Differences in opiate-induced synaptic excitability of hippocampal slices prepared from tolerant and nontolerant rats

Opioid withdrawal and memory consolidation

It is well established that learning and memory are central to substance dependence. This paper specifically reviews the effect of opioid withdrawal on memory consolidation. Although there is evidence that opioid withdrawal can interfere with initial acquisition and retrieval of older memories, there are several reasons to postulate a facilitatory action on the consolidation of newly acquired memories. In fact, there is substantial evidence that memory consolidation is facilitated by the release of stress hormones, that it requires the activation of the amygdala, of central noradrenergic and cholinergic pathways, and that it involves long-term potentiation. This review highlights evidence that very similar neurobiological processes are involved in opioid withdrawal, and summarizes recent results indicating that naltrexone-precipitated withdrawal enhanced consolidation in rats. From this neurocognitive perspective, therefore, opioid use may escalate during the addiction cycle in part because memories of stimuli and actions experienced during withdrawal are strengthened.

Involvement of NMDA receptors and voltage-dependent calcium channels on augmentation of long-term potentiation in hippocampal CA1 area of morphine dependent rats

The involvement of NMDA receptors and voltage-dependent calcium channels on augmentation of long-term potentiation (LTP) was investigated at the Schaffer collateral-CA1 pyramidal cell synapses in hippocampal slices of morphine dependent rats, using primed-bursts tetanic stimulation. The amplitude of population spike was measured as an index of increase in postsynaptic excitability. d, l-AP5 and nifedipine were used as NMDA receptor antagonist and voltage-dependent calcium channel blocker, respectively. The amount of LTP of orthodromic population spike amplitude was higher in slices from dependent rats. Perfusion of slices from control or dependent rats with ACSF containing either D,L-AP5 (25 microM) or nifedipine (10 microM) and delivering tetanic stimulation, showed that D,L-AP5 completely blocked LTP of OPS in slices from both control and dependent rats, while nifedipine attenuated the amount of LTP of OPS in dependent slices and had no effect on control ones. The results suggest that the enhanced LTP of OPS in the CA1 area of hippocampal slices from morphine dependent rats is primarily induced by the NMDA receptors activity and the voltage-dependent calcium channels may also be partially involved in the phenomenon.

Augmentation of LTP induced by primed-bursts tetanic stimulation in hippocampal CA1 area of morphine dependent rats

The effects of chronic morphine administration on the development of Long-term potentiation (LTP) were investigated at the Schaffer collateral-CA1 pyramidal cell synapses of the rat hippocampal slices using primed-bursts tetanic stimulation. Significant enhancement of orthodromic population spike (OPS) was found for all stimulus intensities after tetanic stimulation. OPS enhancement was greatest when tested with low to mid-range stimulus intensities (25 and 50 microA). There was also significant decrease in OPS delay. These responses were similar in slices from both control and morphine dependent rats. At all delivered stimulus intensities, the amount of LTP of OPS in slices from dependent rats was larger than that of control slices. However, these differences in LTP of OPS were significant at low stimulus intensities. These findings suggest that chronic morphine administration had induced changes in CA1 neurocircuitry which modulated synaptic plasticity during high frequency stimulation and appeared as augmented LTP.

Acute tolerance to the excitatory effects of opioids in the rat hippocampus

Prolonged iontophoretic administrations of delta- and mu-selective opioid receptor agonists were conducted in the hippocampus of rats, in order to study the possible development of acute tolerance to the excitatory effects of the opioids. Acute tolerance (AT) to the excitatory effects of the delta-selective opioid receptor agonist Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET) was observed when the drug was applied locally for 3-5 min in the CA1 hippocampal pyramidal neurons. The acute tolerance was expressed as a decrease in the commissurally evoked spike responsiveness during peptide's administration and led to a long-lasting potentiation of the population spike (PS) upon its withdrawal. In all cases, where AT and spike potentiation were evident, the population excitatory postsynaptic potential (pEPSP) remained unaltered. Pharmacological studies of AT and long-lasting spike potentiation showed the following: (1) the nonselective opioid receptor antagonist, naloxone, while effective in blocking the excitatory effects of DSLET when applied prior and during the application of the latter, failed to exhibit any effect on the long-lasting potentiating effect of the opioid; and (2) during the spike potentiation phase, administration of DSLET exhibited a depressant effect towards baseline values. This depressant effect of the opioid was evident 2-3 min from the beginning of the application and was completely antagonized by naloxone. The above results show that the development of acute tolerance to the excitatory effects of the DSLET led to long-lasting spike potentiation, which manifests a withdrawal phenomenon.

Chronic morphine exposure blocks opioid effects on both the early and late inhibitory postsynaptic potentials in hippocampal CA1 pyramidal cells

The mu-opioid agonist, [N-MePhe3,D-Pro4]morphiceptin (PL017), significantly decreased the conductance changes measured during both the early and late inhibitory postsynaptic potentials (IPSP) in CA1 pyramidal cells. Although the conductance change during the early IPSP was much larger than that during the late IPSP, the relative decrease in conductance caused by 1 microM PL017 was similar for both. Chronic morphine treatment of rats prior to hippocampal slice preparation resulted in a loss of PL017 (1 microM) effects on both the early and late IPSPs. These results suggest that opioids have an equal ability to alter both early and late IPSPs in the CA1, that these effects are equally sensitive to chronic morphine, and that these measurements are a sensitive means of determining opioid tolerance in the hippocampus.

Chronic intracerebroventricular morphine and lactation in rats: dependence and tolerance in relation to oxytocin neurones

1. Acutely, opioids inhibit oxytocin secretion. To study the responses of oxytocin neurones during chronic opioid exposure, forty-five lactating rats were infused continuously from a subcutaneous osmotically driven mini-pump via a lateral cerebral ventricle with morphine sulphate solution from day 2 post-partum for 5-7 days; the infusion rate was increased 2- or 2.5-fold each 40 h from 10 micrograms/h initially up to 50 micrograms/h; controls were infused with vehicle (1 microliter/h, twenty-eight rats) or were untreated (eight rats). 2. Maternal behaviour was disrupted in 27% of the morphine-treated rats; in rats that remained maternal morphine did not affect body weight or water intake but increased rectal temperature by 0.82 +/- 0.14 degrees C (mean +/- S.E.M.) across the first 4 days. 3. Weight gain of the litters of maternal morphine-treated rats was reduced by 32% during 7 days, predominantly in the first day of treatment when milk transfer was also reduced. Observation of pup behaviour during suckling showed decreased frequency of milk ejections on only the second day of morphine treatment. Plasma concentration of prolactin after 6 days was similar in maternal morphine-treated and control rats, but reduced by 90% in non-maternal morphine-treated rats, indicating normal control of prolactin secretion by suckling in morphine-treated rats. 4. Oxytocin and vasopressin contents, measured by radioimmunoassay, in the supraoptic and paraventricular nuclei and in the neurohypophysis were similar between fourteen maternal morphine-treated, twelve vehicle-treated and eight untreated lactating rats; thus exposure to morphine did not involve increased production and storage of oxytocin. 5. Distribution of [3H]morphine infused intracerebroventricularly into six virgin female rats for 6 days was measured by scintillation counting of tissue extracts. Morphine concentration in the hypothalamus and neurohypophysis was 2.7 and 12.8 micrograms/g, respectively, and in blood plasma 0.75 micrograms/g. Tolerance was not due to failure of morphine infusion. In addition, naloxone (5 mg/kg s.c.) provoked typical withdrawal reactions ('wet dog' shakes, defaecation, burrowing) in lactating rats infused with morphine for 5 days. 6. Pups were suckled onto seven maternal morphine-infused and five vehicle-infused rats anaesthetized with urethane for recording of intramammary and arterial blood pressures after treatment for 5 days. The incidence and pattern of milk ejections, and mammary gland sensitivity to oxytocin were similar in the two groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal sensitivity to opiate and opioid peptides in the bed nucleus of the stria terminalis. Effects of chronic treatment with morphine

To characterize the phenomenon of morphine tolerance in central nervous system, the neuronal sensitivity to opiate and opioid peptides in the bed nucleus of the stria terminalis (BST) and the effects of chronic treatment with morphine were examined with thin slices of the brain of guinea pigs. Both morphine and enkephalins (D-Ala2-Met5-enkephalinamide and D-Ala2-D-Leu5-enkephalin) suppressed neuronal activity in the stria terminalis. Three days after implantation of morphine pellets the guinea pigs became tolerant to morphine and EC50 (the morphine concentration required for causing 50% of the maximal effect) was 79 microM, nearly six times the value in the control animals (EC50 = 13 microM). Cross-tolerance between morphine and enkephalins was not observed. Results of radioreceptor assay, using [3H]dihydromorphine as a ligand, showed that the binding capacity of the morphine-tolerant animals was not statistically different from that of the controls. These results suggested that tolerance to morphine cannot be explained by changes in number and/or affinity of opiate receptors, and that tolerance probably results from modification of the intracellular mechanisms mediating between the binding of morphine with the receptors and neuronal inhibition.