Lydia Kerkerian

Ultrastructural correlates of functional relationships between nigral dopaminergic or cortical afferent fibers and neuropeptide Y-containing neurons in the rat striatum

This study examines the ultrastructural relationships established by the nigrostriatal dopaminergic and the corticostriatal afferent fibers with neuropeptide Y (NPY)-containing neurons in the rat striatum. By means of dual immunolabeling procedures using peroxidase conjugated F(ab) fragments and 125I-labeled protein A, direct appositions and morphologically defined synaptic contacts of the symmetrical type were visualized between tyrosine hydroxylase-labeled nerve terminals and NPY-labeled neurons. After deafferentation of the striatum from its cortical input direct appositions and asymmetrical synaptic contacts were evidenced between characteristic degenerative boutons and NPY-positive neurons in the striatum. These results suggest that striatal NPY interneurons undergo direct influence from both nigrostriatal dopaminergic and corticostriatal neuronal systems.

Striatal neuropeptide Y neurones are under the influence of the nigrostriatal dopaminergic pathway: Immunohistochemical evidence

Selective unilateral 6-hydroxydopamine lesion of the nigrostriatal dopaminergic neurones in rats resulted, 12 to 90 days later, in a marked increase in the number of neuropeptide Y (NPY)-immunoreactive perikarya in the ipsilateral deafferented striatum compared to the contralateral intact side in the same animal and to both sides in control animals. The staining intensity of most positive cells also appeared to be enhanced in the deafferented striatum. These effects which can be interpreted as resulting from an increase in the intraneuronal levels of NPY may be accounted for by an increased production or a decreased metabolization of the peptide in the striatum deafferented from its dopaminergic input. The results therefore provide morphological evidence that the NPY-containing neuronal system of the striatum may undergo tonic influence from nigrostriatal dopaminergic afferents.

Modulatory effect of dopamine on high-affinity glutamate uptake in the rat striatum.

Abstract: In vivo electrical stimulation of the frontal cortical areas was found to enhance sodium‐dependent high‐affinity glutamate uptake (HAGU) measured in rat striatal homogenates. This activating effect was counteracted by in vivo administration of apomorphine and by in vitro addition of dopamine (DA; 10–8M) in the incubation medium, and potentiated by in vivo haloperidol administration. At the doses used, the dopaminergic compounds had no effect on basal HAGU. α‐Methylparatyrosine pretreatment was found to enhance slightly basal HAGU as well as the activating eifects of cortical stimulation. Interestingly enough, lesion of dopaminergic neurons by substantia nigra injection of 6‐hydroxydopamine (6‐OHDA) did not cause any significant change either in basal HAGU or in the effect of cortical stimulation. Measurement of DA effects in vitro in experiments combined with in vivo manipulations of the dopaminergic nigrostriatal and corticostriatal systems showed that the capacity of DA to inhibit striatal HAGU depends directly on the level of the uptake activation reached over basal value. These results suggest that under physiological conditions, the dopaminergic nigrostriatal pathway exerts a modulatory presynaptic action on corticostriatal glutamatergic transmission, counteracting increasing glutamatergic activity. In the case of chronic DA depletion induced by 6‐OHDA, striatal adaptations may occur modifying the mechanisms acting at corticostriatal nerve terminal level.

Pharmacological characterization of dopaminergic influence on expression of neuropeptide Y immunoreactivity by rat striatal neurons

Selective unilateral lesion of the nigrostriatal dopamine pathway by the cytotoxin 6-hydroxydopamine was previously shown to enhance the number and staining intensity of neurons expressing neuropeptide Y immunoreactivity in the ipsilateral striatum. This effect was completely reversed by treatment of the 6-hydroxydopamine-injected animals with the directly acting dopamine agonist apomorphine. This finding reinforces our previous hypothesis that changes in striatal neuropeptide Y staining subsequent to 6-hydroxydopamine lesions of this kind reflect changes in intraneuronal neuropeptide Y levels which are directly attributable to the suppression of a tonic dopaminergic control. In contrast to the effect of 6-hydroxydopamine lesion, non-destructive impairment of striatal dopamine transmission by treatments with either the dual dopamine D1/D2 receptor antagonist haloperidol or the dopamine synthesis inhibitor alpha-methylparatyrosine induced a decrease in both the number of neuropeptide Y striatal cells (-29.8% and -34.8%, respectively) and in their labeling intensity. The selective D2-antagonist sulpiride also showed a tendency to reduce the number of neuropeptide Y immunoreactive cells, whereas the selective D1 antagonist SCH 23390 induced a small but constant increase in this number. Taken as a whole, these results suggest that the dopaminergic D1 and D2 receptor subtypes play opposite roles in the dopaminergic control of the striatal neuropeptide Y neuronal system, which may account for the different changes in striatal neuropeptide Y immunostaining observed after 6-hydroxydopamine injury and after non-destructive impairment of nigrostriatal dopaminergic transmission.

Ultrastructural features of NPY-containing neurons in the rat striatum

In the present study, we examined the ultrastructure of striatal neurons containing neuropeptide Y (NPY) which were labeled by an immunohistochemical method using peroxidase-conjugated F(ab) fragments in the rat. Each of the 26 neurons identified had a deeply indented oval nucleus. The cytoplasm, which was mainly concentrated at the emergence of the dendrites, contained an abundant Golgi apparatus and a well-developed granular endoplasmic reticulum. Dendrites were poorly branched and rarely exhibited varicosities or dendritic spines. NPY-immunoreactive (Ir) axons were small in diameter and unmyelinated. These features corresponded to a subpopulation of striatal neurons classified as aspiny type IV in previous Golgi studies. Axon terminals forming symmetrical synapses were numerous on the NPY-Ir perikarya and proximal dendrites. On distal NPY-Ir dendrites, synaptic contacts were mainly of the asymmetrical type, suggesting that NPY neurons are contacted by at least 2 categories of afferent fibers. Several NPY-Ir axonal processes and boutons were found to form symmetrical synapses with dendrites, dendritic spines and perikarya belonging to spiny type neurons. These data were consistent with the view that NPY may act as a neurotransmitter of striatal interneurons. Moreover, the frequent observation of NPY axonal processes in the close vicinity of striatal vessels suggested that NPY might also play a role in the control of cerebral vasomotricity. Thirty hours after intranigral injection of 6-hydroxydopamine to induce a degeneration of nigrostriatal dopamine terminals, some characteristic degenerative boutons were observed in close apposition to NPY-Ir cell bodies, suggesting that NPY neurons are under a direct nigrostriatal dopaminergic influence.

Presynaptic dopaminergic control of high affinity glutamate uptake in the striatum

The high affinity glutamate uptake (HAGU) was measured in homogenates obtained from striata of control rats and animals submitted to electrical stimulation of the frontal cortical areas. Results show that the stimulation of the corticostriatal neurons induces an activation of the uptake of glutamate (Glu) in the striatum resulting from an increase in the affinity of the transport site for Glu. In other experiments we have obtained evidence that, when stimulated, presumed dopaminergic receptors can inhibit HAGU. The dopaminergic antagonist haloperidol was shown not to influence that basal Glu uptake but this neuroleptic highly potentiates the excitatory effects of cortical stimulations. The nigrostriatal dopaminergic pathway may exert a modulatory action on the corticostriatal glutamatergic neurons as it counteracts increasing glutamatergic activity.

Expression of neuropeptide Y immunoreactivity in the rat nucleus accumbens is under the influence of the dopaminergic mesencephalic pathway.

SummaryThe density of neuropeptide Y (NPY) immunostained neurons examined in the rat nucleus accumbens (NAcc) was shown to be constant across the anteroposterior extent of the nucleus and did not present any right-left hemispheric difference. Selective unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigral dopaminergic neurons induced, 15 to 21 days later, a bilateral decrease in the NPY neuron density which was, interestingly, more marked in the contralateral than in the ipsilateral NAcc. Dopamine depletion induced by α-methylparatyrosine treatment elicited a decrease in NPY neuronal density similar in amplitude to that induced by the 6-OHDA lesion in the ipsilateral NAcc suggesting that similar mechanisms underly both NPY responses. In both experimental conditions, changes in NPY immunostaining were quite homogeneous in the two antero-posterior NAcc portions arbitrarily considered. Apomorphine treatment in animals with 6-OHDA injury completely reversed the ipsilateral lesion effect in the anterior part of the NAcc but only partially the contralateral one. In contrast, no significant effect of apomorphine was observed in either side of the NAcc posterior portion. This data suggests the involvement of at least 2 components in the NPY neuron responses to the lesion. The component reversed by apomorphine treatment was presumed to be directly linked to the DA depletion, while the second component not antagonized by apomorphine was considered independant on DA transmission. These data therefore provide morphological evidence for the occurence of complex functional interactions between dopaminergic afferents and NPY-containing neurons within the NAcc.

Cortical Regulation of Striatal Neuropeptide Y (NPY)-Containing Neurons in the Rat

This study examined the functional relationships established by nigral, cortical, and thalamic striatal afferent pathways with neuropeptide Y (NPY)‐containing neurons in the rat rostral striatum by coupling selective deafferentation procedures and NPY immunohistochemistry. Previous experiments have shown that after unilateral 6‐hydroxydopamine (6‐OHDA)‐induced degeneration of nigrostriatal dopaminergic neurons, the mean number of NPY‐immunoreactive (lr) neurons per frontal section was increased in the striatum ipsilateral to the lesion side and unaltered in the contralateral striatum. The present topographical analysis of the 6‐OHDA lesion effects led us to state that the increase in NPY‐lr neuron density occurs in restricted ventral and medial zones of the ipsilateral striatum. Unilateral ablation of the frontoparietal cerebral cortex by thermocoagulation was moreover shown to elicit, 20–30 days later, a significant bilateral increase in the number of striatal NPY‐lr cells. The increase was more marked in the striatum ipsilateral to the hemidecortication where it was similar in amplitude to that induced by the 6‐OHDA lesion. The topographical analysis of the cortical lesion effects also revealed an uneven striatal response, but, in contrast to that observed for the 6‐OHDA lesion, changes were restricted to dorsolateral areas of the striatum in both brain sides, revealing an apparent complementarity of nigral dopaminergic and cortical influences over striatal NPY neuronal system. Combined unilateral nigral and cortical lesions surprisingly counteracted in a survival time dependent manner the effects of each lesion considered separately. In that condition topographical changes related to the 6‐OHDA lesion totally disappeared and those related to the cortical lesion were attenuated but still present. These results suggest that expression of striatal dopamine—NPY interaction is dependent on corticostriatal transmission. Interestingly lesion of thalamic areas projecting to the striatum did not significantly modify the mean number of NPY‐lr neurons determined per section from the whole striatal surface, but selectively increased the NPY neuron density in the mediodorsal region of the striatum, suggesting that the striatal NPY‐containing neuronal system is also influenced by thalamostriatal projections.

Inhibitory effects of dopamine on high affinity glutamate uptake from rat striatum

The role of dopamine (DA) input on the activity of glutamate neurons was investigated on rat striatal and cortical tissue using the measurement of sodium-dependent high affinity glutamate uptake (HAGU) as an index. Incubation of the tissue in the presence of DA, apomorphine or bromocriptine produced marked inhibition of 3H-glutamate uptake from rat striatal homogenates. No change occurred with samples from the frontal cortex. Dopaminergic inhibition of HAGU in striatal homogenates was shown to be reversed in the presence of haloperidol or domperidone which act by blocking dopaminergic receptor sites. These results are consistent with the existence of an inhibitory control of the neuronal activity of the glutamatergic neurons in the striatum by the nigro-striatal dopaminergic input. The effects could be due to the activation of D2-like DA receptors located at pre-synaptic levels on cortico-striatal glutamatergic nerve endings.

Topographic changes in high-affinity glutamate uptake in the cat red nucleus, substantia nigra, thalamus, and caudate nucleus after lesions of sensorimotor cortical areas

The distribution of presumed glutamatergic projections from sensorimotor cortical areas to the red nucleus (RN), the substantia nigra (SN), the ventrolateral thalamic complex, and the caudate nucleus (CN) was investigated in the cat. For this purpose, the changes in the sodium-dependent high-affinity glutamate uptake (HAGU) rate were measured in homogenates of tissue microdissected from various parts of these subcortical structures after chronic surgical ablation of sensorimotor cortical areas. After 8 to 10 days survival, significant reductions in HAGU activity were noted in the structures studied on the side ipsilateral to the operated cortex. Within each structure, various quantitative or qualitative changes were observed. Higher decreases in HAGU activity were found in the caudal part of the RN, the ventrolateral thalamic nucleus, and the dorsolateral part of the CN than in the other parts of these structures. The lateral part of the SN showed a large decrease in HAGU rate and its medial part a small but significant increase. Referring to the anatomic data concerning the organization of cortical projections from sensorimotor areas to the structures studied, it was shown that our results support the view that glutamate could act as a neurotransmitter along various corticosubcortical pathways.