Susy A. Kim

Opioid self-administration in the nerve-injured rat : Relevance of antiallodynic effects to drug consumption and effects of intrathecal analgesics

Background:Neuropathic pain is associated with several sensory abnormalities, including allodynia as well as spontaneous pain. Opioid intake in neuropathic pain patients is motivated by alleviation of both pain and allodynia. However, laboratory animal studies rely almost exclusively on reflexive withdrawal measures of allodynia. The authors examined the pharmacology of self-regulated intake of opioids in rats with or without nerve injury and compared the rate of drug intake to reversal of allodynia. Methods:Rats were implanted with intravenous catheters, and the L5 and L6 spinal nerves were ligated in half of these animals. Rats were then trained to self-administer a commonly abused opioid (heroin) and commonly prescribed opioids (morphine, fentanyl, hydromorphone, and methadone). In addition, rats trained to self-administer heroin were given either clonidine or adenosine spinally before self-administration sessions to assess opioid-sparing effects. Results:Nerve injury significantly decreased the reinforcing effects of low doses of opioids, and only doses of each opioid that reduced mechanical hypersensitivity maintained self-administration after spinal nerve ligation. The rate of drug consumption was correlated with the duration of the antiallodynic effect for each dose of opioid. Intrathecal administration of clonidine or adenosine reversed mechanical hypersensitivity, but only clonidine reduced heroin self-administration in rats with spinal nerve ligation. Conclusion:Opioid self-administration is significantly altered by nerve injury, with rate of drug intake being correlated with reversal of allodynia. Intrathecal clonidine, but not adenosine, produces opioid-sparing effects in self-administering rats. The neurobiologic mechanisms that regulate opioid consumption in rats therefore seem to be altered after nerve injury.

Biased agonists of the kappa opioid receptor suppress pain and itch without causing sedation or dysphoria

Biased agonists of the kappa opioid receptor may relieve intractable itch without causing unwanted side effects. Itch relief from biased agonists Activating the kappa opioid receptor (KOR) can relieve itching that is not caused by allergic reactions. However, compounds that activate this receptor also cause unwanted side effects, such as dysphoria and sedation. KOR activation can trigger multiple downstream signaling pathways. Brust et al. characterized a biased agonist of this receptor that preferentially activated one downstream pathway over another. This agonist relieved itch in rodents without causing dysphoria or sedation. Thus, biased KOR agonists may provide a long-sought therapeutic option for intractable itch without the unwanted side effects. Agonists targeting the kappa opioid receptor (KOR) have been promising therapeutic candidates because of their efficacy for treating intractable itch and relieving pain. Unlike typical opioid narcotics, KOR agonists do not produce euphoria or lead to respiratory suppression or overdose. However, they do produce dysphoria and sedation, side effects that have precluded their clinical development as therapeutics. KOR signaling can be fine-tuned to preferentially activate certain pathways over others, such that agonists can bias signaling so that the receptor signals through G proteins rather than other effectors such as βarrestin2. We evaluated a newly developed G protein signaling–biased KOR agonist in preclinical models of pain, pruritis, sedation, dopamine regulation, and dysphoria. We found that triazole 1.1 retained the antinociceptive and antipruritic efficacies of a conventional KOR agonist, yet it did not induce sedation or reductions in dopamine release in mice, nor did it produce dysphoria as determined by intracranial self-stimulation in rats. These data demonstrated that biased agonists may be used to segregate physiological responses downstream of the receptor. Moreover, the findings suggest that biased KOR agonists may present a means to treat pain and intractable itch without the side effects of dysphoria and sedation and with reduced abuse potential.

Clonidine maintains intrathecal self-administration in rats following spinal nerve ligation

&NA; Clonidine is approved for spinal administration against neuropathic pain, and reverses both spontaneous and elicited pain in humans following spinal administration. Rodent studies that seek to model pharmacology in pain states have historically relied on reflexive withdrawal from noxious stimuli as the primary endpoint. Drug self‐administration studies have face validity in the drug abuse field for modeling drug abuse in humans, however, this methodology has not been applied to address issues related to drug seeking behaviors that may be relevant for other human populations, such as patients with neuropathic pain. Rats without spinal nerve ligation (SNL) failed to acquire intrathecal clonidine self‐administration over 10 days of access. Rats were found to self‐administer intrathecal infusions of clonidine following SNL in a stable and dose‐responsive manner, however, and clonidine was self‐administered throughout the day with 66% of total drug intake occurring during the dark cycle. Substitution of clonidine with saline or with clonidine and the &agr;2‐adrenoceptor antagonist idazoxan resulted in extinction of responding in SNL animals. Food reinforcement was initially decreased in SNL rats self‐administering clonidine compared to normal animals, however, tolerance developed to this effect of clonidine in SNL rats after 5 days. These data demonstrate that drug self‐administration can be applied to questions other than drug abuse, and provides an additional measure for development of novel therapeutic strategies for chronic pain treatment.

Effects of β-funaltrexamine on dose–effect curves for heroin self-administration in rats: comparison with alteration of [3H]DAMGO binding to rat brain sections

These studies were undertaken to determine the effects of mu-opioid receptor depletion through irreversible alkylation on the dose-effect curve for heroin self-administration. Heroin maintained responding in rats with an inverted U-shaped dose-effect curve and administration of 10 nmol of beta-funaltrexamine i.c.v. (beta-FNA) significantly increased the ED50 on the ascending limb from 1.9 to 5.3 micrograms/infusion, and from 24.3 to 211.8 micrograms/infusion on the descending limb. Administration of saline i.c.v. produced no effect on heroin self-administration. Administration of 40 nmol of beta-FNA increased the ED50S from 5.1 to 33.9 and from 14.4 to 502.8 micrograms/infusion on the ascending and descending portions of heroins dose-effect curve, respectively. beta-FNA (40 nmol, i.c.v.) had no effect on cocaine self-administration. [3H]DAMGO binding density was decreased in the caudate and nucleus accumbens by 29 or 54% 24 h after administration of 10 or 40 nmol of beta-FNA i.c.v., respectively. The effects of beta-FNA on heroin self-administration were completely overcome by increasing the dose of heroin however, as the shape and slope of the self-administration dose-effect curve was not different when higher doses of heroin were made available for self-administration compared to control data or saline administration. Therefore, there appear to be spare mu-opioid receptors for heroin for the production of its reinforcing effects in rats. Furthermore, the self-administration dose-effect curves returned to control values prior to the return of [3H]DAMGO binding, further suggesting that the full complement of mu-opioid receptors is not necessary for heroin to produce its reinforcing effects. These findings support the existence of spare mu-opioid receptors for heroin in maintaining self-administration in rats.

Dose–Effect Functions for Cocaine Self-Administration: Effects of Schedule and Dosing Procedure

Research related to determining how procedural variables can alter dose-effect functions for cocaine self-administration is limited. Toward clarifying the role of procedural variables, responding was maintained in rats under either variable-interval (VI) or fixed-ratio (FR) schedules of cocaine infusion. In addition to free-operant FR schedules, discrete-trial FR schedules were evaluated. The dose-effect functions were obtained by either substituting a dose for the usual daily dose, instituting a particular dose for several sessions, or making all doses available within a session. Dose-effect functions for response rate (or number of trials with infusions for the discrete-trial FR) were bitonic for the VI and discrete-trial FR schedules but tended to be strictly decreasing for the free-operant FR schedules. Responding was maintained under FR schedules by a low dose (0.083 mg/infusion) if the dose was substituted for a higher daily dose but not when made available daily. Rate of response was higher under ratio schedules at 0.17 mg/infusion when this dose occurred within the context of other higher doses within a session than when the dose was simply substituted for a higher daily dose. These data indicate that procedural variables can alter dose-response curves for cocaine self-administration.

Potent reinforcing effects of dihydroetorphine in rats

Dihydroetorphine is a novel opioid that is an extremely potent analgesic in rodents. The reinforcing potency was determined in rats trained to self-administer heroin and compared to those of fentanyl, heroin, 6-acetylmorphine and morphine for assessment of the abuse potential of dihydroetorphine using a procedure that determines the dose-effect curve in individual sessions. Dihydroetorphine produced a bimodal dose-effect curve similar to that of other opioids. Potency ratios were determined with morphine for the ascending and descending limbs of the dose-effect curve, as well as the dose that yielded maximal response rate. Fentanyl, heroin and 6-acetylmorphine were approximately 100, 8 and 2 times more potent than morphine in maintaining self-administration, respectively. Dihydroetorphine was roughly 1500 to 3000 times more potent than morphine, however, depending upon the limb of the dose-effect curve used for comparison. These potency ratios of dihydroetorphine to morphine were somewhat less than has been reported for analgesia assays, and therefore this compound may have some clinical advantages over other opioids. However, these studies indicate significant abuse liability for dihydroetorphine given its potency in maintaining self-administration in these animals.

Involvement of the Lateral Amygdala in the Antiallodynic and Reinforcing Effects of Heroin in Rats after Peripheral Nerve Injury

Background:Neuropathic pain alters opioid self-administration in rats. The brain regions altered in the presence of neuropathic pain mediating these differences have not been identified, but likely involve ascending pain pathways interacting with the limbic system. The amygdala is a brain region that integrates noxious stimulation with limbic activity. Methods:&mgr;-Opioid receptors were blocked in the amygdala using the irreversible antagonist, &bgr;-funaltrexamine, and the antiallodynic and reinforcing effects of heroin were determined in spinal nerve-ligated rats. In addition, the effect of &bgr;-funaltrexamine was determined on heroin self-administration in sham-operated rats. Results:&bgr;-Funaltrexamine decreased functional activity of &mgr;-opioid receptors by 60 ± 5% (mean ± SD). Irreversible inhibition of &mgr;-opioid receptors in the amygdala significantly attenuated the ability of doses of heroin up to 100 &mgr;g/kg to reverse hypersensitivity after spinal nerve ligation. Heroin intake by self-administration in spinal nerve-ligated rats was increased from 5.0 ± 0.3 to 9.9 ± 2.1 infusions/h after administration of 2.5 nmol of &bgr;-funaltrexamine in the lateral amygdala, while having no effect in sham-operated animals (5.8 ± 1.6 before, 6.7 ± 0.9 after). The antiallodynic effects of 60 &mgr;g/kg heroin were decreased up to 4 days, but self-administration was affected for up to 14 days. Conclusions:&mgr;-Opioid receptors in the lateral amygdala partially meditate heroins antiallodynic effects and self-administration after peripheral nerve injury. The lack of effect of &bgr;-funaltrexamine on heroin self-administration in sham-operated subjects suggests that opioids maintain self-administration through a distinct mechanism in the presence of pain.

Assessment of attention threshold in rats by titration of visual cue duration during the five choice serial reaction time task.

BACKGROUND The 5 choice serial reaction time task (5CSRTT) is commonly used to assess attention in rodents. We sought to develop a variant of the 5CSRTT that would speed training to objective success criteria, and to test whether this variant could determine attention capability in each subject. NEW METHOD Fisher 344 rats were trained to perform a variant of the 5CSRTT in which the duration of visual cue presentation (cue duration) was titrated between trials based upon performance. The cue duration was decreased when the subject made a correct response, or increased with incorrect responses or omissions. Additionally, test day challenges were provided consisting of lengthening the intertrial interval and inclusion of a visual distracting stimulus. RESULTS Rats readily titrated the cue duration to less than 1s in 25 training sessions or less (mean±SEM, 22.9±0.7), and the median cue duration (MCD) was calculated as a measure of attention threshold. Increasing the intertrial interval increased premature responses, decreased the number of trials completed, and increased the MCD. Decreasing the intertrial interval and time allotted for consuming the food reward demonstrated that a minimum of 3.5s is required for rats to consume two food pellets and successfully attend to the next trial. Visual distraction in the form of a 3Hz flashing light increased the MCD and both premature and time out responses. COMPARISON WITH EXISTING METHOD The titration variant of the 5CSRTT is a useful method that dynamically measures attention threshold across a wide range of subject performance, and significantly decreases the time required for training. Task challenges produce similar effects in the titration method as reported for the classical procedure. CONCLUSIONS The titration 5CSRTT method is an efficient training procedure for assessing attention and can be utilized to assess the limit in performance ability across subjects and various schedule manipulations.

Assessment of Behavioral Disruption in Rats with Abdominal Inflammation Using Visual Cue Titration and the Five-choice Serial-reaction Time Task

BACKGROUND Both acute and chronic pain result in a number of behavioral symptoms in patients, including cognitive effects such as decreased attention and working memory. Intraperitoneal administration of dilute lactic acid in rodents has been used to induce abdominal inflammation and produce effects in behavioral assays of both sensory-discriminative and affective pain modalities. METHODS Intraperitoneal injection of dilute lactic acid was used to study the impact of abdominal inflammation on an operant task requiring sustained visual attention in rats (N = 7 to 15/group) that adapts dynamically to performance ability. The effects of ketoprofen and morphine on lactic acid-induced impairment were compared with those on the disruptive effects of scopolamine. RESULTS Lactic acid impaired performance in a concentration-dependent manner, increasing the duration of cue presentation required to maintain optimal performance from 0.5 ± 0.2 s (mean ± SD) to 17.2 ± 11.4 s after the administration of 1.8% (v/v) (N = 13). The latency to emit correct responses and to retrieve the food reward were both increased by lactic acid. All effects of lactic acid injection were reversed by both ketoprofen and morphine in a dose-dependent manner. Scopolamine, however, produced dose-dependent, nonpain-related disruption in sustained attention that was not altered by either ketoprofen or morphine. CONCLUSIONS These data demonstrate that abdominal inflammation induced by lactic acid produces robust disruption in a visual attention-based operant task and that this disruption is reversed by analgesics. Future studies will focus on pain-related circuitry and its impact on both limbic forebrain and frontal cortical mechanisms.

Psychosocial Stress Delays Recovery of Postoperative Pain Following Incisional Surgery in the Rat

Psychosocial factors such as anxiety, depression and catastrophizing, commonly associated with established chronic pain, also may be associated with an increased risk of chronic postsurgical pain (CPSP) when present preoperatively. We used a repeat social defeat (RSD) paradigm to induce psychosocial stress in rodents prior to incisional surgery of the paw. Mixed effects growth curve models were utilized to examine resolution of mechanical hypersensitivity in rats for four weeks following surgery. Eight days following surgery, immunohistochemistry was conducted to examine glial activation as well as evoked neuronal activation in the spinal cord. Here we document that RSD resulted in reduced weight gain and increased depressive symptoms prior to surgery. Rats exposed to RSD displayed delayed resolution of mechanical hypersensitivity in the ipsilateral paw following surgery compared to non-defeated rats. Prior exposure to RSD significantly increased microglial activation and neuronal sensitization (pERK-IR) within the ipsilateral spinal cord. In conclusion, we found that chronic social stress alters the neurobiological response to surgical injury, resulting in slowed recovery. This model maybe useful for future interventional studies examining the mechanistic interactions between depression and risk of CPSP.