Involvement of central cholinergic mechanisms in the effects of oxytocin and an oxytocin receptor antagonist on retention performance in mice

The role of intraamygdaloid oxytocin in spatial learning and avoidance learning

The goal of the present study is to investigate the role of intraamygdaloid oxytocin in learning-related mechanisms. Oxytocin is a neuropeptide which is involved in social bonding, trust, emotional responses and various social behaviors. By conducting passive avoidance and Morris water maze tests on male Wistar rats, the role of intraamygdaloid oxytocin in memory performance and learning was investigated. Oxytocin doses of 10 ng and 100 ng were injected into the central nucleus of the amygdala. Our results showed that 10 ng oxytocin significantly reduced the time required to locate the platform during the Morris water maze test while significantly increasing the latency time in the passive avoidance test. However, the 100 ng oxytocin experiment failed to produce a significant effect in either of the tests. Wistar rats pretreated with 20 ng oxytocin receptor antagonist (L-2540) were administered 10 ng of oxytocin into the central nucleus of the amygdala and were also subjected to the aforementioned tests to highlight the role of oxytocin receptors in spatial- and avoidance learning. Results suggest that oxytocin supports memory processing during both the passive avoidance and the Morris water maze tests. Oxytocin antagonists can however block the effects of oxytocin in both tests. The results substantiate that oxytocin uses oxytocin receptors to enhance memory and learning performance.

The neurohypophyseal hormone oxytocin and eating behaviors: a narrative review

BACKGROUND

The neuropeptide oxytocin (OT) is crucial in several conditions, such as lactation, parturition, mother-infant interaction, and psychosocial function. Moreover, OT may be involved in the regulation of eating behaviors.

METHODS

This review briefly summarizes data concerning the role of OT in eating behaviors. Appropriate keywords and medical subject headings were identified and searched for in PubMed/MEDLINE. References of original articles and reviews were screened, examined, and selected.

RESULTS

Hypothalamic OT-secreting neurons project to different cerebral areas controlling eating behaviors, such as the amygdala, area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus nerve. Intracerebral/ventricular OT administration decreases food intake and body weight in wild and genetically obese rats. OT may alter food intake and the quality of meals, especially carbohydrates and sweets, in humans.

DISCUSSION

OT may play a role in the pathophysiology of eating disorders with potential therapeutic perspectives. In obese patients and those with certain eating disorders, such as bulimia nervosa or binge/compulsive eating, OT may reduce appetite and caloric consumption. Conversely, OT administered to patients with anorexia nervosa may paradoxically stimulate appetite, possibly by lowering anxiety which usually complicates the management of these patients. Nevertheless, OT administration (e.g., intranasal route) is not always associated with clinical benefit, probably because intranasally administered OT fails to achieve therapeutic intracerebral levels of the hormone.

CONCLUSION

OT administration could play a therapeutic role in managing eating disorders and disordered eating. However, specific studies are needed to clarify this issue with regard to dose-finding and route and administration time.

Oxytocin-Cholinergic Central Interaction: Implications for Non-Social Memory Formation

Oxytocin (OT) and vasopressin (AVP) are two closely related neuropeptides implicated in learning and memory processes, anxiety, nociception, addiction, feeding behavior and social information processing. Regarding learning and memory, OT has induced long-lasting impairment in different behaviors, while the opposite was observed with AVP. We have previously evaluated the effect of peripheral administration of OT or its antagonist (AOT) on the inhibitory avoidance response of mice and on the modulation of cholinergic mechanisms. Here, we replicate and validate those results, but this time through central administration of neuropeptides, considering their poor passage through the blood-brain barrier (BBB). When we delivered OT (0.10 ng/mouse) and its antagonist (0.10 ng/mouse) through intracerebroventricular (ICV) injections, the neuropeptide impaired and AOT enhanced the behavioral performance on an inhibitory avoidance response evaluated 48 h after training in a dose-dependent manner. On top of that, we investigated a possible central interaction between OT and the cholinergic system. Administration of anticholinesterases inhibitors with access to the central nervous system (CNS), the activation of muscarinic acetylcholine (Ach) receptors and the increase of evoked ACh release using linopirdine (Lino) (3-10 µg/kg, IP), reversed the impairment of retention performance induced by OT. Besides, either muscarinic or nicotinic antagonists with unrestricted access to the CNS reduced the magnitude of the performance-facilitating effect of AOT's central infusion. We suggest that OT might induce a cholinergic hypofunction state, resulting in an impairment of IA memory formation, a process for which the cholinergic system is crucially necessary.

Reversal of oxycodone conditioned place preference by oxytocin: Promoting global DNA methylation in the hippocampus

Repeated exposure to the opioid agonist, oxycodone, can lead to addiction. Accumulating evidence has shown that oxytocin (OT), a neurohypophyseal neuropeptide, could reduce the abuse potential of drugs. Recent studies suggest that epigenetic regulation through DNA methylation are involved in neuroadaptations. The current study was conducted to investigate the effects of OT on oxycodone conditioned place preference (CPP) and the epigenetic mechanism of OT in the hippocampus. For induction of CPP, oxycodone (3.0 mg/kg, i. p.) was administrated to male Sprague-Dawley rats once every other day during an eight-day conditioning phase. Global 5-methylcytosine (5-mC) level was determined based on CPP procedure, including acquisition, expression, extinction and reinstatement. We also measured mRNA levels of DNA methyltransferases (Dnmts), ten-eleven translocations (Tets) and synaptic genes (Psd95, Shank2, Gap43, etc.), and determined synaptic density after restraint stress-induced reinstatement of oxycodone CPP. The results showed that OT (2.5 μg, i. c.v.) pretreatment specifically inhibited the CPP acquisition and expression, facilitated the CPP extinction, and abolished restraint stress-induced reinstatement of oxycodone CPP. OT markedly inhibited global 5-mC changes induced by oxycodone CPP in the four phases. Following restraint stress-induced reinstatement of oxycodone CPP, OT significantly increased mRNA levels of Dnmt1, decreased Tet1 mRNA, synaptic proteins and synaptic density in the hippocampus. Our study indicated that reversal of global DNA hypomethylation through OT could significantly attenuate the rewarding effects induced by oxycodone. Our results suggested that OT could be specific manipulation on oxycodone addiction.

Oxytocin prevents the increase of cocaine-related responses produced by social defeat

The neuropeptide oxytocin (OXT) plays a critical role in the regulation of social and emotional behaviors. OXT plays a role in stress response and in drug reward, but to date no studies have evaluated its implication in the long-lasting increase of the motivational effects of cocaine induced by repeated social defeat (RSD). During the social defeat procedure, 1 mg/kg of OXT was administered 30 min before each episode of RSD. Three weeks after the last defeat, the effects of cocaine on the conditioned place preference (CPP), locomotor sensitization and the self-administration (SA) paradigms were evaluated. The influence of OXT on the levels of BDNF in the prefrontal cortex (PFC), striatum and hippocampus was also measured. Our results confirm that raising the levels of OXT during social defeat stress can block the long-lasting effects of this type of stress. OXT counteracts the anxiety induced by social defeat and modifies BDNF levels in all the structures we have studied. Moreover, OXT prevents RSD-induced increases in the motivational effects of cocaine. Administration of OXT before each social defeat blocked the social defeat-induced increment in the conditioned rewarding effects of cocaine in the CPP, favored the extinction of cocaine-associated memories in both the CPP and SA, and decreased reinstatement of cocaine-seeking behavior in the SA. In conclusion, the long-lasting effects of RSD are counteracted by administering OXT prior to stress, and changes in BDNF expression may underlie these protective effects.

Intranasal Oxytocin and Vasopressin Modulate Divergent Brainwide Functional Substrates

The neuropeptides oxytocin (OXT) and vasopressin (AVP) have been identified as modulators of emotional social behaviors and associated with neuropsychiatric disorders characterized by social dysfunction. Experimental and therapeutic use of OXT and AVP via the intranasal route is the subject of extensive clinical research. However, the large-scale functional substrates directly engaged by these peptides and their functional dynamics remain elusive. By using cerebral blood volume (CBV) weighted fMRI in the mouse, we show that intranasal administration of OXT rapidly elicits the transient activation of cortical regions and a sustained activation of hippocampal and forebrain areas characterized by high oxytocin receptor density. By contrast, intranasal administration of AVP produced a robust and sustained deactivation in cortico-parietal, thalamic and mesolimbic regions. Importantly, intravenous administration of OXT and AVP did not recapitulate the patterns of modulation produced by intranasal dosing, supporting a central origin of the observed functional changes. In keeping with this notion, hippocampal local field potential recordings revealed multi-band power increases upon intranasal OXT administration. We also show that the selective OXT-derivative TGOT reproduced the pattern of activation elicited by OXT and that the deletion of OXT receptors does not affect AVP-mediated deactivation. Collectively, our data document divergent modulation of brainwide neural systems by intranasal administration of OXT and AVP, an effect that involves key substrates of social and emotional behavior. The observed divergence calls for a deeper investigation of the systems-level mechanisms by which exogenous OXT and AVP modulate brain function and exert their putative therapeutic effects.

Basal Forebrain Cholinergic System and Memory

Basal forebrain cholinergic neurons constitute a way station for many ascending and descending pathways. These cholinergic neurons have a role in eliciting cortical activation and arousal. It is well established that they are mainly involved in cognitive processes requiring increased levels of arousal, attentive states and/or cortical activation with desynchronized activity in the EEG. These cholinergic neurons are modulated by several afferents of different neurotransmitter systems. Of particular importance within the cortical targets of basal forebrain neurons is the hippocampal cortex. The septohippocampal pathway is a bidirectional pathway constituting the main septal efferent system, which is widely known to be implicated in every memory process investigated. The present work aims to review the main neurotransmitter systems involved in modulating cognitive processes related to learning and memory through modulation of basal forebrain neurons.

Autism, oxytocin and interoception

Autism is a pervasive developmental disorder characterized by profound social and verbal communication deficits, stereotypical motor behaviors, restricted interests, and cognitive abnormalities. Autism affects approximately 1% of children in developing countries. Given this prevalence, identifying risk factors and therapeutic interventions are pressing objectives—objectives that rest on neurobiologically grounded and psychologically informed theories about the underlying pathophysiology. In this article, we review the evidence that autism could result from a dysfunctional oxytocin system early in life. As a mediator of successful procreation, not only in the reproductive system, but also in the brain, oxytocin plays a crucial role in sculpting socio-sexual behavior. Formulated within a (Bayesian) predictive coding framework, we propose that oxytocin encodes the saliency or precision of interoceptive signals and enables the neuronal plasticity necessary for acquiring a generative model of the emotional and social 'self.' An aberrant oxytocin system in infancy could therefore help explain the marked deficits in language and social communication—as well as the sensory, autonomic, motor, behavioral, and cognitive abnormalities—seen in autism.

Learning about oxytocin: pharmacologic and behavioral issues

Despite the accumulating evidence suggesting that the neuropeptide oxytocin (OT) plays a role in neuropsychiatric disorders characterized by social dysfunction, the influence of OT on the nonsocial aspects of learning and memory have been less investigated. To foster research in this area, we review the effects of OT on learning and memory in animal models and humans. In healthy animal models, OT improves memory consolidation and extinction, but only if given at a low dose immediately after the acquisition phase. On the contrary, OT effects in healthy humans have been inconsistent; although, in this case, OT was always given before the acquisition phase and no dose-response curves have ever been drawn up. Interestingly, a specific impairment in the reversal of learning has been found in mice devoid of OT receptors and OT has been demonstrated to enhance fear extinction in rodents. All together, these data suggest that OT plays a role in elementary forms of behavioral flexibility and adaptive responses and support its therapeutic potential in neuropsychiatric disorders characterized by cognitive inflexibility and/or impairment (autism, schizophrenia, Alzheimer's disease, Parkinson disease, stroke, posttraumatic stress disorder). Accordingly, OT has been shown to improve cognitive flexibility in OT receptor-deficient mice, and scattered findings indicate that intranasal OT has positive effects on the memory of patients with schizophrenia or posttraumatic stress disorders. Further studies of the therapeutic potential of OT as an enhancer of learning and memory are warranted.

Oxytocin in learning and addiction: From early discoveries to the present

Oxytocin (OXT) has a plethora of effects on brain function. This review provides a historical overview of the development of research on OXT and drug addiction. By focusing on research that has emerged from our laboratories, we describe how early discoveries of the influence of OXT on learning and memory processes and the emerging conceptualization of addiction as 'pathological learning' have contributed to the demonstration that OXT effectively attenuates long-term neuroadaptation related to opiate and psychostimulant addiction. Through integrating earlier evidence with recent discoveries of the social/affiliative role of OXT, we propose that OXT may interfere with reward and addiction by influencing neurobiological processes involved in stress, learning and memory and social/affiliative behavior.

Neonatal tactile stimulation alleviates the negative effects of neonatal isolation on novel object recognition, sociability and neuroendocrine levels in male adult mandarin voles (Microtus mandarinus)

Neonatal isolation results in long-lasting negative alterations to the brain and behavior. Some of these changes include effects on non-spatial learning and memory, sociability and neuroendocrine levels. Theoretically, neonatal tactile stimulation should reverse the impacts of neonatal isolation; however, this remains unknown for changes relating to learning, memory, sociability and hormones in social animals. Using socially monogamous mandarin voles (Microtus mandarinus), the long-lasting effects of these early manipulations on anxiety-like behavior, novel object recognition, sociability, and neuroendocrine levels were investigated. Compared with neonatal-isolated males, males subjected to the same manipulation but accompanied with tactile stimulation had heavier body weights across PND4-18 and displayed significantly less anxiety-like behavior in an open field test. In addition, tactile stimulation increased the preference index for novel object recognition reduced by neonatal isolation. Compared with control males, neonatal-isolated males engaged in less body contact with unfamiliar same-sex individuals and this effect was reversed by neonatal tactile stimulation. Tactile stimulation enhanced aggressive behavior in neonatal-isolated males and increased the levels of AVP and OT in the paraventricular nucleus (PVN) which were decreased by neonatal isolation. This early manipulation also reduced serum CORT levels that were significantly up-regulated by neonatal isolation in both neonatal and adult offspring. These results indicate that adequate tactile stimulation in early life plays an important role in the prevention of behavioral disturbances induced by neonatal isolation, possibly through the alteration of central OT, AVP and the serum corticosterone levels.

Oxytocin reduces background anxiety in a fear-potentiated startle paradigm

Oxytocin reportedly decreases anxious feelings in humans and may therefore have therapeutic value for anxiety disorders, such as post-traumatic stress disorder (PTSD). As PTSD patients have exaggerated startle responses, a fear-potentiated startle paradigm in rats may have face validity as an animal model to examine the efficacy of oxytocin in treating these symptoms. Oxytocin (0, 0.01, 0.1, or 1.0 μg, subcutaneously) was given either 30 min before fear conditioning, immediately after fear conditioning, or 30 min before fear-potentiated startle testing to assess its effects on acquisition, consolidation, and expression of conditioned fear, respectively. Startle both in the presence and absence of the fear-conditioned light was significantly diminished by oxytocin when administered at acquisition, consolidation, or expression. There was no specific effect of oxytocin on light fear-potentiated startle. In an additional experiment, oxytocin had no effects on acoustic startle without previous fear conditioning. Further, in a context-conditioned test, previous light-shock fear conditioning did not increase acoustic startle during testing when the fear-conditioned light was not presented. The data suggest that oxytocin did not diminish cue-specific conditioned nor contextually conditioned fear, but reduced background anxiety. This suggests that oxytocin has unique effects of decreasing background anxiety without affecting learning and memory of a specific traumatic event. Oxytocin may have antianxiety properties that are particularly germane to the hyper-vigilance and exaggerated startle typically seen in PTSD patients.

The peptide that binds: a systematic review of oxytocin and its prosocial effects in humans

Oxytocin is a neuropeptide involved in a wide variety of social behaviors in diverse species. Recent research on its effects in humans has generated an arresting picture of its role in the dynamic function of the social brain. This review presents a broad overview of this uniquely social peptide, with a particular focus on extant studies of its effects in humans. After a short discussion of the evolutionary history of the oxytocin system, critical aspects of its peripheral and central physiology, and several salient technical issues surrounding human oxytocin research, a systematic review of studies of the effects of intranasal oxytocin in humans is presented. These effects include alterations in social decision making, processing of social stimuli, certain uniquely social behaviors (e.g., eye contact), and social memory. Oxytocin's prosocial influence is then framed by an evolutionary perspective on its role in mammalian social bonding and attachment. Finally, limitations in current human oxytocin research and oxytocin's potential therapeutic applications are discussed. Key conclusions are (1) human research with intranasal oxytocin has uniquely enhanced our understanding of the microstructure and function of the human social brain, and (2) the oxytocin system is a promising target for therapeutic interventions in a variety of conditions, especially those characterized by anxiety and aberrations in social function.

Post-learning intranasal oxytocin modulates human memory for facial identity

The nanopeptide oxytocin has physiological functions during labour and lactation. In addition, oxytocin is known to modulate aggression, anxiety, social behaviour and cognition. Little is known about its effects on memory for emotional stimuli. In the present single-blind, placebo-controlled, randomised study we have investigated the short- and long-term effects of a single post-learning dose (20 IU) of intranasal oxytocin on memory for facial identity and expression in 36 healthy young females and males using a face portrait recognition test. In the acquisition phase of the test, 60 different male faces with happy, angry or neutral expressions were presented to the volunteers. Thirty minutes and 24h after oxytocin administration, recognition memory tests were performed using portraits with neutral facial expressions, only. Oxytocin improved identity recognition memory independently of participant's gender, for neutral and angry faces, whereas this effect was not present for happy faces. Oxytocin-treated subjects had a lower bias to judge not previously seen faces as being previously seen. Oxytocin had no effect on facial expression memory. In conclusion, oxytocin has distinct effects on memory performance for facial identity and may contribute to the modulation of social behaviour.

Oxytocin facilitates female sexual maturation through a glia-to-neuron signaling pathway

It has been earlier proposed that oxytocin could play a facilitatory role in the preovulatory LH surge in both rats and humans. We here provide evidence that oxytocin also facilitates sexual maturation in female rats. The administration of an oxytocin antagonist for 6 d to immature female rats decreased GnRH pulse frequency ex vivo and delayed the age at vaginal opening and first estrus. The in vitro reduction in GnRH pulse frequency required chronic blockade of oxytocin receptors, because it was not acutely observed after a single injection of the antagonist. Hypothalamic explants exposed to the antagonist in vitro showed a reduced GnRH pulse frequency and failed to respond to oxytocin with GnRH release. Prostaglandin E(2) (PGE(2)) mimicked the stimulatory effect of oxytocin on GnRH pulse frequency, and inhibition of PG synthesis blocked the effect of oxytocin, suggesting that oxytocin accelerates pulsatile GnRH release via PGE(2). The source of PGE(2) appears to be astrocytes, because oxytocin stimulates PGE(2) release from cultured hypothalamic astrocytes. Moreover, astrocytes express oxytocin receptors, whereas GnRH neurons do not. These results suggest that oxytocin facilitates female sexual development and that this effect is mediated by a mechanism involving glial production of PGE(2).

Glucocorticoid-mediated effects of systemic oxytocin upon memory retrieval

During the last decade, a considerable amount of evidence has accumulated to show that oxytocin (OT) is involved with functions other than its classical roles in reproduction-associated processes, such as social recognition, maternal behavior and neuroendocrine regulation of the stress response. It has been shown, for instance, that post-training systemic administration of oxytocin in mice produces an amnestic effect on the step-through inhibitory avoidance. Since it is still unclear how systemic levels of OT may affect CNS memory processes, our aim here was to investigate the hypothesis that systemic OT effects on memory retrieval might be mediated through an oxytocin-induced decrease in glucocorticoid release. In our first experiment, we have found an amnestic effect of i.p. pre-test 0.4 microg/kg of OT upon memory retrieval in the inhibitory avoidance task (IA); this OT dose was shown to (a) significantly decrease plasma corticosterone levels when compared to the saline group, and (b) not to cause any anxiety effects by itself in a plus-maze task. At last, an ineffective-by-itself dose of dexamethasone was able to reverse the amnestic effect of this OT dose. Our results suggest that the amnestic effect of systemically administered oxytocin upon memory retrieval in the inhibitory avoidance task was probably caused by an oxytocin-induced decrease in glucocorticoid release from the adrenal gland.

NF-kappaB transcription factor is required for inhibitory avoidance long-term memory in mice

Although it is generally accepted that memory consolidation requires regulation of gene expression, only a few transcription factors (TFs) have been clearly demonstrated to be specifically involved in this process. Increasing research data point to the participation of the Rel/nuclear factor-kappaB (NF-kappaB) family of TFs in memory and neural plasticity. Here we found that two independent inhibitors of NF-kappaB induced memory impairment in the one-trial step-through inhibitory avoidance paradigm in mice: post-training administration of the drug sulfasalazine and 2 h pretraining administration of a double-stranded DNA oligonucleotide containing the NF-kappaB consensus sequence (kappaB decoy). Conversely, one base mutation of the kappaB decoy (mut-kappaB decoy) injection did not affect long-term memory. Accordingly, the kappaB decoy inhibited NF-kappaB in hippocampus 2 h after injection but no inhibition was found with mut-kappaB decoy administration. A temporal course of hippocampal NF-kappaB activity after training was determined. Unexpectedly, an inhibition of NF-kappaB was found 15 min after training in shocked and unshocked groups when compared with the naïve group. Hippocampal NF-kappaB was activated 45 min after training in both shocked and unshocked groups, decreasing 1 h after training and returning to basal levels 2 and 4 h after training. On the basis of the latter results, we propose that activation of NF-kappaB in hippocampus is part of the molecular mechanism involved in the storage of contextual features that constitute the conditioned stimulus representation. The results presented here provide the first evidence to support NF-kappaB activity being regulated in hippocampus during consolidation, stressing the role of this TF as a conserved molecular mechanism for memory storage.

Roles of oxytocin in spatial learning and memory in the nucleus basalis of Meynert in rats

The present study was performed to explore the role of oxytocin (OT) in spatial learning and memory in the nucleus basalis of Meynert (NBM) of rats. The latency, distance and swimming path to find the platform were tested by Morris water maze and recorded by a video camera connected to a computer. Intra-NBM injections of 2 or 10 nmol of OT, but not 0.2 nmol of OT, induced significant increase on the latency of spatial learning. Rats receiving intra-NBM administrations of 2 or 10 nmol of OT showed a more random search pattern. There were no significant changes in the swimming speed in Morris water maze test after the injection of OT. Furthermore, the impaired effect of OT on the latency of spatial learning was blocked by intra-NBM injection of the selective OT antagonist Atosiban, indicating that the effect of OT was mediated by OT receptor in the NBM of rats. Moreover, there were no influences of OT or Atosiban on the retention performance in rats. The results suggest that OT plays an inhibitory role in spatial learning in the NBM; the effect is mediated by OT receptor.

Memory consolidation and reconsolidation of an inhibitory avoidance response in mice: effects of i.c.v. injections of hemicholinium-3

The immediate post-training i.c.v. administration of hemicholinium-3 (HC-3) (1 microg), a specific inhibitor of the high-affinity choline uptake (HACU) in brain cholinergic neurons, impaired retention test performance of a one-trial step-through inhibitory avoidance response in adult male CF-1 mice. The effect was observed in mice that received a footshock (0.8 mA, 50 Hz, 1 s) on the learning trial, and not only 48 h after training, but also 7 days after it. After the completion of the retention test at each of the training-test interval that were studied, the HACU in the hippocampus of HC-3-treated mice was not significantly different from that of saline-injected (1 microl) control groups. Mice that were over-reinforced (1.2 mA, 50 Hz, 1 s) on the learning trial, exhibited a high retention performance 48 h after training. The immediate i.c.v. injection of HC-3 (1 microg) after the retention test, that is, after memory reactivation, significantly impaired retention performance over 4 consecutive days, whereas the saline-injected control group shown a slight, but significant performance decrease only at the last retention test. Retention performance was unchanged in HC-3-treated mice not undergoing memory reactivation session. These results, taken together, indicate that HC-3, not only impaired consolidation, but also reconsolidation of an inhibitory avoidance task in mice, suggesting a critical participation of central cholinergic mechanisms in both memory processes.

Atropine, an anticholinergic drug, impairs memory retrieval of a high consolidated avoidance response in mice

Immediate post-training intraperitoneal administration of the centrally acting anticholinesterase physostigmine (70.0, or 150.0 microg/kg) enhanced retention of male CF-1 mice tested 48 h after training in a one-trial step-through inhibitory avoidance task (0.8 mA, 50 Hz, 1 s footshock). The effect was observed in mice that received saline 30 min before the retention test; on the contrary, the pre-test administration of the centrally active muscarinic cholinergic antagonist, atropine (1.0 mg/kg, i.p.), but not methylatropine (1.0 mg/kg, i.p.), instead of saline, prevents the enhancement of retention induced by both doses of the anticholinesterase when given immediately after training. The high retention performance caused by post-training physostigmine was recovered following a second administration of the same doses of the drug, 10 min after the pre-test injections of atropine. Since, physostigmine do not influence memory retrieval when given prior to the retention test, and its post-training effects are not due to the induction of state-dependency, the recover of the high retention performance was probably due to a classical interaction between a muscarinic competitive antagonist and an indirect cholinergic agonist. Further, atropine probably does not modify the memory trace by erasing it, but by producing a poor retrieval.

Memory improving actions of gabapentin in mice: possible involvement of central muscarinic cholinergic mechanism

Male CF-1 mice were tested 48 h after training on a one trial step-through inhibitory avoidance task. Immediately post-training, intraperitoneal (i.p.) injections of the antiepileptic gabapentin (1-(aminomethyl) cyclohexaneacetic acid) (GBP, 10 mg/kg) enhanced retention performance. The effect was prevented by atropine, a central muscarinic cholinergic receptor antagonist (0.5 mg/kg, i.p.) administered after training but 10 min prior to GBP treatment. In contrast, neither methylatropine (0.5 mg/kg, i.p.), a peripherally acting muscarinic receptor blocker, nor mecamylamine (5 mg/kg, i.p.) or hexamethonium (5 mg/kg, i.p.), two cholinergic nicotinic receptor antagonists, prevented the effects of post-training GBP on retention performance. Low subeffective doses of the central acting anticholinesterase physostigmine (35 mg/kg, i.p.) administered immediately after training, and GBP (5 mg/kg, i.p.), given 10 min after training, significantly enhanced retention performance. The effects of GBP (5 mg/kg, i.p.) were not influenced by the peripherally acting anticholinesterase neostigmine (150 mg/kg, i.p.). Considered together, these findings suggest a disinhibitory action of GBP on the activity of central muscarinic cholinergic mechanisms that are involved in memory consolidation.