Chronic, but not acute, dosing of antipsychotic drugs alters neurotensin binding in rat brain regions

A neurotensin agonist and antagonist decrease and increase activity, respectively, but do not preclude discrete cue conditioning

There is evidence to suggest that neurotensin (NT) may enhance cognitive function. For example, in aversive trace conditioning, the NT agonist PD149163 selectively increased trace conditioning (Grimond-Billa, et al., 2008). The present study, therefore, examined the role of NT in associative learning, tested using an appetitive trace conditioning procedure (0-s or 10-s inter-stimulus-interval [ISI]) with a mixed frequency noise as a conditioned stimulus (CS) and food delivery as the unconditioned stimulus (UCS). The effects of an NT agonist (PD149163, 0.125 and 0.25 mg/kg, Experiment 1) and an NT antagonist (SR142948A, 0.01 and 0.1 mg/kg, Experiment 2) were compared. To take nonspecific effects of these compounds into account, conditioning to the CS was measured as a percentage of total responding, during UCS deliveries and in the inter-trial-interval (ITI). In both experiments, associative learning to the contiguously (0-s) presented CS was demonstrated, although there was a relative reduction in this learning under 0.125 mg/kg PD149163. Counter to prediction, the only effect on trace conditioning was some overall reduction in responding to the CS in the 10-s group conditioned under 0.25 mg/kg PD149163. The NT antagonist was without any effect on appetitive conditioning. However, these NT compounds were not ineffective: decreases and increases in responding in the ITI, ISI and during UCS deliveries seen under PD149163 and SR142948A were dissociable from effects on discrete cue conditioning.

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.

Electron microscopic dual labeling of high-affinity neurotensin and dopamine D2 receptors in the rat nucleus accumbens shell

The dopamine D2 receptor (D2R) in the nucleus accumbens (NAc) shell is implicated in schizophrenia and in psychostimulant-induced drug-seeking behavior, both of which are affected by activation of the functionally opposed high-affinity neurotensin receptor (NTS1). To determine the functionally relevant sites, we examined the dual electron microscopic immunocytochemical localization of D2R and NTS1 in the NAc shell of rat brain. Immunolabeling for each receptor was seen in association with cytoplasmic organelles, or more rarely, on the plasma membrane of both axonal and somatodendritic profiles. Some of the axonal and many of the dendritic processes colocalized the two receptors. The dually labeled axon terminals often formed symmetric synapses or appositional contacts with unlabeled dendritic profiles. The morphology of these terminals suggests that they contain either inhibitory amino acids or dopamine. Other axonal profiles expressing exclusively NTS1 or D2R were without synaptic specializations or formed asymmetric, excitatory-type synapses mainly on unlabeled dendritic spines. In addition, however, several D2R-immunoreactive terminals were observed presynaptic to dendrites containing NTS1. The somatodendritic profiles immunolabeled for NTS1 and/or D2R had morphological features typical of inhibitory spiny projection neurons in the NAc. These results suggest that activation of NTS1 and D2R can dually modulate transmitter release from the same or separate phenotypically distinct axon terminals in the NAc shell. These presynaptic receptors as well as the postsynaptic NTS1 distribution in neurons that also contain or receive input from terminals containing D2R may mediate the opposing actions of neurotensin and dopamine in the NAc.

Central administration of the neurotensin receptor antagonist SR48692 attenuates vacuous chewing movements in a rodent model of tardive dyskinesia

Tardive dyskinesia is a movement disorder that develops in 20-30% of patients treated with chronic neuroleptics. Whilst the pathogenesis of tardive dyskinesia remains unclear, altered expression of neuropeptides in the basal ganglia has been implicated in its emergence. The peptide neurotensin is expressed in both dopamine D1 receptor-bearing neurons of the direct striatonigral pathway and dopamine D2 receptor-bearing neurons of the indirect striatopallidal pathway. Increased levels of striatal neurotensin messenger RNA (mRNA) are reported following chronic neuroleptic therapy. Chronic treatment with the typical antipsychotic haloperidol elicits neurotensin immunoreactivity in a large number of striatopallidal and a modest number of striatonigral projection neurons, whilst treatment with the potent dopamine releaser, methamphetamine, induces intense neurotensin immunoreactivity in striatonigral projection neurons. In order to determine whether increased levels of striatal neurotensin mRNA in the direct striatonigral or the indirect striatopallidal pathway play a more influential role in the development of tardive dyskinesia, we explored the effects of a specific neurotensin antagonist in a rodent model (vacuous chewing movements [VCMs] induced by chronic neuroleptics). Three groups of animals received injections of fluphenazine decanoate (25 mg/kg) or its vehicle sesame oil every 3 weeks for at least 18 weeks. They were then surgically implanted with bilateral guide cannulae aimed at the striatum, the substantia nigra pars reticulata, or the globus pallidus respectively. After recovery, animals were infused with 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-imethoxyphenyl)pyrazol-3-yl)carbonylamino]tricyclo(3.3.1.1.(3.7))decan-2-carboxylic acid (SR48692; 0.25, 0.50, and 1.0 nmol/microl), or its vehicle (10% dimethyl sulfoxide [DMSO] in saline) and observed for 60 min. Intra-striatal, intra-nigral or intra-pallidal infusion of SR48692 attenuated neuroleptic-induced VCMs. These findings lend further support to a role for neurotensin in the development of VCMs but do not clarify which pathway plays a more important role. Thus, treatments that reduce or prevent the effects of increased neurotensin expression and release may be useful in the management of tardive dyskinesia.