The effect of substance P and its fragments on passive avoidance retention and brain monoamine activity

Hemokinin-1 mediates anxiolytic and anti-depressant-like actions in mice

The tachykinin NK1 receptor was suggested to be involved in psychiatric disorders, but its antagonists have failed to be effective as antidepressants in clinical trials. Hemokinin-1 (HK-1), the newest tachykinin, is present in several brain regions and activates the NK1 receptor similarly to substance P (SP), but acts also through other mechanisms. Therefore, we investigated the roles of the Tac4 gene-derived HK-1 in comparison with SP and neurokinin A (NKA) encoded by the Tac1 gene, as well as the NK1 receptor in anxiety and depression-like behaviors in mice. Mice lacking SP/NKA, HK-1 or the NK1 receptor (Tac1^-/-, Tac4^-/-, Tacr1^-/-, respectively) compared to C57Bl/6 wildtypes (WT), and treatment with the NK1 antagonist CP99994 were used in the experiments. Anxiety was evaluated in the light-dark box (LDB) and the elevated plus maze (EPM), locomotor activity in the open field (OFT) tests. Hedonic behavior was assessed in the sucrose preference test (SPT), depression-like behavior in the tail suspension (TST) and forced swim (FST) tests. FST-induced neuronal responsiveness was evaluated with Fos immunohistochemistry in several stress-related brain regions. In the LDB, Tac4^-/- mice spent significantly less, while Tacr1^-/- and CP99994-treated mice spent significantly more time in the lit compartment. In the EPM only Tac4^-/- showed reduced time in the open arms, but no difference was observed in any other groups. In the OFT Tac4^-/- mice showed significantly reduced, while Tac1^-/- and Tacr1^-/- animals increased motility than the WTs, but CP99994 had no effect. NK1^-/- consumed markedly more, while Tac4^-/- less sucrose solution compared to WTs. In the TST and FST, Tac4^-/- mice showed significantly increased immobility. However, depression-like behavior was decreased both in cases of genetic deletion and pharmacological blockade of the NK1 receptor. FST-induced neuronal activation in different nuclei involved in behavioral and neuroendocrine stress responses was significantly reduced in the brain of Tac4 ^-/- mice. Our results provide the first evidence for an anxiolytic and anti-depressant-like actions of HK-1 through a presently unknown target-mediated mechanism. Identification of its receptor and/or signaling pathways might open new perspectives for anxiolytic and anti-depressant therapies.

Anxiolytic-like effects of the neurokinin 1 receptor antagonist GR-205171 in the elevated plus maze and contextual fear-potentiated startle model of anxiety in gerbils

Gerbils show a neurokinin (NK)1 receptor pharmacological profile, which is similar to that observed in humans, and thus have become a commonly used species to test efficacy of NK1 receptor antagonists. The aim of this study was to determine whether systemic administration of the NK1 receptor antagonist GR-205171 produced anxiolytic-like effects in the elevated plus maze and in a novel contextual conditioned fear test using fear-potentiated startle (FPS). On the elevated plus maze, treatment with GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing produced anxiolytic-like effects in an increasing dose-response manner as measured by the percentage of open arm time and percentage of open arm entries. For contextual fear conditioning, gerbils were given 10 unsignaled footshocks (0.6 mA) at a 2-min variable interstimulus interval in a distinctive training context. Twenty-four hours after training, gerbils received treatment of GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing in which startle was elicited in the same context in which they were trained. Contextual FPS was defined as an increase in startle over pretraining baseline values. All drug dose levels (0.3, 1.0, and 5.0 mg/kg) significantly attenuated contextual FPS when compared with the vehicle control group. A control group, which received testing in a different context, showed little FPS. These findings support other evidence for anxiolytic activity of NK1 receptor antagonists and provide a novel conditioned fear test that may be an appropriate procedure to test other NK1 antagonists for preclinical anxiolytic activity in gerbils.

Disruption of the neurokinin-3 receptor (NK3) in mice leads to cognitive deficits

RATIONALE

The structurally related neuropeptides, substance P, neurokinin A, and neurokinin B, belong to a family of molecules termed tachykinins and are widely distributed in the central and peripheral nervous systems. These peptides mediate their effects through three G protein coupled receptor subtypes, the neurokinin-1, neurokinin-2 and neurokinin-3 receptors, respectively.

OBJECTIVE

To study the physiological functions of NK3, a line of NK3 knockout mice were generated and characterized in a broad spectrum of well-established behavioral tests.

RESULTS

In several tests, including spontaneous locomotor activity, elevated plus maze, forced swim, and hot plate, wild-type and knockout mice performed similarly. However, in several cognition tests, including passive avoidance, acquisition of conditioned avoidance responding (CAR), and the Morris water maze, NK3 knockout mice displayed deficits compared to wild-type mice. Although NK3 wild-type and knockout mice performed similarly in the training phase of the passive avoidance test, knockout mice had shorter latencies to enter the dark compartment on days 3 and 4, suggesting impaired retention. In the acquisition phase of the conditioned avoidance responding assay, NK3 knockout mice acquired the CAR task at a slower rate than wild-type mice. Once the CAR test was acquired, both NK3 wild-type and knockout mice responded similarly to clozapine and risperidone, drugs which suppress responding in this test. In the Morris water maze, NK3 knockout mice showed increased latencies to find the escape platform on day 3 of training, suggesting a modest, but significant delay in acquisition compared to wild-type mice.

CONCLUSION

These studies suggest a role for NK3 in learning and memory in mice.

Effects of chronic substance P treatment and intracranial fetal grafts on learning after hippocampal kainic acid lesions

The purpose of this experiment was to investigate whether the neurokinin substance P (SP) can enhance adaptive graft effects on learning and memory functions in animals with lesions of the hippocampus. Adult male Wistar rats received a bilateral kainic acid (KA) lesion of the dorsal hippocampus. One week postlesion, bilateral grafts of fetal hippocampal tissue suspension were applied into the damaged region in half of the animals, whereas the other half received sham transplants (physiological saline). Animals of the control group received a bilateral sham lesion of the hippocampus and sham transplants. One week after transplantation surgery, the rats were tested in the place version of the Morris water maze over a period of 9 weeks. Then they were tested for SP-induced conditioned place preference and on a step-through inhibitory avoidance task. All animals received IP injections of either SP (5 or 50 micrograms/kg) or the SP vehicle (0.5 ml/kg). The treatment with SP or the vehicle was begun 1 week after transplantation and was performed 5 days a week over a period of 10 weeks. During behavioral tests in the water maze and avoidance task, application of the substances was performed 5 h after testing. For the conditioned place preference test, the conditioning trials were performed immediately after drug administration; the test trials were given 24 h later. Chronic administration of 50 micrograms/kg SP, but not 5 micrograms/ kg SP, was found to improve water maze performance in lesioned animals with and without grafts. Unexpectedly, the lesion group with the graft without additional SP treatment was not superior to the lesion group devoid of the graft in this task. The rats without lesions of the hippocampus still showed a conditioned place preference to 50 micrograms/kg SP after 9 weeks of repeated SP applications. In the inhibitory avoidance task, the grafts facilitated retention performance independent of whether SP treatment was given. The morphological analysis of the transplants revealed higher graft volumes and a higher diameter of large pyramidal neurons (> 10 microns) in rats chronically treated with 50 micrograms/kg SP.

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.

Sequence-specific effects of neurokinin substance P on memory, reinforcement, and brain dopamine activity

There is ample evidence that the neurokinin substance P (SP) can have neurotrophic as well as memory-promoting effects. This paper outlines a recent series of experiments dealing with the effects of SP and its N- and C-terminal fragments on memory, reinforcement, and brain monoamine metabolism. It was shown that SP, when applied peripherally (IP), promotes memory (inhibitory avoidance learning) and is reinforcing (place preference task) at the same dose of 37 nmol/kg. Most important, however, is the finding that these effects seemed to be encoded by different SP sequences, since the N-terminal SP1-7 (185 nmol/kg) enhanced memory, whereas C-terminal hepta- and hexapeptide sequences of SP proved to be reinforcing in a dose equimolar to SP. These differential behavioral effects were paralleled by selective and site-specific changes in dopamine (DA) activity, as both SP and its C-, but not N-terminus, increased extracellular DA in the nucleus accumbens (NAc), but not in the neostriatum. The neurochemical changes lasted at least 2 h after injection. These results show that the reinforcing action of peripheral administered SP may be mediated by its C-terminal sequence, and that this effect could be related to DA activity in the NAc. Direct application of SP (0.74 pmol) into the region of the nucleus basalis magnocellularis (NBM) was also memory-promoting and reinforcing, and again, these effects were differentially produced by the N-terminus and C-terminus, supporting the proposed structure-activity relationship for SP's effects on memory and reinfrocement. These results may provide a hypothetical link between the memory-modulating and reinforcing effects of SP and the impairment in associative functioning accompanying certain neurodegenerative processes.

Enhanced learning produced by injection of neurokinin substance P into the region of the nucleus basalis magnocellularis: mediation by the N-terminal sequence

The effect of unilateral injection of the neurokinin substance P (SP) and of certain N- or C-terminal SP-fragments into the region of the nucleus basalis magnocellularis (NBM) on inhibitory avoidance learning was investigated. Rats with chronically implanted cannulae were tested on a one-trial uphill avoidance task. Immediately after the training trial, rats were injected with 0.74 pmol SP or equimolar dosed SP(1-7), DIME-C7, or SP(7-11). Control groups included vehicle-injected rats and a group given an injection of SP(1-7) 5-h after the trial. When tested 24 h later, rats treated with SP or SP(1-7), but not with DIME-C7 or SP(7-11), exhibited longer step-up latencies than vehicle-treated controls. The retention latencies for rats in the SP(1-7) 5-h delay group did not differ from those of vehicle-injected animals, ruling out proactive effects of SP(1-7) on performance. The results show that SP facilitates retention of an inhibitory avoidance response when injected into the NBM. Furthermore, the amino acid sequence that encodes this effect may be located in the N-terminal part of the SP-molecule.

The substance P fragment SP(1–7) stimulates motor behavior and nigral dopamine release

Earlier studies have shown that the undecapeptide substance P (SP) alters motor behavior and dopamine metabolism following injection into the substantia nigra (SN) in rat, even though the SN appears largely devoid of SP-specific (NK-1) receptors. In this report, intra-nigral injections of the amino-terminal SP fragment SP(1-7) enhanced rearing, sniffing and locomotor activity, and increased the nigral DOPAC-to-DA ratio. In addition, SP(1-7) increased 3H-DA release from the SN in vitro. These findings suggest that some of the effects of nigral SP on motor behavior and dopamine release are mediated by amino-terminal fragments of SP.

Evidence for dose-dependent positively and negatively reinforcing effects of the substance P C-terminal analog DIME-C7

Possible positive reinforcing and aversive effects of intraperitoneally (ip) administered substance P (SP) and its C-terminal heptapeptide analog [pGlu5, MePhe8, Sar9]-SP5-11 (DIME-C7) were investigated in rats. The 'conditioned corral preference and avoidance procedure' was used for assessing positive and negative reinforcement. Behavioural testing was conducted in a circular open field consisting of four uniform quadrants equally preferred by the rats prior to drug treatment. On 3 consecutive days, rats received an ip injection of either SP (37 nmol/kg), DIME-C7 (3.7, 7.4, 37, 185 nmol/kg) or vehicle (0.01 M acetic acid in saline) and were placed immediately into their assigned treatment corral. During the test for conditioned corral preference and avoidance, when provided a choice between the four quadrants, rats treated with SP and with the equimolar dose of DIME-C7 (37 nmol/kg) spent significant more time in the drug-paired corral, indicative of positive reinforcing effects. The doses of 3.7 and 7.4 nmol/kg DIME-C7 did not influence the preference behaviour. The high dose of 185 nmol/kg DIME-C7 led to a significant decrease in time spent in the previously drug-paired corral, suggestive of aversive effects of the treatment. Gross locomotor activity was not influenced by either treatment. These results are discussed in the framework of a structure/activity relationship for the reinforcing properties of peripheraly applied SP.

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.

Reinforcing effects of peripherally administered substance P and its C-terminal sequence pGlu6-SP6-11 in the rat

Reinforcing effects of intraperitoneally (IP) administered substance P (SP1-11), its amino-terminal fragment SP1-7 (SPN) and an analog of the carboxy terminus (pGlu6-SP6-11: SPC) were studied in rats. Two conditioned place preference paradigms were used. After three pairings of the drug with a certain environment the effect of the treatment was evaluated in the drug-free state during a test trial. The reinforcing effects of SP (37 nmol) and the equimolar dose of SPC were expressed by a significant increase in the amount of time the animals spent in the treatment environment. Other doses of SP (3.7 and 185 nmol) and SPC (7.4 and 185 nmol) and none of the doses of SPN (37, 185, 370 nmol) influenced the place preference behavior of the rats. The reinforcing effects of SP parallel the known facilitating effects of peripherally administered SP on memory. The amino acids that encode the reinforcing effects of SP may lie within the C-terminal sequence of the SP molecule.

Cardiovascular effects of substance P peptides in the nucleus of the solitary tract

Microinjection of the neuropeptide substance P (SP) into the baroreceptor portions of the nucleus of the solitary tract (NTS) caused a dose-dependent decrease in blood pressure (BP) and heart rate (HR), consistent with the putative role for SP as a transmitter in the baroreceptor reflex arc. In contrast, SP elevated BP and HR when microinjected into the adjacent area postrema. Structure-activity studies of effects of SP in the NTS revealed that an aminoterminal heptapeptide fragment of SP could fully reproduce the depressor and bradycardic effects of SP. In contrast, a carboxyterminal hexapeptide fragment of SP significantly elevated both BP and HR. The structural requirements for aminoterminal fragment effects were quite specific in terms of peptide length and sensitivity to D-amino acid substitutions. These findings are consistent with a role for SP as a baroreceptor reflex transmitter and suggest, furthermore, that this action is mediated by the aminoterminal region of SP.

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.

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.

N- and C-terminal fragments of substance P: spinal effects in the rat tail flick test

Evidence exists to suggest that within the CNS, substance P may be enzymatically cleaved into fragments which may mediate some of the effects of substance P. As we have previously reported on the spinal effects of substance P, the present study examines the effects of selected substance P fragments on reaction time in the tail flick test. Peptides were administered via a chronically implanted intrathecal catheter to the L5 vertebral level in the rat. Administration of 6.5 nmoles of SP(1-7) produced a transient decrease in reaction time at 1 min after injection with a return to above control values by 5 min. Similar administration of SP(7-11) produced a smaller decrease in reaction time at 6 min which lasted until 16 min. Administration of 6.5 nmoles of SP(1-9), SP(8-11) and of CSF were without effect. As the effects of SP(1-7) on reaction time resembled those of similar administration of substance P in the earlier experiments, these results suggest that this fragment may be the active component involved in facilitating the tail flick reflex. Substance P may be degraded to the active fragment prior to receptor activation or alternatively, substance P and SP(1-7) may act on the same receptor.

Selective agonists for substance P and neurokinin receptors

A series of neurokinin analogues and fragments have been prepared in an attempt to identify selective agonists for NK-P, NK-A and NK-B receptors. The compounds have been tested on the dog carotid artery (NK-P receptor system), the rabbit pulmonary artery (NK-A) and the rat portal vein (NK-B). C-terminal substituted analogues of the three neurokinins have provided indication that NK-P receptor selectivity is improved by the oxidation of methionine to Met(O2), while selectivity for NK-A is favoured by replacing Met with NIe. Selectivity for NK-P receptors is further improved by the replacement of Gly9 with Sar. Selectivity and affinity for NK-B receptors is markedly increased when Val7 is replaced with MePhe in both the fragment NKB (4-10) and NKB. The results of the present study indicate that a) [Sar9,Met(O2)11]SP is a potent and selective agonist for the NK-P receptors of the dog carotid artery; b) [MePhe7]NKB is a very potent and selective stimulant of receptors for neurokinin B and c) [Nle10]NKA (4-10) is a promising compound, showing some selectivity for NK-A receptor; further modifications are however needed to improve its affinity.