Opioid drugs as reinforcing stimuli in rhesus monkeys

Gail Winger and James H. Woods

Working in a laboratory at the University of Michigan that was originally designed in the
1960s by Dr. Tomoji Yanagita, we have been evaluating opioid drugs for their ability tomaintain behavior in rhesus monkeys. The animals are prepared with intravenous
catheters that can be accessed from the outside of the cage. Inside the cage are two
response levers and several stimulus lights. When one of the lights is turned on, it signals
to the monkey that it can receive an intravenous injection of a centrally acting drug as aconsequence of responses on the lever.
Mu opioid agonists, particularly heroin, present serious human abuse problems in
the United States and other countries. These drugs also maintain lever responding bymonkeys. Kappa opioid agonists do not maintain responding and may be aversive toanimals and to humans. Delta opioid agonists also fail to maintain lever responding bymonkeys. By using pA2 analysis we have determined that the same opioid receptor (mu)mediates the analgesic, respiratory depressant, and reinforcing effects of opioid drugs,
making it unlikely that abuse liability and analgesic effects can be effectively separated.
It is commonly acknowledged that human heroin addicts take increasing amountsof heroin over time. A similar pattern of increasing opioid intake was shown many years
ago in both rats and rhesus monkeys. Although this increasing intake may be the resultof tolerance development, it is not clear that the tolerance was to the reinforcing effects,
as compared with the rate-suppressing effects, of the opioid. In our studies, we foundthat daily administration of morphine produced a slight decrease in the potency ofmorphine and heroin as reinforcers, a much larger decrease in the potency of the partial
agonists buprenorphine and nalbuphine, and no change in the potency of alfentanil. Thissuggests that tolerance to the reinforcing effects of opioids depends on the efficacy of theindividual agents.
We have evaluated the ability of a new treatment agent, the partial agonist
buprenorphine, to modify the reinforcing effects of alfentanil. Although buprenorphine isan effective reinforcer, it is also able to decrease the potency of alfentanil as a reinforcer

in a dose-related manner. In this model, buprenorphine appears to act as an antagonist to
modify alfentanil’s reinforcing effects.
Our recent work has concentrated on experiments that can measure theeffectiveness of drugs as reinforcers. To do this we have borrowed concepts from
economics to evaluate the effects of increased price on the demand for drugs. It is
commonly known using various commodities that demand for a product decreases as
price increases. This is true using drugs as well when the price is adjusted by changing
the number of responses necessary to earn a drug injection. One parameter of demand
curves is the Pmax, the price at which the maximum number of responses are made. Thiscan be used as an indicator of the reinforcing effectiveness of a drug. We have foundusing demand curve analysis that alfentanil is a more effective reinforcer than nalbuphine,a partial mu agonist. Alfentanil is nearly equal in reinforcing effectiveness as cocaine,and more effective than the short-acting barbiturate, methohexital. We have also foundthat, in our procedures, three opioids with different durations of action but similar onsets
of action, fentanyl, alfentanil, and remifentanil, are nearly equally effective as reinforcers,
indicating that onset of action may be more important than duration of action in
determining the abuse liability of a drug.

Anti-depressant activity of delta opioid receptor activation
James H. Woods
Recently, we summarized (Broom etal., 2002) the evidence that supported the
proposition that activation of the delta opioid receptor by direct-acting agonists or byenkephalinase inhibitors conveys behavioral effects in forced swimming assays that predict
antidepressant-like potential.
In addition to this interesting behavioral effect, delta agonists
produce a variety of other receptor-mediated effects in rodents, e.g., antinociception andconvulsions. We have shown that, although different behavioral effects occur at similar doses,
the antidepressant effect is dissociable from other effects of delta agonists. For example,
profound tolerance developes to the convulsant effect in mice and rats without a commensurate
tolerance to the antidepressant-like effect. In addition, the convulsion can be eliminated by
midazolam without interfering with the antidepressant-like effect.
It has been shown by others and us that locomotor activity is increased by delta agonists,
and this effect is associated with evidence from conditioned place preference for rewarding
effects of these compounds (Spina etal..,1998). We (Gomez etal., Submitted)have carried out astudy in rats of self-injection with a delta agonist following prior experience with, in different
groups, milk or milk and nomifensine conditioning. Though the delta agonist produced atransient increase in responding, when substituted for either milk or nomifensine, it failed tosustain responding indicative of a positive reinforcing effect in the rat. This finding supports
earlier research by us in rhesus monkeys suggesting that delta agonists fail to maintain reinforced
responding (Negus etal., 1994).
Delta agonists have been shown to interfere with learning under certain circumstances inprimates and rodents (Pakarinen etal., 1995). We have carried out a study to examine if the
reduced immobility could be due to a learning deficit produced the delta agonist. We compareda delta agonist directly with scopolamine, a drug with well-documented effects on acquisition
processes. The compounds were quite different in their behavioral profiles; the delta agonist
induced reductions in immobility at doses that produced very little effect on acquisition while thereverse was the case with scopolamine (Jutkiewicz etal., In press).
Many of the medically approved antidepressants and non-drug approachs to treating
depression have in common (Duman, 2002) the ability to induce slowly an upregulation ofbrain-derived neurotrophic factor (BDNF). We have recently shown that delta agonists will
upregulate BDNF; its regulation appears not to be dependent upon convulsant effect and isdifferent from other compounds that have antidepressant activity (Torregrossa etal., 2003) in thatthe effect occurs with the first dose.
References
Broom, D.C., Jutkiewicz, E.M., Rice, K.C., Traynor, J.R., and Woods, J.H. Behavioral effects ofdelta-opioid agonists: potential antidepressants? Jpn.J.Pharmacol. 90, 1-6 (2002)

Duman, R.S. Synaptic plasticity and mood disorders. Mol. Psychiat. 7, S29-S34 (2002)
Gomez, F., Rice, K.C.,Shi,Y.-G., Jutkiewicz, E.M., Torregrossa, M., and Woods, J.H. Evaluationof the reinforcing effects of the delta agonist, BW 373U86, in rats. Submitted for publication.
Jutkiewicz,E.M., Rice, K.C., Woods, J.H., and Winsauer, P.J. Effects of the delta opioid receptor
agonist SNC80 on learning relative to its antidepressant-like effects in rats. Behav. Pharmacol.
(In press)
Negus,S.S.,Butelman,E.R.,Chang,K.-J.,DeCosta,B.,Winger,G.D.,and Woods, J.H. Behavioral
effects of the systemically active delta opioid agonist BW373U86 in rhesus monkeys. J.
Pharmacol.Exp.Ther.
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Pakarinen,E.D.,Woods,J.H.,and Moerschbaecher,J.M. Repeated acquisition of behavioral chainsin squirrel monkeys: comparisons of a mu,kappa,and delta opioid agonist. J.Pharmacol.Exp.Ther.
272:552-559 (1995)
Spina,L.,Longoni,R.,Mulas,A.,Chang,K.-J., and DiChiara,G. Dopamine-dependent behavioural
stimulation by non-peptide delta opioids BW373U86 and SNC80:1. Locomotion,rearing, andstereotypies in intact rats. Behav.Pharmacol.9:1-8 (1998)
Torregrossa,M.M., Isgor,C.,Rice,K.C.,Watson,S.J., and Woods, J.H. Stimulation of delta-opioid
receptors increases BDNF mRNA expression in several brain regions. FASEB Absr. 17:
A203-A204 (2003)

Substance P (SP) and its bioactive fragment SP(1-7) in relation to opioid tolerance andwithdrawal.
Fred Nyberg
Dept of Pharmaceutical Biosciences, Uppsala University, Uppsala, Sweden.
The neuroactive undecapeptide substance P (SP; Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-MetNH2) is widely distributed in the central nervous system (CNS). Tissue levels of SP is regulated byseveral enzymes, one of which is substance P endopeptidase (SPE), known to release the bioactive Nterminal fragment SP(1-7) from the parent compound (Persson et al., 1995). SP preferentially acts on
the neurokinin-1 (NK-1) receptor, which is highly expressed in CNS areas that are critical for the
regulation of pain influx, affective behavior and stress. Moreover, earlier studies have implicated thesubstance P (SP) system in opioid reward and dependence. It is thus shown that the NK-1 receptor
has a potential role in opioid reward (Murtra et al., 2000). It is further demonstrated that SP
potentiates the intensity of the reaction to opioid withdrawal, an effect which is counteracted by theheptapeptide fragment SP(1-7). In recent years we have particularly focused our investigation of the
SP system on SP(1-7) and SPE. We observed a concomitant increase in the level of the heptapeptide
and SPE, as well, in brain areas related to opioid reward during naloxone precipitated withdrawal(Zhou et al., 1998, 2001). A significant correlation between several signs of opioid withdrawal andthe SPE activity was found. Recently, we investigated the effect of SP(1-7) on the expression of the
dopamine receptors D1 and D2 receptor mRNA in the brain of morphine dependent male rats (Zhouet al., 2003). We also examined the effect of SP(1-7) on dopamine release using a microdialysis
technique. In the first experiment the peptide was infused through an implanted cannula aimed to thelateral ventricle 30 min before a naloxone challenge in rats, previously treated with twice daily
injections of morphine for 7 days. The animals were decapitated 4h after naloxone injection. The
result indicated that SP(1-7) induced an alteration in the expression of the receptor gene transcripts innucleus accumbens and frontal cortex (Zhou et al., 2003). In the microdialysis experiment the probe
was inserted into the nucleus accumbens. HPLC combined with electrochemical detection was used
to quantitate dopamine and its metabolite DOPAC. It was found that the SP fragment produced asignificant increase in both dopamine and DOPAC in this particular brain area (Zhou and Nyberg,
2002). It was concluded that SP(1-7) may elicit an increased release of dopamine and thereby
counteract the withdrawal reaction. This findings together with other observations showing that SP(17) opposes the effect of SP suggest neuropeptide conversion as an important pathway inneuromodulation (Hallberg and Nyberg, 2003).
References
Hallberg, M. and Nyberg, F. (2003): Neuropeptide conversion to bioactive fragments – an important pathway inneuromodulation. Curr Protein Pept Sci. 4:31-44.
Murtra, P., Sheasby, A.M., Hunt, S.P. & De Felipe, C. (2000) Rewarding effects of opiates are absent in mice lacking thereceptor for substance P. Nature, 405, 180-183.
Persson, S., Le Grevès, P., Thörnwall, M., Eriksson, U., Silberring, J. & Nyberg, F. (1995) Neuropeptide converting andprocessing enzymes in the spinal cord and cerebrospinal fluid. Progr. Brain Res., 104, 111-130
Zhou, Q., Karlsson, K., Liu, Z., Johansson, P., Le Grevès, M., Kiuru, A. & Nyberg, F. (2001) Substance P endopeptidaselike activity is altered in various regions of the rat central nervous system during morphine tolerance and withdrawal.
Neuropharmacology, 41, 246-253.
Zhou, Q., Liu, Z., Ray, A., Huang, W., Karlsson, K. & Nyberg, F. (1998) Alteration in the brain content of substance P(1-7) during withdrawal in morphine-dependent rats. Neuropharmacology, 37, 1545-1552.
Zhou, Q. & Nyberg, F. (2002) Injection of substance P (SP) N-ternminal fragment SP1-7 into the ventral tegmental area
modulates the levels of nucleus accumbens dopamine and dihydroxyphenylacetic acid in male rats during mrophinewithdrawal. Neurosci. Lett., 320, 11.
Zhou, Q., Frandberg, P.-A., Kindlundh, A.M.S., LeGrevés, P. and Nyberg, F. (2003) Substance P(1-7) affects theexpression of dopamine D2 receptor mRNA in male rat brain during morphine withdrawal. Peptides, 24, 147-153.