Signal transduction and functional selectivity of F15599, a preferential post-synaptic 5-HT1A receptor agonist.

BACKGROUND AND PURPOSE: Activation of post-synaptic 5-HT(1A) receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT(1A) receptor agonist. EXPERIMENTAL APPROACH: F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo. KEY RESULTS: F15599 was highly selective for 5-HT(1A) receptors in binding experiments and in [(35)S]-GTPgammaS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT(1A) receptors. In cell lines expressing h5-HT(1A) receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT(1A) receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [(35)S]-GTPgammaS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated G(alphai) than G(alphao) activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT(1A) receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT(1A) receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT(1A) receptors in vivo almost as potently as F13714. CONCLUSIONS AND IMPLICATIONS: F15599 showed a distinctive activation profiles for 5-HT(1A) receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT(1A) receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. III. Activity in models of cognition and negative symptoms.

BACKGROUND AND PURPOSE: The D(2)/D(3) receptor antagonist, D(4) receptor partial agonist, and high efficacy 5-HT(1A) receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the 'serotonin syndrome'. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia. EXPERIMENTAL APPROACH: F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems. KEY RESULTS: Through 5-HT(1A) activation, F15063 partially alleviated (MED: 0.04 mg kg(-1)) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg(-1), F15063 reduced interaction by itself. F15063 (0.16 mg kg(-1)) selectively re-established PCP-impaired 'cognitive flexibility' in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04-0.63 mg kg(-1)) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D(4) partial agonism, as it was blocked by the D(4) antagonist L745,870. Finally, F15063 up to 40 mg kg(-1) did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating. CONCLUSIONS AND IMPLICATIONS: The balance of D(2)/D(3), D(4) and 5-HT(1A) receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia.

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.

5-HT(2A) gene promoter polymorphism as a modifying rather than a vulnerability factor in anorexia nervosa.

The A allele of the 5-HT(2A) gene (-1438A/G polymorphism) has been associated with anorexia nervosa in four studies, but not in three others. One possibility to explain such a discrepancy is that the A allele acts as a modifying rather than a vulnerability allele. To test this hypothesis, we increased our initial sample of 102 trios left open bracket Mol. Psychiatry 7 (2002) 90 right open bracket with 43 new patients with anorexia nervosa and 98 healthy controls. In addition to confirming the absence of association on the global sample of 145 patients, we found that patients with the A allele had a significantly later age at onset of the disease (P = 0.032). Furthermore, the A allele was also transmitted with an older age at onset (P = 0.023) using a quantitative-trait TDT approach. The A allele may thus act as a modifying factor (delaying onset), potentially explaining variations of allele frequency across samples, in which differences in average age at onset are not only possible, but also expected. Taking into account vulnerability genes, but also genes modifying the expression of the disorder, will help to disentangle the complexity of the etiological factors involved in anorexia nervosa.

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.

Study of the addictive potential of modafinil in naive and cocaine-experienced rats.

RATIONALE: Modafinil is a drug that promotes wakefulness and, as such, is used to treat hypersomnia and narcolepsy. Preclinical and clinical studies suggest that modafinil could possess weak reinforcing effects in drug-experienced subjects. However, its abuse potential in drug-naive healthy individuals is still totally uninvestigated, despite the fact that availability of modafinil has recently increased. OBJECTIVES: The purpose of our study was to investigate the potential addictive properties of modafinil by testing its reinforcing effects in naive rats. The interactions of modafinil with the reinforcing effects of cocaine were also tested. METHODS: First, using i.v. self-administration and place conditioning tests, we studied the reinforcing and rewarding effects of a large range of doses of modafinil in naive rats. Second, we tested the influence of modafinil on reinforcing and incentive effects of cocaine in rats trained for cocaine self-administration. The effects of modafinil were compared with those of amphetamine and haloperidol. RESULTS: Modafinil did not produce reinforcing or rewarding effects and did not modify the effects of cocaine. CONCLUSIONS: Our results suggest that modafinil does not possess an addictive potential in naive individuals. Furthermore, it would be behaviorally distinct from classical central nervous system stimulants which are known to alter cocaine-induced effects. However, as shown previously in nonhuman primates and in humans, modafinil could possibly have reinforcing effects in cocaine-experienced individuals.

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.

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.

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.

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.

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.

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.

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.

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.

Lack of association between anorexia nervosa and D3 dopamine receptor gene.

BACKGROUND: Evidence from family and twin studies suggests a genetic contribution to the etiology of anorexia nervosa. Different genes could contribute to the vulnerability to anorexia nervosa, but dopamine could be more specifically implicated in anorexia nervosa because of pharmacologic, endocrine, and neurobiological specificities. The dopamine receptor D3 (DRD3) may be of additional interest, since it is specifically located in the limbic area, an area implicated in reward and reinforcement behavior. METHODS: We performed an association study between 39 patients with severe (requiring hospitalization and with young age at onset) anorexia nervosa (DSM-III-R), and 42 controls, with the Bal I polymorphism in exon I of the DRD3 gene. RESULTS: There was no significant difference between patients with anorexia nervosa and controls in allele frequencies or genotype count. The association was still negative between subgroups separated according to family history of anorexia nervosa or comorbid mood disorders. CONCLUSIONS: Despite the fact that the number of patients tested is small, there is good evidence that the Bal I DRD3 polymorphism does not play a major role in the genetic component of anorexia nervosa. It would be useful to test polymorphisms of the other genes coding for dopamine receptors.