A persistent opioid-addiction state of memory

We determined whether tolerance develops to a morphine-induced state of memory. Rats were injected with 5 mg/kg of morphine and trained to complete a FR-10 schedule of lever presses in daily sessions. The dose-response curve of morphine (1.25-40 mg/kg) in enabling retrieval was tested in one group immediately after criterion had been reached and, in another group, after an additional 40 training sessions. The additional training enhanced, rather than attenuated, the dependence of retrieval on morphine; this was because the further gain in response latency that developed during additional training also became state-dependent. Thus, because tolerance did not develop to the morphine state, an increasingly large body of engrams became encoded in that state, rendering retrieval increasingly dependent.

Experimental conditions for the continuous subcutaneous infusion of four central analgesics in rats

For the analysis of pharmacotherapeutic regimens for chronic pain in animals, it is important to establish delivery methods in which analgesics can be administered continuously and at a constant rate for a prolonged period of time. This allows for the assessment of how drug effects may vary over time in the presence of ongoing pain. The present study determined, for four analgesic compounds, the maximal doses that met all of the following criteria: (i) water-soluble, (ii) stable over 14 days at 38 degrees C, and (iii) devoid of undesirable side-effects in normal rats, as assessed by evolution of body weight and temperature after the subcutaneous implantation of an osmotic mini-pump that continuously infused the compounds over a 14-day period. The results showed the maximal doses to be 5 mg/rat/day for morphine hydrochloride, 2.5 mg/rat/day for imipramine hydrochloride, 20 mg/rat/day for ketamine hydrochloride, and 10 mg/rat/day for gabapentin. These doses were further found to be sufficient to express each compound's representative pharmacological activity. The conditions identified here appear appropriate for future studies of these four compounds in rat models of chronic pain and neuropathic allodynia.

Sign-reversal during persistent activation in mu-opioid signal transduction

A concept of signal transduction in biological systems specifies that any instantaneous input is appreciated by its departure from the moving average of past activity. The concept provides an adequate account of the occurrence of both the one-directional (e.g. analgesic) effects induced by opioid receptor activation, and of the contra-directional (e.g. hyperalgesic) effects that can be observed when activation is discontinued. Following this transduction concept, the numerical simulations reported here revealed, remarkably, that under some parametric conditions, the input's effect may reverse even as input is maintained at a constant magnitude. In in vitro conditions that are proximal to the signal transduction that occurs when an opioid agonist binds to the G-protein coupled opioid receptor, the effects of opioid receptor activation were monitored by measuring time-dependent Ca(2+) responses in CHO-K1 cells transfected with a mu-opioid receptor and G(alpha 15) protein. The results indicate morphine to produce an initial increase in intracellular Ca(2+) concentration followed by a decrease below basal level. The occurrence of a sign-reversal was confirmed in native conditions of receptor-to-G protein coupling; the continuous in vivo infusion over a 2-week period of 0.31 mg rat(-1)day(-1) of fentanyl initially caused an increase of the mechanical threshold to induce a pain response (i.e. analgesia) that was followed by a decrease (i.e. hyperalgesia). The findings indicate that with opioid signaling systems, transduction mechanisms operate that may cause the sign of the effect to reverse not only when activation is discontinued but also whilst it is maintained at a constant magnitude.

Evidence that mnesic states govern normal and disordered memory

Although it is recognized that retrieval may be state-dependent, only recently has a paradigm been identified that allows state-dependence in rats to be demonstrated reliably and at relevant doses of CNS agents. In humans, the effects of scopolamine constitute a valuable model of disordered memory. We used this paradigm to analyze the effects of scopolamine on different memory processes. Rats treated with either saline or 0.01-10 mg/kg doses of scopolamine learned to lever press for milk reward, and were then tested for retrieval while given the same or a different treatment. Saline-to-scopolamine as well as scopolamine-to-saline state changes produced robust failures to retrieve the response. Remarkably, the state produced by 2.5 mg/kg scopolamine, like that produced by saline, produced little intrinsic effect on learning or any other memory process (i.e. when the prevailing state was left unchanged). However, changing the implemented state from one to the other between two different processes, disrupted not only retrieval, but also learning, encoding and retention. We also determined whether the graded state changes produced by 0.01-10 mg/kg doses of scopolamine could mimic the peculiar and poorly understood temporally graded retrograde amnesia that occurs in Alzheimer's disease. In rats that had acquired a complex drug-discrimination task over a 6-month period, scopolamine-induced state changes seemed to produce dose-dependent deficits in the retrieval of recent information while preserving those abilities that had been acquired in the increasingly remote past. Beyond its role in retrieval, the findings implicate state dependence in learning, encoding and retention, and suggest that physiologically defined mnesic states govern each of these. The changes of mnesic state that are likely associated with excessively labile cholinergic neurotransmission may conceivably cause the complex disabilities of Alzheimer's disease.

Opiate self-administration as a measure of chronic nociceptive pain in arthritic rats

The study examined the validity of oral fentanyl self-administration (FSA) as a measure of the chronic nociceptive pain that develops in rats with adjuvant arthritis independently of acute noxious challenges. Arthritic rats self-administered more of a 0.008 mg/ml fentanyl solution (up to 3.4 g/rat per day) than non-arthritic controls (0.5 g/rat per day) and did so with a biphasic time course that reached peak during weeks 3 and 4 after inoculation with Mycobacterium butyricum. The time course paralleled both the disease process and the chronic pain. Continuous infusion of dexamethasone during weeks 3 and 4 via subcutaneous osmotic pumps at 0.0025-0.04 mg/rat per day disrupted the arthritic disease and decreased FSA to a level (i.e. by 65%) similar to that observed in non-arthritic rats. Continuous naloxone (2.5 mg/rat per day) decreased FSA (by 55%) in arthritic but not in non-arthritic animals. Continuous, subcutaneous infusion of fentanyl also decreased arthritic FSA in a manner that varied with dose at 0.04-0.16 mg/rat per day doses, but leveled off at 47% of controls with 0.31 mg/rat per day. The effects of continuous fentanyl on arthritic FSA occurred only with those doses and dose-dependent dynamics with which fentanyl also induced dependence in non-arthritic rats. The findings indicate that pain, rather than the rewarding or dependence-inducing action of fentanyl mediates FSA in arthritic rats. Paralleling patient-controlled analgesic drug intake, FSA offers a specific measure allowing the dynamic effects of neurobiological agents to be studied in this unique animal model of persistent nociceptive pain.

Memory of an operant response and of depressed mood retained in activation states of 5-HT(1A) receptors: evidence from rodent models

Three series of studies were conducted to specify the role of 5-HT(1A) receptors in memory; using selective ligands that differentially activate 5-HT(1A) receptors, it was determined whether a change in the activation state of these receptors can lead to deficient retrieval, and whether a so-produced deficit can occur in an animal model of depression. First, in vitro studies of [35S]GTPgammaS binding responses identified ligands that differentially activate 5-HT(1A) receptors in rat hippocampus. WAY 100635, 8-OH-DPAT and flesinoxan induced 5-HT(1A) receptor activation that amounted to -2, +50 and +63%, respectively, of that produced by 5-HT. Second, we determined whether changes in the activation state of 5-HT(1A) receptors could impair the retrieval of an operant response in vivo. Rats treated with either a 5-HT(1A) receptor ligand or saline were trained to lever press for milk reward, and were then tested for retrieval with either the same or another treatment. Animals trained with 8-OH-DPAT retrieved the response when tested in the same state, but not when tested in the saline state, and vice versa. Rats trained with 0.16 mg/kg of 8-OH-DPAT also retrieved the response when tested with the other intermediate-efficacy ligand flesinoxan (0.63 mg/kg), but not when tested in a state of lower-magnitude activation (i.e. with 0.16 mg/kg of WAY 100635). Animals trained with 0.16 mg/kg of WAY 100635 retrieved the response when tested in this same state or with saline, but not when tested in a state of intermediate-magnitude activation (i.e. with 0.16 mg/kg of 8-OH-DPAT). Finally, studies using the forced swimming paradigm indicated that the retrieval of learned immobility was similarly dependent upon the activation state of 5-HT(1A) receptors. The findings indicate that changes in activation states of 5-HT(1A) receptors can impair the retrieval of learned responses. It is suggested that depression may in part be acquired in the course of ontogeny and may be available for retrieval in the same but not in other states; various biological rhythms conceivably define such states.

Opiate states of memory: receptor mechanisms

The present studies characterized the receptor mechanisms of morphine-induced states of memory. Morphine (5 mg/kg) produced a state in which rats could learn and retrieve an operant response; retrieval was impaired, however, when the rats were tested in the normal state. Conversely, rats that were trained in the normal state failed to retrieve the response in the morphine state. In either case the mnesic state was dose dependent, commencing at morphine doses as low as 0.8 mg/kg. In rats trained with 5 mg/kg of morphine, retrieval was fully adequate when tested with this same dose but not when tested with either lower or higher doses. Naloxone, but not naltrindole, antagonized the morphine-induced state; heroin and (-)-cyclazocine, but not U50,488H, (+)-cyclazocine and SNC80, produced a state in which retrieval occurred at least partially. Time-effect studies in which injections were made from 0 to 240 min before the sessions indicated that the retrieval in saline-to-morphine and morphine-to-saline conditions occurred along different time courses; a theory of opiate signal transduction suggests that these temporal profiles result from morphine producing two bi-directional mnesic states that may differ as much as the analgesia and hyperalgesia that morphine also induces. It appears that a particular magnitude of mu opiate receptor activation produces a state to which a memory trace can be confined in a highly selective manner. The normal and this particular morphine state are only some of the many mutually inaccessible and molecularly definable states of memory that are likely to exist, thus challenging the unitary concept of an individual organism's memory.

Neuroactive steroids and the constraint of memory

Different normo- and pathophysiological conditions are associated with large variations in plasma and brain concentrations of neuroactive steroids. In an attempt to specify the possible role of these steroids in memory processes, we examined the ability of pregnanolone, a positive modulator of the gamma-aminobutyric acid type A (GABAA) receptor complex, to sustain state dependence in rats. Animals treated with either saline or different doses of pregnanolone were trained to complete a fixed ratio 10 (FR10) schedule of lever presses for milk reward within 120 s, and were tested for the retention of this response 48 h later while treated with the same or a different treatment. The data indicate that saline-to-drug as well as drug-to-saline state changes produced robust failures to recall the response. Furthermore, animals trained with pregnanolone showed transfer of the response when tested with the benzodiazepine chlordiazepoxide and vice versa. The partial benzodiazepine inverse agonist N-methyl-beta-carboline-3-carboxamide (FG-7142) antagonized the states produced by both pregnanolone and chlordiazepoxide. State changes constitute a mechanism of action that may operate endogenously; the release of neuroactive steroids in response to various physiological conditions may act to contain but also to constrain memories associated with these events, rendering these memories inaccessible on other occasions. The apparent memory impairment that can so be produced may render the effects of past experience available in a manner that is appropriately selective.

Experimental realization of a signal transduction algorithm

A theory of signal transduction specifies that in biological systems, any instantaneous input is appreciated by its departure from the mean past activity that occurred over a certain time window called the sample period; it also specifies that any input generates two so-called first- and second-order effects that are opposite in sign. We here report the detailed experimental realization of the algorithm that formalizes this signal transduction process; numerical simulations adequately predicted various intricate features of the (first order) analgesia and the paradoxical (second order) hyperalgesia which morphine produces. The data also offer a first estimation of the physiological sample period that may govern the classical rat tail flick response. The signal transduction process appears to operate ubiquitously and provides an unprecedented account of the paradoxical effects that have been observed with different signalling systems. This process may also operate with such phenomena as refractoriness, homeostasis, adaptation, sensitization, dependence, tolerance or resistance and neuronal plasticity.

Recall rendered dependent on an opiate state

Remembering may require that the organism be in a state that is similar to that in which the event was initially experienced (state dependence [StD]). We determined whether morphine induces StD and whether this StD is conceivably involved in the analgesic effects that opiates produce. Rats trained while treated with morphine recalled the learned response when tested with this opiate but not when in their nondrugged state, and vice versa. Furthermore, morphine analgesia occurred in a manner that was similar to StD in terms of both dose and time. In as much as responses to nociceptive stimulation are learned during the course of ontogeny, StD may constitute the psychophysiological mechanism whereby opiates produce their characteristic analgesic effects.

Recall rendered dependent on an opiate state

Remembering may require that the organism be in a state that is similar to that in which the event was initially experienced (state dependence [StD]). We determined whether morphine induces StD and whether this StD is conceivably involved in the analgesic effects that opiates produce. Rats trained while treated with morphine recalled the learned response when tested with this opiate but not when in their nondrugged state, and vice versa. Furthermore, morphine analgesia occurred in a manner that was similar to StD in terms of both dose and time. In as much as responses to nociceptive stimulation are learned during the course of ontogeny, StD may constitute the psychophysiological mechanism whereby opiates produce their characteristic analgesic effects.

Ethanol state dependence involving a lever press response requirement in rats

Previous studies have found that the timely completion of an FR10 schedule of food-rewarded lever pressing in rats demonstrates state dependence in drug-to-saline state changes with benzodiazepines and NMDA antagonists, but not with ethanol. We report here that, using sweetened condensed milk rather than food pellets as a reward, ethanol nonetheless produces a symmetrical state dependence with the lever press response requirement at doses that also impair acquisition. Extensive parametric studies are needed to unravel the apparently subtle conditions that govern the occurrence and features of the state dependence produced by various CNS compounds.