Biochemical changes accompanying unilateral 6-hydroxydopamine lesions in the rat substantia nigra

A synaptic mechanism underlying the behavioral abnormalities induced by manganese intoxication

In the present study we have characterized a rat model of manganese (Mn) intoxication leading to behavioral disinhibition in the absence of major motor alterations. These behavioral changes were associated with significantly increased brain Mn levels but were uncoupled to anatomical lesions of the striatum or to morphological and cytochemical changes of the nigrostriatal dopaminergic pathway. The analysis of this model at cellular level showed an enhanced dopaminergic inhibitory control of the corticostriatal excitatory transmission via presynaptic D2-like dopamine (DA) receptors in slices obtained from Mn-treated rats. Conversely, the use of agonists acting on presynaptic purinergic, muscarinic, and glutamatergic metabotropic receptors revealed a normal sensitivity. Moreover, membrane responses recorded from single dopaminergic neurons following activation of D2 DA autoreceptors were also unchanged following Mn intoxication. Thus, our findings indicate a selective involvement of the D2-like DA receptors located on glutamatergic corticostriatal terminals in this pathological condition and suggest that the behavioral symptoms described in the "early" clinical phase of manganism may be caused by an abnormal dopaminergic inhibitory control on corticostriatal inputs. The identification of the synaptic mechanism underlying the "early" phase of Mn intoxication might have a critical importance to understand the causes of the progression of this pathological condition towards an "established" phase characterized by motor abnormalities and anatomical lesions of the basal ganglia.

Increases in [3H]FK-506 and [3H]L-N(G)-nitro-arginine binding in the rat brain after nigrostriatal dopaminergic denervation

Receptor autoradiographic technique was studied to investigate sequential changes in FK-506 binding proteins, nitric oxide synthase and dopamine uptake sites in the brain 1 week to 8 weeks after unilateral 6-hydroxydopamine injection of the medial forebrain bundle in rats. [3H]FK-506, [3H]L-N(G)-nitro-arginine and [3H]mazindol were used to label FK-506 binding proteins (immunophilin), nitric oxide synthase and dopamine uptake sites, respectively. [3H]FK-506 binding showed about 13-25% increase in the ipsilateral striatum from 2 to 8 weeks after degeneration of nigrostriatal pathway. However, no significant change in [3H]FK-506 binding was observed in the ipsilateral substantia nigra during the postlesion periods. In the contralateral side, [3H]FK-506 binding also showed about 13-25% increase in the striatum from 2 to 8 weeks postlesion. The substantia nigra showed a 21% increase in [3H]FK-506 binding only 2 weeks after the lesioning. On the other hand, [3H]L-N(G)-nitro-arginine binding showed about 21-31% increase in the parietal cortex and striatum 1 week or 2 weeks postlesion. In the contralateral side, a 21% increase in [3H]L-N(G)-nitro-arginine binding was found in the dorsolateral striatum only 1 week postlesion. In contrast, degeneration of nigrostriatal pathway caused a conspicuous loss of [3H]mazindol binding in the ipsilateral striatum (87-96%), substantia nigra (36-73%) and ventral tegmental area (91-100%) during the postlesion periods. In the contralateral side, no significant changes in [3H]mazindol binding were observed in these areas up to 8 weeks after the postlesion. The present study demonstrates that unilateral injection of 6-hydroxydopamine into the medial forebrain bundle of rats can cause a significant increase in [3H]FK-506 and [3H]L-N(G)-nitro-arginine bindings in the brains. In contrast, a marked reduction in [3H]mazindol binding is observed in the brains after the lesioning, indicating severe damage to nigrostriatal dopaminergic pathway. These results suggest that immunophilin and nitric oxide synthase may play some role in the pathogenesis of neurodegenerative disorders such as Parkinson's disease.

Infusions of tyrosine hydroxylase antisense oligodeoxynucleotide into substantia nigra of the rat: effects on tyrosine hydroxylase mRNA and protein content, striatal dopamine release and behaviour

Modulation of the transcriptional message of tyrosine hydroxylase was investigated in vivo in the rat nigrostriatal dopamine system with unmodified antisense oligodeoxynucleotide, mismatch oligodeoxynucleotide or vehicle controls. Oligodeoxynucleotide was infused (0.5 microgram/0.5 microliter/h) unilaterally into the substantia nigra by an osmotic minipump system over 14 days. The presence of oligodeoxynucleotide in the brain was verified by in situ hybridization and fluorescence labelling. Animals treated with unmodified antisense oligodeoxynucleotide showed ipsilateral turning behaviour when challenged systemically with the indirect dopamine agonist amphetamine, whereas mismatch- and vehicle-infused rats showed no such behavioural asymmetries. In the substantia nigra, antisense treatment had no effects on tyrosine hydroxylase mRNA, but it led to a reduction in tyrosine hydroxylase protein content. Tissue levels of dopamine, measured in postmortem tissue punches of the neostriatum and substantia nigra, were reduced in the oligodeoxynucleotide-treated hemisphere. Furthermore, basal extracellular levels of dopamine, monitored by in vivo microdialysis, were also lower in the neostriatum ipsilateral to antisense infusion and showed a weaker response to an amphetamine challenge when compared with the contralateral side. These effects were not observed after infusion of mismatch oligodeoxynucleotide or vehicle into the substantia nigra. Finally, the GABAergic enzyme glutamate decarboxylase was not affected in the antisense-treated substantia nigra, indicating that non-specific damage in this area was not caused by this treatment. Our results indicate that antisense oligodeoxynucleotide treatment against tyrosine hydroxylase in the substantia nigra has behavioural and neurochemical effects that are comparable with known actions of dopamine neurotoxins, which are conventional pharmacological tools for the depletion of dopamine. Furthermore, our data show the potential of antisense targetting to reveal new relationships between neurotransmitter-related enzymes and behavioural parameters, because the possibility of selectively and discretely manipulating tyrosine hydroxylase function is likely to produce new insights into the physiological and behavioural functions of the dopaminergic nigrostriatal system.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Synaptic reorganisation in the rat striatum after dopaminergic deafferentation: an ultrastructural study using glutamate decarboxylase immunocytochemistry

The ultrastructure of GABAergic and non-GABAergic synapses in the adult rat neostriatum was examined 6-8 months after unilateral removal of the nigrostriatal dopaminergic pathway by 6-hydroxydopamine injection into the medial forebrain bundle. GABAergic profiles were identified by preembedding glutamate decarboxylase (GAD) immunocytochemistry performed on parasagittal vibratome sections. In three representative fields of the striatum, the nature and number of boutons and their postsynaptic partners were determined and the differences between the striata ipsi- and contralateral to the lesion analyzed. The percentage of GAD-immunoreactive boutons was increased from 23% on the intact side to 28% on the lesioned side. In addition, the GABAergic boutons underwent significantly more multiple contacts with several independent postsynaptic profiles, preferentially with dendritic spines. This could reflect a lesion-induced sprouting of local GABAergic axon terminals. On the other hand, although the vast majority of GABAergic boutons underwent synaptic contacts with dendrites (77% vs. 80%), the number of boutons per dendrite or per dendritic circumference remained unchanged. Thus, the higher frequency of GABAergic boutons may simply reflect the loss of the dopaminergic nerve endings, without a heterosynaptic replacement by GABAergic boutons. The deafferentation also induced structural changes of the postsynaptic profiles. Some dendritic spines had a shortened neck; others were completely integrated in the dendrite which now contained a spine apparatus and was contacted by boutons with the features of axospinous synapses. The spine retraction resulted in a quantitative decrease in the number of spines. Analysis of the synaptic curvature revealed that only spines with a flat contact zone were lost. Concurrently, the number of dendrites was increased, of the GAD-containing in particular, suggesting that the denrites of GABAergic interneurons tend to elongate and/or arborize. Taken together, the results of the present study show that the dopaminergic denervation caused a remodeling of the postsynaptic neurons. The relative increase of the number of GABAergic boutons and their synaptic contacts suggests that an altered wiring of the intrinsic GABAergic system contributes to the changes in the striatal output activity.

Reduction in cortical 5-HT3 binding sites following a unilateral 6-hydroxydopamine lesion of the medial forebrain bundle in rats

Serotonergic mechanisms have been implicated in pathophysiology of Parkinson's disease, an illness where the dopamine deficiency represents the prime biochemical deficit. Present interest centres on the possible involvement of serotonergic receptors in modulating dopamine transmission. In this paper the binding of the selective 5-HT3 antagonist [3H]GR 65630 was studied in rats with a unilateral 6-hydroxydopamine lesion of the medial forebrain bundle. The maximal density of specific [3H]GR 65630 binding was reduced in homogenates of entorhinal (17.1%, P < 0.05) and prefrontal cortex (27.5%, P < 0.05) on the lesioned side of the rat brain compared to the control tissues. An increase in affinity for [3H]GR 65630 binding was also found in homogenates of prefrontal cortex (33.8%, P < 0.05). No changes in the characteristics of [3H]GR 65630 binding to homogenates from the amygdala and hippocampus were observed. These data suggest that altered dopamine function may affect serotonergic mechanisms in the cortex in Parkinson's disease.

Opposite effects of intranigral ibotenic acid and 6-hydroxydopamine on motor behavior and on striatal neuropeptide Y neurons

Unilateral lesions of the basal ganglia circuit induce a disequilibrium of motor processing, most obviously expressed by the resulting circling behavior. Compensatory events, which reduce the motor asymmetry, could be accompanied by changes in neurotransmitter/modulator parameters in the involved brain regions. In the present investigation, the effects of an interruption of the striato-nigro-thalamic loop by ibotenic acid (IBO)-induced lesions of total substantia nigra (SN) on circling behavior and on striatal neuropeptide Y (NPY) neurons were compared with those after the selective destruction of the dopaminergic nigrostriatal projection with 6-hydroxydopamine (6-OHDA). Directly after the operation, IBO-lesioned rats showed a high circling rate to the side contralateral to the lesion, whereas 6-OHDA-lesioned rats showed ipsiversive circling. With the lesion-induced development of dopamine receptor supersensitivity, 6-OHDA-treated rats, when stimulated with the dopaminergic agonist apomorphine, change their circling direction to the contralateral side. Complete IBO lesions of the SN abolished this effect: rats continued to circle to the contralateral side. These observations suggest that not only the dopaminergic denervation of the striatum but also the imbalance in the activity of the thalamo-cortical projection (reduced after 6-OHDA, augmented after IBO) are instrumental in determining the degree and direction of circling. Quantification of NPY-immunoreactive neurons in striatum revealed a decrease in 6-OHDA lesioned rats after 3 days on the side contralateral to the lesion, an effect even more pronounced after 4 month's survival time. IBO-induced lesions of the SN had an opposite effect on NPY-immunoreactivity in the striatum: neuron counts were lower on the ipsi- than on the contralateral side. In addition, a time-dependent variation in total number of NPY-neurons was noted: during the early postoperative periods an increase, followed by a prolonged decrease to values below 50% of the controls after 4 months. Taken together, these results provide evidence that a dopaminergic deafferentation and its consequences on the nigro-thalamo-cortical loop will determine NPY expression in the striatal interneurons. In particular, it is suggested that the number of striatal NPY-neurons and the imbalance in cortical activity are tightly coupled in terms of a negative correlation.

Comparative effects of electroconvulsive shock and haloperidol on in vivo tyrosine hydroxylation and tetrahydrobiopterin in the brain of rats with 6-hydroxydopamine lesions

We have evaluated the effects of electroconvulsive shock (ECS) and haloperidol treatment on the in vivo tyrosine hydroxylation rate and the tetrahydrobiopterin levels in the nigrostriatal system of 6-OHDA-lesioned rats. The rate of DOPA accumulation was significantly decreased by 96% in the ipsilateral striatum and by 50% in substantia nigra of the 6-OHDA-lesioned rats compared to the control activity of contralateral non-lesioned striatum and substantia nigra. The loss of total biopterin was found to be 75% and 50% in the ipsilateral striatum and substantia nigra, respectively. Following administration of haloperidol, the rate of DOPA accumulation increased significantly in the striatum and substantia nigra on the lesioned side compared to that in the vehicle treatment group. Application of ECS also significantly increased the rate of DOPA accumulation in the ipsilateral striatum and substantia nigra compared to that obtained in the non-shocked rats. The biopterin levels in the nigrostriatal system of 6-OHDA-lesioned were elevated significantly in the striatum after haloperidol treatment; in contrast the biopterin levels were unchanged in response to ECS. Our results show that both haloperidol and ECS significantly enhanced the rate of in vivo tyrosine hydroxylation in the striatum and substantia nigra of rats with greater than 90% lesions. These results suggest that the nigrostriatal system, although up-regulated following 6-OHDA lesions, still maintains the potential for further up-regulation of dopaminergic function in response to haloperidol and ECS treatment.

Lack of change in striatal DARPP-32 levels following nigrostriatal dopaminergic lesions in animals and in parkinsonian syndromes in man

The present study was performed to determine the effect of a nearly complete nigrostriatal dopaminergic denervation on DARPP-32 levels in the striatum from animals and parkinsonian patients. DARPP-32 levels were estimated by in vitro phosphorylation in the presence of cAMP, or after inactivation of endogenous kinases and phosphatases, in the presence of the catalytic subunit of cAMP-dependent protein kinase. Intranigral 6-hydroxydopamine (6-OHDA) infusion in rats, or peripheral administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to common marmosets, did not change striatal DARPP-32 levels. Postmortem studies, carried out on brains obtained shortly after death, from patients with Parkinson disease, or from patients with progressive supranuclear palsy, showed that the levels of striatal DARPP-32 were not different from controls. These results indicate that dopaminergic striatal denervation did not modify the amount of DARPP-32 in the striatum, suggesting that the expression of DARPP-32, a protein which mediates some of the effects of dopamine in striatal neurons, is independent from the dopaminergic innervation.

Association of dopamine D1 and D2 receptors with specific cellular elements in the basal ganglia of the cat: the uneven topography of dopamine receptors in the striatum is determined by intrinsic striatal cells, not nigrostriatal axons

To ascertain the cellular associations of the D1 and D2 dopamine receptor subtypes in components of the basal ganglia, cats were prepared with unilateral, axon-sparing, ibotenic acid lesions of the striatum (n = 6) or lesions of the nigrostriatal dopamine system by intranigral infusion of 6-hydroxydopamine (n = 8). After 42 days survival, tissue sections from the brains were processed for quantitative, in vitro receptor autoradiography with [3H]SCH23390 (D1 radioligand) or [3H]spiroperidol (D2 radioligand). Lesion-induced changes in basal ganglia nuclei were assessed by comparing them to the corresponding nuclei on the intact side and in naive brains. Ibotenate lesions cause a decline in specific D1 and D2 receptor-binding in the area of the striatal lesion of 94% and 85%, respectively, and completely eliminate the uneven patterns of high- and low-density binding that are characteristic of the cat's caudate nucleus. The globus pallidus, entopeduncular nucleus and pars reticulata of the substantia nigra also show marked reductions in binding after striatal ibotenate lesions. Thus, after caudate nucleus lesions, D2 binding in the two pallidal segments declines by approximately 50%, but remains unchanged in the substantia nigra. Binding of the D1 radioligand (which is not measurable in the globus pallidus) declines by about 75% in the affected regions of the entopeduncular nucleus and pars reticulata, and by about 30% in the pars compacta. Lesions of the nigral dopamine neurons reduce D2 receptor-binding by 95% in the pars compacta and 40% in the pars reticulata, but have no effect on the concentration of D1 or D2 radioligand-binding in the striatum or pallidum. Moreover, such lesions failed to alter the uneven patterns of binding in the striatum. These data suggest that most, if not all, D1 receptors in the basal ganglia are associated with cells of the striatum and their axons in the entopeduncular nucleus and substantia nigra, and likewise, a large majority of D2 receptors are associated with striatal cells and their axons in pallidal structures. Nearly all D2 receptors in the substantia nigra are associated with dopamine neurons (autoreceptors). Finally, the heterogeneous patterns of D1 and D2 receptors in the striatum are a consequence of intrinsic neuronal distributions.

Distribution of D1 and D2 dopamine receptors in the basal ganglia of the cat determined by quantitative autoradiography

The patterns of dopamine D1 and D2 receptors were examined in the corpus striatum and related structures in the cat brain by quantitative autoradiography after in vitro radioligand binding with [3H]SCH23390 (D1 antagonist) and [3H]spiperone (D2 antagonist). Highly specific binding for both radioligands occurs in striatal structures known to receive dopaminergic input: the caudate nucleus, putamen, nucleus accumbens, and olfactory tubercle. However, the density of binding varies from one structure to another, and the density distribution within striatal nuclei is heterogeneous. In all but one portion of the striatum, the concentration of bound D1 radioligand ranges from 46 to 230% more than that of the D2 radioligand. The exception to this difference occurs at caudal putamenal levels where the two radioligands bind in equal concentrations (approximately equal to 220 fmol/mg tissue wet-weight). The highest density of both D1 and D2 radioligand binding occurs in irregular zones in the head and body of the caudate nucleus. Such high-density zones of D2 radioligand binding appear mainly in the dorsolateral part of the caudate's head. For the D1 radioligand, the high-density zones are more widespread throughout the caudate nucleus, nucleus accumbens, and putamen. The D2 radioligand binding (but not the D1) also exhibits low-density zones at more caudal levels of the caudate nucleus, and these are often in register with the high-density zones of D1 radioligand binding. In the putamen, inverted concentration gradients exist for the two radioligands: the [3H]SCH23390 gradient runs from higher levels rostrally to lower levels caudally. The lowest levels of bound [3H]spiperone in the striatum occur in the nucleus accumbens-olfactory tubercle area, whereas the lowest binding of [3H]SCH23390 occurs in the caudal putamen. Pallidal and nigral structures show marked disparities in binding of the two different radioligands. The D2 radioligand binding in the globus pallidus (80 +/- 8 fmol/mg tissue wet-weight) is about twice that in the entopedunuclear nucleus and pars reticulata of the substantia nigra, the latter two having equal levels (35 +/- 3 fmol/mg). No specific binding of the D2 radioligand occurs in the ventral pallidum. In contrast, D1 radioligand binding is highest in the entopeduncular nucleus (217 +/- 6 fmol/mg) and in the pars reticulata of the substantia nigra (198 +/- 2 fmol/mg) and moderate in the ventral pallidum (135 +/- 15 fmol/mg). In the globus pallidus, no detectable D1 radioligand binding occurs.(ABSTRACT TRUNCATED AT 400 WORDS)

Opposing roles of dopamine D1 and D2 receptors in nigral gamma-[3H]aminobutyric acid release?

This study examined the effects of dopamine D1 and D2 receptor agonists and antagonists on the spontaneous and calcium-dependent, K+-induced release of gamma-[3H]aminobutyric acid [( 3H]GABA) accumulated by slices of rat substantia nigra. SKF 38393 (D1 agonist) and dopamine (dual D1/D2 agonist) were without effect on [3H]GABA efflux by themselves (1-40 microM), or in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (0.5 mM), but potentiated evoked release in the presence of forskolin (0.5 microM), an adenylate cyclase activator. These increases in release were prevented by the D1 antagonist SCH 23390 (0.5 microM), but not by the D2 antagonist metoclopramide (0.5 microM). Higher concentrations of forskolin (10-40 microM) augmented stimulus-evoked [3H]GABA release directly, whereas dibutyryl cyclic AMP (100-200 microM) depressed it. Apomorphine, noradrenaline, and 5-hydroxytryptamine (1-40 microM) had no effect. The D2 stimulants lisuride, RU 24213, LY 171555, and bromocriptine dose-dependently inhibited depolarisation-induced but not basal [3H]GABA outflow. These inhibitory responses were not modified by the additional presence of SKF 38393 (10 microM) or SCH 23390 (1 microM), or by injection of 6-hydroxydopamine into the medial forebrain bundle 42 days earlier, but were attenuated by metoclopramide (0.5 microM). Higher amounts (10 microM) of SCH 23390, metoclopramide, or other D2 antagonists (loxapine, haloperidol) reduced evoked GABA release by themselves, probably by nonspecific mechanisms. These results suggest D1 and D2 receptors may have opposing effects on nigral GABA output and could explain the variable effects of mixed D1/D2 dopaminomimetics in earlier release and electrophysiological experiments.

Catecholaminergic mechanisms of feeding-related lateral hypothalamic activity in the monkey

The functional role of the catecholaminergic mechanism in the lateral hypothalamus (LHA), in feeding behavior of the monkey was investigated by single neuron activity recording and electrophoretic application of dopamine (DA), noradrenaline (NA) and their antagonists. The feeding paradigm had 4 phases: cue light (CL) signaled start of bar press; bar press (BP, 20-30 times); short cue tone (CT) triggered by last bar press signaled presentation of food; and ingestion-reward (RW). Of 312 neurons tested, 189 (61%) responded in one or more phases of the feeding task. Two types of response were observed: CL- or CT-related transient, and BP- or RW-related long-lasting responses. These feeding-related responses depended on the nature of the food and on the hunger-satiety level. DA excited or inhibited different neurons, while NA mainly inhibited firing. DA-sensitive neurons responded more often in the feeding task than insensitive neurons due mainly to differences in responsiveness to CL on (chi 2 test, P less than 0.01), at motor initiation, and during BP (P less than 0.05). Spiperone blocked the former two responses. NA-sensitive neurons responded more often in the feeding task due to responsiveness during BP and RW (P less than 0.01). Sotalol blocked some BP-related responses, and phenoxybenzamine and sotalol blocked the CT-related responses. The data suggest that dopaminergic and noradrenergic inputs in the LHA are crucial in task initiation and reward processing, respectively. Integration of these catecholaminergic and other inputs in the LHA might be important in accomplishing motivated feeding.

Study on the mechanism of the long-lasting antagonistic effect of caerulein on amphetamine-induced hyperactivity in rats

The long-lasting antagonistic effect of caerulein (CLN) on amphetamine (AMP) hyperactivity was abolished in rats following partial 6-OHDA lesions of the nucleus accumbens (NA) or dopamine (DA) A10 area. CLN showed the long-term antagonistic effect on AMP hyperactivity in rats following complete 6-OHDA lesions of the DA A9 area. Neonatal monosodium L-glutamate-treated rats did not show the CLN effect. These results suggest that opiate receptors presynaptically located on DA neurons in the NA and some modulatory changes in the beta-endorphin system in the arcuate nucleus may play an important role in producing the CLN effect.

Low doses of apomorphine elicit two opposing influences on dopamine cell electrophysiology

Dopamine (DA) cells are known to be very sensitive to direct acting DA agonists, and inhibition of DA cell firing by low doses of DA agonists is generally considered to be an action of these agonists on DA cell autoreceptors. During intracellular recording from spontaneously discharging DA cells in vivo, intravenous administration of apomorphine (20 micrograms/kg i.v.) elicited a hyperpolarization and an increase in input resistance. The calculated reversal potential of the apomorphine effect was approximately -40 mV. However, in non-firing DA cells the reversal potential of these effects was significantly different (P less than 0.01), being close to the reversal potential of responses induced by stimulation of striatonigral pathways (i.e. -67 mV). In addition, haloperidol (0.01 mg/kg i.v.) reversed the hyperpolarization produced by apomorphine but not the increase in input resistance. Transection of striatonigral pathways eliminated most of the increase in input resistance accompanying apomorphine administration, and shifted the apomorphine reversal potential to a value positive to 0 mV. Low doses of apomorphine were also found to affect a class of zona reticulata (ZR) interneurons. Apomorphine caused decreases in ZR cell firing rate, which were abolished by striatonigral pathway transection. Thus, the following mechanism is proposed for the electrophysiological actions of autoreceptor-selective doses of apomorphine on DA cells: (1) apomorphine directly inhibits spontaneous DA cell discharge by inhibiting the slow depolarization preceding action potentials and thereby hyperpolarizes the DA cell, (2) decreased DA cell firing disinhibits GABAergic striatal cells, whose increased firing preferentially (3) inhibits GABAergic ZR interneurons, and thus (4) removes an inhibitory input to DA cells, resulting in an increase in input resistance.

Catecholamines and serotonin in rat frontal cortex after PCPA and 6-OHDA: absolute amounts and ratios

The catecholamines dopamine, epinephrine and norepinephrine, as well as the indoleamine serotonin were measured using radioenzymatic assays, in the frontal cortex of normal rats and of rats in which serotonin synthesis had been inhibited with p-chlorophenylalanine. Besides serotonin, norepinephrine levels were significantly reduced, thus changing the monoamine ratios. To further evaluate these changes, two models of catecholaminergic deafferentation were analyzed, i.e., the massive catecholamine depletion induced by intraventricular 6-OHDA and the more selective norepinephrine deafferentation, obtained by microinjecting 6-OHDA in the dorsal noradrenergic bundle. The results are interpreted in relation to the functional interactions between the catecholamines and between the catecholamines and serotonin.

Partial lesions of the dopaminergic nigrostriatal system in rat brain: biochemical characterization

Various doses of 6-hydroxydopamine injected into the rat substantia nigra produced partial, dose-dependent lesions of the dopaminergic nigrostriatal tract. The resulting reduction in striatal dopamine concentrations and tyrosine hydroxylase activities tended to be proportional, allowing these measurements to serve as indices for lesion severity in any particular animal. Lesions destroying two-thirds or more of the nigrostriatal neurons accelerated dopamine's synthesis in, and release from, surviving neurons, as indicated by increased striatal levels of the dopamine metabolites dihydroxyphenylacetic acid and homovanillic acid. Formation of these metabolites was also enhanced in dendrites of dopaminergic neurons in the substantia nigra. Supersensitivity of striatal postsynaptic receptors, as judged by induction of rotational behavior after apomorphine or L-DOPA administration, occurred when 90% or more of the nigrostriatal neurons had been destroyed. In contrast, rotational behavior could be induced by amphetamine in animals with only 50% of these neurons destroyed.

The central effects of a novel dopamine agonist

A new peripheral dopamine agonist which causes dopaminergic renal vasodilation, was tested for central dopaminergic activity. SK & F 38393 stimulated the dopamine-sensitive adenylate cyclase in homogenates of rat caudate, as a partial agonist, and caused contralateral rotation in rats with unilateral 6-OHDA lesions of substantia nigra. Rotation was shown to be due to a direct effect on supersensitive dopamine receptors. Stimulation of cAMP formation and rotation were blocked by dopamine antagonists. In contrast to other dopamine agonists, SK & F 38393 did not cause stereotypy, emesis or inhibition of prolactin release, nor did SK & F 38393 affect dopamine turnover. The results suggest that SK & F 38393 may selectively stimulate supersensitive central dopamine receptors in vivo or may activate only a certain subclass of dopamine receptors including the receptor in the renal vasculature and the adenylate cyclase coupled postsynaptic receptor in the caudate.