Photometric assessment of glyoxylic acid-induced fluorescence of dopamine in the caudate nucleus

Constant illumination causes spatially discrete dopamine depletion in the normal and degenerate retina

A fully competent retinal dopamine system underpins normal visual function. Although this system is known to be compromised both prior to and during retinal degeneration, the spatial dynamics of dopamine turnover within the degenerate retina are at present unknown. Here, using immunohistochemistry for dopamine in combination with quantitative optical density measurements, we reveal a global decline in retinal dopamine levels in the light adapted RCS dystrophic rat, which is restricted to plexiform layers in the dark. Pharmacological blockade of dopamine production with the drug alpha-methyl-p-tyrosine (AMPT) allows the direct visualisation of dopamine depletion in normal and degenerate retina in response to constant illumination. In normal retinae this effect is spatially discrete, being undetectable in perikarya and specific to amacrine cell fibres in sublamina 1 of the inner plexiform layer. A similar response was observed in the retinae of dystrophic rats but with a reduction in amplitude of approximately 50%. It is suggested that the pattern of dopamine depletion observed in rat retina may reflect an AMPT-resistant pool of perikaryal dopamine and/or a reduction in extrasynaptic release of this neurotransmitter in response to illumination in vivo. We conclude that the visualisation of dopamine depletion reported here represents a release of this neurotransmitter in the response to light. Turnover of dopamine in the dystrophic retina is discussed in the context of surviving photoreceptors, including the intrinsically photosensitive melanopsin ganglion cells of the inner retina.

Partial, graded losses of dopamine terminals in the rat caudate-putamen: an animal model for the study of compensatory adaptation in preclinical parkinsonism

Procedures to lesion dopamine (DA) neurons innervating the rat caudate-putamen (CP) in a partial, graded fashion are described in this study. The goal is to provide a lesion model that supports intra-animal comparisons of voltammetric recordings used to investigate compensatory adaptation of DA neurotransmission. Lesions exploited the topography of mesostriatal DA neurons, microinjections of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial and lateral edges of the ventral mesencephalon containing DA cell bodies and microdissection of the CP into six regions. Analysis of tissue DA content in these regions by HPLC-EC demonstrated that 6-OHDA injected into the lateral substantia nigra results in a significantly greater loss of DA in lateral versus medial regions of the CP. The direction of the graded loss of DA was reversed (i.e. a medial to lateral lesion gradient) by the injection of 6-OHDA into the ventral tegmental area near the medial SN. Extracellular concentrations of electrically evoked DA could be measured across the mediolateral axis of the CP in a single animal using the technique of in vivo voltammetry. More importantly, graded decreases in the amplitude of evoked DA levels generally followed the direction of the tissue DA gradient in lesioned animals. These results suggest that the graded loss of DA terminals in the CP, coupled to a spatially and temporally resolved technique for monitoring extracellular DA, is a viable tool for investigating compensatory adaptation in the mesostriatal DA system.

Quantitative image analysis with densitometry for immunohistochemistry and autoradiography of receptor binding sites--methodological considerations

Major technical progress in the development of computer-based image analysis has made possible the entry of autoradiography and immunohistochemistry into a new era where quantification by densitometry has become easily accessible. Autoradiography could become quantitative and displayed adequate reproducibility with the help of emulsion-coated films and the use of scales of standards of known radioactivity exposed and analyzed in parallel to the tissue sections. Immunohistochemistry after revelation by a color-based enzymatic technique can also become quantitative, providing that standardization of the crucial steps of the procedure and calibration through a parallel treatment of a scale of antigen standards can be ensured. Such an approach is described here in the rat with reference to tyrosine hydroxylase (TH), the main synthesizing enzyme for catecholamines, and with dopamine (DA) itself, a catecholaminergic neurotransmitter. The different parts of the procedure, which can influence the results, such as the fixation of the animals by perfusion and the evaluation of the fluctuations via the calibration curve, are discussed in detail. Biological validation of the proposed procedure is described by reference to experiments already well documented biochemically, such as the induction effect of reserpine on TH in the rat locus coeruleus and the depleting effect of alpha-methyltyrosine (AMPT), a well-known blocker of TH activity, on rat striatal DA content. Finally the importance of restricting the measurements to the (pseudo)linear portion of the calibration curve is illustrated by the autoradiographic identification of the differential intrastriatal repartition of the dopaminergic D1 and D2 receptor sites, particularly the dual patch-matrix compartments.

Dopamine high-affinity transport site topography in rat brain: major differences between dorsal and ventral striatum

Investigations were conducted to determine the topography of the high-affinity dopamine uptake process within the rat striatum. [3H]Dopamine uptake into crude synaptosomes prepared from micropunch samples was found to be two- to three-fold higher in dorsal caudate-putamen relative to nucleus accumbens septi. In contrast, the concentrations of dopamine in the two regions were equivalent. The recognition site associated with high-affinity dopamine uptake was labeled using [3H]mazindol, and the binding of this ligand was also found to be two- to three-fold higher in homogenates from dorsal caudate-putamen samples relative to nucleus accumbens septi. Regional differences in uptake of [3H]dopamine or binding of [3H]mazindol were shown to be due to variations in Vmax or Bmax, not to differences in apparent affinity. Autoradiography of [3H]mazindol binding in rat striatum revealed a decreasing density of the site along the dorsal-to-ventral axis, with the highest binding occurring in the dorsolateral caudate-putamen, lower binding in the ventral caudate-putamen, and lowest levels in the septal pole of the nucleus accumbens septi. Quantification showed that the extent of this gradient was two-fold. Further autoradiographic studies revealed less striatal heterogeneity in the pattern of binding of [3H]ketanserin, another radioligand associated with the striatal dopaminergic innervation but not linked to the dopamine uptake process of the plasma membrane. The findings suggest that the dopaminergic fibers of the ventral striatum, especially the medial nucleus accumbens septi, may be relatively lacking in their capacity for dopamine uptake following its release. This organization may result in regional differences in the time-course of of extraneuronal dopamine following transmitter release and may render the dopamine-containing terminals of the ventral striatum less susceptible to the degenerative influences of neurotoxins that are incorporated by the high-affinity dopamine uptake process.

Quantitative autoradiographical analysis of the age-related modulation of central dopamine D1 and D2 receptors

Quantitative autoradiography of [3H]SCH 23390 and [3H](-)-sulpiride binding was performed in the brain of rats of various ages (3, 11 and 24 months) in order to study the changes in D1 and D2 receptor density with age. Binding of [3H]SCH 23390 in the caudate-putamen decreased progressively and markedly at rostral levels in 11- and 24- compared with 3-month-old rats (max. decrease -63%) while at caudal levels significant decrease was observed only in 24-month-old rats. [3H](-)-Sulpiride binding progressively decreased during aging in the caudate-putamen at rostral levels and the decrease was more pronounced laterally (-70% at 24 months), while at caudal levels no significant decrease was observed. D1 and D2 binding sites also decreased in the nucleus accumbens and olfactory tubercle of aged rats, while in the substantia nigra only the D1 receptors appeared to be modified with aging. No change was found in the entopeduncular nucleus, amygdala, frontoparietal, suprarinal-prefrontal and anterior cingulate cortex. The results indicate that the age-associated decrease of D1 and D2 receptors is not widespread, being confined to dopaminergic areas with high density of dopamine receptors.

Thalamic control of subcortical dopamine function in the rat and the effects of lesions applied to the medial prefrontal cortex

Dopamine (DA) utilisation has been assessed in medial and lateral segments of the caudate-putamen complex (CPM and CPL, respectively) in response to unilateral manipulations aimed at the thalamic mediodorsal nucleus, lateral division (MDL). The ratios of 3,4-dihydroxyphenylacetic acid (DOPAC):DA and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid, HVA):DA are used as indices of DA utilisation and, in the case of HVA:DA, may also reflect DA release. Neither electrical stimulation nor ibotenate (IBO) treatment followed by long recovery periods (2 days or 1 week) had any significant effect on DA utilisation in CPM or CPL. Cell-specific activation of neurones produced by short-term (1 h recovery) infusions of IBO aimed unilaterally at MDL (right side) resulted in bilateral increases of DA utilisation in both CP sectors. These changes tended to be slightly more marked in the hemisphere ipsilateral to the side of IBO infusion. Unilateral infusions of IBO were then aimed at MDL of either (1) the left or right hemisphere of animals which had already received a 1-week-old unilateral (right side) prefrontal cortex (FCx) lesion or (2) the right hemisphere of animals which had previously received a 1 week-old bilateral FCx lesion. The pattern of changes, when expressed relative to the 'sham-operated' animals which received the FCx lesion alone, were similar to those described above following intra-MDL infusions of IBO into animals with an intact cortex. The FCx lesions themselves were shown to have no significant effect on DA utilisation in any CP sector. In view of the known neuroanatomical connections, it is likely that the effects observed in CP are not due to activation of MDL neurones themselves but are more likely the result of activation of neurones in the intralaminar nuclei which border MDL. Nevertheless, these findings support the concept that activation of thalamic nuclei will enhance DA function in a variety of forebrain areas in the rat.

Quantification of the dopamine innervation in adult rat neostriatum

Conditions leading to specific and integral visualization of dopamine axon terminals (varicosities) were tested in adult rat cerebral hemisphere slices incubated with [3H]dopamine and processed for high resolution radioautography. Specific visualization of the dopamine endings was achieved after incubation with 10(-6) M [3H]dopamine in the presence of a monoamine oxidase inhibitor (pargyline 10(-4) M), and of desipramine (5 X 10(-6) M), an inhibitor of catecholamine uptake by noradrenaline and serotonin neurons. [3H]Dopamine varicosity labeling was eliminated by the addition of 5 X 10(-5) M benztropine (an inhibitor of catecholamine uptake by catecholamine neurons), and was almost absent when dopamine nerve cell bodies of the midbrain had been previously destroyed with 6-hydroxydopamine. In dopamine-denervated neostriatum incubated without desipramine, a second set of labeled terminals was also visible. These were identified as serotoninergic, since their labeling was suppressed by citalopram, an inhibitor of monoamine uptake highly specific for serotonin neurons. There was no desipramine-sensitive but citalopram-resistant varicosity labeling suggestive of neostriatal noradrenaline innervation. In normal striatum, incubation at 35 degrees C always resulted in a labeling of dopamine varicosities restricted to a narrow band which followed the contours and cut surface of this anatomical region. This unusual distribution was the result of an uptake barrier generated by the tightly packed dopamine varicosities. Indeed, the striatal dopamine varicosity labeling was more widespread after partial 6-hydroxydopamine denervation or in normal tissue incubated either with a higher [3H]dopamine concentration (5 X 10(-6) M), in the presence of relatively low benztropine concentrations (10(-5) M), or at lower temperature (15 degrees C). Material incubated at 15 degrees C for 90 min was suitable for purposes of quantification: labeled varicosities were then visualized throughout the striatum and across the full thickness of the slices; moreover, the number of labeled varicosities plotted against radioautographic exposure time increased in parallel and reached a plateau at the same time in neostriatal sectors with widely different innervation densities. At a rostral transverse level across neostriatum, the dorsolateral quadrant showed hyperdense "patches" of labeled terminals distinguishable from an already dense surrounding "matrix", whereas, ventromedially, the dopamine innervation appeared more uniform and somewhat less dense.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of intraventricular 6-hydroxydopamine on the dopaminergic innervation of striatum: histochemical and neurochemical analysis

The impact of intracerebroventricular administration of 6-hydroxydopamine (6-HDA) on the dopamine (DA)-containing nigrostriatal projection was determined by regional histochemical and biochemical analyses. One week postinjection, we observed that tyrosine hydroxylase (TH)-positive terminals were almost completely absent from the medial portion of striatum but gradually increased in density toward the lateral margin of this structure. A similar gradient was indicated by fluorescence histochemistry and biochemical analyses of DA. In contrast, the 6-HDA-induced changes in TH activity and in dihydroxyphenylacetic acid content were less severe and showed little or no medial-to-lateral gradient. These high levels of TH activity and DOPAC content, relative to local DA concentrations, suggest an increase in the synthesis and release of DA from residual terminals that may serve to compensate for the brain damage. By 4 months postlesion, both histochemical and biochemical analyses indicated the presence of more DA terminals in striatum than there had been one week postlesion. This change was most markedly obvious in the medial striatum, which had been almost completely devoid of terminals at one week postlesion. Retrograde tracing experiments revealed that terminals appearing in the medial striatum at 4 months postlesion arise from the same region of the substantia nigra that innervates the medial striatum in the intact animal. Thus, no change in the topographic relation between substantia nigra and striatum occurred as a result of the lesion.

Dopamine D-2 receptors in rat caudate-putamen: the lateral to medial gradient does not correspond to dopaminergic innervation

The topography of dopamine D-2 receptors within the rat caudate-putamen (CPU) was characterized by measuring [3H]spiroperidol binding to membranes from dissected regions of the CPU and to thin sections of rat forebrain using quantitative autoradiography. Specific binding, defined using 1 microM (+)-butaclamol, was considerably higher in the lateral CPU than the medial CPU, without any obvious gradient in the dorsoventral axis. The difference in Bmax between the lateral and medial CPU was similar whether determined by digital subtraction autoradiography of coronal forebrain sections or by [3H]spiroperidol binding to membrane homogenates derived from CPU subregions. No regional differences were observed in the affinity of [3H]spiroperidol for D-2 sites. There were no lateral-to-medial differences in the dopaminergic innervation of the CPU, as determined by the measurement of the levels of dopamine, homovanillic acid and dihydroxyphenylacetic acid in subregions of the CPU using high pressure liquid chromatography. Thus, regional variations in the density of D-2 receptors within the CPU do not correspond to the degree of dopaminergic innervation. The functional importance of the organization of this dopamine receptor system is discussed in relation to the lateral-to-medial organization of the CPU.

Brain dopamine and serotonin receptor sites revealed by digital subtraction autoradiography

Autoradiography combined with image analysis permitted quantitative visualization of dopamine (D2) and serotonin (S2) binding sites in rat brain. Forebrain sections were incubated with tritiated spiroperidol alone or with tritiated spiroperidol plus unlabeled compounds that saturated the D2 or S2 sites. By subtracting the digitized image of an autoradiograph derived from the latter sections from that of the former, the D2 or S2 sites were specifically revealed. The resulting quantitative images demonstrate the differing anatomical distributions of these sites. The D2 site is largely restricted to the striatal complex (caudate-putamen, nucleus accumbens septi, and olfactory tubercle), whereas the S2 site is enriched in layer 5 of motor cortex, the perirhinal and cingulate cortices, and the claustrum.

Intracellular and extracellular electrophysiology of nigral dopaminergic neurons--3. Evidence for electrotonic coupling

Using three independent in vivo methods, we have obtained evidence for electrotonic coupling between sets of rat zona compacta dopaminergic neurons: (1) Lucifer yellow injection into single dopamine neurons resulted in labeling of two to five dopamine neurons in 18 out of 33 injections. Similar injections into reticular formation or nigral reticulata cells did not demonstrate multiple labeling. (2) Intracellular recording revealed spontaneously occurring small (3-15 mV) fast potentials that often triggered action potentials in dopamine neurons when the membrane potential was close to firing threshold. These fast potentials had a firing rate and pattern similar to that reported previously for extracellularly recorded dopamine neurons. Fast potentials were activated antidromically from the caudate nucleus at a latency similar to that reported for dopamine neurons, followed high frequency antidromic stimulation at a constant latency, and collided with spontaneously occurring fast potentials. However, directly elicited action potentials would not collide reliably with antidromically activated fast potentials. Intracellular injection of depolarizing or hyperpolarizing current increased and decreased, respectively, the rate of occurrence of these potentials. The firing rate of fast potentials could be increased and decreased by the intravenous administration of dopamine antagonists and agonists, respectively. (3) Simultaneous extracellular recording from pairs of DA neurons revealed numerous instances of synchronized action potentials. This was observed more frequently following intravenous haloperidol administration. Sets of burst firing dopamine neurons recorded simultaneously consistently demonstrated a decrease in the interspike interval as the burst progressed; a phenomenon commonly reported in other electrically coupled systems. Electrical coupling has been suggested to be present in sets of identified nigrostriatal dopamine neurons. Electrical communication between these neurons could be involved in modulating burst firing and in synchronizing dopamine release.

Spontaneous and drug-induced rotation following localized 6-hydroxydopamine and kainic acid-induced lesions of the neostriatum

Twelve groups of rats received small localized lesions produced by either 6-hydroxydopamine (6-OHDA) or kainic acid (KA) in one of six placements in the right neostriatum. All groups lesioned with 6-OHDA showed ipsilateral spontaneous and amphetamine-induced rotation and contralateral apomorphine-induced rotation. All groups lesioned with kainic acid showed ipsilateral spontaneous rotation on day 1 following the lesion but contralateral rotation by day 8, and no significant rotation to injections of either amphetamine or apomorphine. The histology suggested that the lesions were topographically specific, and it is therefore concluded that the neostriatum is topographically homogeneous in the mediation of rotational behaviour as measured in automated rotometers.

Functional validation of projection topography in the nigrostriatal dopamine system

Anatomical investigations have revealed that the nigrostriatal pathway is topographically organised. In two experiments, nigrostriatal topography was investigated with catecholamine specific procedures, using paradigms which reflect the functional activity of dopaminergic neurones. Data were analysed with the intention of discovering possible relationships between the mesencephalic location of stimulating electrodes or injection cannulae, the extent and location of dopamine histofluorescence depletion within the striatum, and the effects of amphetamine and apomorphine on rotational behaviour. In animals pretreated with 250 mg/kg alpha-methyl-p-tyrosine it was found that unilateral stimulation with medially-placed nigral electrodes produced maximal depletion of dopamine histofluorescence in anterior dorso-medial regions of the striatum, while laterally-located electrodes principally depleted posterior, ventro-lateral areas. In the second experiment, 2 micrograms of 6-hydroxydopamine in a volume of 0.5 microliter was injected unilaterally at varying loci within the ventral midbrain of animals pre-treated with desmethylimipramine (25 mg/kg). It was discovered that the lateral injection coordinate was significantly associated with both the extent and location of the depletion of dopamine-related fluorescence from the ipsilateral striatum. Rotational behaviour, induced by dopamine-agonists was related firstly, to the overall extent of dopamine depletion from the striatum, and secondly, the contraversive turning induced by apomorphine in particular was related to the dorsoventral coordinate of the mesencephalic 6-hydroxydopamine injection. The results provide functional validation for the pattern of topographical projection within the nigrostriatal dopaminergic system proposed on the basis of intracellular tracing techniques.

Increased food intake following the manipulation of intracerebral dopamine levels with gamma-hydroxybutyrate

The administration of gamma-hydroxybutyric acid (GHB) initially causes a temporary "sleep-like" state during which there is an increase in forebrain dopamine levels. The present series of experiments examined whether in the period following the GHB-induced behavioural depression, when accumulated dopamine is dispersed, there is any behavioural evidence of increased dopaminergic activity. The first experiment, in which GHB was injected directly into the cerebral ventricles, demonstrated that in the immediate post-recovery period rats exhibited various forms of stereotyped oral behaviour and stereotyped sniffing. Unexpectedly, it was also observed that if food were present animals preferred to eat. The nature of this feeding response was examined in two further experiments. Firstly, it was shown that in the period following the behavioural depression animals would perform, in a dose-dependent fashion, an operant response which was rewarded by food. Secondly, the GHB-induced increase in feeding was abolished by the pre-treatment of animals with either the dopamine synthesis inhibitor alpha-methyl-p-tyrosine, or the dopamine receptor blocker haloperidol. These data indicate that (i) in the period when it is known that the GHB-induced accumulation of dopamine is dispersing, there is behavioural evidence of increased dopaminergic activity; (ii) the feeding response is not a simple oral reflex; and (iii) in addition to being essential for food intake dopaminergic transmission may play a direct role in the production of feeding.