Postnatal development of functional dopamine, opioid and tachykinin receptors that regulate acetylcholine release from rat neostriatal slices. Effect of 6-hydroxydopamine lesion

In the present work we have studied the postnatal development of functional dopamine, opioid and tachykinin receptors, which regulate cholinergic activity in the neostriatum. The release of endogenous acetylcholine from rat striatal slices was measured using a chemiluminescent method. We have observed that the inhibition mediated by dopamine through D₂ receptors was not detectable until postnatal day 10, whereas the inhibition mediated by opioid receptors was detectable at postnatal day 15 for δ-receptors ([D-Pen², D-Pen⁵]-enkephalin) and at postnatal day 21 for μ-receptors ([D-Ala², Gly(ol)⁵]-enkephalin). Excitatory effect mediated by tachykinins through NK₁ ([Sar⁹, Met(O2)¹¹]-Substance P), NK₂ ([Nle¹⁰]-Neurokinin A_4–10), or NK₃ (senktide) receptors was already detectable at postnatal day 5.

In order to examine the influence of dopamine in the development of tachykinin and opioid systems in the neostriatum, we induced dopamine deficiency by intraventricular injection of 6-hydroxydopamine at postnatal day 3. We observed an increase in senktide-evoked acetylcholine release at postnatal day 30. The effect produced by [Sar⁹, Met(O₂¹¹]-Substance P and [Nle¹⁰]-Neurokinin A_4–10 was not modified. Furthermore, at postnatal day 35, we could observed that the two opioid receptor agonists have no effect.

Our results show that dopamine, tachykinins and opioids are already able to mediate the modulation of acetylcholine release in early stages of development with a different pattern of postnatal development. Furthermore, the integrity of a dopaminergic system plays an important role in the functional development of the neostriatal cholinergic neurons which are differentially modulated by opioids or tachykinins.     

Tachykinins protect cholinergic neurons from quinolinic acid excitotoxicity in striatal cultures

The neuroprotective effect of tachykinins against excitotoxic death of cholinergic neurons was studied in rat striatal cell cultures. Quinolinic acid (QUIN) and kainic acid (KA) produced a dose dependent decrease in choline acetyttransferase activity, but KA was more potent. Our results show that substance P (SP) totally reversed the toxicity induced by 125 μM QUIN but not by 40 μM KA. This effect was also observed using protease inhibitors or a SP-analog resistant to degradation, [Sar⁹]-Substance P. The survival of neuron specific enolase- and acetylcholinesterase (AChE)-positive cells after treatment with QUIN alone or in the presence of SP was also examined. We observed that, while a decrease in total cell number produced by QUIN was not prevented by SP treatment, AChE-positive cells were rescued from the toxic damage. To characterize the SP protective effect we used more selective agonists of the three classes of neurokinin (NK) receptors. [Sar⁹, Met(O₂)¹¹]-Substance P (NK₁ receptor agonist), [Nle¹⁰]-Neurokinin A (NK₂ receptor agonist) or [Me-Phe⁷]-Neurokinin B (NK₃ receptor agonist) were all able to block the toxic effect of QUIN on cholinergic activity. These results show that tachykinins provide an important protective support for striatal neurons, suggesting a possible therapeutical benefit in neurodegenerative disorders affecting cholinergic neurons.     

Dopamine antagonists induce fos-like-immunoreactivity in the substantia nigra and entopeduncular nucleus of the rat

Injections of the D₂ receptor antagonists haloperidol (0.5–8 mg/kg) and metoclopramide (6.25–50 mg/kg) in rats resulted in a dose dependent induction of Fos-like-immunoreactivity in the rostral portion of the entopeduncular nucleus (EPN) and in the medial portion of the pars reticulata of the substantia nigra (SNpr). Nigral staining occurred exclusively in neurons which were not immunoreactive for tyrosine hydroxylase and could be antagonized by pretreatment with the anticholinergic drug scopolamine (3 mg/kg). Effects were much less pronounced following injections of the selective D1 antagonist SCH-23390 (2–8 mg/kg). No staining could be observed following administration of the 5HT₃ antagonist MDL-72222 (10 mg/kg) or the 5HT₁/5HT₂ antagonist metergoline (5 mg/kg), suggesting that the effects observed with dopamine antagonists were not secondary to actions at serotonin receptors. These results are consistant with the hypothesis that blockade of dopamine receptors results in a disinhibition of cells within the SNpr and EPN and further suggest that examination of immediate-early gene expression may provide a useful tool for studying the extrastriatal circuitry engaged by manipulations of dopaminergic transmission.     

Cardiovascular effects produced by microinjection of angiotensins and angiotensin antagonists into the ventrolateral medulla of freely moving rats

In this study we determined the cardiovascular effects produced by microinjection of angiotensin peptides [Angiotensin-(1–7) and Angiotensin II] and angiotensin antagonists (losartan, L-158,809, CGP 42112A, Sar¹-Thr⁸-Ang II, A-779) into the rostral ventrolateral medulla of freely moving rats. Microinjection of angiotensins (12.5–50 pmol) produced pressor responses associated to variable changes in heart rate, usually tachycardia. Unexpectedly, microinjection of both AT₁ and AT₂ ligands produced pressor effects at doses that did not change blood pressure in anesthetized rats. Conversely, microinjection of Sar¹-Thr⁸-Ang II and the selective Ang-(1–7) antagonist, A-779, produced a small but significant decrease in MAP an HR. These findings suggest that angiotensins can influence the tonic activity of vasomotor neurons at the RVLM. As previously observed in anesthetized rats, our results further suggest a role for endogenous Ang-(1–7) at the RVLM. The pressor activity of the ligands for AT₁ and AT₂ angiotensin receptor subtypes at the RVLM, remains to be clarified.     

Behavioural effects of selective tachykinin agonists in midbrain dopamine regions.

The effects of selective NK-1, NK-2 and NK-3 tachykinin agonists in midbrain dopamine cell containing regions were investigated in the rat. The NK-3 agonist senktide induced locomotion, rearing and sniffing following infusion into the substantia nigra pars compacta, and to a lesser extent in the ventral tegmental area. These behavioural responses were not seen following infusion of the selective NK-1 agonist [Sar9,Met (O2)11]SP or the NK-2 agonist [N1e10]NKA4-10. In contrast, grooming was induced only by the NK-1 agonist administered into the substantia nigra. Yawning, chewing mouth movements and wet dog shakes were all seen following infusion of senktide into the ventral tegmental area. These findings suggest that (i) dopamine-mediated behavioural responses seen following tachykinin administration into the midbrain are dependent upon stimulation of NK-3 tachykinin receptors, (ii) tachykinin-induced grooming is mediated by stimulation of NK-1 receptors and (iii) some of the previously described 5-HT mediated behaviours seen following administration of NK-3 tachykinin agonists are probably generated by stimulation of 5-HT cell bodies in the ventral tegmental area.     

Release of endogenous dopamine and cholecystokinin from rat striatal slices: effects of amphetamine and dopamine antagonists

The release of immunoreactive cholecystokinin (CCK) and dopamine was monitored simultaneously from superfused rat striatal slices. Exposure of the tissue to medium containing elevated of dopamine (10⁻⁷ and 10⁻⁶)M), the dopamine agonist pergolide (10⁻⁷, M), the D₂-antagonist sulpride (1 μM) or the D₁-antagonist (SCH 23390) had no significant effect on basal overflow or on evoked release of CCK. On the other hand, preincubation of striatal slices withd-amphetamine (10⁻⁵ M) enhanced basal and veratrine-stimulated dopamine release but markedly suppressed evoked CCK release. Sulpiride blocked this action of amphetamine whereas SCH 23390 was ineffective. The data suggests that whereas it is difficult to observe any effects of exogenous dopamine agonists or antagonists on evoked CCK release, endogenously released dopamine appears to interact with D₂-receptors to suppress evoked CCK release from rat striatal slices.     

The neuroleptic-like peptide desenkephalin-γ-endorphin does not antagonize the dopamine receptor agonist-induced inhibition of the release of [H]dopamine from rat nucleus accumbens slices in vitro

In rats, the non-opioid β-endorphin (βE) fragment desenkephalin-γ-endorphin (DEγE, βE_6–17) antagonizes the hypomotility induced by a small dose of dopamine (DA) receptor agonists. It has been suggested that DEγE might act in this respect by a direct or indirect blockade of presynaptically located DA receptors in the nucleus accumbens, thereby causing an increase of DA release. Therefore in the present study the effect of DEγE was examined on DA receptor agonist-induced inhibition of the electrically evoked release of previously accumulated [³H]DA from rat nucleus accumbens slices in vitro. The DA receptor agonists apomorphine, LY 171555 andn,n-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT) inhibited in a concentration-dependent manner the electrically evoked release of [³H]DA. The selective D₂ receptor antagonist (−)-sulpiride blocked the effects of apomorphine, corroborating that the DA receptor involved is of a D₂ type. DEγE was tested at several concentrations (10⁻⁹–10⁻⁶) and under various experimental conditions. DEγE, by itself, did not affect either the electrically stimulated or the basal release of [³H]DA. The inhibiting effect of DA receptor agonists was slightly reduced by DEγE, but this effect was present in some experiments only. It is concluded that DEγE does not function as an antagonist for the DA receptor mediating DA release and that the interaction observed in behavioural experiments between DA agonists and DEγE does not occur at the level of this receptor.     

Presynaptic D2 dopaminergic receptors mediate inhibition of excitatory synaptic transmission in rat neostriatum

The effect of dopamine (DA) on excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular- and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of DA (0.01–10 μM) reversibly decreases EPSP in a concentration-dependent manner and with a estimated IC₅ of 0.3 μM. In addition, the inhibitory effect induced by DA at a low concentratiion (0.1 μM) was antagonized by sulpiride (1–10 nM), a selective D₂ dopaminergic receptor antagonist. However, D₁ dopaminergic receptor antagonist SKF-83566 (1–5 μM) did not affect the blocking effect by DA 0.1 μM. Based on these findings, we conclude that DA at a low concentration ( 0.1 μM) reduced the excitatory response of neostriatal neurons following cortical stimulation via the activation of D₂, but not D₁ dopaminergic receptors, located on the terminals of corticostriatal neurons.     

Sequence of events in substance P-mediated plasma extravasation in rat skin

Using a blister model of inflammation in the rat hind footpad, we have studied the temporal and quantitative contribution of mast cell mediators and prostaglandins to substance P-induced plasma extravasation. In addition substance P-related peptides (neurokinin A, SP_5–11 and SP_1–7) were tested for their ability to induce a plasma extravasation response and the extent of histamine involvement to the response was determined. The present results show that the plasma extravasation response to substance P consists of an early substance P-mediated response that is independent of other mediators and a late response that involves interaction between substance P, mast cell mediators and prostaglandins. An early histamine-independent response was also mediated by neurokinin A, a tachykinin that shares a common C-terminal with substance P and by a C-terminally directed analogue of substance P, namely SP_5–11. On the other hand, a late histamine-dependent response was mediated by the N-terminally directed analogue, SP_1–7. The present data are suggestive of a possible sequence of events that might occur during an inflammatory response to substance P and might involve independent actions of its C- and N-terminal.     

Partial antagonism of 8-OH-DPAT'S effects on male rat sexual behavior with a D2, but not a 5-HT1A, antagonist

The serotonin agonist 8-hydroxy-di-propylaminotetralin (8-OH-DPAT), injected systemically or directly into the medial preoptic area (MPOA), reduces the ejaculatory threshold in male rats. While 8-OH-DPAT has been characterized as an agonist at the 5-HT_1A receptor, it also acts at other receptor sites including the dopamine D₂ receptor. The current experiments investigated whether 8-OH-DPAT injected into the MPOA facilitates male sexual behavior through stimulation of the 5-HT_1A receptor or the dopamine D₂ receptor. Experiment 1 co-administered 8-OH-DPAT (6 μg) with either the 5-HT_1A antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide hydrochloride (MPPI) (10 μg) or the D₂ antagonist raclopride (10 μg). Raclopride blocked 8-OH-DPAT's facilitative effects on ejaculation frequency and latency, while the 5-HT_1A antagonist was ineffective. In Experiment 2, 8-OH-DPAT (500 μM), retrodialyzed into the MPOA through a microdialysis probe, enhanced male copulatory behavior similarly to the microinjection, increasing ejaculation frequency and decreasing ejaculation latency, postejaculatory interval and mount frequency. Retrodialyzing 8-OH-DPAT through a microdialysis probe in the MPOA had been previously shown to increase extracellular levels of dopamine and serotonin. The data from the present studies suggest that the effects of 8-OH-DPAT in the MPOA on male rat copulatory behavior may be mediated, at least in part, either directly through 8-OH-DPAT's activity at D₂ receptors or indirectly through 8-OH-DPAT's ability to increase extracellular dopamine.     

Independent regulation of activation and inactivation phases in non-contractile Ca transients by nicotinic receptor at the mouse neuromuscular junction

Non-contractile Ca²⁺ mobilization (not accompanied by muscle contraction) occurs by the prolonged activation of nicotinic acetylcholine receptor in mouse diaphragm muscles treated with anticholinesterase. To elucidate the regulation properties of non-contractile Ca²⁺ mobilization by nicotinic receptor, the modes of action of competitive and depolarizing neurmuscular blockers were investigated. (+)-Tubocurarine (0.07–0.1 μM), pancuronium (0.05 μM) and -bungarotoxin (0.03–0.06 μM) decreased decay time (T₂, duration of inactivation phase) without changes in rise time (T₁, duration of activation phase) of non-contractile Ca²⁺ transients. These competitive antagonists also suppressed their peak amplitude at higher concentrations than those affectingT₂. Contractile Ca²⁺ transients were not inhibited by these antagonists at the concentrations used. Decamethonium (1 μM), a depolarizing blocker, suppressed the peak amplitude of non-contractile Ca²⁺ transients without affecting their duration. In contrast, succinylcholine (0.3 μM) suppressed both peak amplitude andT₁ without changingT₂, presumably via the receptor desentization. Succinylcholine but not decamthonium inhibited contractile Ca²⁺ transients at the concentrations used. These results demonstrate that the activation and inactivation phase in non-contractile Ca²⁺ transients are independently regulated by nicotinic acetylcholine receptor.     

Localization of striatal and nigral tachykinin receptors in the rat

The effects of lesioning mesostriatal dopamine projections or striatal neurons on tachykinin binding in the basal ganglia were assessed in the rat. 6-Hydroxydopamine lesions of the medial forebrain bundle destroyed striatal dopamine terminals as assessed by [³H]mazindol autoradiography, but did not significantly affect the binding of NK-1 ([³H][Sar⁹, Met(O₂)¹¹]substance P) or NK-3 ([³H]senktide) tachykinin ligands in the striatum. 6-Hydroxydopamine lesions significantly reduced NK-3 binding in the substantia nigra pars compacta, but not the ventral tegmental area. In contrast, striatal quinolinic acid lesions reduced both NK-1 and NK-3 binding in the striatum, but failed to affect NK-3 binding in the substantia nigra. These findings suggest that both NK-1 and NK-3 receptors within the striatum are predominantly post-synaptic with respect to dopamine neurons, whereas nigral NK-3 receptors are located on dopaminergic neurons.     

Dopamine D2 receptor-mediated G-protein activation in rat striatum: functional autoradiography and influence of unilateral 6-hydroxydopamine lesions of the substantia nigra.

Unilateral 6-hydroxydopamine (6-OHDA) lesions of substantia nigra pars compacta (SNPC) neurons in rats induce behavioural hypersensitivity to dopaminergic agonists. However, the role of specific dopamine receptors is unclear, and potential alterations in their transduction mechanisms remain to be evaluated. The present study addressed these issues employing the dopaminergic agonist, quinelorane, which efficaciously stimulated G-protein activation (as assessed by [³⁵S]GTPγS binding) at cloned hD₂ (and hD₃) receptors. At rat striatal membranes, dopamine stimulated [³⁵S]GTPγS binding by 1.9-fold over basal, but its actions were only partially reversed by the selective D₂/D₃ receptor antagonist, raclopride, indicating the involvement of other receptor subtypes. In contrast, quinelorane-induced stimulation (48% of the effect of dopamine) was abolished by raclopride, and by the D₂ receptor antagonist, L741,626. Further, novel antagonists selective for D₃ and D₄ receptors, S33084 and S18126, respectively, blocked the actions of quinelorane at concentrations corresponding to their affinities for D₂ receptors. Quinelorane potently induced contralateral rotation in unilaterally 6-OHDA-lesioned rats, an effect abolished by raclopride and L741,626, but not by D₃ and D₄ receptor-selective doses of S33084 and S18126, respectively. In functional ([³⁵S]GTPγS) autoradiography experiments, quinelorane stimulated G-protein activation in caudate putamen and, to a lesser extent, in nucleus accumbens and cingulate cortex of naive rats. In unilaterally SNPC-lesioned rats, quinelorane-induced G-protein activation in the caudate putamen on the non-lesioned side was similar to that seen in naive animals (50% stimulation), but significantly greater on the lesioned side (80%). This increase was both pharmacologically and regionally specific since it was reversed by raclopride, and was not observed in nucleus accumbens or cingulate cortex. In conclusion, the present data indicate that, in rat striatum, the actions of quinelorane are mediated primarily by D₂ receptors, and suggest that behavioural hypersensitivity to this agonist, induced by unilateral SNPC lesions, is associated with an increase in D₂, but not D₃ or D₄, receptor-mediated G-protein activation.     

Effects of Substance P on Medial Vestibular Nucleus Neurons in Guinea-pig Brainstem Slices

The undecapeptide substance P (SP) has been recently implicated in the control of vestibular function. In particular, it seems to be co-localized with glutamate in approximately half of the primary vestibular afferents in mammals. Using intracellular recordings in guinea-pig brainstem slices, we have investigated the effects of SP and of several agonists of the three known tachykinin receptor subtypes (NK₁, NK₂ and NK₃) on the three main types (A, B and B+LTS) of guinea-pig medial vestibular nucleus neurons (MVNn) that we had previously described. SP could induce two distinct kinds of effects on all types of MVNn. Whereas around half of them were depolarized and had their membrane resistance increased by SP, ∼ 10% of all MVNn were in contrast hyperpolarized and inhibited while their membrane resistance was decreased. Both responses persisted under conditions of blockade of synaptic transmission, and were thus due to the activation of postsynaptic binding sites. The SP-induced membrane depolarization could not be reproduced with any one of the specific agonists of the three tachykinin receptor subtypes, nor was it blocked by the specific NK₁ receptor antagonists GR 82334 and CP 99994. This effect might therefore be due to the activation of a new, pharmacologically distinct, 'NK₁-like' receptor. Only the hyperpolarizing effects, which were in contrast mimicked by the specific NK₁ receptor agonists GR 73632 and [Sar⁹, Met (O₂)¹¹]-SP, would be mediated by the few typical NK₁ receptors which have been demonstrated in the medial vestibular nucleus.     

Differential modulation of NMDA-stimulated [H]dopamine release from rat striatum by neuropeptide Y and σ receptor ligands

Although the identity of the endogenous ligands for sigma (σ) receptors is unknown, neuropeptide Y (NPY) has been named as a possible candidate for a natural transmitter at these receptors. Using a superfusion system, we compared the effect of NPY on NMDA-stimulated [³H]dopamine release in rat striatum to that of the σ agonists (+)-pentazocine and BD737. In contrast to (+)-pentazocine- or BD737-mediated inhibition of release, NPY enhanced release. However, the same σ antagonists (BD1008, DuP734, haloperidol and DTG) that reverse (+)-pentazocine- or BD737-mediated inhibition, as well as a Y receptor antagonist, PYX-1, all reversed the enhancement. PYX-1 also reversed the (+)-pentazocine- and BD737-mediated inhibition of release. Peptide YY (PYY) and [Leu³¹,Pro³⁴]NPY did not mimic the effect of NPY. NPY_13–36 enhanced release to the same extent as NPY but the effect was not reversed by σ antagonists. Our findings are consistent with the potential role of NPY as an endogenous ligand for a subtype of σ receptor with characteristics different from Y₁, Y₂ and Y₃ receptors but sensitive to PYX-1.     

Reduction in opioid and non-opioid forms of swim analgesia by 5-HT2 receptor antagonists

Acute exposure to continuous (CCWS) or intermittent (ICWS) cold-water swims elicits non-opioid and opioid forms of analgesia respectively. Intrathecal administration of methysergide blocks ICWS, but not CCWS analgesia. The present study evaluated the role of serotonin (5-HT) receptor subtypes in the mediation of CCWS and ICWS analgesia on the tail-flick and jump tests following administration of methysergide, a non-specific 5-HT antagonist and pirenpirone and ketanserin, two 5-HT₂ receptor subtype antagonists. Systemic methysergide was more effective in reducing CCWS analgesia (50–58%, 0.1–1.0 mg/kg) than ICWS analgesia (21%, 5 mg/kg) on both pain tests. Systemic pirenpirone (0.04–0.2 mg/kg) and ketanserin (1–5 mg/kg) were also more effective in reducing CCWS analgesia (43–57%) on both tests than ICWS analgesia (pirenpirone: 0.4 mg/kg, 34%; ketanserin: 5 mg/kg, 21%) on the tail-flick test. Indeed, both 5-HT₂ receptor antagonists potentiated ICWS analgesia on the jump test. While serotonin antagonist effects upon hypothermia could not account for CCWS analgesia effects, similar potentiations in ICWS analgesia and hypothermia were observed following pirenpirone and ketanserin. Finally, both 5-HT₂ receptor antagonists differentially reduced CCWS hypothermia and potentiated ICWS hypothermia. These data suggest differential serotonergic modulation of the two forms of swim analgesia with opioid-mediated ICWS analgesia acting through spinal 5-HT₁ receptors and non-opioid-mediated CCWS analgesia acting through supraspinal 5-HT₂ receptors.     

Sequential changes of dopaminergic receptors in the rat brain after 6-hydroxydopamine lesions of the medial forebrain bundle

We investigated the sequential patterns of changes in dopamine uptake sites, D₁ and D₂ receptors in the brain of animals lesioned with 6-hydroxydopamine using quantitative receptor autoradiography. The rats were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks postlesion. Degeneration of the nigrostriatal pathway caused a significant loss of dopamine uptake sites in the ipsilateral striatum, substantia nigra (SN) and ventral tegmental area (VTA) in the lesioned animals. Dopamine D₁ receptors were significantly increased in the ventromedial part of striatum of the ipsilateral side from 2 to 4 weeks postlesion. In the ipsilateral SN, a transient increase in dopamine D₁ receptors was observed only 1 week after lesioning. However, the frontal cortex, parietal cortex and dorsolateral part of the striatum showed no significant change in dopamine D₁ receptors throughout the experiments. On the other hand, dopamine D₂ receptors were decreased increased in the ipsilateral SN and VTA from 1 week to 8 weeks postlesion. In the ipsilateral striatum, dopamine D₂ receptors were increased in the dorsolateral part from 2 weeks to 8 weeks and in the ventromedial part from 2 weeks to 4 weeks. However, the frontal cortex and parietal cortex showed no significant change in dopamine D₂ receptors during postlesion. In the contralateral side, most of regions examined showed no significant change in dopamine uptake sites, dopamine D₁ receptors and dopamine D₂ receptors during postlesion except for a transient change in a few regions. These results demonstrate that 6-hydroxydopamine can cause a severe functional damage in dopamine uptake sites in the striatum, SN and VTA. Our findings also suggest that the up-regulation in dopamine D₂ receptors is more pronounced than that in dopamine D₁ receptors in the brain after 6-hydroxydopamine treatment. Furthermore, our results support the existence of dopamine D₂ receptors on the neurons of SN and VTA. Thus, our findings provide insights into the pathogenesis of Parkinson's disease.     

Rats bred for enhanced apomorphine susceptibility have elevated tyrosine hydroxylase mRNA and dopamine D2-receptor binding sites in nigrostriatal and tuberoinfundibular dopamine systems

From a Wistar population two rat lines were generated using as criterion the behavioral response to the dopamine agonist apomorphine. Rats of the apomorphine-susceptible (apo-sus) line revealed a vigorous gnawing response to apomorphine administration while the other rat line, the apomorphine-unsusceptible (apo-unsus) line, was selected for lack of response to the drug. In the present study using the 12th and 13th generation of these genetically selected lines, we have investigated whether this difference in apomorphine responsiveness was correlated with changes in dopamine neurochemistry. Therefore, we measured tyrosine hydroxylase (TH), the rate limiting enzyme in dopamine synthesis, as well as dopamine D₁ and D₂ receptor mRNA levels in discrete brain regions by in situ hybridization. Dopamine (D₂/D₃) receptor binding was assessed with [¹²⁵I]iodosulpride in a membrane binding assay and by quantitative autoradiography on tissue sections. [³H]SCH 23390 was used to analyze D₁ receptor binding. Apo-sus rats displayed significantly higher TH mRNA levels in the A₉ cell group of the substantia nigra pars compacta and in the A₁₂ cell group of the arcuate nucleus. No difference was found in the A₁₀ cell group of the VTA and the A₆ cell group of the locus coeruleus. The density ofD_2/3 binding sites as well as D₁ receptor mRNA levels in the striatal projection area of the A₉ substantia nigra neurons, were significantly elevated in apo-sus rats. Dopamine D₂ receptor mRNA and D₁ receptor binding levels in caudate putamen and nucleus accumbens, however, were similar in rats of both lines. In conclusion, high apomorphine susceptibility is related to a potentially enhanced dopamine responsiveness selective for the nigrostriatal and tuberoinfundibular pathways.     

D1 And D2 Receptors and circling behavior in rats with unilateral lesion of the entopeduncular nucleus

The role of D₁ and D₂ striatal dopamine receptors on circling behavior was studied in a normosensitive model obtained by unilateral kainic acid lesion of the entopeduncular nucleus. In this model, the sensitivity of striatal dopamine receptors was preserved, because kainic acid destroyed the neurons of the entopeduncular nucleus and left undamage the fibers of passage and axon terminals. Systemic administration of SKF 38393 to these animals fails to induce circling activity. In contrast, administration of quinpirole elicited rotation toward the lesioned side, which was increased by concurrent injection of SKF 38393. This behavior was inhibited by pretreatment with either a specific D₁ (SCH 23390) or D₂ (-sulpiride) antagonist. The apomorphine also induced ipsilateral circling that was abolished by pretreatment with D₁ or D₂ antagonists. The above results suggest that coactivation of both D₁ and D₂ striatal dopamine receptors are necessary to induce rotation in this normosensitive model.     

Effects of REM sleep deprivation on the d-amphetamine-induced self-mutilating behavior

It is well known that self-mutilating behavior (SMB) is developed in rats and humans during the daily treatment with d-amphetamine. Accordingly, in this work it was found that the daily treatment with 7.5 mg/kg d-amphetamine induced in rats a progressive appearance of SMB. Lower doses (5.0 mg/kg) were uneffective and higher doses (10 mg/kg) produced a pattern of SMB in which the mutilation induced at the beginning of the d-amphetamine administration disappears completely as the treatment progresses. Interestingly, it was also found that REM sleep deprivation (48 h) potentiated significantly the SMB induced by the daily administration of 7.5 mg/kg d-amphetamine, and to lesser extent, the SMB induced by the daily treatment with 10 mg/kg d-amphetamine. R(+)-SCH-23390 a D₁ dopamine (DA) receptor antagonist blocked completely or abolished the SMB induced by 7.5 mg/kg d-amphetamine in REM sleep deprived rats while (±)-sulpiride a D₂ DA receptor antagonist had only a partial blocking effect. Haloperidol a D₁/D₂ DA receptor antagonist behaved as a D₁ antagonist. Our results indicate that REM sleep deprivation enhances the SMB induced by the daily administration of d-amphetamine and suggest the involvement of D₁ DA receptors in the mechanism underlying the SMB. A role of REM sleep deprivation is also suggested in the appearance of self-mutilating episodes in d-amphetamine addicts.