N-methyl-D-aspartate receptor antagonist MK-801 impairs learning but not memory fixation or expression of classical fear conditioning in goldfish (Carassius auratus)

Developing zebrafish models relevant to PTSD and other trauma- and stressor-related disorders

While post-traumatic stress disorder (PTSD) and other trauma- and stress-related disorders (TSRDs) represent a serious societal and public health concern, their pathogenesis is largely unknown. Given the clinical complexity of TSRD development and susceptibility, greater investigation into candidate biomarkers and specific genetic pathways implicated in both risk and resilience to trauma becomes critical. In line with this, numerous animal models have been extensively used to better understand the pathogenic mechanisms of PTSD and related TSRD. Here, we discuss the rapidly increasing potential of zebrafish as models of these disorders, and how their use may aid researchers in uncovering novel treatments and therapies in this field.

Active avoidance conditioning in zebrafish (Danio rerio)

The zebrafish represents a potentially useful organism for studying genes involved in learning and memory function in vertebrates, because a number of genetic techniques in zebrafish have been developed to produce a wide variety of genetic mutants. While zebrafish mutants are being developed, behavioral studies on learning and memory function in zebrafish are in urgent need. The present study investigated active avoidance conditioning in normal zebrafish. Zebrafish were trained to swim from a lighted (CS) compartment to a dark compartment to avoid an electrical body shock (US) in a shuttle-box that consisted of a water-filled tank separated by an opaque barrier into two equal compartments. By varying the number of trials per training session and the duration of the intertrial interval, Experiments 1 and 2 showed that, with the CS, US, and intertrial interval being 12s, zebrafish learned avoidance responses within a training session consisting of 30 trials and retained the avoidance responses. Experiment 3 showed that zebrafish learned avoidance responses following the association between the CS of light and the US of shock in the avoidance conditioning paradigm. Using the avoidance conditioning paradigm, Experiment 4 investigated the amnestic effects of N-methyl-D-aspartate receptor antagonist MK-801 and nitric oxide synthase inhibitor L-NAME in zebrafish. Experiment 4 showed that post-training injection of L-NAME significantly impaired retention of avoidance responses while MK-801 did not, confirming previous results with other vertebrates. The results of the present study suggest the similar involvements of neurochemicals in learning and memory among vertebrates. Thus, future studies with zebrafish mutants may identify genes involved in learning and memory in vertebrates.

Reconsolidation: A brief history, a retrieval view, and some recent issues

This review briefly traces some of the history of the phenomenon of what has come to be called "reconsolidation." The early findings of retrograde amnesia for an old but reactivated memory led to several interesting but largely behaviorally oriented studies. With only a few sporadic exceptions, research in the area languished until about 2000, when several articles caught the attention of the neuroscience community and led to a number of studies examining the phenomenon at several different levels of analysis. We consider several of the current issues generated by those studies, present a retrieval based model that may account for some findings, and indicate some possible new directions on this topic.

Vicarious trial-and-error behavior and hippocampal cytochrome oxidase activity during Y-maze discrimination learning in the rat

The present study investigated whether more vicarious trial-and-error (VTE) behavior, defined by head movement from one stimulus to another at a choice point during simultaneous discriminations, led to better visual discrimination learning in a Y-maze, and whether VTE behavior was a function of the hippocampus by measuring regional brain cytochrome oxidase (C.O.) activity, an index of neuronal metabolic activity. The results showed that the more VTEs a rat made, the better the rat learned the visual discrimination. Furthermore, both learning and VTE behavior during learning were correlated to C.O. activity in the hippocampus, suggesting that the hippocampus plays a role in VTE behavior during discrimination learning.

The role of telencephalic NMDA receptors in avoidance learning in goldfish (Carassius auratus)

Studies with goldfish (Carassius auratus) have suggested that N-methyl-D-aspartate (NMDA) receptors are concentrated most densely in the telencephalon, a simple structure homologous to the limbic structure of higher vertebrates. The present study investigated the amnestic effects of microinjections of the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) to the goldfish telencephalon on avoidance conditioning. Results showed that microinjections of D-AP5 before training impaired avoidance learning at doses that did not impair performance processes. High-performance liquid chromatography measurements showed that D-AP5 was detected only in the telencephalon following microinjections. Thus, D-AP5 impaired avoidance learning through its interaction with telencephalic NMDA receptors in goldfish. Furthermore, microinjections of D-AP5 to the goldfish telencephalon immediately following training did not impair memory consolidation of avoidance conditioning.

NMDA receptor antagonist AP5 and nitric oxide synthase inhibitor 7-NI affect different phases of learning and memory in goldfish

The present study investigated the amnestic effects of N-methyl-D-aspartate (NMDA) receptor antagonist AP5 and nitric oxide (NO) synthase inhibitor 7-NI in avoidance conditioning in goldfish. The results showed that both AP5 and 7-NI, without impairing performance processes, produced anterograde amnesia when given before training. Furthermore, 7-NI produced retrograde amnesia when given immediately following training while AP5 did not. Thus, AP5 and 7-NI affected different phases of learning and memory.

Ketamine blocks a taste recognition memory in fetal rats

Decisions about novelty/familiarity are critical in determining whether or not information should be attended to, and possibly encoded, for long-term storage. We have reported that fetal and neonatal rats exhibit an increase in orofacial movements (e.g., perseverative mouthing and mouth movements, and licks) upon tasting saccharin (SAC), if it was experienced previously. E19 rat fetuses can acquire this taste recognition memory and retain it for at least 5 days (P3). In the current study, we sought to evaluate the role of N-methyl-D-aspartate (NMDA) receptors in establishing a taste recognition memory. Pregnant Sprague-Dawley rats received ketamine (NMDA receptor antagonist) (doses: 0, 50, or 100 mg/kg, i.p.). One-half hour later, we performed a reversible spinal block on each pregnant dam, and E19 fetuses received an oral injection of 10 microl, 0.3% SAC or water (control) while in utero. The uterus was replaced and the pups were later born via a normal vaginal delivery. On P3, all pups experienced oral lavage of 10 microl, 0.3% SAC, and motor responses were recorded. As expected, non-drugged control neonates tasting familiar SAC exhibited significantly more perseverative mouth movements, as well as total mouth movements and licks, than did pups tasting novel SAC. However, this taste recognition memory response was not observed in rats exposed to ketamine in utero. The data suggest that early non-associative taste memories may be disrupted by NMDA receptor blockade.

Ketamine blocks a taste-mediated conditioned motor response in perinatal rats

Brain N-methyl-D-aspartate (NMDA) glutamate receptors have been implicated as important mediators of both learning and neuronal development. The current study investigated how ketamine HCl (a well-known NMDA-receptor blocking drug) would influence taste-mediated conditioned motor responses in perinatal rats. Dams pregnant with E19 rat fetuses were injected with 0, 50, or 100 mg/kg ketamine HCl (IP). One-half hour later, a reversible spinal block was performed on the dam, and fetuses received an oral injection of 10 microl 0.3% Saccharin (SAC) or water while in utero. After the oral injection, fetuses received either saline or LiCl (81 mg/kg, IP). The uterus was replaced and, 2 days later (E21), rats received oral lavage with SAC. Rats in other litters were born via a normal vaginal delivery and were exposed to SAC on postnatal day 3 (P3). Observations of motor responses were recorded immediately after the oral lavage of SAC. If SAC had been paired with LiCl in utero, both E21 and P3 pups exhibited a conditioned suppression of orofacial movements (compared to controls). Both doses of ketamine significantly attenuated this taste-mediated conditioned motor response. These data reinforce the current conception of the fetus and neonate as sophisticated sensors and responders to the uterine and extrauterine environment. Further, our findings indicate a role for NMDA receptors in the formation of a conditioned motor response in fetal rats.

MK-801-induced disruptions of one-trial inhibitory avoidance are potentiated by stress and reversed by naltrexone

Five experiments were carried out to investigate opioid and NMDA receptor-mediated responses to one-trial inhibitory avoidance training in CD1 mice. In the first experiment immediate posttraining intraperitoneal administration of the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 impaired the performance of mice. The effects of MK-801 were time-dependent (they were absent in mice injected with the drug starting 120 min after training). No effect was evident in no-foot-shock groups, showing lack of proactive influence of the treatment on performance. In the second experiment preexposure of the mice to the testing apparatus decreased the effects of MK-801. In the the third experiment naltrexone antagonized the effects of MK-801, suggesting an involvement of opioid neurons. In the fourth experiment immediate posttraining immobilization stress exerted a potentiating effect on the performance of MK-801-injected animals. In the fifth experiment the potentiation of the impairing effect of MK-801 induced by immobilization stress was antagonized by naltrexone.

Ketamine blocks a conditioned taste aversion (CTA) in neonatal rats

These experiments explored the effects of glutamate, N-methyl-D-aspartate (NMDA) receptor blockade on the formation, retention, and expression of conditioned taste aversion (CTA) in young rats. Previous data from our laboratory suggested that ketamine administration potentiates a CTA in E18 rat fetuses. The current studies investigated this phenomenon in neonates. High-pressure liquid chromatography (HPLC) methods were used to determine the amount of ketamine that must be injected intraperitoneally (i.p.) to achieve brain ketamine levels in neonates comparable to those found in the fetuses from our previous experiments. Then, on their day of birth, Sprague-Dawley rat pups received injections of either 0.1, 10, or 70 mg/kg of ketamine HCI, i.p. or a Sal control injection. One-half hour later, pups were injected orally with either Saccharin (Sac; 10 microL of 0.3%) or water followed by an injection of either lithium chloride (LiCl; 81 mg/kg) or Sal (i.p.). The CTA was evaluated in two different tests. Two weeks after conditioning, the dam was anesthetized and the frequency with which pups attached to Sac-painted nipples versus nipples painted with water was measured (i.e., the nipple taste test, NTT). Controls for state-dependent learning were run in which 10 mg/kg of ketamine or saline (Sal) was administered before both taste aversion conditioning and the NTT. After weaning, the CTA was also evaluated by measuring the amount of Sac (0.3%) or water consumed during a two-bottle test. Neonates that received Sal control injections before the Sac + LiCl pairing acquired CTAs and avoided Sac-painted nipples. However, the pups injected with ketamine on the conditioning day only (P0) did not avoid Sac-painted nipples (as compared to controls). Pups that had ketamine both at the time of CTA training and testing, or just before the NTT, also failed to avoid Sac-painted nipples. Ketamine's acute effects apparently influenced the outcome of the NTT of state-dependent control subjects. Rat pups that received the highest doses of ketamine (10 or 70 mg/kg) and tasted Sac on P0 later failed to show a neophobia for Sac-painted nipples. Whereas, rat pups that received the high dose of ketamine and water on P0, later exhibited a neophobic response. These data suggest that ketamine did not impair the animal's ability to taste Sac. These data reflecting a ketamine-induced blockade of neonatal CTAs may be contrasted with our previous findings in which ketamine potentiated fetal CTAs. However, they are in consonance with data from adult rats suggesting that ketamine can cause an amnesia for CTAs. NMDA receptor blockade may shape memory formation in a manner that is dependent on the stage of brain development.

Complex roles of glutamate in the Gibbs-Ng model of one-trial aversive learning in the new-born chick

Glutamate is the most widespread excitatory transmitter in the CNS and is probably involved in LTP, a neural phenomenon which may be associated with learning and memory formation. Intracerebral injection of large amounts of glutamate between 5 min and 2.5 min after passive avoidance learning in young chicks inhibits short-term memory, which occurs between 0 and 10 min post-learning in a three-stage model of memory formation first established by Gibbs and Ng(25) [Physiol. Behav. 23:369-375; 1979]. This effect may be attributed to non-specific excitation. Blockade of glutamate uptake by L-aspartic and beta-hydroxamate also abolishes this stage of memory, provided the drug is administered within 2.5 min of learning. Interference with either production of percursors for transmitter glutamate in astrocytes or with glutamate receptors is also detrimental to memory formation, but the effects appear much later. After its release from glutamatergic neurons, glutamate is, to a large extent, accumulated into astrocytes where it is converted to glutamine, which can be returned to glutamatergic neurons and reutilized for synthesis of transmitter glutamate, and partly oxidized as a metabolic substrate. The latter process leads to a net loss of transmitter glutamate which can be compensated for by de novo synthesis of a glutamate precursor alpha-ketoglutarate (alpha KG) in astrocytes, a process which is inhibited by the astrocyte-specific toxin fluoroacetate (R. A. Swanson, personal communication). Intracerebral injection of this toxin abolishes memory during an intermediate stage of memory processing occurring between 20 and 30 min post-training (50) [Cog. Brain Res, 2:93-102; 1994]. Injection of methionine sulfoximine (MSO), a specific inhibitor of glutamine synthetase, which interferes with the re-supply of transmitter glutamate to neurons by inhibition of glutamine synthesis in astrocytes, has a similar effect. This effect of MSO is prevented by intracerebral injection of glutamate, glutamine, or a combination and alpha KG and alanine. MSO must be administered before learning, but does not interfere with acquisition since short-term memory remains intact. Administration of either the NMDA antagonist AP5, the AMPA antagonist DNQX, or the metabotropic receptor antagonist MCPF, also induces amnesia. Memory loss in each case does not occur until after 70 min post-training, during a protein synthesis-dependent long-term memory stage which begins at 60 min following learning. However, to be effective, AP5 must be administered within 60 s following learning, MCPG before 15 min post-learning, and DNQX between 15 and 25 min after learning. Together, these findings suggest that learning results in an immediate release of glutamate, followed by a secondary release of this transmitter at later stages of processing of the memory trace, and that one or both of these increases in extracellular glutamate concentration are essential for the consolidation of long-term memory. Since both fluoroacetate and MSO act exclusively on glial cells, the findings also show that neuronal-glial interactions are necessary during the establishment of memory.

Participation of the glutamatergic input of the nucleus accumbens in the regulation of the synaptic release of dopamine during associative learning

The changes in the synaptic release of dopamine in the medial division of the nucleus accumbens in the course of a conditioned emotional response and the influence on this process of the blockade of N-methyl-D-aspartate (NMDA) receptors of this structure were investigated in awake Lister-hooded rats, using the method of intravital intracerebral dialysis in combination with high-pressure liquid chromatography and electrochemical detection. It was established that situational stimuli, previously combined with painful reinforcement, but not acoustic conditional signals, lead to a slow increase in the level of dopamine in the extracellular space of the nucleus accumbens. This process reaches a maximum 40 min after the beginning of testing and lasts 80 min. Dialysis perfusion of the nucleus accumbens with a solution of MK-801 (50 mumole/liter) does not alter the magnitude of the slow rise in the synaptic release of dopamine in this structure in the course of the conditioned emotional response, but completely blocks the late components of the release. It is concluded that the glutamatergic input of the nucleus accumbens participates in the regulation of the late components of the synaptic release of dopamine, governed by the conditioned emotional response, in this structure through NMDA receptors.

Conditioned dopamine release: dependence upon N-methyl-D-aspartate receptors

The study have investigated the effect of a conditioned emotional response using a contextual cue on dopamine release in the rat nucleus accumbens, measured with in vivo microdialysis, and its inhibition by N-methyl-D-aspartate antagonist dizocilpine maleate. The extracellular level of dopamine in the medial nucleus accumbens markedly increased for up to 40 min when rats were given mild footshock in the testing box. When the rats were returned to the testing box, but not given footshock (conditioned emotional response), there was an immediate and long-lasting (80 min) increase in extracellular dopamine. Dizocilpine maleate (50 mumol/l) administered into the nucleus accumbens through the dialysis probe had no significant effect on the immediate increase in dopamine induced by conditioned emotional response but completely prevented the later phase. Dizocilpine maleate had no effect on basal dopamine release in control rats but decreased basal dopamine in rats exposed to footshock 2 h previously. The alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate/kainate antagonist 6-cyano-7-nitroquinnoxaline-2,3-dione (100 mumol/l) had no effect on the increase in dopamine release in response to conditioned emotional response. The results indicate that the acquisition of conditioned emotional response causes long-lasting changes in the mechanisms involved in the glutamatergic control of dopamine release in the nucleus accumbens. Furthermore glutamate inputs into the nucleus accumbens may also regulate the delayed phase of conditioned dopamine release during expression of conditioned emotional response to a contextural cue through activation of N-methyl-D-aspartate receptors.

Comparative anterograde amnestic and anticonvulsant effects of two types of NMDA receptor antagonists: MK-801 and HA-966

The anterograde amnestic effects of non-competitive NMDA antagonists MK-801 and HA-966 on classic fear conditioning in goldfish (Carassius auratus) were examined in a series of experiments. Experiments 1 and 2 contrasted the anterograde amnestic effects of MK-801, (+)HA-966, and (-)HA-966. Experiment 3 examined the effects of MK-801 and (+)HA-966 on the expression of conditioned responses. Experiments 4 and 5 investigated whether the potency of MK-801, (+)HA-966 or (-)HA-966 in blocking NMDA-induced convulsions paralleled their potency in producing amnesia. The results showed that MK-801 was more potent than (+)HA-966 in producing anterograde amnesia and impairing expression, while (-)HA-966 did not produce anterograde amnesia. The anticonvulsant potency of MK-801, (+)HA-966, and (-)HA-966 paralleled their amnestic potency. These findings suggested that MK-801 and (+)HA-966 produced anterograde amnesia by their specific antagonism of the NMDA receptor complex.

Dizocilpine antagonizes the effect of chronic imipramine on learned helplessness in rats

Dizocilpine coadministered with imipramine (IMI) through an SC-implanted osmotic minipump completely prevents the occurrence of behavioral supersensitivity to quinpirole, as well as the decrease of dopamine D1 and beta-adrenergic receptor function. The present report shows that, in the same experimental conditions, dizocilpine completely antagonized the capacity of IMI to prevent the development of the learned helplessness behavior in rats. Thus suggesting that the blockade of NMDA receptors also antagonizes the antidepressant effect of IMI. Interestingly, rats acutely treated with dizocilpine 30 min before the inescapable shock session behaved similarly to naive animals during the escape test session.

Cognitive aspects of classical conditioning

Cognitive processes have been increasingly implicated in Pavlovian conditioning. Research in the past year has focused on questions of stimulus selection and the internal representation of events and the relations between them. Recent data support negative feedback models of selection that assume conditioning-dependent changes in processing of conditioned and unconditioned stimulus events, and suggest potential neural mechanisms that may underlie these processes. New models of conditioning propose a more detailed representation of individual conditioning episodes than traditionally assumed. The results of investigations into conditional discrimination learning imply a hierarchical organization of event representations, and illustrate the importance of conditioned modulatory processes as distinct from response elicitation.

Long-term imipramine effects are prevented by NMDA receptor blockade

Long-term exposure to different antidepressant treatments induces increased motor response to central stimulants, due to a selective supersensitivity of dopamine D2 receptors in the limbic areas. Such an effect is accompanied by down-regulation of dopamine D1 receptor number, and by a decreased response of adenylyl cyclase to dopamine stimulation in the limbic system. Moreover, the number of beta-adrenergic receptors and the response of adenylyl cyclase to beta-adrenergic stimulation in the cortex result to be reduced. The present data confirms that imipramine (10 mg/kg twice a day for 3 weeks) produces such effects, and shows that the co-administration of imipramine with MK-801 (administered by a subcutaneously implanted osmotic minipump delivering 0.05 mg/kg/day of the compound) prevented the occurrence of both the behavioral supersensitivity to quinpirole, and the decrease of dopamine D1 and beta-adrenergic receptor function.