Evidence that the stimulatory effect of neurotensin on dopamine release in rat nucleus accumbens slices is independent of dopamine D2-receptor activation

Gut-brain peptides in corticostriatal-limbic circuitry and alcohol use disorders

Peptides synthesized in endocrine cells in the gastrointestinal tract and neurons are traditionally considered regulators of metabolism, energy intake, and appetite. However, recent work has demonstrated that many of these peptides act on corticostriatal-limbic circuitry and, in turn, regulate addictive behaviors. Given that alcohol is a source of energy and an addictive substance, it is not surprising that increasing evidence supports a role for gut-brain peptides specifically in alcohol use disorders (AUD). In this review, we discuss the effects of several gut-brain peptides on alcohol-related behaviors and the potential mechanisms by which these gut-brain peptides may interfere with alcohol-induced changes in corticostriatal-limbic circuitry. This review provides a summary of current knowledge on gut-brain peptides focusing on five peptides: neurotensin, glucagon-like peptide 1, ghrelin, substance P, and neuropeptide Y. Our review will be helpful to develop novel therapeutic targets for AUD.

Neurotensin depolarizes globus pallidus neurons in rats via neurotensin type-1 receptor

The globus pallidus is a major component in the indirect pathway of the basal ganglia. There is evidence that neurotensin receptors exist in this nucleus. To determine the electrophysiological effects of neurotensin on pallidal neurons, whole-cell patch-clamp recordings were performed in the acutely prepared brain slices. Under current-clamp recordings, neurotensin at 1 microM depolarized pallidal neurons. Voltage-clamp recordings also showed an inward current induced by neurotensin. The depolarizing effect of neurotensin could be mimicked by the C-terminal fragment, neurotensin (8-13), but not by the N-terminal fragment, neurotensin (1-8). Both SR 142948A, a non-selective neurotensin receptor type-1 and type-2 antagonist, and SR 48692, a selective type-1 receptor antagonist, blocked the depolarizing effect of neurotensin, and which themselves had no effect on membrane potential. Thus, neurotensin type-1 receptors appear to mediate the effect of neurotensin. The depolarization evoked by neurotensin persisted in the presence of tetrodotoxin, ionotropic and metabotropic glutamate and GABA receptor antagonists, indicating that neurotensin excited the pallidal neurons by activating the receptor expressed on the neurons recorded. Current-voltage relationship revealed that both the suppression of a potassium conductance and the activation of a cationic conductance are involved in the neurotensin-induced depolarization. Based on the action of neurotensin in the globus pallidus we hypothesize that alterations of the striatopallidal neurotensin system contribute to symptoms of basal ganglia motor disorders.

Differences between electrically-, ritalin- and D-amphetamine-stimulated release of [3H]dopamine from brain slices suggest impaired vesicular storage of dopamine in an animal model of Attention-Deficit Hyperactivity Disorder

The spontaneously hypertensive rat (SHR) has behavioural characteristics which make it a suitable animal model for Attention-Deficit Hyperactivity Disorder (ADHD). The drugs of choice in the treatment of ADHD are methylphenidate and D-amphetamine. Using an in vitro superfusion system, we showed that both drugs released [3H]dopamine (DA) (and metabolites) from prefrontal cortex, nucleus accumbens and caudate-putamen slices, but methylphenidate was from 7- to 17-fold less potent than D-amphetamine. The similarity in the drug effects on SHR and WKY [3H]DA release is in accordance with the fact that there is no 'paradoxical effect' of psychomotor stimulants on ADHD behaviour. Methylphenidate released significantly less [3H]DA from nucleus accumbens slices obtained from SHR than from their normotensive Wistar-Kyoto (WKY) controls. Electrical stimulation released less [3H]DA from prefrontal cortex and caudate-putamen slices of SHR, while D-amphetamine, in contrast to methylphenidate, released more [3H]DA from prefrontal cortex, nucleus accumbens and caudate-putamen slices of SHR compared to WKY. Inhibition of the DA uptake carrier by low concentrations of methylphenidate increased the electrically-stimulated release of [3H]DA to the same extent in SHR and WKY tissue, suggesting that the DA transporter was not responsible for the differences between SHR and WKY. The present results suggest that SHR may have impaired vesicular storage of DA causing leakage of DA into the cytoplasm, since SHR released less [3H]DA from vesicular stores in response to methylphenidate or electrical stimulation and released more [3H]DA from cytoplasmic stores via the uptake carrier in response to D-amphetamine. Methylphenidate might be the drug of choice in the treatment of ADHD because it releases DA from vesicular stores only and is less potent than D-amphetamine, thus making it possible to adjust the dose and thereby 'normalise' reduced DA function more precisely than is possible with D-amphetamine. There was no difference between SHR and WKY with respect to D-amphetamine-stimulated release of [14C]acetylcholine (ACh) or methylphenidate-induced inhibition of the electrically-stimulated release of [14C]ACh from nucleus accumbens or caudate-putamen slices, suggesting that there is no major change in cholinergic transmission in SHR.

Differential distribution, affinity and plasticity of dopamine D-1 and D-2 receptors in the target sites of the mesolimbic system in an animal model of ADHD

The distribution of dopamine (DA) D-1 and D-2 receptors has been studied by autoradiography in the anterior forebrain of the pre-hypertensive spontaneously hypertensive rat (SHR) as an animal model of attention-deficit hyperactivity disorder (ADHD) in children. Juvenile male SHR and Wistar Kyoto (WKY) controls were given either vehicle or the DA re-uptake blocker methylphenidate (MP; 3 mg/kg, i.p.), daily during a 2-week period. A saturation analysis for the D-1 receptor subfamily was carried out with 0.1-5.0 nM of [3H]SCH23390 and two competition studies for the D-2 receptor subfamily with 4 nM of [3H]raclopride or 5 nM of [3H]quinpirole were carried out with unlabelled spiperone and 7-OH-DPAT as unlabelled displacers on cryostat coronal sections of the anterior forebrain. Quantitative receptor autoradiography and computer-assisted image analysis with reference to co-exposed 3H-microscale standards showed in vehicle-treated SHR higher density of DA D-1/D-5 receptor subtypes in the caudate-putamen (CPU), the nucleus accumbens (ACB) core and shell and the olfactory tubercle (OT), which was associated to a lower affinity. MP treatment normalised the DA D-1/D-5 receptors by decreasing the number of binding sites and increasing the affinity to control level. In addition, MP treatment 'down-regulated' DA D-2/D-4 subtypes in the CPU, ACB and OT, and 'up-regulated' mostly D-3 subtype in CPU, ACB, OT in both rat lines and in the globus pallidus, ventral pallidum and lateral septum in WKY rats only. In contrast, D-3 receptors were 'down-regulated' in the islands of Calleja in both rat lines. Moreover, regional cross-correlative analyses revealed a modulatory influence of DA receptors in the cross-talk within the anterior forebrain, which was altered in the SHR. Thus, the differential distribution and regulation of DA receptor subtypes following DA re-uptake blocker as well as the different regional cross-talk in the target sites of nigrostriatal and mesolimbic DA systems lend support to the DA hypothesis of ADHD in children.

Kindled seizures do not affect adenosinergic inhibition of DA or ACh release in rat accumbens or PFC

Epileptic seizures are thought to terminate largely as a result of the extracellular accumulation of the purinergic neuromodulator, adenosine, released by discharging neurons. However, the postictal surge in extracellular adenosine and its widespread inhibitory effects are limited in time to only a few minutes and cannot directly account for increased resistance to seizures and the complex behavioural and motivational effects that may persist for hours or days after a seizure. The present study examined whether kindled seizures might alter the sensitivity or efficacy of inhibitory presynaptic adenosine receptors, and thereby induce more enduring changes in downstream transmitter systems. Rats were kindled in the amygdala of the dominant cerebral hemisphere, contralateral to the preferred direction of rotation, and their brains were removed either 2 h or 28 days after completion of kindling. Inhibition of electrically stimulated release of dopamine (DA) and acetylcholine (ACh) by the A1 adenosine-receptor agonist, R-phenylisopropyladenosine (R-PIA) was then measured in the prefrontal cortex (PFC) and nucleus accumbens. R-PIA (1.0 microM) inhibited [1H]DA release from PFC and nucleus accumbens tissue, and [14C]ACh release from nucleus accumbens tissue, but release was unaffected by prior kindling, regardless of the intervening interval. These results do not support suggestions that DA or ACh might mediate the effects of seizure-induced changes in purinergic inhibitory tone so as to cause long-term shifts in seizure threshold and postictal behavior.

Neurotensin, N-acetyl-aspartylglutamate and beta-endorphin modulate [3H]dopamine release from guinea pig nucleus accumbens, prefrontal cortex and caudate-putamen

Dopaminergic hyperactivity in nucleus accumbens and dopaminergic hypoactivity in prefrontal cortex are thought to underlie positive and negative symptoms of schizophrenia, respectively. The caudate putamen is the neuroanatomical substrate for extrapyramidal side effects resulting from chronic antipsychotic treatment. We sought to identify potential endogenous regulators of dopamine release that might produce differential effects in these brain areas. We tested neurotensin, N-acetyl-aspartyl-glutamate and beta-endorphin for potential regulation of [3H]dopamine release in these regions of guinea pig brain. All three peptides stimulated dopamine release, above basal activity, at all concentrations tested in the three regions. Neurotensin significantly enhanced and N-acetyl-aspartyl-glutamate had no significant effect on N-methyl-D-aspartate-stimulated release from all three regions. In contrast, beta-endorphin significantly inhibited N-methyl-D-aspartate-stimulated release in nucleus accumbens and caudate putamen. These results suggest that these neuropeptides may regulate endogenous dopamine release and therefore may be potential therapeutic targets for antipsychotic drug development.

Receptor-receptor interactions and their relevance for receptor diversity. Focus on neuropeptide/dopamine interactions

Receptor diversity in combination with receptor-receptor subtype specific interactions, which can be antagonistic or synergistic in character, markedly increase plasticity in WT and VT in the nervous system. In this way switching among transmission lines for the various DA receptor subtypes becomes possible. Some of these aspects are supported by our work on selective modulation of D2 receptors by CCK and NT. Selective regulation of D2 receptors via CCK-8 receptor subtypes and NT receptors may underlie CCK/DA interactions and NT/DA interactions in the basal ganglia. These studies underline the importance of receptor-receptor interactions exerted at the membrane level between neuropeptide receptors and D2 receptors, which are determined at least in part by the ongoing activity at D1 receptors. In the case of both CCK/D2 and NT/D2 receptor interactions, it has been possible, by means of intrastriatal and intraaccumbens microdialysis, to obtain a functional correlate to the receptor interactions found in the membrane preparations from the striatum. Schizophrenia may be in part related to reduced release of CCK and/or NT peptides or to alterations in their receptor interactions with the D2 receptor. This view may lead to new therapeutic approaches.

Altered dopaminergic function in the prefrontal cortex, nucleus accumbens and caudate-putamen of an animal model of attention-deficit hyperactivity disorder--the spontaneously hypertensive rat

The spontaneously hypertensive rat (SHR) has been proposed as an animal model for Attention-Deficit Hyperactivity Disorder (ADHD). The behavioural problems of ADHD have been suggested to be secondary to altered reinforcement mechanisms resulting from dysfunction of the mesolimbic and mesocortical dopaminergic systems. The present study therefore investigated whether there are regional differences in dopamine (DA) and acetylcholine (ACh) release and DA D2-receptor function in SHR compared to their normotensive Wistar-Kyoto (WKY) controls. The DA D2-receptor agonist, quinpirole, caused significantly greater inhibition of DA release from caudate-putamen but not from nucleus accumbens or prefrontal cortex slices of SHR relative to WKY. DA D2-receptor blockade by the antagonist, sulpiride, caused a significantly greater increase in DA release from nucleus accumbens slices of SHR compared to WKY suggesting increased efficacy of DA autoreceptors at low endogenous agonist concentrations in the nucleus accumbens of SHR. The electrically-stimulated release of DA was significantly lower in caudate-putamen and prefrontal cortex slices of SHR than in slices of WKY. This could be attributed to increased autoreceptor-mediated inhibition of DA release in caudate-putamen slices but not in the prefrontal cortex. No difference was observed between SHR and WKY with respect to DA D2-receptor-mediated inhibition of ACh release from caudate-putamen or nucleus accumbens slices, suggesting that postsynaptic DA D2-receptor function is not altered in SHR relative to WKY.

Regional effects of neurotensin on the electrically stimulated release of [3H]dopamine and [14C]acetylcholine in the rat nucleus accumbens

This study has shown that neurotensin (NT) increases the electrically stimulated release of [3H]DA to a similar extent in all but the extreme caudolateral area of the rat nucleus accumbens and appears to modulate DA release equally in the medial and lateral zones of this brain area. The simultaneous release of ACh was not significantly affected by NT.

Muscarinic antagonists attenuate neurotensin-stimulated accumbens and striatal dopamine metabolism

The effect of scopolamine and atropine upon the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by central injection of neurotensin was examined in the nucleus accumbens and the striatum of anaesthetized rats using in vivo differential pulse voltammetry with carbon fibre electrodes. Scopolamine (1 and 3 mg/kg, i.p.) and atropine (20 micrograms, i.c.v.) did not alter the 3,4-dihydroxyphenylacetic acid level in the nucleus accumbens or the striatum, measured for 60 min after administration. Neurotensin (10 micrograms, i.c.v.) increased the 3,4-dihydroxyphenylacetic acid peak height in both regions. Pretreatment with scopolamine (1 mg/kg) 15 min before neurotensin injection blocked the increase in extracellular 3,4-dihydroxyphenylacetic acid in the striatum but not in the nucleus accumbens whilst scopolamine (3 mg/kg) partially attenuated the effect of neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid in the striatum. Atropine partially attenuated the effect produced by neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid induced by the peptide in the striatum. However, the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by haloperidol (1 mg/kg, s.c.) was not altered by scopolamine (1 mg/kg) or atropine. Also, the increase in dopamine metabolism in the nucleus accumbens and the striatum after centrally injected haloperidol (10 micrograms, i.c.v.) was not altered by atropine (20 micrograms, i.c.v.). Together, the results demonstrate a functional interaction between muscarinic antagonists and neurotensin on in vivo dopamine metabolism in the nucleus accumbens and the striatum but with a greater effect in the latter region.