending on the monoaminergic antidepressant used. The level of this interaction remains to be elucidated.

Differential effects of 3-isobutyl-1-methylxanthine injected intrathecally or intracerebroventricularly on antinociception induced by opioids administered intracerebroventricularly in the mouse

Various doses of 3-isobutyl-1-methylxanthine (IBMX), a cAMP phosphodiesterase inhibitor, injected intrathecally (i.t.) or intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. IBMX (0.01 to 1 ng) pretreatment i.t. for 10 min dose-dependently attenuated the inhibition of the tail-flick response induced by i.c.v. administered morphine (2 micrograms), beta-endorphin (1 microgram), and U50, 488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide), 60 micrograms. However, pretreatment with IBMX i.c.v. did not affect the inhibition of the tail-flick response induced by morphine, beta-endorphin, and U50, 488H administered i.c.v. Neither i.c.v. nor i.t. pretreatment with IBMX attenuated the inhibition of the tail-flick response induced by D-Pen2-D-Pen5-enkephalin (DPDPE; 10 micrograms) administered i.c.v. Our results suggest that spinal but not supraspinal cAMP phosphodiesterases are involved in mediating antinociception induced by morphine, beta-endorphin and U50, 488H administered supraspinally. However, neither spinal nor supraspinal cAMP phosphodiesterase is involved in mediating antinociception induced by DPDPE administered supraspinally.

Effects of intraventricular injection of morphine and beta-endorphin on serotonin release from the spinal cord in rats

Effects of intraventricular (third ventricle) injection of morphine and beta-endorphin on the release of serotonin (5-HT; 5-hydroxytryptamine) and 5-HIAA (5-hydroxy indolacetic acid) from the spinal cord were studied using urethane anesthetized spinally perfused rats. Intraventricular injection of morphine (25 micrograms) increased the 5-HT level in the perfusate about threefold. The increase of 5-HT release reached at peak between 30 and 60 min after the first injection of morphine. However, the levels of 5-HIAA, a metabolite of 5-HT, was not significantly altered by intraventricular injection of morphine. Furthermore, second intraventricular injection of morphine at the same dose did not increase 5-HT level in the spinal perfusate. In contrast to the results with morphine, beta-endorphin (10 micrograms) administered intraventricularly did not alter the release of 5-HT and 5-HIAA from the spinal cord. In addition, acute antinociceptive tolerance to intraventricular morphine induced by a prior intraventricular injection of morphine was studied in pentobarbital anesthetized rats. Acute tolerance was induced by intraventricular pretreatment with morphine (20 micrograms) for 120 min and the same dose of morphine was injected intraventricularly. The tail-flick test was used as an antinociceptive test. Pretreatment of rats with morphine intraventricularly reduced inhibition of the tail-flick response to intraventricularly injected morphine. The results support our previous hypothesis that beta-endorphin and morphine administered supraspinally activate separate descending systems. Spinopetal serotonergic descending pathway is selectively activated by intraventricularly injected morphine but not beta-endorphin.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of protection by D-Pen2-D-Pen5-enkephalin or D-Ala2-NMePhe4-Gly-ol-enkephalin against beta-chlornaltrexamine in the spinal cord on the antinociception induced by beta-endorphin administered intracerebroventricularly in the mouse

Chlornaltrexamine (beta-CNA, 0.5 micrograms) alone or beta-CNA plus either mu-agonist, D-Ala2-NMePhe4-Gly-ol-enkephalin (DAMGO, 500 ng) or delta-agonist, D-Pen2-D-Pen5-enkephalin (DPDPE, 10 micrograms) was injected intrathecally (i.t.) to protect mu- or delta-opioid receptors, respectively, for 24 h in male ICR mice. The antinociception was assessed by the tail-flick and hot-plate test. DPDPE or DAMGO injected i.t. increased inhibition of the tail-flick and hot-plate response in a dose-dependent manner. The dose-response curve for tail-flick and hot-plate response induced by DPDPE or DAMGO in i.t. saline-treated group significantly shifted to the right in i.t. beta-CNA alone treated group but returned to the control level in the group treated with i.t. beta-CNA coadministered with DPDPE or DAMGO, respectively. The effects of protection of mu- and delta-opioid receptor in the spinal cord on inhibition of the tail-flick and hot-plate response induced by beta-endorphin and morphine administered intracerebroventricularly (i.c.v.) were then studied. Intrathecal pretreatment with beta-CNA or beta-CNA coadministered with DAMGO attenuated inhibition of the tail-flick response induced by beta-endorphin administered i.c.v. However, i.t. treatment with beta-CNA coadministered with DPDPE did not affect inhibition of the tail-flick response induced by beta-endorphin administered i.c.v. Intrathecal pretreatment with beta-CNA or beta-CNA coadministered with either DPDPE or DAMGO did not alter inhibition of the hot-plate response induced by beta-endorphin administered i.c.v.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intrathecal or intracerebroventricular pretreatment with pertussis toxin on antinociception induced by beta-endorphin or morphine administered intracerebroventricularly in mice

We have previously demonstrated that beta-endorphin and morphine, when administered supraspinally, produce antinociception by activating different descending pain inhibitory systems in both rats and mice. However, the signal transduction mechanisms involved in the descending pain-inhibitory systems that are activated by beta-endorphin and morphine administered intracerebroventricularly (i.c.v.) have not been characterized. Therefore, in the present study, the effects of intrathecal (i.t.) and i.c.v. pretreatments with pertussis toxin (PTX) on antinociception induced by beta-endorphin or by morphine administered i.c.v. were studied in ICR mice. Antinociception was assessed by the tail-flick assay and by the hot-plate assay. Intrathecal pretreatment with PTX (0.5 microgram) for 6 days effectively reduced the inhibition of the tail-flick response induced by beta-endorphin (1 microgram) or by morphine (1 microgram) administered i.c.v. However, i.t. pretreatment with PTX was not effective in reducing the inhibition of the hot-plate response induced by beta-endorphin or by morphine administered i.c.v. Intracerebroventricular pretreatment with PTX (0.5 microgram) for 6 days effectively reduced the inhibition of the tail-flick and hot-plate responses induced by morphine (1 microgram), but not that induced by beta-endorphin (1 microgram), administered i.c.v. Our results suggest that there are PTX-sensitive G proteins coupled to the spinal descending pain inhibitory systems that are activated by beta-endorphin and morphine administered i.c.v. At a supraspinal level, i.c.v. morphine- but not beta-endorphin-induced antinociception is mediated by PTX-sensitive G proteins.

Effects of a single oral dose of desipramine on postoperative morphine analgesia

Drugs that block norepinephrine reuptake offer promise as opioid potentiators, because norepinephrine mediates opioid analgesia but not side effects such as sedation or nausea. In a two-by-two factorial design, we randomized 62 inpatients with pain following major surgery to receive either desipramine, 50 mg by mouth, or placebo at 6 a.m. on the first day after surgery. At their first request of pain medication after 8 a.m., they were given intravenous morphine, either 0.033 mg/kg or 0.10 mg/kg. Pain relief and side effects were assessed for 4 hr; peak relief on the visual analog scale (VAS) was the primary outcome variable. Pain relief, side effect scores, and time to remedication were significantly greater with the higher dose than with the lower dose of morphine, verifying assay sensitivity, but desipramine pretreatment did not significantly enhance morphine analgesia. The mean increase in peak VAS relief score after desipramine pretreatment, relative to placebo, was 6%; the 95% confidence interval for this estimate ranged from a 21% reduction to a 34% increase in pain relief. These results differ from a previous report that 1 week of pretreatment with desipramine, 75 mg per day, potentiated postoperative morphine analgesia. We conclude that if desipramine potentiation of opioid analgesia occurs in humans, its demonstration may require higher doses or chronic treatment.