5-HT1A agonists disrupt memory of fear conditioning in mice

A series of experiments was carried out to analyze the effects of the 5-HT1A agonists tandospirone or buspirone on the retention of fear conditioning in mice. Fear was produced by pairing tone and shock in a conditioned emotional response (CER) paradigm and strength of conditioning was assessed by measuring suppression of drinking in presence of tone. Fear conditioning was disrupted if tandospirone and buspirone were administered before the conditioning session but not before the test trial. Diazepam disrupted conditioning at both times. Tandospirone did not disrupt performance if conditioning was tested 1 hr rather than 24 hr after training, suggesting that disrupted memory rather than impaired acquisition was responsible for the deficit. The effect of tandospirone on fear conditioning could be reversed by administration of d-amphetamine prior to the retention test, which suggests that information was stored but is inaccessible to normal retrieval cues. Tandospirone and buspirone also retarded extinction, a clear indication that the disruption caused by these drugs is unrelated to their anxiolytic action.

Effect of propofol on memory in mice

The amnestic effects of the intravenous hypnotic anesthetic agent 2,6-diisopropylphenol (propofol; Diprivan) were studied in a single-trial passive avoidance task. Mice were injected with propofol 10 min before or immediately after training. Memory was impaired in a dose-dependent fashion when the anesthetic was administered before learning, but no amnesia was apparent with posttraining injections. Examination of the acquisition of passive avoidance using a multitrial task showed that propofol-treated mice learned the response normally but forgot the learning significantly faster than vehicle-treated controls. The anterograde amnesia was not the result of state-dependent learning. Propofol also disrupted extinction of fear conditioning when the anesthetic was given during the extinction session. Propofol-induced amnesia could be attenuated by amphetamine (1 mg/kg) injected 30 min before the retention test.

Halothane anesthesia causes state-dependent retrieval failure in mice

Effects of exposure to halothane on memory processing was studied using single-trial inhibitory avoidance learning to measure retention. Mice were anesthetized with halothane either before training, immediately after training, or both before training and before testing. Results showed that memory was not impaired by posttraining halothane exposure, indicating that the anesthetic does not cause retrograde amnesia. Mice trained after recovery from halothane showed a robust memory loss 24 h later. This deficit could be alleviated by reexposure to the anesthetic before the retention test. Mice given multiple training trials following recovery from the anesthetic showed a normal rate of learning when compared with controls, but deficient retention. This indicates that the performance deficit was the result of impaired retention (anterograde amnesia) rather than disrupted acquisition. Anterograde amnesia occurred when training was delayed up to 2 h after recovery from anesthesia. These findings indicate that the memory impairment following halothane anesthesia is the result of a state-dependent retrieval failure.

Milacemide enhances memory storage and alleviates spontaneous forgetting in mice

The objective of this study was to evaluate the effectiveness of milacemide as a memory-enhancing drug in mice. Experiment 1 showed that forgetting of active avoidance learning produced by a 14-day training to test delay could be alleviated by milacemide (10 mg/kg) administered before the retention test. Experiment 2 demonstrated that the same dose of milacemide could also attenuate spontaneous forgetting of passive avoidance learning, thereby ruling out nonspecific effects as an explanation for the enhancement of performance following pretesting drug administration. A third experiment showed that the facilitation of retrieval induced by milacemide could be blocked by the NMDA receptor antagonist AP-7, suggesting that the effects of milacemide on memory may be mediated by NMDA receptor activation. A final experiment demonstrated that retention was improved when milacemide was administered immediately following active avoidance training, indicating that the drug can also facilitate remembering by its actions on consolidation and storage processes.

Prenatal treatment with a selective D1 receptor agonist (SKF 38393) alters adult [3H]paroxetine binding and dopamine and serotonin behavioral sensitivity

We have previously shown that the development of serotonin neurons can be affected by various pharmacological agents acting on the serotonin system. Receptor stimulation by high doses of 5-methoxytryptamine (5-MT) causes increased outgrowth, through release of an astroglial growth factor, while a low concentration of 5-MT has a direct inhibitory effect on neuronal outgrowth. Since 5-MT is known to be a release-regulating autoreceptor agonist, the present study was aimed at testing the hypothesis that inhibition of serotonin release causes the inhibition of outgrowth. We used the D1 receptor agonist SKF 38393 as an inhibitor of serotonin release. Pregnant Sprague-Dawley rats were treated with SKF 38393 (1 mg/kg; subcutaneously) from gestational day 12 until parturition. Development of serotonin terminal outgrowth was evaluated in the offspring using the selective uptake marker [3H]paroxetine in brainstem and frontal cortex. In saline and SKF 38393 treated animals, the developmental pattern in the frontal cortex showed the highest terminal density at day 60 and a return to normal by day 90, with no statistically significant differences between the groups. Conversely, in the brainstem, [3H]paroxetine binding developed normally until postnatal day 90, when the SKF 38393 pretreated animals showed only 58% of the binding observed in saline animals. In a fixed interval responding task, given at day 90, both dopamine and serotonin receptor systems have significantly decreased sensitively after the SKF 38393 pretreatment. In conclusion, our results show that the inhibitory effects of serotonin on the growth of serotonin neurons, may be through inhibition of neurotransmitter release. Moreover, we propose a means by which serotonin and dopamine systems could be interdependent during development.

Persistence of retrieval enhancement by amphetamine following scopolamine-induced amnesia

Little information is available on the permanence of pharmacologically-induced retrieval enhancement following amnesia. This was studied by comparing the rate of forgetting of a memory reactivated by d-amphetamine after amnesia with spontaneous forgetting of undisturbed fear conditioning. Mice were treated with either saline or scopolamine before conditioning and retention was tested three days later. Scopolamine-treated mice received either saline or amphetamine before testing while the saline controls received a second saline injection. The scopolamine-saline group exhibited robust amnesia, whereas both saline-saline and scopolamine-amphetamine groups showed good retention. To test the persistence of these effects mice in the three groups were subdivided and given a second retention test either 1 day, 1 week or 1 month after the first test. Amphetamine was not administered before the second test. The scopolamine-saline mice continued to exhibit amnesia for up to 1 month while the scopolamine-amphetamine and saline-saline groups continued to show strong memory with only a modest decrement in performance by 1 month after the first test. These results show that amphetamine results in a permanent recovery from scopolamine amnesia.

Strength of scopolamine-induced amnesia as a function of time between training and testing

Variations in the strength of scopolamine-induced amnesia as a function of age of the habit were studied in Swiss Webster mice. Animals were trained in an active avoidance task to a criterion of 9/10 avoidances and immediately following training injected with scopolamine hydrochloride (1.0 mg/kg) or saline. Retention of the avoidance learning was evaluated by testing different groups of animals 1, 3, 7, 10, 14, and 28 days following training. The retention test consisted of five trials in which the CS but not the UCS was presented. Results indicated that saline-treated mice exhibited near-perfect retention up to 14 days post-training with forgetting beginning to be apparent at 28 days. Scopolamine treatment produced strong amnesia in animals tested 1 and 3 days post-training but normal retention in animals tested 7 and 10 days after learning. The amnesia abruptly reappeared at 14 days after which time it remained stable. The marked similarity of the scopolamine retention curve to changes in the strength of memory of discrimination learning in undertrained rats reported by Deutsch suggested that scopolamine resulted in the storage of a weak memory of the avoidance response. To explore this idea further we trained mice to a criterion (4/5) which would result in a weak avoidance response and tested different groups 1, 3, 10, 14, and 28 days following learning. Results showed that strength of the memory of avoidance learning increased up to 10 days and then decreased abruptly at 14 days thus replicating the general shape of the retention curve produced by injecting scopolamine following strong training. These data suggest that scopolamine disrupts processes essential for the formation of durable memories.

Alleviation of scopolamine amnesia by different retrieval enhancing treatments

Mice were trained in a one-way active avoidance task to a criterion of 9/10 avoidances. Immediately following training they were injected with scopolamine hydrochloride (1 mg/kg SC) or with saline. Retention was assessed 3 days after training by 5 test trials on which the UCS was not present. Thirty min prior to the test, groups were injected with different doses of arecoline, d-amphetamine sulphate or with saline. Other scopolamine-treated mice were exposed to the CS or the UCS 24 hr prior to the test. The scopolamine-induced amnesia was attenuated by both 0.5 and 1.0 mg/kg arecoline and by 2.0 mg/kg d-amphetamine. Retention was also improved by exposure to the CS and the UCS. These data show that scopolamine amnesia can be alleviated by treatments which activate retrieval processes.

Attenuation of forgetting by pharmacological stimulation of aminergic neurotransmitter systems

Mice were trained in one-way active avoidance to a criterion of 3/4 avoidances and tested under extinction conditions one week later when substantial forgetting had occurred. Thirty min prior to testing animals were injected with either saline or different doses of drugs which activate the noradrenergic (phenylephrine, salbutamol, clonidine) dopaminergic (L-dopa(Sinemet) transdihydrolisuride, apomorphine) and serotonergic (fluoxetine, 5-methoxy DMT) neurotransmitter systems. Results showed that all agents alleviated forgetting in a dose dependent fashion. Untrained mice treated with the most effective dose of representative drugs from each class did not exhibit avoidance behavior at testing indicating that the improved performance of trained animals was probably not the result of increased activity or other non-memorial effects of the drugs. It was concluded that pharmacological agents which stimulate monoamine systems may improve memory retrieval by activating a non-specific neural system which controls arousal, attention and motor readiness.

Amphetamine enhances retrieval following diverse sources of forgetting

The generality of amphetamine-induced retrieval enhancement was investigated by determining whether pretest administration could alleviate different types of forgetting. Thirsty mice were punished for licking a water tube following a period of free drinking. Forgetting of the conditioned drink suppression was induced in different groups of animals by; protein synthesis inhibition, cholinergic receptor blockade, inhibition of norepinephrine synthesis, stimulation of serotonin receptors, electroconvulsive shock, a 2.5 month training to test interval and the use of senescent animals with an endogenous memory defect. Thirty min prior to testing mice were injected with either saline or with 2 mg/kg d-amphetamine sulphate. Results showed that amphetamine produced a highly significant improvement in remembering in all of the forgetting treatment groups. It is concluded that amphetamine can alleviate forgetting caused by widely diverse etiologies probably by activating a nonspecific general retrieval system.

Reversal learning in senescent rats

The ability of old (24 months) and young (3 months) male rats to reverse a previously acquired discrimination was compared in 5 experiments. The old rats did not need more trials to learn a position habit in a T-maze to obtain water reward, but required more trials to reverse the position habit. The old rats showed a similar deficit in a second, but not in subsequent reversals of the position habit. In a second experiment, old rats were slower in learning to operate one of two levers in an operant chamber to obtain food reward on a CRF schedule, but by the session prior to reaching criterion for acquisition they showed response rates similar to the young animals. When the rats were required to operate the alternative lever to obtain reward, the young rats emitted 70% of their responses during the first reversal session on the newly-correct lever, but the old rats only 35%. Nevertheless, the groups were similar in the number of sessions required to reach a criterion of 95% of responses on the correct lever. In 3 subsequent reversals, old and young rats did not differ nor were there differences in the number of responses in 4 extinction sessions in the rats which had received reversal training. In experiment 3 with old and young rats which had received only acquisition training, old rats emitted fewer responses than young animals during extinction. From these experiments it was hypothesized that the apparent difficulty of old rats in learning a reversal task was due to the low probability of their emitting spontaneously a novel or previously unrewarded response, and not to a difficulty in forming a new association. This hypothesis was tested in two further experiments in which rats were required to learn a brightness discrimination in a T-maze. Old and young rats which had learned and reversed position habits in the T-maze in experiment 1, did not differ in either acquisition or reversal of the brightness discrimination, suggesting that old rats do not differ from young animals in reversal tasks when the motor response requirements for the task are already within the animals' behavioural repertoire. Consistent with this hypothesis, naive old rats were slower than young rats in acquiring a similar brightness discrimination but did not differ in the reversal task.

Age differences in within-session habituation of exploratory behavior: effects of stimulus complexity

The effects of age on the habituation of exploratory behavior of 8-month- and 28-month-old male C57BL/NNia mice were examined under three different stimulus complexity conditions. Increases in the degree of stimulus complexity resulted in an attenuation of between-session habituation and an initial disruption of within-session habituation by 8-month-old mice. Although increases in stimulus complexity also resulted in an increase in the overall level of exploration by aged mice, stimulus complexity was not found to have a systematic effect on between- or within-session habituation by aged mice. No between-session habituation was observed in aged mice under any of the stimulus complexity conditions. Further, aged mice exhibited significant within-session increases, rather than decreases, in exploration under each stimulus complexity condition. This disruption of within-session habituation in aged mice was found to persist over four daily test sessions. In view of the specific patterns of exploration by aged mice, the disruption of within-session habituation was attributed to age-related differences in reactivity to the arousal-inducing properties of novel stimuli.

Recovery of memory following forgetting induced by depletion of biogenic amines

Following depletion of biogenic amines by reserpine, mice were trained to avoid one compartment of a shuttle box by employing the procedures of Pavlovian fear conditioning. Retention was tested one week later using both an active and a passive measure. A robust amnesia was apparent in reserpine-treated animals on both retention measures. Treatment with the mixed dopamine-serotonin agonist lisuride 30 min prior to the test alleviated the memory loss. Since improved retention in the drug treated mice was indexed by increased response latencies in the passive test and decreased latencies in the active test it is unlikely that the improvement in performance was the result of non-specific effects on activity. The results are consistent with the hypothesis that lisuride treatment before testing facilitates retrieval processes.

Facilitation of memory retrieval by centrally administered catecholamine stimulating agents

Amnesia for inhibitory avoidance learning induced in mice by a post-training injection of the protein synthesis inhibitor anisomycin was alleviated in a dose-dependent fashion by introcerebroventricular injections of D-amphetamine (20 micrograms), dopamine (10, 25 and 50 micrograms) and the dopamine agonist lisuride (0.5 and 1.0 micrograms), given 5 min before the retention test. Animals which received avoidance training in a different apparatus did not show increased test latencies following drug treatment thus eliminating non-specific behavioral suppression as an explanation for the findings. Neither norepinephrine nor the norepinephrine agonist clonidine was able to reverse the amnesia. These findings indicate that central dopamine systems may play a role in the retrieval of inhibitory avoidance learning.

Brain cyclic AMP and memory in mice

A phosphodiesterase inhibitor 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (Rolipram, 10 mg/kg IP) administered immediately, but not 3 hr post-training, reversed an amnesia for an inhibitory avoidance response induced by the protein synthesis inhibitor anisomycin. Immediate post-training administration of Rolipram also enhanced retention for a weakly learned avoidance response. Unshocked animals did not show increased test latencies thus ruling out conditioned aversion as an explanation for the enhanced avoidance. Mice treated with Rolipram (10 mg/kg after training showed elevated cyclic AMP but not cyclic GMP in frontal cortex, thalamus, and hypothalamus. These results support the suggestion that cyclic AMP may play a role in memory processes.

Effects of cycloheximide, a protein synthesis inhibitor, on mouse brain catecholamine biochemistry

Cycloheximide (CXM), a protein synthesis inhibitor, has been shown to result in a marked inhibition of central catecholamine (CA) synthetic mechanisms at doses that cause amnesia in animals. Unlike other inhibitors of CA synthesis no significant depletion of whole brain NE or DA concentrations was observed 0.75, 1, 2, 3, 4, 6, 17, or 24 hours after administration of CXM (120 mg/kg) to C57BL/6J mice. In order to investigate the underlying basis of maintenance of CA levels in face of CA synthesis inhibition, the effects of CXM on in vitro release of 3H-NE was studied in mouse hypothalamic slices. CXM, in a dose related manner, significantly inhibited the potassium stimulated release of NE from hypothalamic slices. Anisomycin, another protein synthesis inhibitor, similarly inhibited NE release. These studies further document the effects of protein synthesis inhibitors on CA mechanisms and suggest that disruption of CA biochemistry may play a role in the amnesia observed after administration of protein synthesis inhibitors.

Characteristics of retrograde amnesia following reactivation of memory in mice

Amnesia for approach-avoidance learning was induced in mice by injecting the protein synthesis inhibitor anisomycin (ANI) immediately, 1, or 2 hours, but not 3 hours after training. A robust amnesia could be demonstrated if ANI was administered 3 hours after training, immediately following a 60 second exposure to the training apparatus or to a structurally similar environment. The temporal gradient of effectiveness of amnesia production by ANI was significantly steeper following reactivation treatment than it was following initial training. In addition, while amnesia produced by the conventional procedure remained stable for 6 days, the amnesia induced following reactivation treatment spontaneously recovered 4 days after training. These findings are discussed in terms of their relevance to interpretations of retrograde amnesia studies.

Alleviation of anisomycin-induced amnesia by pre-test treatment with lysine- vasopressin

Amnesia in mice for a passive avoidance response induced by anisomycin injection immediately after training was reversed by 40 micrograms of lysine-vasopressin given one hour before testing. Control groups receiving non-contingent shock instead of training were used to demonstrate that the effects of vasopressin were due to memory of shock received in a particular place, rather than non-specific suppression of locomotion. The effects of vasopressin on retention were not mimicked by either pentylenetetrazol or epinephrine suggesting that the enhanced latencies were probably not the result of increases in fear or arousal. These data support the hypothesis that the retrieval of memory can be facilitated by vasopressin. The possibility of a relationship between the effects of vasopressin and those of catecholamine manipulations on memory is discussed.

Reversal of anisomycin-induced amnesia by the ergot derivative hydergine

The ergot alkaloid Hydergine was tested for its ability to reverse an amnesia for approach-avoidance training. Thirsty mice were trained to drink in a test chamber and then punished with brief electric shocks for drinking. Those mice injected with the protein-synthesis inhibitor anisomycin immediately after training were amnesic for the shock when tested 48 h later. Pre-test injection of 10.0 or 1.0 mg/kg of Hydergine effectively reversed the amnesia while 0.1 mg/kg was ineffective. Non-contingent shock control groups ruled out the possibility that the effect was due to non-specific effects of the drug or training stimuli.

Clonidine reverses the amnesia induced by dopamine beta hydroxylase inhibition

The role of noradrenergic (NE) mechanisms in amnesia induced by the dopamine-beta-hydroxylase (DBH) inhibitor, diethyldithiocarbamate (DEDTC) was examined by studying the antiamnestic characteristics of the alpha-NE receptor stimulator clonidine. DEDTC (250 mg/kg) administered 3 hr prior to training to C57BL/6J mice resulted in marked deficits when retention of a multiple trial food motivated spatial discrimination task was measured 24 hr after learning. Investigation of the temporal aspects of recovery indicated that the agonist was an effective antiamnestic agent when administered 0, 1, 3, 21 and 23 hr after training. No recovery was observed when the drug was administered 6 and 18 hr posttraining. A dose response study of the effectiveness of clonidine administered 1 hr prior to testing indicated recovery of memory at doses ranging from 10-500 microgram/kg. The clonidine induced recovery was not a result of general performance facilitation, but specific to the memory tested. In addition, the clonidine effect was pharmacologically specific to its actions on NE receptors, as recovery was blocked by pre-treatment with the alpha-NE antagonist, phentolamine. No recovery from DEDTC induced amnesia was seen with post-training or pre-test injection of d-amphetamine.

Reversal of cycloheximide-induced amnesia by adrenergic receptor stimulation

Amnesia for a multiple trial appetitive spatial dicrimination habit induced by the protein synthesis inhibitor cycloheximide (CXM) was reversed by peripheral injections of both alpha (clonidine) and beta (isoproterenol) norepinephrine receptor stimulators. Stimulation of dopamine receptors with piribedil and acetylcholine receptors with pilocarpine was ineffective in reversing amnesia. The clonidine-induced recovery was blocked by phentolamine and the isoproterenol recovery by propranolol. Examination of the temporal parameters of clonidine-induced recovery indicated that the amnesia was prevented if the agonist was injected either before training and CXM treatment, up to 1 hr after training and up to 3 hr prior to testing. Clonidine also alleviated amnesia induced by another protein synthesis inhibitor anisomycin, for a shock motivated brightness discrimination habit. These data suggest that the transient amnesia induced by CXM may be a consequence of disruption of adrenergic mechanisms and more specifically that norepinephrine may play an important role in memory retrieval.

Some characteristics of amnesia induced by FLA-63 an inhibitor of dopamine beta hydroxylase

The amnesic effects of FLA-63, a potent dopamine-beta-hydroxylase (DBH) inhibitor, were investigated in a food motivated spatial discrimination task. Groups of C57BL/6J mice were injected with either 5 mg/kg, 15 mg/kg, 25 mg/kg, 35 mg/kg or physiological saline 4 hr prior to training. Amnesia was observed 24 hr following training at all dose levels except 5 mg/kh. The performance deficit was specific to memory of the discrimination and not the result of state-dependency. Training conditions which produce an increase in habit strength prevented the amnestic effects of FLS-63. Spontaneous recovery of memory occurred 48 hr following drug administration. Recovery from amnesia was also induced by injections of a monoamine oxidase inhibitor, pargyline, administered 2 hr prior to the retention test. These data suggest that amnesia induced by norpinephrine (NE) depletion is the result of impairment of mechanisms necessary for memory retrieval.