Repeated administration of the neurotensin analogue NT69L induces tolerance to its suppressant effect on conditioned avoidance behaviour

Antipsychotic drug-like facilitation of latent inhibition by a brain-penetrating neurotensin-1 receptor agonist

Latent inhibition (LI) is a measure of cognitive gating and refers to reduced conditioned learning when there is pre-exposure to the conditioned stimulus (CS) before it is paired with the unconditioned stimulus (US). Dysregulation of LI is associated with some neuropsychiatric disorders, including schizophrenia, and the ability to facilitate LI in rodents is a reasonably good predictive test for antipsychotic drugs. Converging evidence supports neurotensin-1 receptor (NTS1) agonists as novel drugs for schizophrenia. Therefore, we investigated the ability of a brain-penetrating, selective NTS1 agonist, PD149163, to facilitate LI in heterozygous Brattleboro rats, a strain that exhibits naturally low LI. Conditioned taste aversion to flavored water (FW; 0.1% saccharin) was induced by pairing it with malaise-inducing injections of lithium chloride (LiCl). Prior to LiCl-FW pairing, rats received subcutaneous injections of saline, or PD149163 (100 µg/kg or 200 µg/kg). Half the rats in each drug group had been allowed to drink FW the day before the LiCl-FW pairing (pre-exposed rats). Two days after pairing, the amount of FW each rat consumed was recorded. LI, defined as significantly greater FW drinking in the pre-exposed group compared with the non pre-exposed group, was exhibited only among rats that received 200 µg/kg of PD149163. These results further support NTS1 agonists as potentially novel drugs for the treatment of schizophrenia.

Systemic administration of the neurotensin NTS₁-receptor agonist PD149163 improves performance on a memory task in naturally deficient male brown Norway rats

Agonists for the neurotensin NTS₁ receptor consistently exhibit antipsychotic effects in animal models without producing catalepsy, suggesting that NTS₁-receptor agonists may be a novel class of drugs to treat schizophrenia. Moreover, studies utilizing NTS₁ agonists have reported improvements in some aspects of cognitive functioning, including prepulse inhibition and learning procedures, which suggest an ability of NTS₁-receptor agonists to diminish neurocognitive deficits. The present study sought to assess both baseline delay-induced memory performance and the effects of NTS₁-receptor activation on learning and memory consolidation in male Long-Evans and Brown Norway rats using a delayed nonmatch-to-position task radial arm-maze task. In the absence of drugs, Brown Norway rats displayed a significant increase in spatial memory errors following 3-, 7-, and 24-hr delay, whereas Long-Evans rats exhibited an increase in spatial memory errors following only a 7-, and 24-hr delay. With Brown Norway rats, administration of PD149163 before or after an information trial significantly reduced errors during a retention trial after a 24 hr delay. Administration of the NTS(1/2)-receptor antagonist SR142948 prior to the information trial did not affect retention-trial errors. These data are consistent with previous findings that Brown Norway rats have natural cognitive deficits and that they may be useful for assessing putative antipsychotic drugs for cognitive efficacy. Moreover, the results of this study support previous findings suggesting that NTS₁-receptor agonists may improve some aspects of cognitive functioning.

Repeated effects of the neurotensin receptor agonist PD149163 in three animal tests of antipsychotic activity: assessing for tolerance and cross-tolerance to clozapine

Neurotensin is an endogenous neuropeptide closely associated with the mesolimbic dopaminergic system and shown to possess antipsychotic-like effects. In particular, acute neurotensin receptor activation can inhibit conditioned avoidance response (CAR), attenuate phencyclidine (PCP)-induced prepulse inhibition (PPI) disruptions, and reverse PCP-induced hyperlocomotion. However, few studies have examined the long term effects of repeated neurotensin receptor activation and results are inconsistent. Since clinical administration of antipsychotic therapy often requires a prolonged treatment schedule, here we assessed the effects of repeated activation of neurotensin receptors using an NTS1 receptor selective agonist, PD149163, in 3 behavioral tests of antipsychotic activity. We also investigated whether reactivity to the atypical antipsychotic clozapine was altered following prior PD149163 treatment. Using both normal and prenatally immune activated rats generated through maternal immune activation with polyinosinic:polycytidylic acid, we tested PD149163 in CAR, PCP (1.5mg/kg)-induced PPI disruption, and PCP (3.2mg/kg)-induced hyperlocomotion. For each paradigm, rats were first repeatedly tested with vehicle or PD149163 (1.0, 4.0, 8.0mg/kg, sc) along with vehicle or PCP for PPI and hyperlocomotion tests, then challenged with PD149163 after 2 drug-free days. All rats were then challenged with clozapine (5.0mg/kg, sc). During the repeated test period, PD149163 exhibited antipsychotic-like effects in all three models. On the PD149163 challenge day, prior drug treatment only caused a tolerance effect in CAR. This tolerance in CAR was transferrable to clozapine, as it enhanced clozapine tolerance in the same group of animals. Although no tolerance effect was seen in the PD149163 challenge for the PCP-induced hyperlocomotion test, the clozapine challenge showed increased sensitivity in groups previously exposed to repeated PD149163 treatment. Our findings suggest that repeated exposure to NTS1 receptor agonists can induce a dose-dependent tolerance and cross-tolerance to clozapine to some of its behavioral effects but not others.

Acute, but not repeated, administration of the neurotensin NTS1 receptor agonist PD149163 decreases conditioned footshock-induced ultrasonic vocalizations in rats

Neurotensin is an endogenous neuropeptide that has significant interactions with monoamine neurotransmitter systems. To date, neurotensin NTS1 receptor agonists, such as PD149163, have been primarily evaluated for the treatment for schizophrenia, drug addiction, and pain. Recently, PD149163 was found to attenuate fear-potentiated startle in rats, an experimental procedure used for screening anxiolytic drugs. The present study sought to assess these findings through testing PD149163 in a conditioned footshock-induced ultrasonic vocalization (USV) model. Conditioning was conducted in male Wistar rats using chambers equipped with shock grid floors and an ultrasonic vocalization detector. PD149163 and the 5-HT1A receptor partial agonist buspirone produced a statistically significant reduction of 22kHz USV counts. The typical antipsychotic haloperidol also reduced 22kHz USV counts, but did so at cataleptic doses. Ten days of repeated administration of PD149163 abolished the inhibitory effects of PD149163 on 22kHz USVs. These findings further support an anxiolytic profile for PD149163. However, tolerance to these effects may limit the utility of these drugs for the treatment of anxiety.

Effects of the neurotensin NTS₁ receptor agonist PD149163 on visual signal detection in rats

Antipsychotic drugs provide limited efficacy for cognitive impairment in schizophrenia. Recent studies have found that the neurotensin NTS1 receptor agonist and putative atypical antipsychotic drug PD149163 reverses deficits in sensory-gating and novel object recognition, suggesting that this compound may have the potential to improve cognitive functioning in schizophrenia. The present study sought to extend these investigations by evaluating the effects of PD149163 on sustained attention using a visual signal detection operant task in rats. PD149163, the atypical antipsychotic drug clozapine, and the dopamine D2/3 receptor antagonist raclopride all significantly decreased percent "hit" accuracy, while none of these compounds altered "correct rejections" (compared to vehicle control). Clozapine and raclopride significantly increased response latency, while high doses of PD149163 and raclopride significantly increased trial omissions. Nicotine, which was tested as a positive control, significantly improved overall performance in this task and did not affect response latency or trial omissions. The present findings suggest that neurotensin NTS1 receptor agonists, like antipsychotic drugs, may inhibit sustained attention in this task despite having different pharmacological mechanisms of action.

The role of NTS2 in the development of tolerance to NT69L in mouse models for hypothermia and thermal analgesia

NT69L is a neurotensin (NT)(8-13) analog that binds the two major NT receptors, NTS1 and NTS2, and elicits similar behavioral effects as endogenous NT. Tolerance develops rapidly to some, but not to all of NT69L's effects, and to date, little is known about the mechanisms responsible for this tolerance. The development of tolerance appears to be more prevalent in behavioral effects mediated by NTS1 than by those mediated by NTS2, including hypothermia and thermal analgesia. However, we hypothesize that both NTS1 and NTS2 have important roles in mediating the effects of NT69L. Here, we investigate the role of NTS2 on NT69L-mediated hypothermia and thermal analgesia with the use of NTS2 knock-out mice. We show that tolerance develops to NT69L-mediated hypothermia and thermal analgesia following sub-chronic treatment in wild-type (WT) mice, and that NTS2 is necessary for the development of that tolerance. Additionally, we suggest potential means by which NTS2 influences these NT69L-mediated behaviors.

The neurotensin-1 receptor agonist PD149163 inhibits conditioned avoidance responding without producing catalepsy in rats

Agonists for neurotensin (NT)-1 receptors have produced antipsychotic-like effects in many animals, including reversal of prepulse inhibition deficits and psychostimulant-induced increases in spontaneous activity. The present study sought to provide a basic assessment of the putative antipsychotic effects of PD149163 in rats using a two way conditioned avoidance response task, which is highly validated for screening antipsychotic drugs, and an inclined grid assessment, which is used to assess extrapyramidal side effect liability. PD149163 (0.0625-8.0 mg/kg) significantly suppressed conditioned avoidance responding (CAR) following administration of a 1.0 or 8.0 mg/kg dose. PD149163 failed to significantly increase catalepsy scores. The typical antipsychotic drug haloperidol (0.01-1.0 mg/kg) significantly suppressed CAR at a 0.1, 0.3, and 1.0 mg/kg dose, and a significant increase in catalepsy scores was found at the 1.0 mg/kg dose. The atypical antipsychotic drug clozapine (2.5-10.0 mg/kg) also produced a significant inhibition of CAR, which occurred following administration of a 10.0 mg/kg dose. Clozapine failed to significantly increase catalepsy scores. Finally, D-amphetamine (1.0 mg/kg), serving as a negative control, failed to suppress CAR or increase catalepsy scores. These data further suggest that PD149163 may have atypical antipsychotic-like properties.

Chronic NT69L potently prevents drug-induced disruption of prepulse inhibition without causing tolerance

NT69L is a neurotensin receptor agonist with antipsychotic-like activity. NT69L blocks apomorphine-induced climbing in rats with no effect on stereotypic behavior, attenuates d-amphetamine-induced hyperactivity, and blocks pharmacologically induced disruption of prepulse inhibition (PPI) of the startle response. Repeated administration of NT69L results in tolerance to some, but not to all of its effects. Because schizophrenic patients require long-term treatment, chronic (21-day) administration of NT69L was tested in PPI with comparisons to chronic haloperidol and clozapine treatment. Sprague-Dawley rats received acute or 21 daily, subcutaneous injections of NT69L (1.0mg/kg). On days 1 and 21 the NT69L injection was followed 30 min later by treatment with either saline; the dopamine agonist, d-amphetamine (5.0mg/kg); or the serotonin 5-HT(2A) psychotomimetic receptor agonist [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] DOI (0.5mg/kg). Experiments were repeated with either haloperidol (1mg/kg) or clozapine (20mg/kg) in place of NT69L. Acute injection of NT69L significantly blocked d-amphetamine and DOI disruption of PPI. As with the acute injection, 21 daily administrations of NT69L also blocked d-amphetamine- and DOI-induced disruption of PPI. The data show that animals do not develop tolerance to the antipsychotic-like effects of NT69L when tested in the PPI of the startle response. The persistent efficacy of NT69L with chronic treatment provides further support for the therapeutic use of neurotensin (NT) agonists to treat schizophrenia and possibly other disorders that are characterized by PPI deficits. The modulatory role of NT69L on the dopaminergic and serotonergic neurotransmission systems both of which are implicated in the pathophysiology of schizophrenia is discussed.

The reversal of amphetamine-induced locomotor activation by a selective neurotensin-1 receptor agonist does not exhibit tolerance

RATIONALE

Neurotensin-1 (NT1) receptor agonists have been proposed as putative antipsychotic drugs. Recently, brain-penetrating NT analogs produced by stability-enhancing modification of the smallest NT fragment, NT(8-13), have demonstrated antipsychotic-like efficacy after acute systemic injection in several preclinical animal tests predictive for antipsychotic efficacy. However, the evidence regarding the persistence versus tolerance of these effects after repeated administration is ambiguous. Previous studies have used compounds that nonselectively activated both NT1 and NT2 receptors or used continuous slow, central infusion of doses rather than daily acute administration, both factors which may have contributed to the ambiguity in the literature regarding the emergence of tolerance.

OBJECTIVES

To determine if tolerance develops to the antipsychotic-like effects of NT1 receptor agonists, we investigated the effects of subchronic daily systemic administration of PD149163, a brain-penetrating NT analog with selectivity for the NT1 receptor, on amphetamine-induced locomotor activation, a classic preclinical test of antipsychotic efficacy.

MATERIALS AND METHODS

Sprague-Dawley rats were pretreated with eight consecutive daily subcutaneous (SC) injections of PD149163 or saline. On the ninth day, rats received a pair of SC injections consisting of PD149163 or saline, followed by amphetamine (0.5 mg/kg) or saline. Locomotor activity was then measured in photobeam-equipped cages.

RESULTS

The results indicated that repeated daily administration of PD149163 was able to antagonize amphetamine's locomotor-activating effect comparable to that of the first dose, despite that repeated administration of PD149163 produced an increase in baseline locomotor activity not seen after the first dose.

CONCLUSIONS

The results do not support the development of tolerance for the acute antipsychotic-like effect of NT1 agonists and thus lend support to the contention that NT1 agonists are viable candidates as putative novel antipsychotic drugs.

Effects of chronic infusion of neurotensin and a neurotensin NT1 selective analogue PD149163 on amphetamine-induced hyperlocomotion

Neurotensin (NT) has been proposed as an endogenous antipsychotic based in part on the similarity in behavioural effects to antipsychotic drugs, for example, attenuation of both amphetamine-induced hyperlocomotion (AH) and amphetamine disrupted pre-pulse inhibition in the rat. However, there is some evidence that repeated administration of NT or an analogue produces behavioural tolerance to such effects. The present experiments sought to confirm and extend these findings by testing the effects on AH of 7 days central administration of NT and the NT1 selective analogue PD 149163 and the effects of 21 days central administration of NT. NT and PD149163 continuously administered for 7 days produced no effect on AH (in contrast to attenuation with a single injection here and previously reported), whereas 21 days of NT administration potentiated AH. Together, these studies report that the effects of NT or a NT analogue on AH depends on the duration of administration of peptide. The results are discussed in comparison with the reported antipsychotic properties of acute administration of NT and possible mechanisms involving NT1 receptors.

Selectively increased trace conditioning under the neurotensin agonist PD 149163 in an aversive procedure in which SR 142948A was without intrinsic effect

There is evidence to suggest that neurotensin (NT) may enhance cognitive function. The present study, therefore, examined the role of NT in associative learning between a conditioned stimulus (CS) and an unconditioned stimulus (UCS). This was tested in a trace procedure using conditioned suppression of drinking with a noise CS and foot shock UCS. We compared the effects of an NT agonist (PD 149163, 0.25 and 1 mg/kg) with those of an NT antagonist (SR 142948A, 0.01 and 0.1 mg/kg). Conditioning after drug treatment was followed by drug-free tests of associative learning. At 0.25 but not 1 mg/kg, PD 149163 selectively increased conditioning over the trace interval: there was no such increased conditioning in the 0s group. This increased conditioning over the trace is an effect that is reliably produced by dopamine (DA) agonists in the same procedure. However, dissimilar to the effects of DA agonists, conditioning to box context, was reduced under PD 149163. Doses of SR 142948A, selected on the basis of their effects in similar aversively motivated tests of latent inhibition, were without intrinsic effect in the present procedure. The dose-related dissociation between trace and contextual conditioning effects under PD 149163 is considered as cognitive enhancement.

The effects of chronic administration of established and putative antipsychotics on natural prepulse inhibition deficits in Brattleboro rats

We previously reported that vasopressin deficient Brattleboro (BRAT) rats exhibit deficits in prepulse inhibition (PPI) of the startle reflex that are consistent with PPI deficits exhibited by patients with schizophrenia and other neuropsychiatric disorders. Preliminary evidence indicates that this may be the basis of a predictive model for antipsychotic drug efficacy. Here we report the effects of acute and chronic administration of established and putative antipsychotics on these PPI deficits. BRAT rats, compared to their derivative strain, Long Evans rats, exhibited significantly decreased PPI and startle habituation consistent with patients with schizophrenia and other neuropsychiatric disorders. The second generation antipsychotics, risperidone and clozapine as well as a neurotensin agonist (PD149163) increased BRAT rat PPI, whereas saline, the typical antipsychotic, haloperidol, and a vasopressin analog (1-desamino-D-arginine vasopressin) did not. Similar to their effects in humans, chronic administration of antipsychotic drugs produced stronger effects than acute administration. These results further support the BRAT rat as a model of sensorimotor gating deficits with predictive validity for antipsychotics. The model appears to be able to differentiate first generation from second generation antipsychotics, identify putative antipsychotics with novel mechanisms (i.e., peptides) and reasonably model the therapeutic time course of antipsychotic drugs in humans.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Virally mediated increased neurotensin 1 receptor in the nucleus accumbens decreases behavioral effects of mesolimbic system activation

Dopamine receptor agonist and NMDA receptor antagonist activation of the mesolimbic dopamine system increases locomotion and disrupts prepulse inhibition of the acoustic startle response (PPI), paradigms frequently used to study both the pharmacology of antipsychotic drugs and drugs of abuse. In rats, virally mediated overexpression of the neurotensin 1 (NT1) receptor in the nucleus accumbens antagonized d-amphetamine- and dizocilpine-induced PPI disruption, hyperlocomotion, and D-amphetamine-induced rearing. The NT receptor antagonist SR 142948A [2-[[5-(2,6-dimethoxyphenyl)-1-(4-N-(3-dimethylaminopropyl)-N-methylcarbamoyl)-2-isopropylphenyl)-1H-pyrazole-3-carbonyl]amino] adamantane-2-carboxylic acid, hydrochloride] blocked inhibition of dizocilpine-induced hyperlocomotion mediated by overexpression of the NT1 receptor. Together, these results suggest that increased nucleus accumbens NT neurotransmission, via the NT1 receptor, can decrease the effects of activation of the mesolimbic dopamine system and disruption of the glutamatergic input from limbic cortices, resembling the action of the atypical antipsychotic drug clozapine. In contrast to clozapine, virally mediated overexpression of the NT1 receptor in the nucleus accumbens had prolonged protective effects (up to 4 weeks after viral injection) without perturbing baseline PPI and locomotor behaviors. These data further confirm the NT1 receptor as the receptor mediating the antistimulant- and antipsychotic-like properties of NT and provide rationale for the development of NT1 receptor agonists as novel antipsychotic drugs. In addition, the NT1 receptor vector might be a valuable tool for understanding the mechanism of action of antipsychotic drugs and drugs of abuse and may have potential therapeutic applications.

Neurotensin agonists: possible drugs for treatment of psychostimulant abuse

Although many neuropeptides have been implicated in the pathophysiology of psychostimulant abuse, the tridecapeptide neurotensin holds a prominent position in this field due to the compelling literature on this peptide and psychostimulants. These data strongly support the hypothesis that a neurotensin agonist will be clinically useful to treat the abuse of psychostimulants, including nicotine. This paper reviews the evidence for a role for neurotensin in stimulant abuse and for a neurotensin agonist for its treatment.