Nicotine dependence: studies with a laboratory model

Simple, rapid preclinical models of nicotine physical dependence and abstinence syndrome are needed to identify underlying neurobiological mechanisms and screen potential therapies. One such model induces dependence by 7 days of continuous subcutaneous nicotine infusion in the rat. Abstinence is initiated through termination of infusion or injection of nicotinic antagonist drugs. The result is an abstinence syndrome involving a pattern of behaviors somewhat resembling opiate abstinence in the rat as well as weight gain and depressed locomotor activity. The model has met a number of validity criteria and its essential features have been replicated in several laboratories. Several research groups have modified or extended the model by measuring emotional/motivational changes associated with nicotine abstinence such as conditioned aversion, intracranial self-stimulation (ICSS) thresholds and the startle response. Dependence models have been used to identify neurobiological systems that contribute to nicotine dependence, particularly endogenous opiate systems and the mesolimbic dopamine pathway. It is hypothesized that these different systems contribute to different behavioral aspects of nicotine abstinence syndrome. Increasingly used as a preclinical screening tool, the model has proved sensitive to various abstinence-alleviating therapeutic approaches, including some with already demonstrated clinical effectiveness.

Hippocampal injections of amyloid beta-peptide 1-40 impair subsequent one-trial/day reward learning

The injection of amyloid beta-peptide (Abeta) into rat CNS has been reported to induce cellular neuropathology. The present study investigated whether multiple intrahippocampal injections of Abeta 1-40 would impair one-trial/day reward learning 14 days later. Twenty-four male Sprague-Dawley rats, 3-4 months old, were injected with either Abeta 1-40 or distilled water into seven hippocampal sites bilaterally. Ten rats received 3 nmol Abeta 1-40 in 2 microl of distilled water per injection site, while 14 rats received distilled water alone. Following a 9-day recovery period, rats were gradually food deprived to 82% of their initial body weight. Fourteen days after the intrahippocampal injection, all rats received an initial training trial and three subsequent daily retention trials. Rats receiving Abeta 1-40 were significantly impaired on the second retention trial in terms of accuracy (number of unbaited alleys entered) and on the second and third retention trials in terms of speed (reciprocal of latency to reward). Histological analysis showed that Abeta 1-40 injections produced significant neuronal loss and gliosis. Abeta 1-40 immunoreactivity persisted locally at the injection site and in macrophages 2 weeks following the hippocampal injections. These effects appear to be sequence-specific; rats receiving Abeta 1-42 with a scrambled peptide sequence did not differ significantly from rats receiving distilled water alone in retention of the learning task or degree of histological damage.

Passive immunization against nicotine prevents nicotine alleviation of nicotine abstinence syndrome

Passive immunization against nicotine interferes with its locomotor and pressor effects. The current study determined whether immunization could prevent another nicotine action: the reversal of nicotine abstinence syndrome. IgG containing 4.4-5.6% nicotine-specific antibody was isolated from rabbits immunized with 3'-amino-methyl-nicotine conjugated to a carrier protein. Twenty rats were rendered dependent by 7 days of subcutaneous infusion of 3.15 mg/kg/day nicotine (expressed as the base). Upon termination of nicotine infusion, each rat was injected intraperitoneally with 150 mg of IgG from normal serum (n=13) or from nicotine antiserum (n=7). Twenty-two and one-half hours later, all rats were observed over 15 min for baseline nicotine abstinence signs. Two and one-half hours after baseline observations, seven of the 13 rats pretreated with control IgG and all seven rats pretreated with nicotine-specific IgG were then challenged by 0.12 mg/kg (sc) nicotine. The remaining six rats pretreated with control IgG were challenged with saline alone. All rats were then observed again for abstinence signs. Nicotine injection caused significantly less reduction of abstinence signs in the immunized rats. The nicotine effect in immunized rats was comparable to the saline effect in nonimmunized rats. Immunization also significantly reduced free serum nicotine concentration and nicotine distribution to the brain. These results raise the possibility that immunization might prevent nicotine consumption from relieving the discomforts of smoking cessation.

Nociceptin/orphanin FQ (N/OFQ) induces a quasi-morphine abstinence syndrome in the rat

Nociceptin/orphanin FQ (N/OFQ) is a neuropeptide that exerts antiopiate effects under some circumstances, and there is evidence that it contributes to opiate tolerance. This raises the question, might N/OFQ also contribute to opiate dependence and abstinence? Twenty-two male Sprague-Dawley rats were cannulated in the third ventricle and challenged 7 days later by third ventricle injection of 50, 200 or 1,000 ng N/OFQ or saline alone. Each rat was observed under "blind" conditions for 30 min beginning 15 min after onset of the third ventricle injection. There was a significant positive linear trend of signs as a function of N/OFQ dose. Subjects receiving saline had 18.0+/-2.0 (mean+/-SEM) overall abstinence-like signs, whereas subjects receiving 50, 200 or 1000 ng N/OFQ had 35.2+/-3.6, 49.8+/-2.6 and 63.5+/-9.7 signs, respectively. In 16 additional rats, abstinence-like signs induced by 1000 ng N/OFQ were significantly attenuated by low SC doses of morphine or clonidine. These results raise the possibility that N/OFQ might contribute to opiate dependence and subsequent abstinence syndrome. On the other hand, N/OFQ over a wide dose range induced abstinence signs with similar potency in morphine dependent and non-dependent rats.

A rodent model of cocaine abstinence syndrome

This study introduces a rat model of cocaine abstinence syndrome based on quantitation of spontaneously emitted behaviors following termination of continuous drug exposure (analogous to established methods of assessing morphine and nicotine abstinence). Groups of eight male S-D rats were infused SC for 7 days via an osmotic minipump with saline alone or with 40 or 60 mg/kg/day cocaine HCl. Pumps were removed and rats were observed at 12, 24, 36, and 48 h postremoval. Each 15-min observation employed a checklist of abstinence signs including ptosis, chews, teeth chatters, gasps, writhes, seminal ejaculations, head shakes, and tremors. The high infusion rate group displayed significantly more signs than the low infusion rate group, which in turn, displayed significantly more signs than the saline group. Cocaine injection significantly reduced signs by 83.3%, while saline injection reduced them by only 4.9%. In another experiment, rats infused with 60 mg/kg/day showed significantly more signs 36 h postinfusion than before infusion, during infusion and 84 h postinfusion. Finally, 6.5 days of infusion resulted in significantly more abstinence signs than did 1.5 days of infusion. This rapid and simple model quantitated cocaine abstinence syndrome in a manner that was cocaine-reversible and related to the rate and duration of drug infusion.

A nicotine conjugate vaccine reduces nicotine distribution to brain and attenuates its behavioral and cardiovascular effects in rats

Vaccination of animals to elicit drug-specific antibodies, or the passive transfer of such antibodies from other animals, can reduce the behavioral effects of drugs such as cocaine and heroin. To study the potential application of this approach to treating nicotine dependence, IgG was isolated from rabbits immunized with a nicotine-protein conjugate vaccine. Anesthetized rats received immune IgG containing nicotine-specific antibodies (Nic-IgG) or control-IgG i.v.. Thirty minutes later, rats received nicotine at 0.03 mg/kg i.v., equivalent on an mg/kg basis to the nicotine intake from two cigarettes by a smoker. Compared to control-IgG, Nic-IgG reduced the brain nicotine concentration in a dose-related manner (65% reduction at the highest IgG dose). Pretreatment with Nic-IgG also reduced the distribution to brain of five repeated doses of nicotine (equivalent to the nicotine intake from 10 cigarettes) administered over 80 min. To study blood pressure effects, rats received control-IgG or Nic-IgG 1 day prior to administering nicotine. Nicotine-induced systolic blood pressure increases were attenuated by Nic-IgG in a dose-related manner, and were almost completely blocked by the highest Nic-IgG dose. Pretreatment with Nic-IgG also completely prevented the nicotine-induced stimulation of locomotor activity observed in rats receiving control-IgG. Nic-IgG did not prevent locomotor activation from cocaine, demonstrating its specificity for nicotine. These data demonstrate that the administration of nicotine-specific antibodies can reduce or prevent some of the pharmacokinetic, cardiovascular, and behavioral consequences of nicotine in rats. Effects were observed at nicotine doses and nicotine serum concentrations equal to or exceeding those typically associated with nicotine exposure in cigarette smokers. A potential role for immunization in the treatment of nicotine dependence is suggested.

The nitric oxide synthesis inhibitor nitro-L-arginine (L-NNA) attenuates nicotine abstinence syndrome in the rat

Nitric oxide synthesis contributes to opiate tolerance and dependence. Nicotine dependence and abstinence syndrome in the rat appear to involve opiate mechanisms. Therefore, it was postulated that nitric oxide synthase (NOS) activity might be essential for the expression of nicotine abstinence syndrome. Twenty-one rats were rendered dependent by SC infusion of 9 mg/kg per day nicotine tartrate via Alzet osmotic minipump. Rats were pretreated SC with vehicle alone, or with 18 or 30 mg/kg of the NOS inhibitor L-NNA (nitro-L-arginine). Thirty minutes later, rats were challenged by 1 mg/kg of the nicotinic antagonist mecamylamine SC and observed for 30 additional minutes. Rats pretreated with vehicle displayed a total of 68.7+/-8.0 mecamylamine-precipitated abstinence signs (mean+/-SEM), while those receiving 18 or 30 mg/kg L-NNA had 12.7+/-2.0 and 5.1+/-1.7 signs, respectively. All three groups differed significantly from one another according to Dunn's post-hoc procedure. Rats pretreated with L-NNA combined with an excess of the NOS substrate L-arginine had significantly more mecamylamine-precipitated abstinence signs than rats receiving L-NNA combined with D-arginine. Also, D-NNA, which does not selectively bind to NOS, was significantly less effective than L-NNA in preventing mecamylamine-precipitated abstinence syndrome. Additional studies determined the effect of L-NNA on spontaneous nicotine abstinence syndrome. Rats were assessed for abstinence signs at 17 and 20 h after termination of nicotine infusion. They received injections of 9, 18, or 30 mg/kg L-NNA SC or vehicle alone immediately before the 20-h observation; all rats were observed for 30 min. Signs at 20 h (post-injection) as a percentage of signs at 17 h (pre-injection) declined significantly as a function of L-NNA dose. Once again, this effect was attenuated significantly more by co-administration of L-arginine than by D-arginine. The overall pattern of results suggests that nitric oxide synthesis is critical to the expression of nicotine abstinence syndrome.

Nicotine abstinence syndrome precipitated by the competitive nicotinic antagonist dihydro-beta-erythroidine

Rats infused subcutaneously with 9 mg/kg/day nicotine tartrate for 7 days exhibit behavioral abstinence signs following either termination of nicotine infusion or injection of the noncompetitive nicotinic antagonists mecamylamine (s.c.) or hexamethonium (ic.c.v.). This study examined the abstinence precipitating effects of dihydro-beta-erythroidine (DHbetaE), a competitive nicotinic antagonist. Twenty-four nicotine-dependent rats were injected in the third ventricle with 10, 18, or 25 microg DHbetaE in 20 microl saline or with saline alone and observed for abstinence signs over a 20-min period. There was a significant positive linear trend of overall abstinence signs as a function of dose, p < 0.01. In 12 nondependent rats, the high dose of DHbetaE did not induce more abstinence-like signs than saline alone. In a second experiment, 18 nicotine-dependent rats were injected s.c. with 1 or 6 mg/kg of the muscarinic antagonist scopolamine or with saline alone. Few abstinence signs were observed in any group: there was no significant drug effect. The results suggest that nicotine abstinence signs observed in the rat are specific to reduced stimulation of previously overstimulated nicotinic receptors.

Nicotine abstinence syndrome precipitated by central but not peripheral hexamethonium

A rodent model of nicotine dependence has been developed based on continuous subcutaneous (s.c.) infusion of nicotine tartrate. Nicotine abstinence syndrome was precipitated by s.c. injection of the nicotinic antagonist mecamylamine, which freely crosses the blood-brain barrier. In contrast, the nicotinic antagonist hexamethonium crosses the blood-brain barrier very poorly. This study determined whether central or peripheral administration of hexamethonium could precipitate nicotine abstinence. In the first experiment, 26 nicotine-dependent rats were injected s.c. with 0.5, 5 or 10 mg/kg hexamethonium dichloride or saline alone and observed for 20 min. Few abstinence signs were observed in any group; there was no significant drug effect. In the second experiment, 18 rats were cannulated in the third ventricle and rendered nicotine dependent. One week later, rats were injected through the cannula with 12 or 18 ng hexamethonium or saline alone and observed for 20 min. Both dose groups differed significantly from the saline-injected group, and there was a significant positive linear trend of signs as a function of dose. The high dose had no significant effect in 14 nondependent rats. We conclude that hexamethonium is much more potent by the central route, and there is a major central nervous system component in nicotine dependence.

Nicotine abstinence syndrome precipitated by an analog of neuropeptide FF

In a recently introduced rodent model of nicotine abstinence syndrome the observed behavioral signs closely resembled those typical of rat opiate abstinence syndrome. Nicotine-induced release of endogenous opioids may contribute to nicotine dependence; morphine potently reverses nicotine abstinence signs, while naloxone precipitates abstinence signs and prevents nicotine from alleviating them. Considerable evidence suggests that neuropeptide FF, an endogenous antiopiate peptide, contributes to opiate dependence. Third ventricle injection of neuropeptide FF precipitates abstinence syndrome in morphine-dependent rats, as does SC injection of its lipophilic analogs, dansyl-PQRFamide and dansyl-RFamide. Might NPFF also play a role in nicotine dependence? In the present study, SC injection of 15 or 25 mg/kg dansyl-RFamide or vehicle alone dose dependently precipitated an abstinence syndrome in nicotine-dependent rats. There was a significant, p < 0.01, positive linear trend of abstinence signs as a function of dose. Categories of abstinence signs had the same rank ordering by frequency as observed in spontaneous nicotine abstinence. Injection of 25 mg/kg dansyl-RFamide SC had no significant effect in nondependent rats.

Nicotine alleviation of nicotine abstinence syndrome is naloxone-reversible

In a recently introduced rodent model of nicotine abstinence syndrome, the observed signs closely resembled those typical of rat opiate abstinence syndrome. Signs were precipitated by naloxone and potently reversed by morphine as well as nicotine itself, suggesting that nicotine might relieve nicotine abstinence syndrome through releasing endogenous opioids. To test this hypothesis, rats were continuously infused subcutaneously (SC) for 7 days with 9 mg/kg per day nicotine tartrate. Each rat was observed for abstinence signs at 18 and 21 h after termination of infusion. Three minutes before the 21-h test, all rats received 0.35 mg/kg nicotine tartrate, SC; 5 min before the nicotine injection, subjects received 9 or 4.5 mg/kg naloxone or saline alone, SC. Abstinence reversal scores were calculated as signs at 21 h as a percentage of signs at 18 h. Naloxone prevented nicotine alleviation of nicotine abstinence in a dose-related manner. However, naloxone in the absence of a nicotine injection had no effect on abstinence severity in either highly dependent or moderately dependent rats (infused with 9 or 5 mg/kg per day nicotine tartrate, respectively). These results support the hypothesis that endogenous opioids play a role in nicotine dependence and abstinence.

Nitric oxide synthesis inhibition attenuates behavioral actions of neuropeptide FF

Neuropeptide FF (NPFF) has certain antiopiate actions and may play a role in opiate tolerance and dependence. Third ventricle injection of 10 micrograms NPFF induces a quasimorphine abstinence syndrome in opiate-naive rats. Nitric oxide synthesis may also contribute to opiate tolerance and dependence. The present study tests the hypothesis that NPFF acts through stimulation of nitric oxide synthase (NOS). Third ventricular injection of 10 micrograms NPFF precipitated an average of 46 abstinence-like signs during a 20-min observation. Pretreatment (30 min earlier) with 7.5 or 15 mg/kg s.c. of the NOS inhibitor nitro-L-arginine (L-NNA) resulted in a significant and dose-dependent alleviation of NPFF-induced abstinence-like signs. The anti-NPFF activity of 15 mg/kg L-NNA was blocked by 750 mg/kg L-arginine, but not by the same amount of D-arginine, indicating that L-NNA attenuates NPFF activity through a stereospecific inhibition of NOS.

Enhanced antiopiate activity and enzyme resistance in a peptidomimetic of FMRFamide containing E-2,3-methanomethionine and E-2,3-methanophenylalanine

FMRFamide is a molluscan peptide that has shown antiopiate activity in a number of mammalian test systems. Peptidomimetics of FMRFamide substituted with conformationally constrained stereoisomers of Z-2,3-methanomethionine or E-2,3-methanomethionine precipitated abstinence syndrome far more potently than FMRFamide itself. The current study determined the effect on antiopiate potency of an additional rigid substitution. A peptidomimetic containing a stereoisomer of E-2,3-methanomethionine was compared with a peptidomimetic additionally substituted at the C-terminal with E-2,3-methanophenylalanine. Morphine abstinence signs were observed after varying doses (0.125-25.0 micrograms) of these two peptidomimetics were injected into the third ventricle of morphine-dependent rats. The peptidomimetic containing both rigid substitutions was far more potent than the peptidomimetic of FMRFamide containing methanomethionine alone. The increased potency appears to be related to enzyme resistance rather than receptor affinity.

Subcutaneous injection of an analog of neuropeptide FF prevents naloxone-precipitated morphine abstinence syndrome

There is evidence that neuropeptide FF (NPFF) has antiopiate activity and may play a role in opiate dependence and subsequent abstinence syndrome. A fragment of NPFF was modified at the C-terminal in an effort to convert it to an NPFF antagonist. It was also dansylated at the N-terminal in an effort to render it more lipophilic and increase its penetration of the blood-brain barrier. Third ventricle administration of the resulting compound, dansyl-PQRamide (0.75 microgram and 1 microgram), dose-dependently antagonized the quasi-morphine abstinence activity of NPFF (10 micrograms) in opiate-naive rats. Subcutaneous injection of dansyl-PQRamide (13 mg/kg) in chronically morphine-infused rats attenuated opiate dependence as indicated by prevention of naloxone-precipitated abstinence syndrome. Dansyl-PQRamide displaced radiolabelled ligand from NPFF receptors in a concentration-dependent manner with a Ki of 13 microM, and had a half-life over 300 times longer than NPFF under aminopeptidase digestion.

The nicotinic antagonist mecamylamine precipitates nicotine abstinence syndrome in the rat

Recently, a rodent model of nicotine abstinence syndrome has been developed based on observing the frequency of spontaneous behavioral signs following termination of continuous subcutaneous infusion of nicotine tartrate. In the present study, the nicotinic antagonist mecamylamine precipitated an abstinence syndrome in nicotine-dependent rats. Twelve rats were each infused for 7 days with 9 mg/kg per day nicotine tartrate in saline via Alzet osmotic minipumps; another 12 rats were sham-operated and remained nicotine-naive. Six rats from each group received 1 mg/kg mecamylamine in saline SC immediately before a 30-min observation, while the remaining six rats from each group received saline alone. Nicotine-infused rats receiving mecamylamine exhibited significantly more (P < 0.01), overall abstinence signs than all other groups. In terms of categories of signs, they displayed significantly more gasps/writhes, teeth chatter/chews, shakes/tremors and ptosis. In a second experiment utilizing only nicotine-naive rats, a far higher dose of mecamylamine (5 mg/kg sc) induced a quasi-nicotine abstinence syndrome. The results provide further validation for this rodent model of nicotine abstinence syndrome.

Chronic methanesulfonyl fluoride enhances one-trial per day reward learning in aged rats

Aged (24-month-old) rats were treated chronically with methanesulfonyl fluoride (MSF), an acetylcholinesterase (AChE) inhibitor with selectivity for central nervous system AChE, or with injection vehicle alone. Twelve 0.22 mg/kg IP injections were given over 4 weeks. MSF rats showed significantly greater speed and accuracy on a 1 trial/day discriminative reward learning task. The chronic MSF treatment resulted in a 56% decrease in brain AChE activity but no discernable locomotor side effects and no liver damage as indicated by aspartate transferase activity.

Enhanced antiopiate activity and enzyme resistance in peptidomimetics of FMRFamide containing (E)-2,3-methanomethionine

FMRFamide is a molluscan peptide that has shown antiopiate activity in a number of mammalian test systems. The current study determined the antiopiate potency of FMRFamide and two conformationally constrained peptidomimetics of FMRFamide containing stereoisomers of (E)-2,3-methanomethionine. Morphine abstinence signs were observed after varying doses (0.25-25.0 microgram) of these substances were injected into the third ventricle of morphine-dependent rats. Both peptidomimetics were far more potent than FMRFamide itself. In addition, although both peptidomimetics bound with lower affinity than FMRFamide to rat spinal cord receptors for NPFF (the mammalian FMRFamide-like peptide), they were far more resistant than FMRFamide to enzymatic degradation by leucine aminopeptidase.

Naloxone precipitates nicotine abstinence syndrome in the rat

Recently, a rodent model of nicotine abstinence syndrome has been developed based on continuous subcutaneous infusion of nicotine tartrate and observing the frequency of spontaneous behavioral signs following termination of infusion. The observed signs closely resemble those commonly seen in rat opiate abstinence syndrome, raising the possibility that there is an endogenous opioid component in nicotine dependence. The present study demonstrates that the opiate antagonist naloxone can precipitate an abstinence syndrome in nicotine-dependent rats. Fourteen rats were infused for 7 days with 9 mg/kg/day nicotine tartrate in saline via an Alzet osmotic minipump. Fourteen rats were sham-operated and remained nicotine-naive. Half of each group received 4.5 mg/kg naloxone SC immediately before a "blind" 15-min observation, while the other half received saline alone. ANOVA revealed significant nicotine infusion, naloxone injection and interaction effects. Post-hoc analysis showed that the nicotine-infused rats injected with naloxone had significantly more signs than all other groups (P < 0.01). In a second experiment, 2 mg/kg morphine sulfate SC produced a significant (P < 0.01) 91.2% reduction of spontaneous abstinence signs observed 21 h after termination of nicotine infusion. These results are consistent with the hypothesized endogenous opioid component in nicotine dependence and abstinence syndrome.

Enhanced antiopiate activity in peptidomimetics of FMRFamide containing Z-2,3-methanomethionine

FMRFa is a molluscan peptide that has shown antiopiate activity in a number of mammalian test systems. The current study determined the antiopiate potency of FMRFa and two conformationally constrained peptidomimetics of FMRFa containing stereoisomers of Z-2,3-methanomethionine. Morphine abstinence signs were observed after varying doses (0.25-25.0 micrograms) of these substances were injected into the third ventricle of morphine-dependent rats. Although both peptidomimetics were far more potent than FMRFa itself, they bound with lower affinity than FMRFa to rat spinal cord receptors for the mammalian FMRFa-like peptide, NPFF.

Subcutaneous injection of an analog of neuropeptide FF precipitates morphine abstinence syndrome

Neuropeptide FF (NPFF) has been shown to exert various antiopiate actions, including precipitation of opiate abstinence syndrome by third ventricle injection in morphine dependent rats. In the present study, dansyl-Pro-Gln-Arg-Phe-amide, a lipophilic analog of NPFF, was injected into morphine dependent rats and appropriate sham controls at a dose of 9 mg/kg s.c. Comparison groups were injected with ethanol/water vehicle alone. The NPFF analog precipitated a vigorous opiate abstinence syndrome in morphine dependent rats, but not in sham controls.

Analog of neuropeptide FF attenuates morphine tolerance

Previous studies suggest that neuropeptide FF (NPFF) plays a role in opiate dependence and subsequent abstinence syndrome. Endogenous NPFF also appears to play a role in opiate tolerance since third ventricle injection of IgG from NPFF antiserum selectively restores morphine sensitivity in morphine-tolerant rats. The NPFF analog, desamino YFLFQPQRamide (daY8Ra) has previously antagonized behavioral effects of NPFF and has attenuated morphine dependence. The present study assessed whether daY8Ra could similarly attenuate morphine tolerance. Third ventricle (i.c.v.) injection of daY8Ra restored the analgesic response to i.c.v. morphine in morphine-tolerant rats (radiant heat tail flick test). Saline injection failed to produce this effect. In opiate-naive rats, however, the same treatment with daY8Ra did not affect the analgesic response to i.c.v. morphine. Thus, daY8Ra appears to selectively restore morphine sensitivity in opiate-tolerant animals. These results further support the hypothesis that endogenous NPFF contributes to opiate tolerance.

Rodent model of nicotine abstinence syndrome

Few animals models are currently in use for the recognized clinical problem of nicotine dependence and abstinence. This study introduces a rapid and convenient model using the rat. Sixteen male rats were rendered nicotine dependent by 7 days of continuous subcutaneous infusion of either 3 mg/kg/day (n = 8) or 9 mg/kg/day (n = 8) nicotine tartrate salt; 8 control rats were infused with saline alone. Rats were observed for 15 min before, during, and after the drug infusion period using a tally sheet modified from a standard checklist of opiate abstinence signs. There were few signs observed in any group at baseline and at the end of the infusion period. However, nicotine-infused rats showed a significant, dose-related increase over the control group at 16 h after the end of infusion, largely subsiding by 40 h. The most frequently observed signs during withdrawals included: teeth-chattering/chews, writhes/gasps, ptosis, tremors/shakes, and yawns. A significant drop in locomotor activity and increase in weight gain following termination of nicotine infusion provided additional evidence of an abstinence syndrome. This syndrome was alleviated by SC administration of 0.4 mg/kg nicotine tartrate.

Galanin attenuates retention of one-trial reward learning

Galanin is a neuropeptide that coexists with acetylcholine in the septohippocampal pathway. Galanin appears to have a negative modulating influence on cholinergic transmission, suggesting that it might interfere with memory formation on a one-trial discriminative reward learning task. The apparatus was a starburst maze with five radiating alleys, one an ascending baited alley. The subjects were 38 two to three month old Sprague-Dawley rats cannulated in the body of the lateral ventricles and deprived to 80% of initial weight. Ten rats were infused i.c.v. over six mins. with 8 micrograms galanin in 24 microliters saline and 10 with saline alone. Twenty mins. after completion of infusion, each rat was placed in the maze and observed under "blind" conditions for number of errors (blind alleys entered) and latency to reach reward. Each rat's speed score was 100 sec./latency. One day later, each rat was retested in the maze. Each rat's retention scores were its decrease in errors and increase in speed between the single training trial and the retention trial. Galanin infused rats showed significantly less retention by both measures. In a second experiment, either the same dose of galanin or saline alone was infused 20 mins. before the retention trial. There was no significant effect, suggesting that galanin may interfere with memory formation rather than memory retrieval or task performance.

IgG from neuropeptide FF antiserum reverses morphine tolerance in the rat

Previous studies suggest that neuropeptide FF (NPFF) plays a role in opiate dependence and subsequent abstinence syndrome. The present study assessed the role of NPFF in opiate tolerance. Third ventricular injection of IgG from NPFF antiserum restored the analgesic response to i.c.v. morphine in morphine-tolerant rats (radiant heat tail flick test). IgG from control serum failed to produce this effect. In opiate-naive rats, however, the same treatment with IgG from NPFF antiserum did not affect the analgesic response to i.c.v. morphine. Thus, immunoneutralization of NPFF appears to selectively restore morphine sensitivity in opiate-tolerant animals. These results support the hypothesis that endogenous NPFF contributes to opiate tolerance.

Analog of neuropeptide FF attenuates morphine abstinence syndrome

The octapeptide FLFQPQRFamide (neuropeptide FF or F8Fa) may play a role in opiate dependence and subsequent abstinence syndrome. Previously, NPFF precipitated opiate abstinence syndrome, while IgG from NPFF antiserum attenuated subsequent naloxone-precipitated abstinence signs in dependent rats. The peptide desamino YFLFQPQRamide (daY8Ra) was synthesized as a possible NPFF antagonist. At a dose of 600 ng ICV, daY8Ra significantly attenuated (p less than 0.001) the number of abstinence-like signs subsequently induced by 10 micrograms NPFF ICV, suggesting that daY8Ra does have antagonist activity against NPFF. Pretreatment of morphine-dependent rats with the same dose of daY8Ra also significantly attenuated (p less than 0.001) the abstinence signs subsequently precipitated by 10 micrograms naloxone ICV. Pretreatment with 600 ng of NPFF itself, or of NPFF modified at the N-terminal only (daY9Fa), failed to attenuate subsequent naloxone-precipitated abstinence, suggesting that the C-terminal modification is critical for NPFF antagonist activity. It should be noted, however, that higher doses of daY8Ra (2 micrograms or more) can precipitate some abstinence signs in a manner similar to NPFF.

Methanesulfonyl fluoride enhances one-trial reward learning in mid-aged rats

In previous studies, 18-month-old rats have shown no significant retention 24 hours after the single acquisition trial in a one-trial discriminative reward learning task. In the present study, ten 18-month-old rats pretreated with 0.5 mg/kg MSF IP showed significantly better retention in terms of speed and errors than eleven 18-month-old rats pretreated with injection vehicle alone. However, twelve two-three-month-old rats pretreated with the same dose of MSF failed to show better retention than eleven two-three-month-old rats pretreated with vehicle alone.

FMRF-NH2-like mammalian octapeptide: possible role in opiate dependence and abstinence

Yang et al. have isolated from bovine brain an octapeptide, FLFQPQRF-NH2 (F-8-F-NH2), with certain antiopiate properties. Malin et al. previously found that ICV injection of this peptide could precipitate an opiate abstinence syndrome in dependent rats. RIA revealed significantly higher levels of F-8-F-NH2 immunoreactivity in CSF withdrawn from the cisterna magna of morphine-dependent rats as opposed to CSF withdrawn from sham-implanted controls. ICV infusion of IgG from antiserum against F-8-F-NH2 significantly reduced the number of abstinence signs subsequently precipitated by naloxone in morphine-dependent rats.

FMRF-NH2-like mammalian peptide precipitates opiate-withdrawal syndrome in the rat

Yang et al. (14) have isolated from mammalian brain an octapeptide FLFQPQRF-NH2 (F-8-F-NH2) with certain antiopiate properties. Third ventricular injection of 2 micrograms of this peptide together with the aminopeptidase inhibitor bestatin precipitated an opiate-withdrawal syndrome in morphine-dependent but not in nondependent rats. Third ventricular injection in nondependent rats of 15 micrograms of the peptide together with bestatin induced a morphine-withdrawal-like behavioral syndrome. This syndrome was not produced by injection of bestatin or saline vehicle alone and was preventable by injection of 3.5 mg/kg morphine sulphate SC.

Augmented analgesic effects of L-tryptophan combined with low current transcranial electrostimulation

The analgesic effects of low current transcranial electrostimulation are both naloxone and pCPA-reversible, suggesting that they may be mediated in part by endogenous opioid and serotonergic activity. The present experiments indicate that pretreatment with the serotonin precursor L-tryptophan results in an increased analgesic effect of electrostimulation as measured by the 50 degrees C wet tail flick test in the rat. Rats receiving both L-tryptophan and electrostimulation displayed significantly more analgesia than rats receiving electrostimulation and injection vehicle alone, rats receiving drug and sham stimulation or rats receiving vehicle and sham stimulation.

Low current electrostimulation produces naloxone-reversible analgesia in rats

A new form of transcranial electrostimulation (TE) has been shown to induce analgesia in rats, as measured by the wet tail flick test. Charge-balanced rectangular current pulses of very low amplitude were delivered bilaterally into low impedance regions of the rat pinnae. The resultant analgesia was studied as a function of systematic variations in stimulus frequency, amplitude and duration. The optimal current for inducing analgesia was found to be 10 microA, well below the startle threshold, and several orders of magnitude below effective stimulus current levels required for other treatment modalities. The optimal stimulation duration was 30 min, during which time a slow onset of analgesia was noted. Significant analgesia persisted for at least 200 min after stimulation ended, and no evidence was found of tolerance developing in the course of 5 daily stimulation sessions. Consistent with findings for other forms of electrostimulation, the analgesic effect of TE was abolished by subcutaneous injection of 3 mg/kg naloxone, suggesting that the mechanism of TE analgesia has an endogenous opioid component.

Augmented analgesic effects of enkephalinase inhibitors combined with transcranial electrostimulation

The analgesic effects of very low current transcranial electrostimulation are naloxone-reversible and thus presumably mediated by endogenous opioid activity. The present experiments indicate that blocking enkephalinase activity by i.c.v. thiorphan or i.p. acetorphan results in an increased analgesic effect of electrostimulation as measured by the 50 degrees C wet tail flick test. In the case of each drug, rats receiving both drug and electrostimulation displayed significantly more analgesia than rats receiving electrostimulation and injection vehicle alone, rats receiving drug and sham stimulation or rats receiving vehicle and sham stimulation.

Auricular microelectrostimulation: naloxone-reversible attenuation of opiate abstinence syndrome

This study evaluated the effects in rats of very low amplitude (10 mu amp) charge-balanced 10-Hz stimulation delivered bilaterally to low impedance points on the outer ear. This microelectrostimulation markedly and significantly reduced the number of opiate abstinence signs observed following a week of continuous morphine infusion. This effect was prevented by subcutaneous injection of 3 mg/kg naloxone, suggesting that stimulation of endogenous opioid activity plays a major role in the actions of auricular microelectrostimulation.

Cerebrospinal fluid from morphine-dependent rats precipitates opiate abstinence syndrome

Cerebrospinal fluid (CSF) was withdrawn from opiate-dependent rats following six hours of abstinence. It was infused into the third ventricle of opiate-dependent rats, precipitating immediate abstinence signs. The effect was similar to that of infusing the opiate antagonist naloxone, suggesting that opiate-dependent organisms may secrete an endogenous opiate antagonist substance. CSF withdrawn from non-dependent rats failed to precipitate an abstinence syndrome in morphine-dependent recipients. Conversely, CSF withdrawn from opiate-dependent rats following six hours of abstinence failed to precipitate an abstinence syndrome in non-dependent recipients. The active factor in the CSF is probably a peptide since it is filterable through a 10,000 MW filter and its activity is destroyed by three different proteolytic enzymes.

Clonidine reverses the behavioral and respiratory effects of continuous naloxone infusion

Opiate naive rats received 24 hours of continuous subcutaneous infusion of 0.67 mg/kg/hr naloxone via osmotic minipump. As in previous studies, this induced an opiate-abstinence-like syndrome of significantly increased oxygen consumption and behavioral signs (wet-dog shakes, abdominal writhes, etc.). Clonidine, which selectively reduces central noradrenergic activity, has been shown to reverse opiate abstinence syndrome. Subcutaneous injection of 0.033 and 0.01 mg/kg clonidine totally reversed the abstinence-like behaviors and respiratory activity induced by naloxone infusion. This constitutes an additional point of similarity between opiate abstinence syndrome and the "endorphin blockade syndrome" or withdrawal from endogenous opioids resulting from chronic naloxone treatment. It is consistent with the hypothesis that hyperactivity of central noradrenergic mechanisms may contribute to both phenomena.

Behavioral alterations produced by chronic naloxone injections

Repeated blockade of the endorphin receptors eventually induces symptoms resembling an opiate abstinence syndrome, despite the complete absence of opiate narcotics. Rats were injected with 0.6 mg/kg naloxone or with injection vehicle alone twice a day for six days. They were observed twice a day for four subsequent days. Body shakes, head shakes, scratches and total symptoms were significantly elevated in the naloxone treated group over controls. Symptoms were completely reversed by a small dose of morphine but not by naloxone. In a second experiment, rats were injected for ten days with the same dosage of naloxone. The abstinence-like syndrome began after six days of naloxone and continued for several days after cessation of injections. Total symptoms, body shakes, scratches and aggression were significantly elevated over controls and were reversed by morphine but not by naloxone.

Stimulus specific effect of scotophobin on mouse plasma corticoids

Scotophobin is a peptide previously extracted from brains of rodents shocked in the dark compartment of a white/dark apparatus and identified as the behaviorally active dark avoidance-inducing factor. This study attempted to ascertain whether a stable synthetic analog of scotophobin induced no adrenocortical stress reaction, a generalized, unconditioned stress reaction, or a stress reaction selectively triggered by the dark compartment of the apparatus. Twenty eight mice were injected IP with the synthetic substance and 28 with placebo. A day later half of each group was placed in the dark compartment and the other half in the light compartment. Fifteen min afterward, the animals were bled, and serum samples were obtained. Plasma corticoid levels were determined by radioassay. Only the animals injected with scotophobin analog and exposed to the dark box had elevated levels. The interaction effect between drug treatment and light or dark environment was highly significant.

Synthetic rat scotophobin induces dark avoidance in mice

Two independent research groups replicate alteration of dark preference to dark avoidance by mice injected with synthetic scotophobin, a pentadecapeptide.