Adaptive down-regulation of the serotonin transporter in the 6-hydroxydopamine-induced rat model of preclinical stages of Parkinson's disease and after chronic pramipexole treatment.

Our recent study has indicated that a moderate lesion induced by bilateral 6-hydroxydopamine (6-OHDA) injections into the ventrolateral region of the caudate-putamen (CP) in rats, modeling preclinical stages of Parkinson's disease, induces a "depressive-like" behavior which is reversed by chronic treatment with pramipexole (PRA). The aim of the present study was to examine the influence of the above lesion and chronic PRA treatment on binding to the serotonin transporter (SERT) in different brain regions. As before, 6-OHDA (15 µg/2.5 µl) was administered bilaterally into the CP. PRA (1mg/kg) was injected subcutaneously twice a day for 2 weeks. Serotonergic and dopaminergic neurons of the dorsal raphe (DR) were immunostained for tryptophan hydroxylase and tyrosine hydroxylase, respectively, and were counted stereologically. Binding of [(3)H]GBR 12,935 to the dopamine transporter (DAT) and [(3)H]citalopram to SERT was analyzed autoradiographically. Intrastriatal 6-OHDA injections decreased the number of dopaminergic, but not serotonergic neurons in the DR. 6-OHDA reduced the DAT binding in the CP, and SERT binding in the nigrostriatal system (CP, substantia nigra (SN)), limbic system (ventral tegmental area (VTA), nucleus accumbens (NAC), amygdala, prefrontal cortex (PFCX), habenula, hippocampus) and DR. A significant positive correlation was found between DAT and SERT binding in the CP. Chronic PRA did not influence DAT binding but reduced SERT binding in the above structures, and deepened the lesion-induced losses in the core region of the NAC, SN, VTA and PFCX. The present study indicates that both the lesion of dopaminergic neurons and chronic PRA administration induce adaptive down-regulation of SERT binding. Moreover, although involvement of stimulation of dopaminergic transmission by chronic PRA in its "antidepressant" effect seems to be prevalent, additional contribution of SERT inhibition cannot be excluded.

The influence of acute and chronic administration of 1,2-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline on the function of the nigrostriatal dopaminergic system in rats.

The contribution of (R)-enantiomer of N-methyl-salsolinol (1,2-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline; NMSal) to the degeneration of dopaminergic neurons in the course of Parkinson's disease (PD) has been predominantly suggested by in vitro experiments in cell culture and by an in vivo study in which this compound has been directly injected into the rat striatum. The aim of the present study was to examine the effects of racemic NMSal (50 and 100 mg/kg) administered systemically, acutely and chronically for 21 days to rats, on the neurochemical and behavioral markers of PD. Our results showed that racemic NMSal easily penetrated the blood-brain barrier. Its brain level was relatively high 2-6 h after a single injection than gradually decreased. NMSal was quickly eliminated from the rat brain, its concentration 24 h after withdrawal from chronic treatment was very low. NMSal at both examined doses did not affect striatal and nigral levels of dopamine (DA) 2 h after the first and last chronic injections, however, it markedly changed DA catabolism. In the striatum both its doses evoked a significant acceleration of the total and oxidative, monoamine oxidase (MAO)-dependent DA catabolism without affecting the catechol-O-methyltransferase (COMT)-dependent O-methylation. In the substantia nigra (SN), only the higher dose of NMSal produced such effect. DA catabolism in either structure was the same as in control 24 h after cessation of chronic treatment. The second characteristic marker of PD, the number of tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the SN, was not affected by chronic NMSal treatment as revealed by the stereological counting. In the behavioral study, it was found that racemic NMSal significantly suppressed spontaneous locomotor activity and effectively prevented that stimulated by apomorphine. Our results suggest that NMSal may play an important role in the regulation of dopaminergic activity rather than in inducing changes of parkinsonian type.

Degeneration of dopaminergic mesocortical neurons and activation of compensatory processes induced by a long-term paraquat administration in rats: implications for Parkinson's disease.

A deficiency of the dopaminergic transmission in the mesocortical system has been suggested to contribute to cognitive disturbances in Parkinson's disease. Therefore, the aim of the present study was to examine whether the long-term administration of a commonly used herbicide, paraquat, which has already been found to induce a slowly progressing degeneration of the nigrostriatal neurons, influences mesocortical dopaminergic neurons in rats. Paraquat at a dose of 10 mg/kg i.p. was injected either acutely or once a week for 4, 8, 12 and 24 weeks. Acute treatment with this pesticide increased the level of homovanillic acid (HVA) and HVA/dopamine ratio in the prefrontal cortex. After 8 weeks of administration paraquat increased the number of stereologically counted tyrosine hydroxylase-immunoreactive (TH-ir) neurons and their staining intensity in the ventral tegmental area (VTA), which is a source of the mesocortical dopaminergic projection. At the same time, few TH-ir neurons appeared in different regions of the cerebral cortex: in the frontal, cingulate, retrosplenial and parietal cortices. Chronic paraquat administration did not influence the level of dopamine in the prefrontal cortex but increased the levels of its metabolites: 3,4-dihydroxyphenylacetic acid (after 8-12 weeks), HVA (after 4 and 12 weeks) and HVA/dopamine ratio (4 weeks). After 24 weeks this pesticide reduced the number of TH-ir neurons in the VTA by 42% and of the Nissl-stained neurons by 26%, and induced shrinkage of this structure by ca. 25%. Moreover, TH-ir neurons in the cortex were no more visible after such a long period of administration and levels of dopamine metabolites returned to control values. The present results suggest that the long-term paraquat administration destroys dopaminergic neurons of the VTA. However, compensatory activation of the VTA neurons and cortex overcomes progressing degeneration and maintains cortical dopaminergic transmission.

A slowly developing dysfunction of dopaminergic nigrostriatal neurons induced by long-term paraquat administration in rats: an animal model of preclinical stages of Parkinson's disease?

The aim of the present study was to examine the influence of the long-term paraquat administration on the dopaminergic nigrostriatal system in rats. Paraquat was injected at a dose of 10 mg/kg i.p. for 4-24 weeks. We found that this pesticide reduced the number of tyrosine hydroxylase-immunoreactive neurons of the substantia nigra; after the 4-week treatment the reduction (17%, nonsignificant) was confined to the rostrocentral region of this structure but, after 24 weeks, had spread along its whole length and was approximately 37%. Moreover, it induced a biphasic effect on dopaminergic transmission. First, levels of dopamine, its metabolites and turnover were elevated (4-8 weeks) in the caudate-putamen, then all these parameters returned to control values (12 weeks) and dropped by 25-30% after 24 weeks. The binding of [3H]GBR 12,935 to dopamine transporter in the caudate-putamen was decreased after 4-8 weeks, then returned to control values after 12 weeks but was again decreased after 24 weeks. Twenty-four-week paraquat administration also decreased the level of tyrosine hydroxylase (Western blot) in the caudate-putamen. In addition, paraquat activated serotonin and noradrenaline transmission during the first 12 weeks of treatment but no decreases in levels of these neurotransmitters were observed after 24 weeks. The above results seem to suggest that long-term paraquat administration produces a slowly progressing degeneration of nigrostriatal neurons, leading to delayed deficits in dopaminergic transmission, which may resemble early, presymptomatic, stages of Parkinson's disease.

Alterations in striatal neuropeptide Y system activity of rats with haloperidol-induced behavioral supersensitivity.

The study was conducted to determine whether the expression of behavioral supersensitivity induced by haloperidol (HAL) administered once daily (2 mg/kg i.p.) for 14 days is associated with the alterations in activity of neuropeptide Y (NPY) system in the striatum (caudate-putamen) and nucleus accumbens. Dopamine supersensitivity was tested by measurement of locomotor activity and stereotyped behavior after administration of the dopamine D2/D3 receptor agonist quinpirole (1 mg/kg i.p.) on day 1, 3 and 7 after HAL withdrawal. Neuropeptide Y-like immunoreactivity (NPY-LI) was determined in the striatum and nucleus accumbens isolated 6 h after quinpirole injection on day 1, 3 and 7 after the end of HAL treatment. NPY mRNA was quantified in these structures on day 7 after HAL withdrawal. HAL increased spontaneous locomotor activity and prevalence of rearing, grooming and head-down sniffing. At the same time, striatal NPY-LI increased progressively from the reduced level found on day 1 of haloperidol withdrawal. NPY mRNA remained unchanged. In saline-treated rats, quinpirole enhanced locomotion, rearing, and induced intense head-down sniffing and oral activity. These behavioral effects were accompanied by a decrease in striatal NPY-LI. NPY mRNA was slightly increased. HAL treatment altered response to quinpirole, namely it increased locomotion, intensified oral activity and reduced rearing and head-down sniffing. The second and the third quinpirole injection decreased NPY-LI levels. NPY mRNA was unchanged. In the nucleus accumbens, apart from a decrease in NPY-LI on day 1 after the last haloperidol dose, the level of NPY-LI and NPY mRNA in any experimental group did not differ from the control value. The presented results suggest that the alterations in the activity of the striatal but not nucleus accumbens NPY system contribute to adaptive changes induced by long-term haloperidol treatment and may be of significance to the motor hyperactivity induced by intermittent stimulation of postsynaptic dopamine D2 receptors.

Morphine and cocaine influence on CRF biosynthesis in the rat central nucleus of amygdala.

The central nucleus of the amygdala is a CRF-containing limbic brain site which mediates both fear-like and avoidance behaviors; moreover it has been hypothesized that atypical stress responses may contribute to compulsive drug use. Therefore, we studied in rat amygdala the level of CRF mRNA by in situ hybrydization, and the level of the peptide using immunocytochemistry after acute and chronic administration of morphine and cocaine and after their withdrawal. Acute injection of morphine (20 mg/kg i.p.) increased CRF mRNA level, but did not change significantly CRF immunoreactivity in the central nucleus of the amygdala. Chronic morphine administration significantly increased the level of CRF mRNA 3, 24 and 48 h after the last dose. Both, acute and chronic cocaine administration increased CRF mRNA, but the peptide level was decreased only after acute cocaine administration. However, in the late withdrawal (48 h after the last dose of cocaine) both mRNA and the peptide levels tended to decrease. The above data suggest that amygdalar CRF system activity is potently activated after administration of morphine and cocaine, and that activation of this system observed at the time of withdrawal from morphine may be responsible for aversion and anxiety related to these states; therefore a CRF1 receptor may be a target for prospective pharmacotherapies of the withdrawal from abused drugs.

Effect of chronic imipramine or electroconvulsive shock on the expression of mGluR1a and mGluR5a immunoreactivity in rat brain hippocampus.

Previous studies showed that chronic electroconvulsive shock (ECS) or imipramine treatment induced a subsensitivity of group I metabotropic glutamate receptors (mGluR) in hippocampus. In the present study effects of antidepressant treatment on the expression of mGluR1a and mGluR5a, belonging to the group I mGluR, were investigated in rat brain hippocampus using immunohistochemical and Western blot methods, respectively. Male Wistar rats were treated singly or chronically for 21 days with imipramine, 10 mg/kg, twice daily; with ECS (90 mA, 50 Hz, 0.5 s) every second day; or with haloperidol, 1.2 mg/kg, once daily. Appropriate controls were injected with saline. Rats were sacrificed 24 h after the last treatment and their hippocampi were taken out for analysis. It was found that the mGluR1a-immunoreactivity expression increased significantly in Ammon's horn (CA) regions after chronic ECS. The most pronounced effect was observed in the CA3. No significant effects were found after single treatment or after haloperidol. The expression of mGluR5a increased significantly after chronic imipramine in the CA1 and after chronic ECS in the CA3 region. The results obtained indicate an influence of antidepressant treatment on group I mGluR. This increase in the receptor protein level may be a compensatory mechanism developing after chronic treatment.

A search for colocalization of mglula receptors with CRF or NPY in the rat brain amygdala.

Excitatory neurotransmitter glutamate, as well as corticoliberin (CRF) and neuropeptide Y (NPY) play an important role in fear and anxiety. Among the brain structures engaged in these effects the important one is amygdala. In the present study, a single and double immunohistochemical staining techniques were used in order to visualize CRF, NPY and metabotropic glutamate receptors (mGluR1a) in rat amygdala. MGluR1a belongs to class of postsynaptic excitatory receptors and has a preferable somatic localization. CRF and NPY were localized using rabbit polyclonal antibodies, and mGluR1a using a mouse monoclonal one. Then, ABC-peroxidase and DAB or benzidine were used. Upon single immunostaining, NPY and CRF were found in some nerve cell bodies and fibres in the amygdala. The immunoreactivity of mGluR1 a was observed in some nerve cells, processes and fibres, especially on the border between the central and the basolateral nuclei and ventrally to that region. Double staining revealed mGluR1 a-IR on some CRF- and NPY-immunoreactive nerve cell bodies and processes. The obtained results indicate that mGlu1a receptors may control at least some NPY and CRF neurons in the amygdala.

Decline in motor functions in aging is related to the loss of NMDA receptors.

The aim of the study was to assess the contribution of central dopaminergic and glutamatergic systems to the age-dependent loss of motor functions in rats. Rats of three age groups were compared: young (3-5-month-old), middle-aged (20-21-month-old) and old (29-31-month-old). The obtained results showed an age-dependent decline in the electromyographic (EMG) resting and reflex activities in the gastrocnemius and tibialis anterior muscles, as well as in the T-maze performance. Although these disturbances were accompanied with significant age-dependent decreases in the binding to NMDA, AMPA and dopamine D2 receptors, and a decline in the number of nigral dopamine neurons, they were significantly correlated with the loss of the binding to NMDA receptors only. The reduction in T-maze performance with aging was additionally correlated with a decrease in motor functions (EMG activity). The study suggests a crucial role of the loss of NMDA receptors in age-dependent motor disabilities, as well as in disturbances measured in the T-maze.

Pharmacological studies on the monoaminergic influence on the synthesis and expression of neuropeptide Y and corticotropin releasing factor in rat brain amygdala.

Our earlier findings concerning the 6-OHDA lesion suggested dopaminergic regulation of neuropeptide Y (NPY) and corticotropin releasing factor (CRF) synthesis and expression in amygdala neurons. On the other hand, some other studies indicated that not only dopamine, but also other monoamines may modulate peptidergic neurons. Therefore the present study examined the effect of pharmacological deprivation of monoaminergic influences on NPY and CRF neurons in rat brain amygdala by means of in situ hybridization and immunohistochemical methods. It was found that NPY mRNA expression in the amygdala decreased after 24h blockade of dopaminergic D1 and D2 receptors, by haloperidol or SCH23390. At the same time the NPY-peptide expression measured immunohistochemically was not significantly changed. A prolonged, 14-day, blockade of dopaminergic receptors by haloperidol induced an opposite effect, an increase in NPY mRNA expression. Impairment of the serotonergic transmission by blockade of 5-HT synthesis using p-chlorophenylalanine, as well as attenuation of the noradrenergic transmission by NA depletion from terminals by DSP4, did not significantly change NPY mRNA expression or the mean number of NPY-immunoreactive neurons in the amygdala. Only a decrease in the staining intensity observed as a decreased number of darkly stained neurons was found after both compounds. Neither the dopamine receptor blockade nor the impairment of serotonergic or noradrenergic transmission changed CRF mRNA or the peptide expression in the amygdala. The obtained results indicate that in rat brain amygdala, of all the monoamines, dopamine seems to be the most important modulator of NPY biosynthesis and expression. The effect of blockade of dopaminergic receptors is biphasic: first it induces a decrease and then - after prolonged treatment an increase in NPY mRNA. Serotonergic and noradrenergic systems in the amygdala seem to be connected with regulation of NPY release rather than the biosynthesis.

The effect of competitive and non-competitive NMDA receptor antagonists, ACPC and MK-801 on NPY and CRF-like immunoreactivity in the rat brain amygdala.

Amygdala is the brain structure responsible for integrating all behavior connected with fear, and in this structure two neuropeptides, neuropeptide Y (NPY), corticoliberin (CRF) and the most abundant excitatory neurotransmitter glutamate seem to take part in the regulation of anxiety behavior. Our previous studies showed the modulation of NPY and CRF expression by classical neurotransmitters in some brain structures, therefore in the present study we investigated the effect of NMDA receptor antagonists on the expression of NPY and CRF immunoreactivity in the rat brain amygdala. A non-competitive NMDA receptor antagonist, MK-801, or a functional NMDA antagonist, ACPC were used. Brains were taken out and processed by immunohistochemical method using specific NPY or CRF antibodies. The staining intensity and density of IR neurons were evaluated under a microscope in amygdala sections. It was found that both MK-801 and ACPC induced a significant decrease in NPY-immunoreactivity in amygdala nerve cell bodies and terminals, which may suggest an increased release of this peptide. CRF-IR was decreased after ACPC only. The obtained results indicate that in the amygdala, the NMDA receptors mediated glutamatergic transmission may regulate NPY neurons.

Effect of acute and chronic administration of 1,2,3,4-tetrahydroisoquinoline on muscle tone, metabolism of dopamine in the striatum and tyrosine hydroxylase immunocytochemistry in the substantia nigra, in rats.

The effects of acute and chronic administration of 1,2,3,4-tetrahydroisoquinoline, an endogenous substance suspected of producing parkinsonism in humans, on the muscle tone and metabolism of dopamine in the striatum, and on the number of tyrosine hydroxylase-immunoreactive cells in the substantia nigra were investigated in rats. Muscle tone was examined using a combined mechanomyographic and electromyographic method which measured simultaneously the muscle resistance of the rat's hind foot to passive extension and flexion in the ankle joint and electromyographic activity of the antagonistic muscles of that joint: gastrocnemius and tibialis anterior. 1,2,3,4-Tetrahydroisoquinoline administered at doses of 50 and 100 mg/kg intraperitoneally for 19 days increased muscle resistance 1 h after the first injection (acute treatment), 1 h after the last injection (chronic treatment) and three days after compound withdrawal. Rigidity observed on the third day of 1,2,3,4-tetrahydroisoquinoline withdrawal was accompanied by an increased tonic (resting) electromyographic activity of the gastrocnemius and tibialis anterior muscles. At the same time, a significant reduction in the number of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra and a decrease in the dopamine level in the striatum were also found. A declining number of tyrosine hydroxylase-immunoreactive neurons in the whole substantia nigra showed a significant negative correlation with the enhanced muscle resistance, as well as with the tonic electromyographic activity recorded at rest, i.e. before the start of movements, from the gastrocnemius and tibialis anterior muscles. Our results suggest that 1,2,3,4-tetrahydroisoquinoline may be one of the endogenous substances involved in the progress of Parkinson's disease.

Effect of chronic antidepressant or electroconvulsive shock treatment on mGLuR1a immunoreactivity expression in the rat hippocampus.

The effect of chronic imipramine (IMI) or electroconvulsive shock (ECS) treatment on the expression of group Ia metabotropic glutamate receptors (mGluR1a) was studied in the rat hippocampus by an immunohistochemical method, using a specific monoclonal antibody. It was found that both those treatments increased the number of mGluR1a immunoreactive neurons in a pyramidal layer of the CA3 hippocampal field. Moreover, IMI, but not ECS, increased the density of mGluR1 a positive neurons in the hilus. The obtained results indicate a possible influence of the antidepressive treatment on the mGluR1a expression in some hippocampal fields.

Effect of 6-hydroxydopamine on neuropeptide Y and corticotropin-releasing factor expression in rat amygdala.

The influence of dopaminergic denervation on neuropeptide Y and corticotropin-releasing factor-containing neurons in the amygdala was investigated in rats by examining the effects of a selective, unilateral 6-hydroxydopamine lesion of mesencephalic dopaminergic neurons in both the substantia nigra and the ventral tegmental area on these peptides and their messenger RNA expression, observed eight to 10 days after the lesion. The studies were conducted by immunocytochemical and in situ hybridization methods. Neuropeptide Y or corticotropin-releasing factor-immunoreactive neurons were counted in sections of the amygdala under a microscope, and the messenger RNA expression was measured as optical density units in autoradiograms. A significant increase in both neuropeptide Y and corticotropin-releasing factor messenger RNA expression was found in the amygdala on the lesioned side in comparison with the contralateral one, as well as with the ipsilateral side of vehicle-injected controls. Immunohistochemical studies showed that the number of neuropeptide Y-immunoreactive neurons increased in the whole amygdala on the lesioned side. At the same time, the number of corticotropin-releasing factor-immunoreactive neurons grouped in the central amygdaloid nucleus declined, and so did the staining intensity. The obtained results indicate that dopaminergic denervation stimulates the synthesis of neuropeptide Y and corticotropin-releasing factor in rat amygdala, but the peptide levels are differently regulated, which points to a diverse release of these peptides.

Reserpine induces increase in neuropeptide Y immunoreactivity in rat amygdala neurons.

The effect of reserpine, a monoamine-depleting agent, on the neuropeptide Y (NPY) immunoreactivity was studied immunohistochemically in the amygdaloid complex (amygdala) of rat brain. It was found that reserpine, at a dose of 10 mg/kg i.p., after 24 h, enhanced the NPY immunoreactivity in amygdala neurons, which was expressed as an increase in the staining intensity and in the number of immunoreactive neurons visible in that structure. The obtained results suggest that monoamines take part in the regulation of NPY expression in amygdala neurons of rat brain, and that elimination of these monoaminergic regulation leads to an increase in the NPY content in that structure.

Clonidine administration increases neuropeptide Y immunoreactivity and neuropeptide Y mRNA in the rat cerebral cortex neurons.

The effect of alpha 2 adrenoceptor stimulation on neuropeptide Y (NPY) and NPYmRNA expression was studied in the rat cerebral cortex. For receptor stimulation clonidine was used in a dose of 50 micrograms/kg s.c., 3 times at every 8 h; brains were studied 30-40 min after the last dose using radioimmunoassay (RIA), immunocytochemistry and in situ hybridization methods. The RIA of NPY did not show any significant changes in the NPY immunoreactivity (IR) level in the whole cortex, whereas the immunohistochemical analysis demonstrated an increase in the number of NPY-IR neurons in ventral cortical regions, especially in external cortical layers. In situ hybridization histochemistry of NPYmRNA also performed in ventral cortical sections showed that clonidine increased NPY synthesis in some cortical neurons. The obtained results indicate that the alpha 2 adrenoceptor stimulation by clonidine increases the NPY content and synthesis in rat cortical neurons.

Activation of metabotropic glutamate receptors increases neuropeptide Y expression in the rat hippocampus. Immunohistochemical studies.

Neuropeptide Y (NPY), a 36-amino acid peptide, is present in some hippocampal interneurons and nerve terminals and seems to modulate glutamatergic transmission in this structure. Earlier studies of some other authors showed an increase in NPY expression in the hippocampus during seizures and activation of ionotropic glutamate receptors. In the present study the effect of metabotropic glutamate receptor (mGluR) stimulation was investigated in rat hippocampus by immunohistochemical methods. It was found that (1S,3R)1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist of mGluRs injected into the rat lateral ventricle (1 mumol/10 microliters) or hippocampus (0.1 mumol/1 microliter), 24 h before taking the brains for immunohistochemical studies, induced a significant increase in NPY-immunoreactivity in the hippocampus, especially in the hilar region. The obtained results indicate that mGluRs positively modulate the NPY content in the hippocampal neurons.

Influence of 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway on the muscle tone and electromyographic activity measured during passive movements.

The aim of the present study was to find out whether a 6-hydroxydopamine-induced lesion of the substantia nigra in rats would evoke muscular rigidity of the parkinsonian type. Simultaneous measurements of muscle resistance (mechanomyogram) of the hind foot to passive flexion and extension at the ankle joint, as well as of the electromyographic activity of the antagonistic muscles of the ankle joint--the gastrocnemius and tibialis anterior--in rats were carried out one, two and four weeks after bilateral injections of 6-hydroxydopamine (6.5 micrograms/microliter) into the substantia nigra. After immunohistochemical staining of brain sections for tyrosine hydroxylase, the rats were divided into two groups in which, on average, either 70% (63-80%) or 89% (81-96%) of nigral cells degenerated. Larger lesions increased the resistance (mechanomyogram) of the rat's hind leg to passive movements two weeks after 6-hydroxydopamine injection, whereas smaller lesions did not. Muscle rigidity was accompanied by an increase in the movement-induced reflex electromyographic activity in both muscles, mainly in long-latency components which are most probably influenced by supraspinal mechanisms. However, in spite of relatively large lesions of nigral dopamine cells, already four weeks after the lesion, muscle rigidity and the respective electromyographic activity diminished dramatically, which seems to result from very effective compensatory mechanisms operating in young lesioned rats. The results suggest that the muscle rigidity induced by the 6-hydroxydopamine nigral lesion seems to be a good model of parkinsonian rigidity.

Inhibitory effect of NPY on the picrotoxin-induced activity in the hippocampus: a behavioural and electrophysiological study.

The effect of neuropeptide Y (NPY) on the picrotoxin-induced activity was studied in rat brain hippocampal slices in vitro and after intrahippocampal injection in vivo. In the hippocampal slices, NPY (0.1-0.5 microM) inhibited the picrotoxin-induced epileptiform activity recorded extracellularly in CA1 and CA3 hippocampal pyramidal cells. Similar inhibition was induced by the Y2 receptor agonist NPY13-36, which indicates that the effect of NPY was due to activation of Y2 receptors. In behavioural studies, rats with chronically implanted cannulae were injected unilaterally into the CA1 hippocampal region with a 1 ml volume of the studied substances. Picrotoxin in a dose of 1 mg (1.6 nmol) induced behavioural excitation, shakes and weak signs of epileptic behaviour. NPY in a dose of 2 mg (470 pmol), but not 1 mg, inhibited some excitatory effects of picrotoxin, but did not change the epileptic symptoms. The obtained results suggest that NPY has an inhibitory action in the hippocampus, which can be observed in vitro and also in a behavioural study.

Inhibitory effect of intrahippocampal NPY injection on amphetamine-induced behavioural activity.

The aim of the present study was to investigate the influence of intrahippocampal neuropeptide Y (NPY) administration on the rats' behaviour. The CA1 field or dentate area (GD) of the dorsal hippocampus was chronically implanted with intracerebral cannulae. NPY (or redistilled water in the control group) was injected bilaterally or unilaterally in a volume of 1 microliter to each hippocampus, in a dose of 1 or 2 micrograms per rat. A separate group of rats was pretreated with amphetamine (1 mg/kg, s.c.), 15 min before a bilateral microinjection of NPY (2 micrograms) into the CA1 region. Immediately after the intrahippocampal injection, the rats' behaviour was tested in an open field box. It was found that NPY did not change the locomotor and the exploratory activity after either CA1 or GD administration to non-pretreated animals. In amphetamine pretreated rats, NPY injected into the CA1 field inhibited the amphetamine induced increase in sniffing and rearing and, to a lesser extent, the number of line-crossings. The obtained results may suggest an inhibitory action of NPY in the CA1 hippocampal field on the behavioural hyperactivity.