Sensitization to the hyperthermic and catecholamine-releasing effects of morphine

Pavlovian conditioning of morphine hyperthermia: assessment of interstimulus interval and CS-US overlap

The present study examined the effect of interstimulus interval on acquisition of conditioned thermal responses produced by trials in which a light/noise stimulus (CS) was repeatedly paired with infusion of morphine sulphate (US). Rats were implanted with a chronic intravenous catheter for drug delivery and a biotelemetry device for remote monitoring of core body temperature. In experiment 1, different groups received morphine either 0.5 (group P0.5) or 15 min (group P15) after onset of the 15-min CS. A third group was exposed to an identical number of CS and US presentations but in an explicitly unpaired manner (group UP). After repeated exposure to morphine, all groups showed a more rapid rise in body temperature in response to drug infusion. Test presentations of CS alone revealed conditioned hyperthermic responses to CS in groups P0.5 and P15. However, the response of the P15 group was smaller than that of the P0.5 group, suggesting weaker conditioning at the longer interstimulus interval. The contribution of CS-US overlap to the diminished associative strength observed in the P15 group was assessed in experiment 2. Groups P0.5/15 and P0.5/30 received infusions of morphine 0.5 min after onset of a 15- or 30-min CS, respectively. Group P15/30 received morphine 15 min after onset of a 30 min CS, whereas group UP/30 received explicitly unpaired presentations of the US and a 30-min CS. Enhancement of the hyperthermic effect of morphine was observed in all groups after ten conditioning trials. Test presentations of the CS without drug revealed that all paired groups had acquired conditioned hyperthermic responses. These results support the conclusion that drug-induced conditioning can occur at relatively long interstimulus intervals when there is sufficient temporal overlap between the CS and unconditioned response evoked by the drug US.

State-dependent peculiarities of cocaine-induced behavioral sensitization and their possible reasons

To study determinants and possible reasons of cocaine-induced behavioral sensitization, locomotor effects of the drug (COC, 15 mg/kg, IP; 5 daily injections followed by a "challenge," four days later) and various stimuli associated (procedure of injection, placement in the test cage) and nonassociated (tail-pinch) with its administration were measured in rats under different experimental conditions. When COC was injected in quite conditions, two hours after animal habituation to the test cages, locomotor response to the first drug injection was only slightly higher compared to one of saline and these responses were relatively stable following repeated treatment (no sensitization). An enhanced locomotor response to the procedure of injection (needle prick) without changes in responsivity to other stimuli used were found in these COC-treated rats. When COC was injected in activated conditions (animal placement in test cages), locomotor response to the first drug administration was significantly higher compared to one of saline and COC used in the quiet state and it was enhanced following repeated treatment (sensitization). More intense locomotion associated with placement in test cages without any changes in responses to other stimuli was found in rats after this procedure of treatment. Thus, locomotor effects of COC and dynamics of its changes following repeated treatment depend upon the organism's functional state accompanying the drug's effect. Reinforcing action of COC determining development of interinfluence of its effects and ones triggered by environmental variables (learning based on conditioning) is considered as a proposed initial cause and a main mechanism both for enhanced reactivity to cocaine administered under conditions of its previous use and to various environmental events associated with previous cocaine effects.

Amphetamine administered to the ventral tegmental area but not to the nucleus accumbens sensitizes rats to systemic morphine: lack of conditioned effects

Different groups of rats received conditioning training with bilateral injections of amphetamine (2.5 micrograms/0.5 microliters/side) either into the ventral tegmental area (VTA) or the nucleus accumbens (NAC) and were tested on separate occasions for conditioned locomotor activity with saline and cross-sensitization to the locomotor activating effects of morphine (1.0 mg/kg, i.p.). Intra-VTA injections of amphetamine had no effect acutely but subsequently produced a sensitized locomotor response to morphine. On the other hand, intra-NAC amphetamine produced increased locomotion but with no evidence of sensitization either during training or on the test with morphine. No evidence for conditioning of the locomotor activating effects of amphetamine or for stimulus control of the expression of the sensitized response to morphine was obtained with either the intra-VTA or the intra-NAC injections. These results confirm earlier findings indicating that the site of action of amphetamine critical for the development of behavioral sensitization is the VTA and not the NAC and extend them to the case of cross-sensitization to morphine. They also demonstrate that behavioral sensitization can be obtained in the absence of conditioned effects and, therefore, that the two phenomena are distinct. Implications of the present findings for an understanding of the mode and site of action of amphetamine in the development of conditioned locomotor activity are discussed.

Conditioned stimulus control of morphine hyperthermia

Classical conditioning of morphine hyperthermia was examined using an explicit conditioned stimulus (CS) paired with intravenous (IV) morphine administration. Rats were implanted with a jugular vein cannula and a biotelemetry device for monitoring body temperature. The animals were housed 24 h/day in the chambers in which all testing occurred. The CS was a 15-min light/noise stimulus. The unconditioned stimulus (US) was an infusion of morphine (5 mg/kg). Rats were assigned to either the Paired group, which received morphine with the CS, or the Unpaired group, which received explicitly unpaired presentations of the CS and US. The CS-morphine pairings resulted in development of a conditioned hyperthermic response in the Paired group evoked by the CS in the absence of morphine. The development of morphine hyperthermia was more rapid in the Paired group in the presence of the CS than in its absence in the same group and more rapid in the Paired group than in the Unpaired group during the CS. These results clearly show that learning affects the response to morphine administered repeatedly. In contrast to previous studies, conditioned hyperthermia was elicited within 15 min by a discrete CS in a situation where the response was not confounded by handling or the stress of injection.

Environment-specific cross-sensitization between the locomotor activating effects of morphine and amphetamine

Groups of eight rats each were preexposed on four occasions to 10 or 20 mg/kg morphine sulfate, IP, either in activity boxes where activity was measured for two hours (COND, conditioning groups) or in their home cages (UNPAIRED groups). On alternate days these groups were administered saline in the other environment. Two groups of eight rats each served as CONTROL groups (one for each preexposure dose) and were administered saline in both environments. On the day following morphine preexposure, all animals were administered 0.5 mg/kg d-amphetamine sulfate, IP, prior to being tested in the activity boxes. On this test, the COND group preexposed to 10 mg/kg morphine showed higher levels of activity than either of its respective UNPAIRED or CONTROL groups. The COND group preexposed to 20 mg/kg morphine was significantly more active than its unpaired group, but not more active than its CONTROL group. The implications of such environment-specific cross-sensitization between the activity effects of opiate and stimulant drugs are discussed.

Tolerance and sensitization to the heart-rate effects of morphine

The effect of daily exposure to one of several doses of morphine (0, 2.0, 4.0 and 8.0 mg/kg IV) on heart rate was assessed in restrained (R) and unrestrained (U) rats. Initially, morphine produced a biphasic heart-rate response; bradycardia followed by tachycardia. Tolerance to the bradycardic effect was established in the 4 and 8 mg/kg U groups and in the 2 and 4 mg/kg U groups. Sensitization developed to the tachycardic effect in the 2 and 4 mg/kg U groups but not in the 8 mg/kg U group or any of the R groups. After several exposures to morphine, mean preinfusion heart rate increased in the 4 and 8 mg/kg dose groups but not in the 0 and 2 mg/kg dose groups. These results are generally consistent with the other data suggesting that tolerance develops only to the depressant effects of morphine, and either no change or sensitization develops to its stimulant effects. The development of higher preinfusion heart rates in the higher dose groups may represent a learned anticipatory response.

Single-trial conditioned place preference using intravenous morphine

Experiments were performed to investigate single-trial conditioned place preference (CPP) using intravenous morphine in rats. Single-trial CPP was obtained when morphine (8 mg/kg) was paired for either 15 or 30 min with a distinct white compartment. When morphine administration was delayed for either 15 or 25 min after the beginning of a 30-min exposure to the white compartment, single-trial CPP was not obtained. Intravenous naloxone (2 mg/kg) also blocked single-trial CPP when administered 15 min after the beginning of the 30-min exposure to the white compartment with morphine, but naloxone by itself did not alter place preference. The results from these experiments indicate that single-trial CPP using intravenous morphine may offer a useful animal model to assess the reinforcing efficacy of the initial drug experience.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents: 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 since 1979, but data from many earlier papers are also tabulated.

Conditioned place preference with morphine: the effect of extinction training on the reinforcing CR

Rats were injected with either morphine (5 mg/kg) or saline in association with one set of distinct environmental stimuli, and injected with saline in association with a different set of stimuli. After four conditioning trials, animals were given a 15-minute free-choice test to determine which stimulus environment was preferred. Animals displayed CPP as a significant increase in duration spent within the morphine-associated environment, but did not display any change in number of entries into that environment. In contrast, when extinction training was given following CPP, animals displayed a significant decrease in duration spent per entry into the morphine-associated environment, but did not display any change in total duration spent in that environment. These results suggest that assessment of the reinforcing conditioned response (CR) in the CPP model may require measurement of both duration spent in and number of entries into the drug-associated environment.

Differential sensitivity to morphine in nervous and normal pointer dogs

The effects of morphine and pentobarbital on body temperature and heart rate were studied in a line of pointer dogs characterized as genetically nervous. Intravenous administration of cumulative doses of morphine to the conscious, restrained, nervous dogs produced less tachycardia and hypothermia than were observed in control dogs. Nervous and control dogs did not differ in their responses to pentobarbital. These data are consistent with recent findings that nervous dogs have a lower density of opioid receptors in the brain than do control dogs.

Morphine suppresses plasma catecholamine responses to laparotomy but not to 2-deoxyglucose

The increase of plasma catecholamines that occurs during surgery can be reduced by administration of morphine. To test the hypothesis that morphine specifically blocks nociceptive stimulation during surgery, we compared the effects of morphine administration on the plasma catecholamine response to a laparotomy in pentobarbital-anesthetized dogs with the effect of morphine on the plasma catecholamine response to the neuroglucopenic agent, 2-deoxy-D-glucose (2DG, 300 mg/kg iv). In control dogs, plasma epinephrine (Epi) and plasma norepinephrine (NE) both increased progressively with time following a midline laparotomy (delta Epi by 50 min, +133 +/- 42 pg/ml, P less than 0.01 and delta NE by 50 min, +108 +/- 38 pg/ml, P less than 0.01, mean +/- SE, n = 12). 2-Deoxy-D-glucose produced a similar increase of both plasma NE and Epi. In dogs that received the anesthesia alone, plasma catecholamines did not increase from base line during the experiment. The analgesic morphine (15 mg iv), given 15 min after the completion of laparotomy, not only prevented the progressive rise of plasma catecholamines after laparotomy, but also caused a small but significant decline (P less than 0.05). Naloxone (0.4 mg iv) totally reversed the suppressive effects of morphine, restoring both catecholamines to the levels of their time-related control. In marked contrast, neither morphine nor naloxone affected the plasma NE and Epi increases following the administration of 2DG. These data suggest that morphine suppression of plasma catecholamines during surgery is not due to a generalized attenuation of sympathetic outflow, but rather to a specific interaction with an opiate receptor that either mediates analgesia or lies within the neural pathway stimulated by laparotomy but not by 2DG.