Partial lesions of the nigrostriatal pathway in the rat. Acceleration of transmitter synthesis and release of surviving dopaminergic neurones by drugs

In rats with partial, unilateral lesions of the dopaminergic nigrostriatal pathway, synthesis of dopamine (DA) per surviving neurone was assessed by measuring the ratio of DOPA accumulated after inhibition of aromatic amino acid decarboxylase to dopamine (DOPA/DA ratio). Release of DA per surviving neurone was assessed by measuring the ratios of the concentrations of 3,4-dihydroxyphenylalanine (DOPAC) and homovanillic acid (HVA) to DA. In striata ipsilateral to the lesion, the DOPA/DA ratio was elevated 4-fold, and the DOPA/DA and HVA/DA ratios were elevated 2-fold as compared to values in contralateral striata. Administration of gamma-butyrolacetone (750 mg/kg, 30 min), i.e. of a drug which accelerates synthesis of DA, further increased the DOPA/DA ratio in lesioned striata to levels 37 times higher than those measured on the control side of saline-injected controls. Morphine (20 mg/kg, 30 min) and haloperidol (2.5 mg/kg, 60 min), i.e. drugs known to accelerate the turnover and release of DA, further elevated the DOPAC/DA and HVA/DA ratios in lesioned striata to levels approx. 5 times higher than the ratios measured on contralateral sides of saline-treated controls. The data indicate that dopaminergic neurones surviving partial lesions of the nigrostriatal pathway synthesize and release DA at an elevated but submaximal rate. Synthesis and release of DA can be further enhanced to a large extent by drugs.     

Brain pharmacokinetics of a nonpeptidic corticotropin-releasing factor receptor antagonist.

Corticotropin-releasing factor (CRF) is known to play an important role in the body response to stress. Butyl-[2,5-dimethyl-7-(2,4,6-trimethylphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]-ethylamine (CP-154,526) is a CRF(1) antagonist showing anxiolytic activities in rats in behavioral models, suggesting that CP-154,526 crosses the blood-brain barrier. However, there is no direct evidence for this. This study determined the pharmacokinetic profile of CP-154,526 in rats after i.v. and p.o. application. After i.v. bolus, the concentration declined in a biphasic manner, the first half-life being 0.9 h and the terminal one being 51 h. Systemic clearance was 36 ml/min/kg, and the volume of distribution was 105 l/kg. Oral bioavailability reached 27%. To study brain pharmacokinetics, rats were given a single dose of CP-154,526 p.o. or i.v. and sacrificed after different post-treatment times. Plasma, cortex, striatum, hypothalamus, hippocampus, and cerebellum concentrations were measured. After i.v. bolus, maximal brain concentration was reached after 20 min. The hypothalamus displayed significantly lower concentrations compared with the other brain tissues. In the p.o. study, the maximal plasma concentration was reached after 30 min, whereas the maximal brain concentration was observed after 1 h and remained stable until 2 h post-treatment, without significant differences between the brain tissues. The unidirectional brain extraction ratio was 27 and 9% at vascular concentrations of 0.08 and 16 nmol/ml, respectively. These results demonstrate a large brain penetration of CP-154,526 after i.v. and p.o. applications and a comparable distribution among the brain regions, except for the hypothalamus, which displayed lower concentrations after i.v. bolus.     

Biochemical analysis of catecholamines in small intensely fluorescent (SIF) cell clusters of the rat superior cervical ganglion

The microlaser technique of isolating small cell clusters has been applied to groups of small intensely fluorescent (SIF) cells in rat superior cervical ganglion. Alternate cryostate sections were either incubated in glyoxylic acid monohydrate or freeze-dried. SIF cell clusters, recognized by glyoxylic acid-induced fluorescence, were re-identified in the consecutive freeze-dried section through dark-field microscopy. These clusters were dissected with a BTG microlaser unit and collected for biochemical assay. The catecholamine content of the specimens was measured by gas chromatography/mass fragmentography, using 3 deuterated catecholamines as an internal standard and calibration curves of each catecholamine. The findings indicate the presence of these 3 catecholamines in rat SIF cell clusters in a varying amount: in probes, each consisting of 5 cell clusters, the content of norepinephrine averaged approximately 7.3 pmol, of epinephrine below 1 pmol, and of dopamine from below 1 pmol to 14.6 pmol.     

Synaptic clustering of GABAC receptor ρ‐subunits in the rat retina

Polyclonal antibodies which recognize the ρ-subunits of the GABA_C receptor were applied to sections of the rat retina. Strong punctate immunoreactivity was found in the inner plexiform layer (IPL), which was shown by electron microscopy to represent a clustering of the GABA_C receptors at synaptic sites. During postnatal development diffuse ρ-immunoreactivity was first observed at postnatal day P3. Distinct labelling of bipolar cells appeared at P7 and punctate, synaptic labelling was observed at P10. In order to show that the ρ-immunoreactive puncta coincide with the axons of bipolar cells, double immunostainings of retinal sections with an antiserum against syntaxin 3 and with the ρ-antiserum were performed. The experiments showed that ρ-immunoreactive puncta are preferentially located on the axon terminals of rod and cone bipolar cells. In order to determine whether GABA_C receptor ρ-subunits coassemble with GABA_A receptor subunits, double-labelling experiments were performed with subunit specific antisera. Punctate, putative synaptic clustering was observed with all antisera applied, however, GABA_C receptor expressing puncta did not coincide with GABA_A receptor containing puncta. This suggests that there are no synaptic GABA receptors in the retina in which GABA_A and GABA_C receptor subunits are coassembled. Similar double-labelling experiments were also performed to find out whether GABA_C receptors and glycine receptors are colocalized. They were clustered at different synapses. This suggests that synaptic GABA_C receptors consist of ρ-subunits and are not coassembled with GABA_A- or glycine-receptor subunits.     

Partial dopamine-agonistic and atypical neuroleptic properties of the amino-ergolines SDZ 208-911 and SDZ 208-912

The aminoergolines SDZ 208-911 [N-[(8-alpha)-2,6-dimethylergoline-8-yl]-2,2- dimethylpropanamide] and SDZ 208-912 [N-[8-alpha)-2-chloro-6-methylergoline-8-yl]- 2,2-dimethylpropanamide] exhibit nonclassical, neuroleptic-like properties in rodents. Thus, they are equipotent to haloperidol as inhibitors of apomorphine-induced gnawing behavior and conditioned avoidance responding, but are essentially devoid of cataleptogenic activity. In addition, they show high affinity for central D-2 receptors in vitro and elevate striatal homovanillic acid levels. In contrast to haloperidol, however, SDZ 208-911 and 208-912 strongly inhibit prolactin secretion and induce contralateral circling behavior in 6-hydroxydopamine-lesioned animals. These profiles are consistent with the drugs exhibiting varying degrees of partial agonistic activity at dopamine D-2 receptors, with SDZ 208-911 being considerably more agonistic than SDZ 208-912. Support for this contention stems from the ability of SDZ 208-911 to reduce the elevation of striatal L-dopa formation induced by gamma-butyrolactone, and SDZ 208-912's partial reversal of apomorphine's inhibitory action on gamma-butyrolactone activity. SDZ 208-911's effects are reduced after the partial alkylation of D-2 receptors with N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline, confirming its partial agonistic properties. SDZ 208-911 and SDZ 208-912 could be effective against both the positive and negative symptoms of schizophrenia, while exhibiting a reduced incidence of dystonic and parkinsonian side-effects. In addition, their clinical testing might throw more light on the central dopaminergic status of schizophrenic subjects.     

Biochemical,behavioural, and endocrine effects of CK 204-933, a novel 8β-ergolene

Summary CK 204-933 displaces [³H]dopamine and [³H]spiperonene with high affinity from D-1 and D-2 recognition sites in membranes of calf caudate. Results from functionalin vitro tests suggest that it is a partial agonist at D-1 receptors and an antagonist at D-2 receptors. These opposite effects at dopamine receptor subtypes are also expressedin vivo. For instance, in 6-hydroxydopamine lesioned rats, CK 204-933 induces contralateral rotations which are antagonised by SCH 23390 but not by sulpiride. On the other hand, CK 204-933 induces a long lasting increase of dopamine turnover in rat striatum and antagonises apomorphine-induced gnawing behaviour in rats. CK 204-933 increases prolactin serum levels in rats after subcutaneous administration, whereas after oral administration a moderate decrease of prolactin serum levels was seen. The latter effect is probably due to the formation of active metabolites. CK 204-933 exhibits also a high affinity to [³H]prazosin binding sites and antagonises serotonin-mediated stimulation of adenylate cyclase in rat hippocampus. On the other hand, CK 204-933 has no effect of only very weak effects on noradrenaline and serotonin release from rat cerebral cortex slices, which is consistent with its weak effects on noradrenaline- and serotonin-turnover in rat brain. Based on these properties it is suggested that CK 204-933 could be of therapeutic value in brain diseases associated with disturbances of monoaminergic neurotransmission.