The effect of protein kinase C and G protein-coupled receptor kinase inhibition on tolerance induced by mu-opioid agonists of different efficacy.

Differences in the mechanisms underlying tolerance and mu-opioid receptor desensitization resulting from exposure to opioid agonists of different efficacy have been suggested previously. The objective of this study was to determine the effects of protein kinase C (PKC) and G protein-coupled receptor kinase (GRK) inhibition on antinociceptive tolerance in vivo to opioid agonists of different efficacy. A rapid (8-h) tolerance-induction model was used where each opioid was repeatedly administered to naive mice. Animals were then challenged with the opioid after injection of a kinase inhibitor to determine its effects on the level of tolerance. Tolerance to meperidine, morphine, or fentanyl was fully reversed by the PKC inhibitor 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)carbazole (Gö6976). However, in vivo tolerance to [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) was not reversed by PKC inhibition. The novel small-molecule GRK inhibitors beta-adrenergic receptor kinase 1 inhibitor and 2-(8-[(dimethylamino) methyl]-6,7,8,9-tetrahydropyridol[1,2-a]indol-3-yl)-3-(1-methylindol-3-yl)maleimide (Ro 32-0432) did not reverse the tolerance to meperidine, fentanyl, or morphine but did reverse the tolerance to DAMGO. To correlate GRK-dependent DAMGO-induced tolerance with mu-opioid receptor desensitization, we used in vitro whole-cell patch-clamp recording from mouse locus coeruleus neurons and observed that the GRK inhibitors reduced DAMGO-induced desensitization of mu-opioid receptors, whereas the PKC inhibitor had no effect. These results suggest that tolerance induced by low- and moderate-efficacy mu-opioid receptor agonists is dependent on PKC, whereas tolerance induced by the high-efficacy agonist DAMGO is dependent on GRK.

Role of protein kinase C and mu-opioid receptor (MOPr) desensitization in tolerance to morphine in rat locus coeruleus neurons.

In morphine tolerance a key question that remains to be answered is whether mu-opioid receptor (MOPr) desensitization contributes to morphine tolerance, and if so by what cellular mechanisms. Here we demonstrate that MOPr desensitization can be observed in single rat brainstem locus coeruleus (LC) neurons following either prolonged (> 4 h) exposure to morphine in vitro or following treatment of animals with morphine in vivo for 3 days. Analysis of receptor function by an operational model indicated that with either treatment morphine could induce a profound degree (70-80%) of loss of receptor function. Ongoing PKC activity in the MOPr-expressing neurons themselves, primarily by PKCalpha, was required to maintain morphine-induced MOPr desensitization, because exposure to PKC inhibitors for only the last 30-50 min of exposure to morphine reduced the MOPr desensitization that was induced both in vitro and in vivo. The presence of morphine was also required for maintenance of desensitization, as washout of morphine for > 2 h reversed MOPr desensitization. MOPr desensitization was homologous, as there was no change in alpha(2)-adrenoceptor or ORL1 receptor function. These results demonstrate that prolonged morphine treatment induces extensive homologous desensitization of MOPrs in mature neurons, that this desensitization has a significant PKC-dependent component and that this desensitization underlies the maintenance of morphine tolerance.

Region-dependent attenuation of mu opioid receptor-mediated G-protein activation in mouse CNS as a function of morphine tolerance.

BACKGROUND AND PURPOSE: Chronic morphine administration produces tolerance in vivo and attenuation of mu opioid receptor (MOR)-mediated G-protein activation measured in vitro, but the relationship between these adaptations is not clear. The present study examined MOR-mediated G-protein activation in the CNS of mice with different levels of morphine tolerance. EXPERIMENTAL APPROACH: Mice were implanted with morphine pellets, with or without supplemental morphine injections, to induce differing levels of tolerance as determined by a range of MOR-mediated behaviours. MOR function was measured using agonist-stimulated [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) and receptor binding throughout the CNS. KEY RESULTS: Morphine pellet implantation produced 6-12-fold tolerance in antinociceptive assays, hypothermia and Straub tail, as measured by the ratio of morphine ED(50) values between morphine-treated and control groups. Pellet implantation plus supplemental injections produced 25-50-fold tolerance in these tests. In morphine pellet-implanted mice, MOR-stimulated [(35)S]GTPgammaS binding was significantly reduced only in the nucleus tractus solitarius (NTS) and spinal cord dorsal horn in tissue sections from morphine pellet-implanted mice. In contrast, MOR-stimulated [(35)S]GTPgammaS binding was significantly decreased in most regions examined in morphine pellet+morphine injected mice, including nucleus accumbens, caudate-putamen, periaqueductal gray, parabrachial nucleus, NTS and spinal cord. CONCLUSIONS AND IMPLICATIONS: Tolerance and the regional pattern of apparent MOR desensitization were influenced positively by the level of morphine exposure. These results indicate that desensitization of MOR-mediated G-protein activity is more regionally widespread upon induction of high levels of tolerance, suggesting that this response contributes more to high than low levels of tolerance to CNS-mediated effects of morphine.

Involvement of phospholipid signal transduction pathways in morphine tolerance in mice.

1. Opioid tolerance involves an alteration in the activity of intracellular kinases such as cyclic AMP-dependent protein kinase (PKA). Drugs that inhibit PKA reverse morphine antinociceptive tolerance. The hypothesis was tested that phospholipid pathways are also altered in morphine tolerance. Inhibitors of the phosphatidylinositol and phosphatidylcholine pathways were injected i.c.v. in an attempt to acutely reverse morphine antinociceptive tolerance. 2. Seventy-two hours after implantation of placebo or 75 mg morphine pellets, mice injected i.c.v. with inhibitor drug were challenged with morphine s.c. for generation of dose-response curves in the tail-flick test. Placebo pellet-implanted mice received doses of inhibitor drug having no effect on morphine's potency, in order to test for tolerance reversal in morphine pellet-implanted mice. Injection of the phosphatidylinositol-specific phospholipase C inhibitor ET-18-OCH3 significantly reversed tolerance, indicating a potential role for inositol 1,4,5-trisphosphate (IP3) and protein kinase C (PKC) in tolerance. Alternatively, phosphatidylcholine-specific phospholipase C increases the production of diacylglycerol and activation of PKC, without concomitant production of IP3. D609, an inhibitor of phosphatidylserine-specific phospholipase C, also reversed tolerance. Heparin is an IP3 receptor antagonist. Injection of low molecular weight heparin also reversed tolerance. PKC was also examined with three structurally dissimilar inhibitors. Bisindolylmaleimide I, Go-7874, and sangivamycin significantly reversed tolerance. 3. Chronic opioid exposure leads to changes in phospholipid metabolism that have a direct role in maintaining a state of tolerance. Evidence is accumulating that opioid tolerance disrupts the homeostatic balance of several important signal transduction pathways.

Role of specific dopamine receptor subtypes in amphetamine discrimination.

Biochemical, electrophysiological, and behavioral experiments suggest that the dopamine D-1 and D-2 receptor subtypes functionally interact. In rats trained to discriminate 1.0 mg/kg d-amphetamine, substitution with the D-2 agonist quinpirole (0.1-2.0 mg/kg) produces amphetamine-lever responding, whereas the D-1 agonist SKF 38393 (0.3-10.0 mg/kg) elicits only saline-appropriate responding. Combining either quinpirole (0.05-0.5 mg/kg) or SKF 38393 (0.5-10.0 mg/kg) with 0.3 mg/kg d-amphetamine results in dose-dependent increases in amphetamine-lever responding. Conversely, the D-1 antagonist SCH 23390 (0.02-0.1 mg/kg) antagonizes the discrimination produced by 0.7 mg/kg d-amphetamine. Additional combination studies examined the effect of DA receptor drugs on discrimination when quinpirole is substituted in d-amphetamine trained rats. SKF 38393 (0.5-7.0 mg/kg) fails to increase the amphetamine-appropriate lever response produced by either 0.05 or 0.2 mg/kg quinpirole. Similarly, SCH 23390 (0.01-0.1 mg/kg) fails to antagonize the amphetamine-lever responding produced by either 0.2 or 0.5 mg/kg quinpirole. Haloperidol (0.02-0.2 mg/kg) does antagonize the amphetamine-appropriate response produced by quinpirole substitution. The d-amphetamine discrimination studies indicate that stimulating D-2 receptors alone or D-1 receptors in the presence of d-amphetamine yields d-amphetamine-lever responding, and suggests that D-1/D-2 receptors can functionally interact to alter discrimination behavior. Quinpirole substitution, on the other hand, shows an insensitivity to D-1 receptor manipulations.

Long-term alterations in opiate antinociception resulting from infant fentanyl tolerance and dependence.

Postnatal day-14 (P14) infant rats remained naive or were implanted with osmotic minipumps infusing saline or fentanyl (50 microg kg(-1) h(-1)). Fentanyl was administered 72 h later for measurement of antinociception in the tail-flick test. The potency of fentanyl was 3.0-fold lower in fentanyl-infused compared to saline-infused P17 rats. Fentanyl-infused P17 rats injected with naloxone underwent withdrawal characterized by increases in spontaneous activity, wall climbing, diarrhea, abdominal stretching, forepaw treading/tremors, wet-dog shakes, jumping, ptosis, rhinorrhea and hypothermia. Other naive, saline-infused and fentanyl-infused P17 rats not challenged with fentanyl or naloxone were housed until maturing into P42 juveniles. Fentanyl's potency was equal among each treatment group. However, morphine's potency was reduced in juveniles tolerant to fentanyl as infants. Morphine was also less potent in P90 adults tolerant to fentanyl as infants. Thus, chronic opiate exposure during infancy may affect the developing central nervous system, and desensitize animals and humans to opiate analgesia throughout life.

Characterization of neonatal rat morphine tolerance and dependence.

The administration of morphine and fentanyl by continuous intravenous infusion has been shown to produce analgesic tolerance and physical dependence in human neonates. In animals, daily repeated morphine bolus injections is a common method of inducing neonatal rat tolerance and dependence. Yet this method differs from the intravenous route reported to affect human neonates. Alzet osmotic minipumps were implanted in postnatal day 14 rats to provide a continuous morphine infusion more closely mimicking the clinical picture. Rats remained naive or were infused with saline or morphine (0.7 mg/kg/h) for 72 h. Morphine's antinociceptive potency was similar between naive and saline-infused animals, while morphine-infused animals were tolerant. Gender did not contribute to the degree of tolerance observed. Naloxone precipitated withdrawal in the morphine pump-implanted rats was similar to that reported by others. Thus, minipumps provide a useful model for assessing the tolerance and dependence liability of different opioids.

Ontogeny of mu opioid agonist anti-nociception in postnatal rats.

Mu opiate agonists morphine, fentanyl and meperidine are administered short-term to pediatric patients, from the neonatal period through adolescence. However, there has been no assessment of the effect of age on the analgesic efficacy or the concentration-response relationship for these opioids in human pediatric patients. Few studies in animals have correlated opioid anti-nociception and tissue levels of these opioids commonly administered to pediatric patients. The present study was conducted to examined the role of age on opioid anti-nociceptive potency and efficacy and brain and plasma opioid levels to provide predictive information on the effect of opioids in developing humans. Administration of trace amounts of tritiated drug with anti-nociceptive doses of unlabeled drug was used for the assessment of anti-nociception in the tail-flick test and for the measurement of brain and plasma drug equivalent levels in postnatal rats (PND 3-21). Morphine and fentanyl were completely efficacious in all postnatal ages examined, although age-related differences in drug potency, as well as, differences in brain and plasma levels were observed. There was a good correlation between morphine (r = 0.96) and fentanyl (r = 0.89) ED(50) values and their respective brain and plasma EC(50) equivalent levels. Meperidine had limited efficacy in young rats (PND 3-9) but was completely efficacious in older rats (PND 14-17). However, PND 21 rats experienced tonic-clonic seizures which limited its efficacy to 70% anti-nociception. Our data suggest that pharmacokinetics, the development of the blood-brain barrier and ontogeny of opioid receptor function may play important roles in the sensitivity of postnatal rats to mu receptor agonists.

Morphine tolerance-induced modulation of [3H]glyburide binding to mouse brain and spinal cord.

Modulation by opioids of ATP-gated potassium channels, which regulate in part intracellular calcium levels, was measured by the binding of [3H]glyburide. Scatchard analyses generated a KD for whole brain of vehicle-pretreated mice of 288 pM with a Bmax of 694 fmol/mg protein. In the spinal cord the KD was 0.94 nM and the Bmax was 184 fmol/mg protein. Acute morphine decreased the KD in brain and spinal cord with no change in Bmax. Morphine tolerance increased the KD in brain and spinal cord 2.6- and 2.9-fold, respectively, concurrent with 1.6- and 3.4-fold increases in Bmax. Modulation by morphine of glyburide-sensitive binding sites may contribute at least in part to tolerance to morphine via alterations in intracellular calcium levels in neurons.

Regional cutaneous differences in the duration of bupivacaine local anesthesia in mice.

At this time little is know about local anesthesia of the skin, and the sensitivity of different cutaneous regions to local anesthetics. Studies were conducted in mice to test the hypothesis that cutaneous regions are differentially sensitive to bupivacaine, and to the ability of epinephrine (EPI) to prolong local anesthesia. Infiltration of 0.25% and 0.5% bupivacaine s.c. in the dorsal aspect of the mouse tail produced local anesthesia that lasted 15 and 45 min, respectively, against radiant heat nociception. EPI (1:200,000) prolonged the local anesthetic effects of the 0.25% concentration by 45 min, whereas the effect of the 0.5% concentration was extended by only 15 min. Bupivacaine (0.25% and 0.5%) infiltrated in the dorsal aspect of the hind-paw produced local anesthesia that lasted 5 and 30 min, respectively. EPI prolonged the local anesthetic effects of the 0.25% concentration by only 10 min, whereas EPI did not prolong anesthesia but appeared to increase the efficacy of the 0.5% concentration. These results provide evidence of regional differences in cutaneous sensitivity to local anesthetics, and the ability of EPI to extend the duration of anesthesia.

Morphine self-administration in the rat during adjuvant-induced arthritis.

Rats injected with Freund's adjuvant develop a syndrome resembling human rheumatoid arthritis complete with paw swelling, edema and persistent pain. At the onset of pain, arthritic rats and their pain-free littermate controls (vehicle injection) were allowed to self-administer intravenous morphine (5.0 mg/kg/injection) in a 24 hr/day schedule. Self-injected morphine appeared to provide analgesia in arthritic rats as demonstrated by a decreased sensitivity to applied tail pressure. Arthritic rats self-inject significantly less morphine than pain-free animals. Injection of indomethacin, which alleviates the pain and inflammation of the adjuvant-induced disease, reduces, at least initially, morphine self-injection in the arthritic but not pain-free animals. As the adjuvant-induced inflammation and pain dissipated, arthritic rats rapidly began to increase opioid intake. The presence of persistent pain apparently reduces the addictive properties of morphine.

Fluoxetine-induced attenuation of amphetamine self-administration in rats.

Daily injections of fluoxetine (5.0 mg/kg i.p.) to rats trained to self-administer intravenous d-amphetamine produced marked decreases in drug intake on three successive days of treatment. After fluoxetine injections were stopped, the number of daily amphetamine self-injections was still significantly reduced for an additional 2 days. When trained amphetamine self-administration animals were placed in an apparatus which delivered i.v. saline with each lever press, increased self-injection is observed. Acute fluoxetine injection did not alter this response. However, if fluoxetine is given prior to amphetamine exposure for 1 day and animals are then tested for the saline response, lever pressing activity is significantly reduced. These data might suggest that 5-hydroxytryptaminergic neurons mediate some aversive or negative reinforcing property of amphetamine. If true, this finding could be exploited clinically in cases of human psychomotor stimulant addiction.

The hypothermic and antinociceptive effects of intrathecal injection of CGRP (8-37) in mice.

Calcitonin gene-related peptide (CGRP) and the selective antagonist properties of the peptide fragment of CGRP [CGRP (8-37)] have been the subjects of numerous investigations. These data represent the first demonstration of the hypothermic and antinociceptive effects of CGRP (8-37). Intrathecal injection of CGRP (8-37) in mice produced hypothermia which differed from that produced by CGRP in time course, duration of action and potency. CGRP and CGRP (8-37) did not alter blood flow. Thus, a direct vasodilatory action was not responsible for the acute hypothermic effects of the drugs. The combination of CGRP and CGRP (8-37) resulted in a decrease in body temperature which was no greater than that of either drug alone. We failed to observe any significant antinociceptive effect in the tail-flick assay after i.t. injection of CGRP (8-37), but dose-dependent antinociception was produced by CGRP (8-37) in the p-phenylquinone (PPQ) assay, with an ED50 value of 6.0 micrograms. However, CGRP (8-37) failed to block or enhance the antinociception produced by CGRP over a wide dose range. In addition to demonstrating an agonist-like effect for CGRP (8-37), these data also indicate that CGRP and CGRP (8-37) may not act through a common mechanism.

Influence of dopaminergic and serotonergic neurons on intravenous ethanol self-administration in the rat.

Rats implanted with chronic indwelling intravenous catheters and allowed access to a self-administration apparatus learned to self-inject intravenous ethanol. Ethanol concentrations of 0.5, 1.0, and 2.0%, corresponding to a dose/injection of 1, 2, and 4 mg/kg, respectively, were consistently self-injected. Self-injection was not acquired or maintained with ethanol doses of 0.5 or 8 mg/kg/injection. Saline replacement of ethanol reservoirs led to marked increases in lever-pressing response in animals self-injecting 1, 2, and 4 mg/kg ethanol/injection but not with 0.5 or 8 mg/kg/injection. Neurotoxin-induced lesions of dopamine-(DA) containing neurons in nucleus accumbens septi failed to alter the acquisition or maintenance of ethanol self-injection. Pretreatment with haloperidol (0.05 and 0.1 mg/kg, SC) failed to alter hourly or daily self-injection rates. On the other hand, p-chlorophenylalanine pretreatment increased, while fluoxetine (2.5 and 5.0 mg/kg) administration significantly reduced, self-injected intravenous ethanol. These data suggest that ethanol is self-injected by the rat in a narrow dose range and that 5-hydroxytryptamine (5-HT), but not DA-containing neurons, subserves some function in the reinforcing or aversive affects of ethanol.

Endogenous dynorphin modulates calcium-mediated antinociception in mice.

We previously reported that calcium administered IT produces antinociception by stimulating spinal Met-enkephalin release. However, at times the antinociceptive effects of calcium in the tail-flick test are greatly diminished. The results of this study indicates that during these periods calcium also stimulates endogenous dynorphin release. Dynorphin has been reported to block opiate-induced antinociception. Calcium-injected mice (150-600 nmol, IT) pretreated with vehicle IP displayed a poor degree of antinociception. Alternatively, pretreating mice with pentobarbital (45 mg/kg, IP) restored the antinociceptive effects of calcium. Low doses of naloxone and norbinaltorphimine (BNI) did not produce antinociception but restored the antinociceptive effects of calcium. Dynorphin (1-17) (Dyn 1-17), and Dyn (1-13), but not Dyn (1-8), blocked the antinociceptive effects of calcium restored with pentobarbital. These results indicate that calcium-mediated antinociception was sensitive to injected dynorphins. In additional experiments, antiserum to Dyn (1-13) was found to restore the antinociceptive effects of calcium, presumably by binding dynorphin released by calcium.

The enhancement of morphine antinociception in mice by delta9-tetrahydrocannabinol.

We have previously reported that intracerebroventricular or intrathecal administration of inactive doses of delta9-tetrahydrocannabinol (THC) greatly enhance the antinociceptive potency of morphine in the mouse tail-flick test. Experiments were conducted to test the hypothesis that morphine's potency would be enhanced in mice receiving THC and morphine by conventional per os (p.o.) and subcutaneously (s.c.) routes of administration. Antinociception was measured in the tail-flick test of radiant heat after administration of different combinations of THC and morphine p.o. and s.c. Subcutaneous administration of THC (4 and 25 mg/kg) increased the potency of s.c. morphine 8.5- and 22.3-fold, respectively, while s.c. THC (25 mg/kg) increased the potency of p.o. morphine 3.1-fold. Per os administration of THC (10 and 20 mg/kg) increased the potency of s.c. and p.o. morphine 11.4-fold and 7.6-fold, respectively. Thus, morphine's potency was significantly increased regardless of the enteral and parenteral routes of THC and morphine administration. The synthetic receptor selective cannabinoid CP-55, 940 (0.1 mg/kg, s.c.) also enhanced morphine's potency. Finally, the ability of the CB1 receptor antagonist SR141716A to antagonize the enhancement of morphine by THC indicates that THC was acting through a cannabinoid receptor mechanism.

Characterization of delta9-tetrahydrocannabinol and anandamide antinociception in nonarthritic and arthritic rats.

Little is known about the effectiveness of delta9-tetrahydrocannabinol (THC) and anandamide in blocking mechanical nociception. Even less is known about their antinociceptive efficacy in chronic inflammatory arthritis induced by Freund's complete adjuvant. The hypothesis was tested that THC and anandamide elicit antinociception in the paw pressure test, and that arthritic rats would exhibit a different response. In nonarthritic rats, THC- and anandamide-induced antinociception lasted 90 min and 15 min, respectively, while antinociception lasted 90 min and 30 min, respectively, in arthritic rats. Area under the curve calculations revealed no effect of arthritis on THC- and anandamide-induced antinociception. Another hypothesis was that paw pressure thresholds in arthritic rats reflect chronic cannabinoid receptor stimulation due to elevations in free anandamide levels. Yet, the CB1 receptor antagonist SR141716A failed to alter paw pressure thresholds in either nonarthritic or arthritic rats. Further investigation revealed that SR141716A significantly blocked THC antinociception, with no effect on anandamide. Thus, anandamide's effects did not result from CB1 receptor stimulation, and any potential contribution of endogenous anandamide in arthritis was not revealed. Finally, THC and anandamide appear to release an as yet unknown endogenous opioid, because naloxone significantly blocked their effects. This study indicates that anandamide and THC may act at different receptor sites to modulate endogenous opioid levels in mechanical nociception.

Involvement of intracellular calcium in morphine tolerance in mice.

Opioid analgesic tolerance is associated with a disruption in Ca++ homeostasis. Drugs reducing Ca++ influx can prevent and reverse tolerance. The hypothesis was tested that both Ca++ influx and mobilization from intracellular pools maintains the expression of morphine tolerance. Ca++ modulating drugs were injected ICV at doses not affecting morphine's potency in placebo pellet-implanted mice, in order to determine whether tolerance would be reversed in morphine pellet-implanted mice. The Ca++ chelator EGTA significantly reversed tolerance. The Ca++ channel antagonists nifedipine and omega-conotoxin GVIA also reversed tolerance. The role of intracellular Ca++ was investigated using the membrane permeable intracellular Ca++ chelator EGTA-AM. EGTA-AM reversed tolerance at lower morphine doses, but not at higher morphine doses. Thus, mobilization of intracellular Ca++ contributes to the expression of tolerance. Finally, 1,4-dihydropyridine-sensitive Ca++ channels are known to stimulate Ca++-induced Ca++ release (CICR) from Ca++/caffeine-sensitive microsomal pools possessing ryanodine receptors. We examined whether blocking Ca++ mobilization from these pools with ryanodine would reverse morphine tolerance. Ryanodine's effects were similar to EGTA-AM. Tolerance was reversed at lower morphine doses, but not at higher doses. Thus, morphine tolerance appears to be associated with increases in Ca++ influx and mobilization from Ca++/caffeine-sensitive pools.

Tyrosine influence on amphetamine self-administration and brain catecholamines in the rat.

Earlier work had shown that L-tyrosine administration, precursor to both dopamine (DA) and norepinephrine (NE), could increase brain DA metabolite concentrations after amphetamine treatment and restore amphetamine-induced decreases in whole brain NE. Both monoamines have been suggested to participate in some aspects of continued drug abuse. Rats trained to self-administer IV d-amphetamine were treated with IP tyrosine during test sessions to examine the behavioral and neurochemical response. In animals with less than 35 days of amphetamine exposure, L-tyrosine treatments did not alter amphetamine self-administration. Experiments using a computer-controlled injection apparatus which administered IV amphetamine to naive rats in patterns mimicking those of self-administration animals indicated tyrosine could antagonize amphetamine-induced NE depletions. The increases in DA metabolite dihydroxyphenylacetic acid (DOPAC) were found limited to the striatum, an area not involved in the positive reinforcing effects of amphetamine. Concentrations of DOPAC in nucleus accumbens septi were unchanged by the amphetamine or the amphetamine-tyrosine regimen. In rats with 4-6 months of chronic amphetamine exposure, however, L-tyrosine administration significantly reduced daily drug self-injection. While neurochemical responses to tyrosine could not be performed, it is speculated that chronic long-term amphetamine abuse might alter the tyrosine-induced changes in DA and/or NE synthesis and release compared to that in the acute or short-term amphetamine abuse animals. These data suggest that the success or failure of an experimental pharmacologic treatment strategy in psychomotor stimulant abusers might be dependent on the subjects history of drug abuse.

Dietary tryptophan supplements attenuate amphetamine self-administration in the rat.

Previously, it had been shown that lesions of cerebral 5-hydroxytryptamine (5-HT)-containing neurons and injections of drugs affecting 5-HT synthesis or receptor mediated function would alter amphetamine self-administration in the rat. The present study sought to ascertain whether diets enriched in L-tryptophan (L-TRY), the amino acid precursor to 5-HT, would: elevate cerebral 5-HT concentrations and affect amphetamine self-administration behavior. Diets containing 2.0 and 4.0% L-TRY increased cerebral 5-HT concentrations above those of rats on normal rat chow (0.26% L-TRY). The 4.0% diet elevated brain 5-HT to the same degree in rats exposed to the diet for 1, 2 or 3 days. When normal diets were restored, brain 5-HT concentrations rapidly returned to normal. Animals trained to self-administer d-amphetamine, when given access to the L-TRY enriched diets, significantly reduced their daily amphetamine self-injection during exposure periods. When normal rat chow was restored a delay in recovery to pre-diet amphetamine self-administration was observed: 1 day with the 2.0% L-TRY diet and 2 days with the 4.0% L-TRY diet. The 4.0% L-TRY diet failed to alter saline-frustration responding indicating the diet did not produce decrements in motor performance. When animals were placed on the 4.0% L-TRY diet and allowed access to amphetamine for 1 day then exposed to saline, a profound decrease in saline-frustration responding was observed.(ABSTRACT TRUNCATED AT 250 WORDS)