Differential effects on learning by ventromedial vs lateral hypothalamic posttrial injection of substance P

Substance P facilitates memory in goldfish in an appetitively motivated learning task

The aim of this study was to investigate if the neuropeptide substance P (SP) can improve learning and memory in goldfish (Carassius auratus). Four groups of fish were trained for seven days to find food in one out of two compartments until discrimination was achieved. On the last training day, they were injected (intra-abdominal) with haloperidol or vehicle before the training, and with SP or vehicle immediately after the training session. Each group of fish received either: (1) vehicle+vehicle (n = 18); (2) vehicle + SP, (n = 20, SP 50 mg/kg); (3) haloperidol+ vehicle (n = 15, haloperidol 2 mg/kg); or (4) haloperidol+ SP (n = 14, haloperidol 2 mg/kg, SP 50 mg/kg). Twenty-four hours later, the time spent to find the food was recorded. Reversal training was done for four consecutive days after this post-injection test and the time spent to find the food was recorded again. The results indicate that only the group treated with vehicle + SP needed more time to reach reversal training than control fish (Mann-Whitney U-test, P = 0.0009). It is suggested that SP can enhance memory in fish and that this effect may have a dopaminergic mediation in discrimination learning task.

Reinforcing properties of the neuropeptide substance P in Carassius auratus: evidence of dopaminergic system involvement

The aim of the present study was to investigate whether neuropeptide substance P (SP) has reinforcing effects in Carassius auratus and whether this effect could be related to dopaminergic neurotransmission. For this purpose fishes were put in a three-compartment box in which one compartment gave access to two others that did not directly link. The time spent in each compartment was registered for 10 min to determine a possible preferred compartment. Twenty-four hours later, the fish were given one of the following intraperitoneal treatments: a) Group VEH, injected with the vehicle of substance P; b) Group SP25, injected with SP, 25 micrograms/kg body wt.; c) Group SP50, injected with SP, 50 micrograms/kg; or d) Group HALO, injected with haloperidol (2 mg/kg) 30 min before an injection of SP (50 micrograms/kg). Immediately after the treatment the fish were kept for 30 min in the compartment preferred least the day before. On the next day the fish were retested for 10 min to verify the time spent in each compartment. The results indicate that SP at the dose of 50 micrograms/kg enhanced the time spent on the paired compartment, and that the pretreatment with haloperidol abolished this enhancement. It is suggested that SP has reinforcing effects in C. auratus that may be mediated by the dopaminergic system.

Effects of naloxone treatment on memory-enhancing properties of post-training administration of substance P

The present study investigated the effects of post-trial systemic injections of the neurokinin substance P (SP) on inhibitory avoidance learning in rats treated with naloxone before conditioning and/or test trials. Rats were trained in a step-down or uphill inhibitory avoidance task and tested 24 h later. The animals received, 30 min before training and/or testing an i.p. injection of saline or naloxone (Nx) at doses of 0.5; 1.0; 5.0 or 50 mg/kg. Immediately after training they were administered SP 50 micrograms/kg or vehicle. Animals that received Nx both before conditioning and test trials (5.0 and 50 mg/kg), in combination with SP, showed better test performance for the uphill and step-down avoidance then those treated only with SP. Nx administered only before training (5.0 and 50 mg/kg) or before test (0.5 to 50 mg/kg) reduced the effects of SP. These data are discussed in terms of the possibility that Nx produces a state-dependent learning when combined with SP.

Facilitation of tunnel maze performance by systemic injection of the neurokinin substance P

The effect of peripheral injections of substance P (SP) on performance in two different configurations of an automated tunnel maze was examined in three experiments. In two experiments, the effect of pretrial SP injections (10-1000 micrograms/kg) on performance in the hexagonal and radial maze configuration of an automated tunnel maze was investigated. In the hexagonal maze, which measures activity, exploratory efficiency, habituation, and perimeter walking, injection of SP affected perimeter walking only. In the radial maze, SP produced a facilitation of measures of efficiency and long-term and short-term memory without affecting activity. In the third experiment, the effect of pre- and posttrial injections of SP (50 or 500 micrograms/kg) on performance in the radial maze configuration was tested. Again, pretrial injections of 500 micrograms of SP facilitated performance with respect to measures of efficiency and short- and long-term memory; 50 micrograms produced a weaker effect. Virtually no effect was seen with posttrial injections.

Facilitation of memory by peripheral administration of substance P and naloxone using avoidance and habituation learning tasks

This paper summarizes results of a series of experiments dealing with the effects of the neuropeptide substance P (SP) on avoidance learning and habituation. Several doses of SP (0.5, 5, 50, 100, 250, 500 micrograms/kg) were administered posttrial intraperitoneally (IP). Three inhibitory one-trial avoidance tasks were used; uphill, step-down and step-through (alcove). Habituation was measured in an open field by recording the number of rearings. The posttrial injection of SP facilitated avoidance responses as well as reduced rearing in a dose- and time-dependent way. Pretraining and pretest injections (IP) of naloxone facilitated avoidance behavior and potentiated the action of SP, also in a dose-dependent manner. These results demonstrate that: a) peripheral posttraining administration of SP enhances memory; b) SP facilitates not only aversive or positively motivated learning tasks, but also habituation, which is a form of learning that involves neither positive nor negative reinforces; c) SP does not exert its effect by a long-lasting proactive action on performance during the testing trial; d) naloxone potentiates the SP posttraining effect. These data, therefore, suggest that memory-enhancing effects of SP are, at least in part, mediated via interactions between this peptide and endogenous opioid systems.

Substance P enhancement of inhibitory avoidance learning: mediation by the N-terminal sequence

Experiments were performed to investigate the effects of intraperitoneally administered undecapeptide substance P (SP), its N-terminal fragment SP(1-7) (SPN) and the C-terminal analog [pGlu6]-SP(6-11) (SPC) on inhibitory avoidance learning, using a one-trial up-hill avoidance task. In Experiment 1 rats were injected with either SP (50 micrograms/kg), SPN (3.3, 33, 167, 333 micrograms/kg) or SPC (2.7, 27, 134, 268 micrograms/kg) immediately after the training trial. Controls received the diluent vehicles. When tested 24 hr later, rats injected with 50 micrograms/kg SP (37 nmol/kg) and 167 micrograms/kg SPN (185 nmol/kg) exhibited longer step-up latencies than vehicle-treated controls. None of the other doses of SPN nor of the C-terminal fragment influenced performance. In Experiment 2, 167 micrograms/kg SPN or vehicle was injected posttrial either immediately or 5 hr after the training trial. Retention latencies 24 hr later were longer for rats treated with 167 micrograms/kg SPN immediately after the training trial. Performance of the SPN 5-hr delay group did not differ from that of the vehicle-injected controls, ruling out proactive effects of SPN on recall.

The relationship between reinforcement and memory: parallels in the rewarding and mnemonic effects of the neuropeptide substance P

A theory of reinforcement is presented which accounts for the backward action of a reinforcer on operant behavior in terms of its effect on memory traces left by the operant. Several possible ways in which a reinforcer could strengthen the probability of recurrence of an operant are discussed. Predictions from the model regarding general memory-promoting effects of reinforcers presented posttrial in various learning paradigms are outlined. The theory also predicts a parallelism in reinforcing and memory-promoting effects of stimuli, including drugs. The second part of the chapter outlines experiments investigating memory modulating and reinforcing effects of the neuropeptide substance P. In general, injection of SP is positively reinforcing when injected into parts of the brain where it has been shown to facilitate learning. Peripheral injection of SP is also reinforcing at the dose known to promote passive avoidance learning when presented posttrial.

Place aversion induced by the substance P analogue, dimethyl-C7, is not state dependent: implication of substance P in aversion

Intraventricular infusion of the stable substance P analogue, DiMethyl-C7, was found to produce aversion in a place conditioning paradigm. This effect was independent of state dependent learning since the place aversion was also seen when the subjects were tested under the influence of the drug. A role for substance P in aversion is discussed.

Nigral 5-HT and substance P-induced enhancement of passive avoidance retention

Peripheral, posttraining injection of substance P (SP) has been shown to facilitate the retention of aversive and appetitive learning tasks, suggesting that SP may play a role in information processing. In addition, SP may modulate the release of nigrostriatal monoamines, which have also been linked with avoidance learning. This paper examines the interaction between SP and nigrostriatal monoamines by observing the behavioral effects of neurochemical lesions on SP-induced avoidance retention, and by measuring changes in nigrostriatal monoamine activity and receptor regulation following avoidance training and SP injection. In Expt. 1, 5,7-dihydroxytryptamine lesions of the substantia nigra, but not the caudate, attenuated the retention-enhancing effects of posttraining SP injection. Further, 6-hydroxydopamine lesions of the substantia nigra produced a deficit in avoidance conditioning that was reversed by posttraining SP injection. Expts. 2 and 3 demonstrated that although passive avoidance training and posttraining SP injections did not significantly alter nigral 5-hydroxytryptamine (5-HT) activity, SP increased 5-HT1 receptor density. It was concluded that SP may affect avoidance retention by modulating nigral 5-HT activity.

Enhanced inhibitory avoidance learning produced by post-trial injections of substance P into the basal forebrain

The effects of injections of the neuropeptide substance P or the GABA agonist muscimol on performance of a step-down inhibitory avoidance task were examined. Immediately after the training trial, rats with chronically implanted cannulas were injected with 100 or 10 ng of substance P or 500 or 50 ng of muscimol into the region of the nucleus basalis magnocellularis. Control groups included vehicle-injected rats, a sham-operated group, a substance P 5-h delay group, and a substance P no-footshock group. Rats injected with 100 ng of substance P exhibited longer step-down latencies when tested 24 h later than did vehicle-injected rats. The retention latencies for rats in the substance P 5-h delay group did not differ from those of vehicle-injected animals, indicating that proactive effects on performance were not responsible for the effect. In contrast to injections of SP, injections of 500 or 50 ng of muscimol disrupted performance. However, in the absence of a delayed-injection control group, proactive effects cannot be ruled out.

Reinforcing properties of substance P in the lateral hypothalamus revealed by conditioned place preference

Reinforcing properties of substance P (SP) were investigated in rats using a conditioned place preference paradigm. After three baseline trials one conditioning trial of 10 min duration was performed. Either SP (100 pg, 1 ng, 100 ng) or saline (SI) was injected unilaterally into the lateral hypothalamus or a sham injection (OC) was given. Pairing one compartment with SP (100 pg, 1 ng) significantly increased the time spent in this compartment. Microinjections of 100 ng SP, saline or sham injection had no effect. Locomotor activity was not influenced by either treatment. These data are discussed in terms of (a) the possibility that SP has a role in mediating reinforcement and (b) the relationship between reinforcing effects and post-trial effects on learning and memory of SP applied into the lateral hypothalamus.

Facilitation of conditioned inhibitory avoidance by post-trial peripheral injection of substance P

Experiments were undertaken to investigate the effects of the neuropeptide Substance P (SP) on performance of a conditioned inhibitory avoidance response in rats. A single-trial inhibitory avoidance task was employed. In Experiment 1 SP was injected IP immediately after the training trial in doses of 0.5, 5, 50, 100, 250 or 500 micrograms/kg; control animals were injected with diluent vehicle. The group treated with 50 micrograms SP/kg exhibited better avoidance than the other groups. In Experiment 2 the doses of SP used were 1, 50, 250 micrograms/kg, and the control animals were injected with vehicle or not injected at all. Only the 50 micrograms SP/kg treatment group showed significantly better performance. In Experiment 3 50 micrograms/kg SP or vehicle was injected post-trial immediately or 5 hr after the trial. Only the group in which SP was injected immediately after the training trial showed significantly better performance when tested 24 hr later. This result rules out the possibility that SP exerts its effect by a long lasting proactive action on performance during the testing trial 24 hr later.

Substance P facilitation of memory: effects in an appetitively motivated learning task

Food deprived, heterogeneous strain (HS/IBG) mice were trained on two different discrimination tasks for food reinforcement. In one experiment animals were trained to make spatial discriminations in a T maze. Immediately after training they were given subcutaneous injections of either substance P (1 ng/g) or vehicle. Twenty-four hours later the animals were given reversal training in the same maze. The results showed that substance P-treated animals took significantly longer to acquire the reversal habit than did control mice. In a second experiment, animals were trained to make visual discriminations in a T maze. Immediately after reaching acquisition criterion animals were injected with either substance P (1 ng/g) or vehicle. Different groups of mice were retrained on the same task either 1, 2, 3, or 7 days after original learning. Savings scores were calculated and, at every interval, substance P-treated mice retained the task better than control animals. One interpretation of these data is that substance P-treated mice remembered the original task significantly better than vehicle-injected control animals.

Central action of substance P: possible role in reward

A series of studies had revealed a dualistic role of post-trial injections of substance P in affecting avoidance behavior depending on the site of the brain in which it is applied. Based on these data, the hypothesis was formulated that substance P has brain site-dependent rewarding and punishing properties, a possibility which was assessed in rats trained on a modified T-maze task. Injections of substance P into the medial forebrain bundle (100 ng) or medial septal nucleus (500 ng) served as a positive reinforcer for conditioned place preference learning in the T maze. Injections into the amygdala (50 ng) or substantia nigra (100 ng) did not have such reinforcing properties.

Substance P enhancement of passive and active avoidance conditioning in mice

In a series of seven experiments we explored the effects of peripherally administered substance P on passive and active avoidance conditioning in mice of two genotypes. The peripheral post-trial administration of substance P significantly enhanced the retention of a single-trial passive avoidance task. This effect was dose dependent; 1 ng/g of substance P enhanced the retention of this habit, whereas higher and lower doses were either less effective or ineffective. In heterogeneous strain (HS) mice, substance P administered before training on an active avoidance task did not alter the rate at which these animals learned this habit. However, animals that had been trained with substance P were significantly more resistant to extinction than were animals that had been injected with vehicle. Similarly, C57Bl/6J mice that had been treated with substance P immediately after active avoidance training were more resistant to extinction than were mice that had been given control injections. The enhancement of retention of the passive avoidance habit with substance P was reversed in animals that had been pretreated with naltrexone. Substance P enhancement of the retention of the passive avoidance habit, and its reversal with naltrexone, was observed in both sham operated and adrenalectomized mice.

Substance p reversal of electroconvulsive shock and cycloheximide-induced retrograde amnesia

Subcutaneous post-trial administration of the neuropeptide substance P was found to reverse the amnestic effects of both electroconvulsive shock and cycloheximide. Substance P was observed to reverse the amnestic effects of cycloheximide in both C57B1/6J and heterogeneous strain (HS) mice. Substance P was found to reverse the amnestic effects of electroconvulsive seizures in C57B1/6J animals. Peripheral injections of substance P were also found to facilitate the retention of a single-trial passive avoidance habit in animals of both genotypes, provided a weak footshock was used during training.