Pierre-Paul Rompré

Role of Calcium in Neurotensin-Evoked Enhancement in Firing in Mesencephalic Dopamine Neurons

Neurotensin (NT) increases neurotransmission within the mesolimbic dopamine system by enhancing the firing rate of dopaminergic (DAergic) neurons and by acting at the nerve terminal level. The signal transduction pathways involved in these effects have not been characterized, but NT receptors are coupled to the phospholipase C pathway and Ca2+ mobilization. However, an enhancement of intracellular Ca2+ concentration ([Ca2+]i) evoked by NT in DAergic neurons has yet to be demonstrated. Furthermore, the hypothesis that the excitatory effects of NT in DAergic neurons are Ca2+ dependant is currently untested. In whole-cell recording experiments, DAergic neurons in culture were identified by their selective ability to express a cell-specific green fluorescent protein reporter construct. These experiments confirmed that NT increases firing rate in cultured DAergic neurons. This effect was Ca2+ dependent because it was blocked by intracellular dialysis with BAPTA. Using Ca2+ imaging, we showed that NT caused a rapid increase in [Ca2+]i in DAergic neurons. Most of the Ca2+ originated from the extracellular medium. NT-induced excitation and Ca2+ influx were blocked by SR48692, an antagonist of the type 1 NT receptor. Blocking IP3 receptors using heparin prevented the excitatory effect of NT. Moreover, Zn2+ and SKF96365 both blocked the excitatory effect of NT, suggesting that nonselective cationic conductances are involved. Finally, although NT can also induce a rise in [Ca2+]i in astrocytes, we find that NT-evoked excitation of DAergic neurons can occur independently of astrocyte activation.

Evidence for a role of endogenous neurotensin in the initiation of amphetamine sensitization

This study was aimed at testing the hypothesis that endogenous neurotensin plays a role in the initiation of sensitization to the locomotor activating effect of amphetamine. During an initial training phase, different groups of male rats were injected on four occasions (every second day: Days 1, 3, 5 and 7) with one of three doses (40, 80 or 160 microg/kg, ip) of the neurotensin antagonist, SR-48692, or its vehicle, followed 30 min later by amphetamine (1.5 mg/kg, ip), or saline. Ambulatory, non-ambulatory, and vertical movements were measured for 2 h in photocell cages immediately following the second injection. One week after the training phase, sensitivity to amphetamine (0.75 mg/kg, ip) was tested in all the rats (sensitization test). The results show that SR-48692, when given alone, produced levels of locomotor activity that were not statistically different from control. At the low dose, it potentiated amphetamine-induced ambulatory and non-ambulatory movements, an effect observed on Day 7 but not on Day 1. On the day of the sensitization test, rats pre-exposed to amphetamine alone displayed stronger ambulatory and non-ambulatory movements than vehicle pre-exposed rats, a sensitization effect that was attenuated and prevented by SR-48692 at 80 and 160 microg/kg, respectively. The present results demonstrate that activation of neurotensin receptors by endogenous neurotensin is required for the initiation of amphetamine sensitization. They provide additional evidence that an increase in central neurotensinergic neurotransmission may lead to a lasting increased sensitivity to psychostimulant drugs.

Facilitation of brain stimulation reward by mesencephalic injections of neurotensin-(1–13)

The effects on brain stimulation reward of neurotensin-(1-13) microinjected at different concentrations (2.5, 5, 10 and 20 micrograms/0.5 microliters) into the ventral mesencephalic region containing mesocorticolimbic dopamine neurons were tested in 12 male rats. Neurotensin lowered the stimulation frequency required to sustain threshold levels of responding for brain stimulation reward, suggesting that this neuropeptide is involved in modulating the activity of dopamine neurons that mediate behaviors motivated by positive reinforces. The magnitude of the facilitatory effect of neurotensin on brain stimulation reward was dependent on the concentration injected and to a significant extent also on whether the peptide was administered in an ascending or a descending order of concentration. The different effects of neurotensin depending on the order of administration may suggest long-lasting effects on the responsiveness of neurotensin receptors in this region after injection of high concentrations of the peptide. Subsequent injection of morphine (2.5-5 micrograms/0.5 microliter) into the same site produced a weaker facilitation of brain stimulation reward than expected, suggesting that local damage after multiple central injections or prior injections of neurotensin itself reduced the responsiveness of dopamine neurons to opiates. Taken together, the results are consistent with data indicating that activation of neurotensin receptors in the ventral mesencephalon stimulates dopamine cell firing and axonal dopamine release in limbic terminal fields and suggest that endogenous neurotensin is involved in the control of behavior motivated by positive reinforcement.

Electrophysiological characteristics of neurons in forebrain regions implicated in self-stimulation of the medial forebrain bundle in the rat

In an attempt to identify neurons likely to play a role in self-stimulation of the medial forebrain bundle (MFB), action potentials of single neurons in the septum and basal forebrain of anesthetized rats were recorded by means of extracellular electrodes. Refractory period estimates were obtained from cells antidromically activated by stimulation of the lateral hypothalamus or ventral tegmental area, and estimates of interelectrode conduction time were obtained from cells that were driven by stimulation of both sites. The results show that some descending MFB axons arising in the medial septum, diagonal band of Broca and neighboring forebrain structures have characteristics comparable to properties of MFB reward neurons inferred from behavioral experiments.

Opioid-neuroleptic interaction in brainstem self-stimulation

Injection of morphine into the ventral tegmental area (but not dorsal to it) induced a dose-dependent decrease in the frequency threshold for midline metencephalic brain stimulation reward. Facilitating doses of ventral tegmental morphine also reversed, in 4 of 6 animals, the threshold-increasing effects of pimozide (0.35 mg/kg, i.p.). This reversal was itself reversed by naloxone (2 mg/kg, i.p.), suggesting a direct action of morphine at ventral tegmental opiate receptors. These data fit with electrophysiological evidence that ventral tegmental morphine stimulates or disinhibits dopamine impulse flow, which would result in increased synaptic dopamine concentrations and decreased synaptic pimozide effectiveness. In the remaining two animals, the combined neuroleptic-opiate treatment resulted in a complete cessation of responding that was not reversed by a 5-fold increase in stimulation frequency. This finding suggested a complete inactivation of the reward mechanism, which might be expected from the interaction of high doses of two drugs that are each known to be capable of producing depolarization inactivation of dopaminergic neurons. These data confirm that brainstem self-stimulation, like medial forebrain bundle self-stimulation, depends critically on the function of the mesocorticolimbic dopamine system.

Glutamate injection into the suprachiasmatic nucleus stimulates brown fat thermogenesis in the rat

Injection of glutamate (100 mM to 1 M, in 0.25 micrograms saline) into the hypothalamic suprachiasmatic nucleus (SCN) dose-dependently increased interscapular brown adipose tissue (IBAT) and core temperatures in the urethane-anaesthetized rat. This effect was more pronounced in rats tested during the light-off period than in animals tested during the light-on period. Prior injection of the local anaesthetic, procaine (5% in 0.5 microliter saline), into the ipsilateral ventromedial hypothalamic nucleus (VMH) attenuated the increases in IBAT and core temperatures induced by intra-SCN glutamate. The VMH has previously been implicated in the central regulation of BAT thermogenesis; the present results suggest the pathway arising in the SCN exerts an excitatory influence on VMH neurons involved in the control of BAT function.

Behavioral evidence for midbrain dopamine depolarization inactivation

High (2.5-5 micrograms) doses of ventral tegmental morphine, which normally facilitate brain stimulation reward, were found to cause a complete cessation of bar pressing for brainstem stimulation in animals pretreated with systemic pimozide (0.175-0.35 mg/kg). It was hypothesized that the behavioral failure was due to depolarization inactivation of the dopamine system. Electrophysiological evidence indicates that sufficient doses of morphine or neuroleptics can each cause inactivation by themselves. The behavior was reinstated by ventral tegmental muscimol, which normally suppresses both the behavior and dopamine cell firing but which reinstates dopamine cell firing in depolarization-inactivated cells. This behavioral reinstatement appears to confirm the hypothesis that depolarization inactivation of the dopamine system caused the behavioral failure, and appears to establish depolarization inactivation as a phenomenon of behavioral, and thus potential clinical, importance.

Repeated activation of neurotensin receptors sensitizes to the stimulant effect of amphetamine

Effects of repeated intracerebroventricular microinjections of 18 nmol/10 microl of neurotensin, [D-Tyr11]neurotensin, or saline were tested on motor activity in different groups of rats. One week after the fourth central injection, sensitivity to the behavioral stimulant effect of amphetamine (1 mg/kg, i.p.) was tested. As previously reported, neurotensin attenuated motor activity while [D-Tyr11]neurotensin when compared to saline produced an initial suppression followed by an excitation. Despite such different behavioral effects, both peptides produced sensitization to the stimulant effect of amphetamine. These results show that repeated activation of neurotensin receptors produces long-lasting changes in responsiveness to a psychostimulant drug.

Psychostimulant-like effect of central microinjection of neurotensin on brain stimulation reward

The curve shift method and the brain stimulation reward paradigm were used to dissociate reward and performance changes and determine whether unilateral ICV microinjection of neurotensin (3, 10, and 30 micrograms/10 microliters) produces neuroleptic- or psychostimulant-like effect on a dopamine-dependent behavior. At the highest dose tested, neurotensin potentiated brain stimulation reward, producing a significant time-dependent decrease in frequency threshold. Neurotensin also suppressed maximal rate of responding at every dose tested, suggesting that it was more effective at attenuating performance capability. These results suggest that a centrally acting neurotensin receptor agonist may specifically stimulate dopamine-dependent behaviors, producing psychostimulant-like effect that can be attenuated or masked by a suppression of performance capability.

Forebrain neurons driven by rewarding stimulation of the medial forebrain bundle in the rat: comparison of psychophysical and electrophysiological estimates of refractory periods

Psychophysically derived estimates of recovery from refractoriness were obtained at self-stimulation sites in the lateral hypothalamus and ventral tegmental area. The refractory periods of single units driven by the same stimulation electrodes and stimulation fields were then measured electrophysiologically. Antidromically driven units with refractory periods longer than those of the neurons responsible for the rewarding effect were concentrated in the septal complex. Units with refractory periods that overlapped the estimates for the reward-related neurons were found in this region as well but were also encountered in neighboring structures lateral, ventral, and/or caudal to the septal nuclei. It is argued that this latter class of units should be considered as possible constituents of the directly stimulated substrate for the rewarding effect because they are driven by rewarding stimulation, have refractory periods similar to those of the reward-related neurons and arise in or near regions in which lesions have been effective in decreasing the rewarding effect of stimulating the medial forebrain bundle.