Autoradiographic localization of mu-opioid and neurotensin receptors within the mesolimbic dopamine system

Dopamine depletion produces augmented behavioral responses to a mu-, but not a delta-opioid receptor agonist in the nucleus accumbens: lack of a role for receptor upregulation

Microinjection of either mu- or delta-opioid agonists into the nucleus accumbens produces an increased locomotor activity, and when the dopaminergic innervation of the nucleus accumbens is bilaterally lesioned, the locomotor response to the microinjection of mixed mu- and delta-opioid agonists is augmented. To determine whether the lesion-induced augmentation to opioids is specific to mu- or delta-opioid receptor activation, dopamine innervation of the nucleus accumbens was lesioned with 6-hydroxydopamine (6-OHDA), and the motor stimulant response to intra-accumbens microinjection of the selective mu-opioid agonist, Tyr-D-Ala-Gly-mePhe-Gly-OH (DAMGO), was compared to that of the delta-opioid agonist, [D-penicillamine2,5]-enkephalin (DPDPE). The lesions caused a 95% depletion of tissue dopamine levels in the nucleus accumbens of the DAMGO-injected rats compared to sham-lesioned rats. Horizontal and vertical photocell counts were significantly increased in response to DAMGO in 6-OHDA-lesioned compared to the sham-lesioned rats. This behavioral augmentation was dose dependent and blocked by naloxone. In rats with similar accumbal dopamine depletions (94%), the locomotor response to DPDPE was not enhanced. The augmentation in the behavioral response to DAMGO was not associated with a change in the Bmax or Kd of [125I]DAMGO binding in nucleus accumbens homogenates from lesioned rats. Likewise, using quantitative receptor autoradiography, no difference between 6-OHDA- and sham-lesioned rats was observed in [125I]DAMGO or [125I]DPDPE binding. Therefore, the augmented behavioral response to opioids in the nucleus accumbens following dopamine depletion relies predominately on mu-opioid receptor stimulation. However, this augmentation is not mediated by an alteration in the number or affinity of these receptors.

Two types of neurone in the rat ventral tegmental area and their synaptic inputs

1. Intracellular recordings were made from 241 ventral tegmental neurones in slices of rat midbrain. Seventy-seven per cent of neurones were hyperpolarized by dopamine (principal cells); 16% were hyperpolarized by opioid peptides (secondary cells). 2. Most principal cells fired spontaneously (1-3 Hz) with a threshold of -53 mV; most secondary cells did not fire spontaneously. Action potentials of principal cells were longer (0.9 ms) than those of secondary cells (0.5 ms). 3. Focal electrical stimulation within the ventral tegmental area evoked a biphasic synaptic potential, depolarization followed by hyperpolarization, with a duration of about 200 ms. Experiments with receptor antagonists showed that the depolarizing component resulted from activation of both N-methyl-D-aspartate (NMDA) and non-NMDA receptors and the hyperpolarizing component resulted from activation of GABAA receptors. 4. A later hyperpolarizing synaptic potential developed after a latency of 50 ms, reached its peak in 250 ms and had a duration of about 1 s. It reversed polarity at -108 mV (external potassium concentration was 2.5 mM), was blocked by phaclofen (30 microM-1 mM) or 2-hydroxysaclofen (100-300 microM). In some cells, a phaclofen-resistant component remained that was increased by cocaine and blocked by sulpiride (1 microM). 5. It is concluded that the ventral tegmental area contains two types of neurone having properties similar to those in the substantia nigra. The cells receive synaptic inputs mediated by excitatory amino acids acting at NMDA and non-NMDA receptors, GABA acting at GABAA and GABAB receptors, and dopamine acting at D2 receptors.

Autoradiographic localization of gamma-aminobutyric acidA receptors within the ventral tegmental area

Destruction of intrinsic neurons in the ventral tegmental area (VTA) with the excitotoxin, quinolinic acid produced a significant decrease (80%) in [3H]muscimol binding to GABAA receptors within the parabrachial pigmented and paranigral nuclei of the VTA. Selective destruction of the dopaminergic neurons with 6-hydroxydopamine (6-OHDA) did not reduce [3H]muscimol binding within the VTA. However, the destruction of dopaminergic neurons did produce an increase (20%) in [3H]muscimol binding contralateral to the lesion, suggesting a reduction in the GABAergic innervation to this region. Additionally, destruction of the VTA afferents with quinolinic acid injections in the medial accumbens failed to produce alterations in [3H]muscimol binding within the VTA. These results are consistent with the predominant localization of GABAA receptors to non-dopaminergic neurons intrinsic to the VTA.

Lesion of dopamine mesolimbic neurons blocks behavioral effects induced by the endogenous enkephalins but not by a mu-opioid receptor agonist

Lesioning of dopamine neurons by injection of 6-hydroxydopamine into the nucleus accumbens blocked the increased rearing activity measured in the open-field and induced by injection into the ventral tegmental area of: [R)-3-(N-hydroxylcarboxamido-2-benzyl-propanoyl)-L-alanine), kelatorphan (complete inhibitor of enkephalin catabolism) or by (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)): BUBU (selective delta agonist) but not the hypolocomotion evoked by the mu agonist (Tyr-D-Ala-Gly-N(Me)-Phe-Glyol): DAMGO. This suggests the involvement of different neuronal pathways in mu and delta effects.

Bilateral modulation of [3H]neurotensin binding by unilateral intrastriatal 6-hydroxydopamine injections: evidence from a receptor autoradiographic study

The present study was carried out in order to re-evaluate the issue of the localization of neurotensin receptors in the caudate-putamen and in the nucleus accumbens of rat. Intrastriatal injections of 6-hydroxydopamine which cause almost complete destruction of the mesostriatal dopaminergic pathway also caused a marked loss of neurotensin receptors in the caudate-putamen (CPu), the nucleus accumbens (NAc) and in the olfactory tubercle (OT). These decreases corresponded to a mean loss of 98, 93 and 41% in the CPu, the NAc, and the OT, respectively. There were corresponding decreases in the substantia nigra pars compacta (SNpc) (-94%) and pars reticulata (SNpr) (-97%), and in the ventral tegmental area (-78%). Moreover, there were also decreases in neurotensin receptors on the contralateral side of the intrastriatal injections which occurred in the CPu but not in the NAc nor in the OT. These results indicate that almost all NT receptors measured within the CPu and the NAc are located on the terminals of dopaminergic neurons within those structures. The bilaterality of the changes which occur in the CPu provide further support for the notion of the interdependence of the two nigrostriatal dopaminergic projections and the peptidergic systems with which they interact.

Dopamine transmission in the initiation and expression of drug- and stress-induced sensitization of motor activity

Progress has been made over the last 10 years in determining the neural mechanisms of sensitization induced by amphetamine-like psychostimulants, opioids and stressors. Changes in dopamine transmission in axon terminal fields such as the nucleus accumbens appear to underlie the expression of sensitization, but the actions of drugs and stressors in the somatodendritic regions of the A10/A9 dopamine neurons seem critical for the initiation of sensitization. Manipulations that increase somatodendritic dopamine release and permit the stimulation of D1 dopamine receptors in this region induce changes in the dopamine system that lead to the development of long-term sensitization. However, it is not known exactly how the changes in the A10/A9 region are encoded to permit augmented dopamine transmission in the terminal field. One possibility is that the dopamine neurons of sensitized animals have become increasingly sensitive to excitatory pharmacological and environmental stimuli or desensitized to inhibitory regulation. Alternatively, changes in cellular activity or protein synthesis may result in a change in the presynaptic regulation of axon terminal dopamine release.

GABAA receptors containing alpha 1 and beta 2 subunits are mainly localized on neurons in the ventral pallidum

The gamma-aminobutyric acid (GABA) projection from the nucleus accumbens to the ventral pallidum (VP) is important in the regulation of locomotion. Thus, stimulation and inhibition of GABAA receptors in the VP can alter locomotor activity. To determine whether the GABAA receptors are located presynaptically on accumbens efferents to the VP or postsynaptically on neurons intrinsic to the VP two experiments were performed. In the first, quinolinic acid lesions of the nucleus accumbens did not alter [3H]muscimol binding in the VP, while lesions in the VP significantly reduced (60-80%) binding as measured by light microscopic receptor autoradiography. In the second experiment, in situ hybridization with oligonucleotide probes for mRNAs of the alpha 1 and beta 2 subunits of the GABAA receptor was examined in the nucleus accumbens and VP. No mRNA for either subunit was observed in the nucleus accumbens, although many positively labeled neurons were present within the VP. By contrast, a moderate to high density of cells in both the nucleus accumbens and VP contained mRNA for glutamic acid decarboxylase. These data argue that the majority of GABAA receptors in the VP are not located presynaptically on axonal terminals originating from neurons in the nucleus accumbens.

Neurotensin regulation of endogenous acetylcholine release from rat striatal slices is independent of dopaminergic tone

The effects of neurotensin (NT) alone or in combination with the dopamine antagonist sulpiride were tested on the release of endogenous acetylcholine (ACh) from striatal slices. NT enhanced potassium (25 mM)-evoked ACh release from striatal slices in a dose-dependent manner. This effect was tetrodotoxin-insensitive, suggesting an action directly on cholinergic elements. The dopamine antagonist sulpiride (5 x 10(-5) M) significantly increased (63%) potassium-evoked ACh release from striatal slices; potassium-evoked ACh release was further increased (90%) in the presence of NT (10(-5) M) and sulpiride (5 x 10(-5) M). The second set of experiments tested the effects of 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra on NT-induced increases of potassium-evoked ACh release. These lesions did not alter the NT regulation of potassium-evoked ACh release from striatal slices, but did significantly increase spontaneous (33%) and potassium-evoked (40%) ACh release from striatal slices. Striatal choline acetyltransferase activity was not affected by 6-OHDA lesions. In addition, following 6-OHDA lesions, sulpiride was ineffective in altering ACh release from striatal slices. Furthermore, evoked ACh release in the presence of the combination of NT and sulpiride was not different from that in the presence of NT alone. These results suggest that in the rat striatum, NT regulates cholinergic interneuron activity by interacting with NT receptors associated with cholinergic elements. Moreover, the NT modulation of cholinergic activity is independent of either an interaction of NT with D2 dopamine receptors or the sustained release of dopamine.

Neurotensin receptor binding levels in basal ganglia are not altered in Huntington's chorea or schizophrenia

Autoradiographic techniques were used to examine the distribution and levels of neurotensin receptor binding sites in the basal ganglia and related regions of the human brain. Monoiodo ([125I]-Tyr3)neurotensin was used as a ligand. High amounts of neurotensin receptor binding sites were found in the substantia nigra pars compacta. Lower but significant quantities of neurotensin receptor binding sites characterized the caudate, putamen, and nucleus accumbens, while very low quantities were seen in both medial and lateral segments of the globus pallidus. In Huntington's chorea, the levels of neurotensin receptor binding sites were found to be comparable to those of control cases. Only slight but not statistically significant decreases in amounts of receptor binding sites were detected in the dorsal part of the head and in the body of caudate nucleus. No alterations in the levels of neurotensin receptor binding sites were observed in the substantia nigra pars compacta and reticulata. These results suggest that a large proportion of neurotensin receptor binding sites in the basal ganglia are located on intrinsic neurons and on extrinsic afferent fibers that do not degenerate in Huntington's disease.

Effects of local delta and mu opioid receptor activation on basal and stimulated dopamine release in striatum and nucleus accumbens of rat: an in vivo electrochemical study

The magnitude and duration of spontaneous and of potassium-stimulated dopamine release were electrochemically measured in striatum and nucleus accumbens of chloral hydrate-anesthetized rats following [D-Pen2-D-Pen5]enkephalin, a delta opioid receptor agonist, or [Tyr-D-Ala-MePhe-Gly-ol], a mu opioid receptor agonist, microinjected directly into the voltammetric recording sites. The data show that delta receptor activation potentiated potassium-stimulated dopamine efflux in striatum and in nucleus accumbens but had no effect on spontaneous dopamine release in either region, whereas mu receptor activation produced unreliable effects in both regions, either having no effect or inhibiting stimulated dopamine efflux without affecting basal levels of extracellular dopamine in either region. The data suggest that some delta opioid receptors in the caudate-putamen and in the nucleus accumbens presynaptically enhance impulse-dependent dopamine release from nigrostriatal and mesolimbic dopamine terminals.

Opioid delta agonists and endogenous enkephalins induce different emotional reactivity than mu agonists after injection in the rat ventral tegmental area

The possible role of opioid receptor heterogeneity in the biphasic changes in locomotion (activation and inhibition) induced by non-selective opiates such as morphine, has been investigated by measuring the behaviour of rats exposed to different environments after injection into the ventral tegmental area, of selective mu (DAGO) or delta (DTLET, DSTBULET, BUBU) opioid agonists and of kelatorphan, a complete inhibitor of enkephalin metabolism. delta agonists or kelatorphan-induced hyperactivity in a familiar (actimeter), unfamiliar (four-hole box) and a fear inducing (open-field) environment. These effects were suppressed by naloxone and delta selective antagonists (ICI 174, 864 2 mg/kg SC, naltrindole 7 nmol in the ventral tegmental area). Moreover, the delta agonists and endogenous enkephalins protected by kelatorphan did not affect the emotional state of rats measured in an elevated plus maze. Infused into the ventral tegmental area, DAGO also enhanced locomotion in the actimeter but in contrast to delta agonists and kelatorphan, the mu agonist decreased activity in the open-field and the four-hole box. The hypoactivity observed in these tests could be related to an enhanced emotionality produced by mu receptor stimulation, as shown by the significant decrease in the number of visits and time spent in open arms of the elevated plus maze. Naloxone (0.3 mg/kg SC) but not delta selective antagonists, blocked the various responses induced by DAGO.

Neurotensin and cholecystokinin microinjected into the ventral tegmental area modulate microdialysate concentrations of dopamine and metabolites in the posterior nucleus accumbens

Neurotensin and cholecystokinin, neuropeptides which coexist with dopamine in many ventral tegmental neurons, were microinjected into the ventral tegmental area during in vivo microdialysis in the posterior nucleus accumbens. Neurotensin significantly elevated concentrations of dopamine and its metabolites at doses of 10 pmol, 1 nmol, and 10 nmol, while cholecystokinin significantly elevated dopamine metabolite concentrations only at a dose of 10 nmol. These data suggest that neurotensin potently mediates the release of dopamine from the mesolimbic pathway via direct actions on the cell body.

Neurotensin binding to dopamine

Rotating disk electrode voltammetry at glassy carbon electrodes and ultraviolet/visible spectroscopy were used to demonstrate that dopamine binds to neurotensin with a dissociation constant of 7.5 x 10(-8). By measuring the binding constants of various neurotensin analogs, it was found that the -Arg8-Arg9-portion of the neurotensin sequence is critical for binding dopamine. Neurotensin also formed a complex with 4-ethylcatechol, 4-methylcatechol, 3-methoxytyramine, and norepinephrine. Although changes in the side chain did not alter the binding constant, methoxylation of the catechol moiety significantly increased the dissociation constant. These data along with additional studies of dopamine interactions with arginine derivatives suggest that the guanidino groups of arginine and the catechol hydroxyls of dopamine are responsible for mediating the observed binding. It is hypothesized that the capacity of neurotensin to bind directly to dopamine may be partly responsible for its previously observed antagonism of dopamine-induced locomotor activity.

Changes in gamma-aminobutyric acid, mu-opioid and neurotensin receptors in the accumbens-pallidal projection after discrete quinolinic acid lesions in the nucleus accumbens

Discrete quinolinic acid lesions in the nucleus accumbens altered [3H]muscimol binding to gamma-aminobutyric acid receptors, [125I]neurotensin binding to neurotensin receptors, [125I]Tyr-D-Ala-Gly-NMePHe-Gly-OH binding to mu-opioid receptors, and [3H]quinuclidinyl benzilate binding to muscarinic receptors. Within lesions of the lateral accumbens core, [3H]muscimol binding increased and [125I]Tyr-D-Ala-Gly-NMePhe-Gly-OH, [125I]neurotensin and [3H]quinuclidinyl benzilate binding decreased. Lesions of the medial nucleus accumbens resulted in decreased [125I]Tyr-D-Ala-Gly-NMePhe-Gly-OH and [3H]quinuclidinyl benzilate binding while no alterations were observed for [3H]muscimol or [125I]neurotensin binding. These data support anatomical distinctions between medial and lateral nucleus accumbens. Destruction of intrinsic neurons in the dorsomedial nucleus accumbens core increased [3H]muscimol binding in the dorsal rim of the ventral pallidum and the rostral globus pallidus without altering [125I]Tyr-D-Ala-Gly-NMePhe-Gly-OH binding. Destruction of neurons in the lateral nucleus accumbens core or medial shell did not alter [3H]muscimol binding in the ventral pallidum. The lack of upregulation in gamma-aminobutyric acid receptors suggests that the gamma-aminobutyric acid-containing projection from the dorsomedial core to the dorsal rim of the ventral pallidum differs from the projection from the lateral accumbens core and medial shell to the more ventral regions of the pallidum. Fluoro-gold retrograde tracer histochemistry confirmed the specific projection from the dorsomedial core to the dorsal ventral pallidum; and from the shell of the nucleus accumbens to more ventral regions of the ventral pallidum.