Effects of rho-chloroamphetamine on locomotor activity and brain 5-hydroxyindoles

p-Chloroamphetamine (PCA) suppresses ingestive behavior in male rats

Ingestive behavior was activated in male rats by intraoral intake and intake from a bottle of 1-M solution of sucrose. Intraperitoneal injection of p-chloroamphetamine (PCA), releasing central 5-hydroxytryptamine (5-HT) from serotonergic nerve terminals, inhibited ingestion of the sucrose solution. Significant inhibition of sucrose intake by PCA was observed at 1.25 and 2.5 mg/kg dose in a bottle intake test, and at 5.0 mg/kg dose in an intraoral intake test. These findings suggested that 1.25 and 5.0 mg/kg of PCA suppressed appetitive ingestive behavior and consummatory ingestive behavior in male rats, respectively.

Acute and chronic fluoxetine treatment decreases the sensitivity of rats to rewarding brain stimulation

The effects of fluoxetine on rewarding brain stimulation were determined in eight Wistar rats using a rate-independent discrete-trial threshold measure. Rats were implanted with bipolar, stainless steel electrodes either into the ventral tegmental area (VTA) or medial forebrain bundle (MFB). Acute administration of fluoxetine significantly raised the reward threshold (decreased sensitivity) at doses of 2.5, 5.0, 10.0, and 20.0 mg/kg, i.p., without altering latency of response. There were no significant differences between VTA and MFB groups. To determine the effects of chronic treatment, daily injections of 5.0 mg/kg fluoxetine were administered to rats for 21 days. Chronic treatment of fluoxetine continued to significantly elevate reward thresholds with no evidence of tolerance. The results of these experiments suggest that fluoxetine does not possess abuse potential and that serotonin produces an inhibitory effect on the mesolimbic dopaminergic reward system. Furthermore, these results suggest that the antidepressant effects of fluoxetine are not the direct result of excitation of brain reward systems, at least in the same manner as abused substances, for example, cocaine.

Fluoxetine at anorectic doses does not have properties of a dopamine uptake inhibitor

Although fluoxetine is a highly selective inhibitor of serotonin uptake in vitro and in vivo, some investigators have suggested that dopamine uptake inhibition may contribute to anorectic actions of fluoxetine. The present experiments were done to determine fluoxetine's effects in some animal protocols in which dopamine uptake inhibitors have characteristic actions. Mazindol prevented the depletion of striatal dopamine and its metabolites by amphetamine in iprindole-pretreated rats, but fluoxetine had no effect. Mazindol prevented the depletion of striatal dopamine and its metabolites by 6-hydroxydopamine injected intracerebroventricularly into rats, but fluoxetine had no effect. Mazindol enhanced the elevation of 3,4-dihydroxyphenylacetic acid concentration in rat brain after spiperone injection, but fluoxetine did not cause that effect. Fluoxetine did not mimic amfonelic acid in antagonizing the retention of alpha-methyl-m-tyramine invant striatum after the injection of alpha-methyl-m-tyrosine. These results show that fluoxetine, at doses that are effective in blocking the serotonin uptake carrier and causing anorexia, does not block the dopamine uptake carrier.

Effects of 5,7-dihydroxytryptamine injections in the fornix-fimbria on locomotor activity in photocell cages and the open field

Microinjections of 5,7-dihydroxytryptamine (5,7-DHT) into the fornix-fimbria (FF) reduced dorsal hippocampal [3]5-HT uptake to 40% of control levels. The FF 5,7-DHT lesions increased nocturnal activity in photocell cages, but reduced central ambulation in diurnal open field tests. The lesions also disrupted both habituation of rearing across days in the open field and alternation in a Y-maze. Hence FF-derived hippocampal 5-HT terminals participate in controlling activity, but their role depends on the test apparatus and procedures.

Sex differences in passive avoidance depend on the integrity of the central serotonergic system

Effects of the neurotoxin para-chloroamphetamine (PCA) on sex differences in passive avoidance were studied. Seven days prior to passive avoidance training and testing, male and female rats were injected with PCA (5 mg/kg) or physiological saline (SAL). Treatment effects on brain monoamines levels were evaluated in brains collected shortly after the passive avoidance test. Compared to SAL-treated control groups PCA severely reduced both serotonin (5-HT) and 5-hydroxyindole-acetic acid (5-HIAA) in the frontal cortex of males and females. Levels of dopamine (DA) and homovanillic acid (HVA) in the frontal cortex were not affected. These data are indicative of a strong and selective depression of the central 5-HT activity. PCA- and SAL-treated male and female rats were trained and tested in a two-compartment step-through passive avoidance apparatus. Sex differences in passive avoidance were clearly observed in the SAL-treated control groups; a higher number of males did not enter either compartment within the maximum test duration. After PCA treatment sex differences in passive avoidance were abolished, mainly resulting from an increase in the number of PCA-males reentering. Irrespective of sex or treatment subjects seldom failed to choose the nonshock compartment when entering during the passive avoidance test, indicating that disturbance of memory or learning cannot explain for the present results. Rather, the data are discussed in terms of a sex-specific role of central 5-HT in punishment-induced behavioral suppression.

The relative contribution of ascending and descending serotonergic pathways in p-chloroamphetamine-induced antinociception

Systemic administration in rats of p-chloroamphetamine (PCA; 2 x 10 mg/kg) reduced the in vitro uptake of 14C-5-hydroxytryptamine (14C-5-HT) in cortical synaptosomes by 76% and in spinal cord synaptosomes by 35%. Intrathecal injection of 5,6-dihydroxytryptamine (20 micrograms/rat) selectively lesioned the descending serotonergic pathways (83% reduction in uptake of 14C-5-HT in spinal synaptosomes, no significant change in uptake in cortical synaptosomes). Administration of PCA or 5,6-DHT did not significantly alter the uptake of 3H-noradrenalin into cortical or spinal synaptosomes. The response thresholds of the rats in the increasing temperature hot plate test (1 to 7 days after administration) were unaffected by either type of lesion. Interference with the antinociceptive effect of PCA (2.5 mg/kg) was evaluated 7 days after administration of the neurotoxins. PCA pretreatment strongly reduced the peak of the PCA-induced antinociception while 5,6-DHT reduced its duration. Thus, both ascending and descending serotonergic pathways contribute to PCA-induced antinociception.

Evidence against serotonin involvement in the hyperactivity produced by injections of muscimol into the median raphe nucleus

Microinjections of muscimol into the median raphe nucleus were found to result in pronounced hyperactivity which could not be attenuated by the serotonin depletion produced either by systemic treatment with p-chlorophenylalanine or by intra-raphe injections of 5,7-dihydroxytryptamine. Furthermore, hyperactivity could not be produced by intra-median raphe injections of serotonin or of fenfluramine, compounds which would be expected to inhibit serotonergic raphe cells. These results argue strongly against an essential involvement of serotonin in mediating the effects of intra-median raphe muscimol injections. Muscimol failed to produce hyperactivity, however, when injected into rats who had previously received an electrolytic median raphe lesion. This finding suggests that muscimol injected into the median raphe produces hyperactivity as a result of an action on local cell bodies, rather than by diffusion to a distant site. The simplest explanation of the current results is that muscimol injected into the median raphe produces hyperactivity as a result of an inhibition of nonserotonergic cells within the median raphe nucleus.

Changes in body temperature after administration of adrenergic and serotonergic agents and related drugs including antidepressants: II

This survey continues a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published from 1980 to 1984 but data from many earlier papers are also tabulated.

Serotonin-lesion myoclonic syndromes. II. Analysis of individual syndrome elements, locomotor activity and behavioral correlations

This study evaluated the behavioral elements of three 5-HT-related syndromes (intraperitoneal 5-hydroxytryptophan after intracisternal 5,7-dihydroxytryptamine (DHT), p-chloroamphetamine (PCA), fenfluramine (FF), or combinations of drugs) scored from video-tapes and their relationship to locomotor activity (LMA) photocell recording, regional monoamine concentration and S-1 receptor binding. Rearing was eliminated by drugs which produce the myoclonic syndrome and was the single best indicator of control treatments (saline or 5-HTP in unlesioned rats and saline in DHT-lesioned rats). Global 'abnormality', hunching (rigid arching of back), hindlimb abduction, forepaw myoclonus, stereotyped lateral head movements, backing, and immobility occurred significantly only in drug-treated rats. Multiple forms of myoclonus (appendicular and truncal) and convulsions were dose-dependent drug effects. Both 5-HTP (after DHT) and PCA increased LMA significantly, but hyperactivity induced by PCA could be blocked by giving 5-HTP concomitantly. Substantial 5-HT presynaptic destruction by DHT prevented backing but not other behavioral or locomotor effects of FF and PCA. Drug combinations did not produce additive behavioral effects. Backing, immobility, and locomotor activity best differentiated between drug treatments, and could be used to correctly allocate animals to drug groups. Drug treatments also could be differentiated by reducing the number of behavioral variables into summary variables (principal components) and by discriminant analysis. Only forepaw myoclonus and total behavioral score were correlated with 5-HT concentrations (brainstem), indicating behavioral heterogeneity. Our study suggests that there is a common core 'myoclonic-serotonergic' syndrome (forepaw myoclonus, head weaving, hindlimb abduction, hunching) of stimulation of 5-HT receptors plus additional drug-specific elements (backing, LMA). Although brainstem receptors appear to be an important locus for some of these behaviors, S-1 receptors do not explain the behavioral supersensitivity to 5-HTP in our DHT-lesioned rats.

Behavioral and biochemical studies of the substrates of median raphe lesion induced hyperactivity

Many authors have demonstrated that electrolytic lesions of the median raphe nucleus lead to dramatic hyperactivity, but little is known as to the neural substrates of this effect. In the current series of experiments we investigated this question by examining locomotor activity and forebrain serotonin levels after the placement of wire knife cuts in various locations around the median raphe. Activity was measured in a five minute open field test and a one hour tilt cage test. Knife cuts designed to transect the major ascending serotonergic projections of the median raphe led to a pronounced depletion of forebrain serotonin, but had no effect on locomotor activity in either testing situation. Knife cuts located antero-ventral to the median raphe, designed to interrupt raphe connections with the ventral tegmental area and interpeduncular nucleus, increased activity in the tilt cage but not in the open field test. These cuts produced only small effects on forebrain serotonin levels. Knife cuts caudal to the median raphe failed to influence forebrain serotonin levels, but produced a significant increase in both open field and tilt cage activity. The effects of the posterior and the anteroventral cuts on tilt cage locomotion were additive, suggesting that different fiber systems were damaged by the two cuts. These results demonstrate that it is possible to double dissociate changes in forebrain serotonin levels and locomotor activity with lesions in the vicinity of the median raphe and further show that ascending projections are unlikely to be the only pathways involved in the effects of median raphe lesions on locomotor behavior.

Ejaculations induced by p-chloroamphetamine in the rat

The ejaculatory response following acute injections of p-chloroamphetamine (PCA) and several other drugs was measured by weighing the compact seminal material accumulated over 2 hr. p-Chloroamphetamine caused a dose-dependent ejaculatory response that was inhibited by the inhibitor of the synthesis of 5-hydroxytryptamine (5-HT), p-chlorophenylalanine (PCPA), neurotoxic doses of PCA, reserpine, DSP 4 a selective noradrenergic neurotoxin given 48 hr before PCA, the inhibitor of synthesis of noradrenaline (NA) FLA 63, the specific inhibitors of uptake of 5-HT, alaproclate, fluoxetine and norzimeldine and the selective inhibitor of the uptake of NA, CPP 199, the E form of norzimeldine. The doses of several receptor antagonists producing a 50% decrease in the weight of seminal material were determined. The non-selective 5-HT receptor antagonists, metitepine and methergoline, the selective alpha 1-adrenoreceptor antagonists, prazosin and phenoxybenzamine and the non-selective alpha-adrenoreceptor antagonist, phentolamine, had strong effects, followed by the selective 5-HT2 antagonists, ketanserin and pirenperone. Yohimbine, an alpha 2-adrenoreceptor antagonist and atropine, a muscarinic receptor antagonist, only produced a partial blockade. The rank order of potency for some dopamine (DA) receptor antagonists was chlorpromazine, domperidone, haloperidol, pimozide. Remoxipride, a selective DA2 receptor antagonist and the selective DA1 antagonist, Sch 23390, had no effect. The following drugs had no effect: propranolol, naloxone, picrotoxin, cimetidine and mepyramine. The 5-HT receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT 3 mg/kg, i.p.) produced a small effect on the weight of seminal material, although 72% of the rats ejaculated. d-Amphetamine did not induce ejaculation at 5 mg/kg but had a marked effect at 15 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Test-dependent antinociceptive effect of spinal serotonin release induced by intrathecal p-chloroamphetamine in mice

The effect of direct intrathecal injection of p-chloroamphetamine (PCA) into the lumbar subarachnoid space was investigated in mice. PCA (0.6 - 20 micrograms) induced a dose-related prolongation of response latencies in the tail-flick test, but failed to affect the hind-paw lick response in a hot-plate test employing slowly rising temperature. PCA (5 micrograms) given intracerebroventricularly did, however, significantly elevate the response temperature in the hot-plate test. The antinociceptive effect of PCA in the tail-flick test was prevented by spinalization, by pretreatment with the selective serotonergic re-uptake blocker zimelidine (20 mg X kg-1 i.p.) and by the serotonin synthesis inhibitor p-chlorophenylalanine (300 + 300 + 150 mg X kg-1 i.p. 72, 48 and 24 h before test). It is concluded that PCA given intrathecally releases serotonin from spinal terminals, which may under certain conditions induce antinociception.

Effects of metergoline and quipazine on locomotor activity of rats in novel and familiar environments

Many studies have investigated the effects on locomotor activity of various manipulations of the brain's serotonin (5-hydroxytryptamine, 5-HT) systems but the results have not been consistent. However, besides employing different techniques for manipulating brain 5-HT, previous studies have differed in size of apparatus, amount of apparatus pre-exposure and length of session. To test the possibility that apparatus familiarity interacts with the effects of 5-HT manipulations on locomotor activity, this variable was manipulated in groups of rats treated with the 5-HT receptor blocker, metergoline or the agonist, quipazine. Within each drug treatment group, 18 rats had prior experience with the activity monitoring photocell chambers (pre-exposed condition) and 18 were not previously exposed (novel condition); each condition was further subdivided into 3 dose subgroups (n = 6). Testing consisted of 3 30-min sessions with subgroups receiving metergoline (0, 2.5, 5.0 mg/kg) or quipazine (0, 2.5, 5.0 mg/kg) 30 min before. Results with metergoline treatment revealed no significant drug effect in the pre-exposed groups but a decrease in activity in the novel condition. Quipazine, on the other hand, had no significant effect in the novel condition but produced a time-dependent effect on activity in the pre-exposed condition. These results suggest that the effects on locomotor activity of compounds affecting 5-HT neurotransmission may interact with the familiarity of the test apparatus and with the duration of testing. Interexperiment differences in these variables may account for some of the inconsistencies previously reported.

Attenuation of morphine-induced analgesia by p-chlorophenylalanine and p-chloroamphetamine: test-dependent effects and evidence for brainstem 5-hydroxytryptamine involvement

A number of studies on the role of 5-HT in morphine analgesia and regulation of nociception are reviewed. Highly divergent conclusions are found in the literature with regard to the importance of serotonergic structures. Several methodological differences, particularly with regard to lesion and depletion techniques and testing procedures, may account for the controversies in the literature. The experimental findings presented demonstrate attenuation of morphine-induced analgesia in the hot-plate and tail-flick tests, and increased responsiveness to noxious electrical shock following depletion of 5-HT in ascending and descending 5-HT pathways by PCPA (200 + 100 + 100 mg/kg on 3 consecutive days prior to testing) as well as following destruction of cerebral 5-HT terminals by PCA (2 X 10 mg/kg, 7 and 8 days before testing). This was also the case when the neurotoxic effect of PCA was largely restricted to the brainstem by pretreatment with the 5-HT reuptake inhibitor zimelidine (20 mg/kg prior to each PCA injection). Attenuation of morphine analgesia was not found in the flinch-jump test or in the hot-plate test when conducted immediately after flinch-jump testing. It is concluded that brainstem 5-HT connections may contribute to the analgetic effect of morphine, but only under certain test conditions.

Environmental determinants of parachloroamphetamine toxicity in rats

The present investigation assessed PCA toxicity at 0.0, 5.0, and 10.0 mg/kg, in both social (4 rats per cage) and non-social (acrylic tube-restraint or tube restraint-plus-tail shock) circumstances with 16 rats per drug-environment condition. The results indicated that no dose of PCA alone yielded mortality under individual housing, and similarly no environmental circumstance by itself yielded mortality in the absence of PCA. However, various drug-environment interactions produced a dose-related enhancement of PCA toxicity. For both 5.0 mg/kg and 10 mg/kg parachloroamphetamine dose levels, restraint-plus-shock generated the highest percent mortality, followed by restraint-only, with conspecific aggregation producing a mortality incidence lower still. Further, the mortality displayed under each of these environmental conditions was greater for the 10.0 mg/kg PCA treatment than for the 5.0 mg/kg treatment. The results are discussed in terms of the relative aversiveness of the environmental setting and it is suggested that stress-related drug toxicity may be further analyzed in non-social settings. It is proposed that toxic environment-PCA interactions may result from altered cardiovascular and/or thermoregulatory processes, mediated by enhanced catecholaminergic activity.

Mechanisms of PCA-induced hypothermia, ejaculation, salivation and irritability in rats

Injections of p-chloramphetamine (PCA, 5 mg/kg) induced hypothermia, ejaculation, salivation and irritability in male rats kept at an ambient temperature of 20 +/- 1 degree C. PCA-induced hypothermia was attenuated by pretreatment with the 5-hydroxytryptamine (5-HT) uptake blockers Lundbeck 10-171 (Lu 10-171, 10 mg/kg) and chlorimipramine (CMI, 20 mg/kg) and the 5-HT synthesis inhibitor parachlorophenylalanine (PCPA, 150 mg/kg daily for 3 days); it was potentiated by pretreatment with the noradrenaline uptake blocker Lundbeck 5-003 (Lu 5-003, 10 mg/kg) and the catecholamine synthesis inhibitor alpha-methyl-p-tyrosine (AMPT, 50 mg/kg every 3 hr for 9 hr). PCA- induced ejaculation was attenuated by pretreatment with Lu 10-171 and CMI. PCA-induced salivation was attenuated by pretreatment with Lu 10-171 and CMI and potentiated by pretreatment with Lu 5-003. PCA-induced irritability was potentiated by pretreatment with PCPA. These results suggest that both 5-HT and the catecholamines play a role in PCA-induced hypothermia, ejaculation, and salivation.

Disproportionate increases in locomotor activity in response to hormonal and photic stimuli following regional neurochemical depletions of serotonin

The role of forebrain serotonin in behavior-related energy output was assessed in two locomotor activity tests conducted 3 and 6 months after bilateral, intrahypothalamic microinfusion of the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). The serotonin-depleted animals exhibited a long-lasting and stable increase in energy expenditure as locomotor activity. This increased activity was investigated at the behavioral level by relating the hyperactivity to estrous cycle, photoperiod and body weight. Although the serotonin depletion-induced hyperactivity occurred in all photoperiod and estrous cycle stages, its magnitude was disproportionately increased during light and estrus. This hyperactivity could not be related to decreases in body weight because the serotonin-depleted animals weighed significantly more than the control animals. These animals responded to the weight loss that normally accompanies wheel running by increasing their activity to the same proportion as the other groups. The neuroanatomical and neurochemical substrate of the increased locomotor activity was investigated with a regional neurochemical assay for serotonin, dopamine and norepinephrine. This assay revealed that the toxin had no effect on dopamine or norepinephrine in any structure analyzed; however, serotonin was depleted in the hippocampus, septum and, to a lesser degree, in the hypothalamus. Serotonin levels were negatively correlated with overall activity. The magnitude of the disproportionate increase in activity during light and estrus was negatively correlated with hippocampal serotonin level. These results indicate that forebrain depletions of serotonin differentially affect the control of activity exerted by the phases of the photoperiod and estrous cycle. However, the modulation of activity levels by decreases in body weight remains intact.

PCA: effects on ejaculation, thermoregulation, salivation, and irritability in rats

The short term monoamine releaser p-chloroamphetamine (PCA) was injected intraperitoneally in male rats housed at 20 degrees C. Within 2 hr of PCA injections (2.5, 5.0, 8.0 or 10.0 mg/kg), rats showed ejaculation, decreased colonic temperature, increased salivation, and increased irritability. Ejaculation and salivation scores were considerably lower in the 2.5 mg/kg than in the higher dose groups, but otherwise were not dose dependent at the doses used. Hypothermia was of similar magnitude in all groups, but lasted longer in the higher dose groups. Irritability increased with dose size. In order to study the role of ambient temperature in PCA-induced behavioral changes, observations were made on an additional group or rats housed at the higher ambient temperature of 25 degrees C. In these rats an increase, rather than a decrease, in mean colonic temperature was observed following PCA injection (5 mg/kg). Ejaculation and irritability scores were similar to those observed at the lower ambient temperature, but salivation was enhanced. It is suggested that PCA induces ejaculation, salivation, irritability and, depending on the ambient temperature, either hypothermia or hyperthermia.

Facilitation of avoidance acquisition in rats produced by P-chlorophenylalanine or P-chloroamphetamine

The effects of reducing brain serotonin using p-chlorophenylalanine (PCPA) were examined as a follow up to our previous report that reducing serotonin with p-chloroamphetamine (PCA) facilitated Y-maze avoidance acquisition and reduced open field activity. In the current work, PCPA was also found to facilitate Y-maze avoidance acquisition, while open field activity, although reduced, was not reduced significantly. In a second experiment, we re-examined PCA, except that the apparatus was changed in order to test the generality of the effect of PCA on avoidance performance in a task other than the Y-maze. Testing was also run at varying shock intensities to determine if this was a significant determinant of the effect. PCA reliably facilitated shuttle-box avoidance acquisition and did so at all shock intensities tested. Finally, in a third experiment, the time course of the onset of the PCA-induced avoidance facilitation was examined and found to develop 8--10 hours following drug treatment and not at a shorter drug to test interval of 4 hours. The present data, in conjunction with our previous data support the concept that lowered brain serotonin content facilitates avoidance acquisition regardless of the specific method used to reduce serotonin or to assess avoidance acquisition.

Synergistic action of p-chloroamphetamine and fluoxetine on food and water consumption patterns in the rat

The distribution of eating, drinking and body weight changes during the 24 hr day were examined following brain 5-HT depletion with p-chloroamphetamine (PCA). Following a baseline period, measurements of food and water intakes and body weights were recorded 1, 2, 3, 6, 12, 20 and 24 hr following PCA, 5.0 mg/kg, or saline. Other animals were pretreated with fluoxetine, 10.0 mg/kg, prior to either PCA or saline in an attempt to block the PCA effects. The results indicate acute hypophagia, hypodipsia, and body weight losses. These decreases were not influenced by the time of day when PCA was administered. Pretreatment with fluoxetine enhanced rather than blocked these effects. No long term changes in ingestive behavior were seen. These results are discussed with respect to the possible role of 5-HT in the control of ingestive behavior.

Short- and long-term effects of para-chloroamphetamine on ingestive behavior

Para-chloroamphetamine (PCA) produces short-term decreases in eating and drinking. PCA also chronically decreases brain serotonin concentration following a single peripheral injection. The present investigation assessed short- and long-term effects of PCA on ingestive behavior and body weight in greater detail. Following an adaptation period, PCA, 0.0, 1.0, 2.0, 5.0 and 10.0 mg/kg, were administered IP, to free feeding rats. A decrease in food and water consumption was observed during the 0--24 hr postinjection period. During the 24--48 hr period, water consumption was significantly increased compared to baseline. Food intakes during this same period returned to baseline levels. No long-term effects on ingestive behavior or body weight were seen during the following 30 days.

Serotonin-containing neurons: their possible role in pain and analgesia

Experimental evidence is reviewed showing that brain and spinal cord serotonergic neurons are involved in nociceptive responses, as well as in the analgesic effects of opiate narcotics. This evidence, based on studies employing pharmacological, surgical, electrophysiological, and dietary manipulations of central nervous system serotonergic neurotransmission, suggests that increases in the activity of brain and spinal cord serotonin neurons are associated with analgesia and enhanced antinociceptive drug potency, whereas decreases in the activities of these neurons correlate with hyperalgesia and diminished analgesic drug potency.