Dopamine and serotonin metabolism in striatum and in the septohippocampal pathway of the Snell dwarf mouse

Differential effects of delayed aging on phenotype and striatal pathology in a murine model of Huntington disease

The common neurodegenerative syndromes exhibit age-related incidence, and many Mendelian neurodegenerative diseases exhibit age-related penetrance. Mutations slowing aging retard age related pathologies. To assess whether delayed aging retards the effects of a mutant allele causing a Huntington's disease (HD)-like syndrome, we generated compound mutant mice, placing a dominant HD knock-in polyglutamine allele onto the slow-aging Snell dwarf genotype. The Snell genotype did not affect mutant huntingtin protein expression. Bigenic and control mice were evaluated prospectively from 10 to 100 weeks of age. Adult HD knock-in allele mice lost weight progressively with weight loss blunted significantly in male bigenic HD knock-in/Snell dwarf mice. Impaired balance beam performance developed significantly more slowly in bigenic HD knock-in/Snell dwarf mice. Striatal dopamine receptor expression was diminished significantly and similarly in all HD-like mice, regardless of the Snell genotype. Striatal neuronal intranuclear inclusion burden was similar between HD knock-in mice with and without the Snell genotype, whereas nigral neuropil aggregates were diminished in bigenic HD knock-in/Snell dwarf mice. Compared with control mice, Snell dwarf mice exhibited differences in regional benzodiazepine and cannabinoid receptor binding site expression. These results indicate that delaying aging delayed behavioral decline with little effect on the development of striatal pathology in this model of HD but may have altered synaptic pathology. These results indicate that mutations prolonging lifespan in mice delay onset of significant phenotypic features of this model and also demonstrate dissociation between striatal pathology and a commonly used behavioral measure of disease burden in HD models.

A neurotoxic lesion of serotonergic neurones using 5,7-dihydroxytryptamine does not disrupt latent inhibition in paradigms sensitive to low doses of amphetamine

Testing the effects of low doses of d-amphetamine on latent inhibition (LI) in two different conditioning paradigms, passive avoidance and conditioned taste aversion, provided evidence of their pharmacological equivalence. For passive avoidance, LI was expressed by the decreased latency to enter a shock compartment in preexposed rats placed 5 min in the compartment during 3 consecutive days before conditioning. In the conditioned taste aversion paradigm, a group of rats was preexposed to a solution of sucrose also for 3 consecutive days prior to the establishment of an association between sucrose and sickness elicited by an injection of LiCl. On the following day, the preexposed rats drunk more sucrose when allowed to choose between one tube containing water and an other containing sucrose. In both paradigms, 0.25 mg/kg d-amphetamine, injected daily on the 3 preexposure days and on the conditioning day, decreased LI. A dose of 0.5 mg/kg suppressed LI in the passive avoidance paradigm. The effect of a serotonergic lesion induced by i.c.v. injection of 5,7-dihydroxytryptamine (5,7-DHT) was evaluated in the same paradigms. The lesion procedure that lowered hippocampal serotonin and 5 HIAA levels by more than 80% did not affect LI. Taken together, the present results lessens the hypothesis that LI is prone to an opposing influence of the two monoaminergic systems considered in this work.

Effect of 5,7-dihydroxytryptamine lesion on mianserin-induced conditioned place aversion and on 5-hydroxytryptamine1C receptors in the rat brain

The lesion of serotonergic neurons (by an intraventricular injection of 5,7-dihydroxytryptamine) potentiated the conditioned place aversion induced by the 5-hydroxytryptamine1C/5-hydroxytryptamine2 antagonist mianserin in rats. This effect was selective for mianserin as the same lesion suppressed the conditioned place aversion induced by the benzodiazepine inverse agonist FG-7142. Previous results had shown the involvement of the 5-hydroxytryptamine1C receptors in the conditioned place aversion induced by mianserin [Rocha et al. (1993) Behav. Pharmac. 4, 101-106]. It was thus of interest to investigate the effect of the lesion on these receptor binding sites. Autoradiographic binding studies showed that the lesion significantly increased the concentration of the 5-hydroxytryptamine1C binding sites in various brain regions, including the amygdala, the hippocampus and the nucleus accumbens. Contrastingly, in these same brain regions, in situ hybridization histochemistry did not reveal an alteration of the level of messenger RNA coding for these receptors. On the one hand, correlating potentiation of the aversive effects of mianserin and increase of 5-hydroxytryptamine1C binding sites in the limbic system represent an interesting step in the comprehension of the molecular and motivational effects of serotonergic drugs. On the other hand, showing a dissociation between the expression of 5-hydroxytryptamine1C receptors and their corresponding messenger RNA, suggest that post-transcriptional mechanisms are involved in the regulation of these receptors.

Effect of age at the time of pituitary grafting on behavioral recovery in dwarf mice

The effects of ectopic pituitary isograft implanted in 13- or 21-day-old Snell dwarf mice on their freely expressed behavior were examined over a 14- or 31-day period. This mutant strain has severe anterohypophyseal deficiency which causes high levels of inactivity and abnormally low exploratory activity. The graft totally normalized these behaviors. Results showed that the time necessary for inducing behavioral changes is independent of age of the animals at the time of grafting, but the age at which implantation occurs influences the speed of full behavioral normalization. The possible involvement of prolactin and growth hormone in such behavioral improvement is discussed.

Involvement of the nigral output pathways in the inhibitory control of the substantia nigra over generalized non-convulsive seizures in the rat

Activation of GABAergic transmission within the substantia nigra has been shown to suppress several forms of generalized seizures in experimental models of epilepsy. More especially, such pharmacological manipulations suppress spontaneous and chemically-induced generalized non-convulsive seizures in the rat. The aim of the present study was to examine the role of the dopaminergic and GABAergic thalamic and collicular nigral outputs in this antiepileptic effect. For this purpose, we examined the effects of output destruction on the antiepileptic effect of intranigral injections of a GABA agonist or pharmacological blockade of the neurotransmission at the nerve terminal level in rats with spontaneous absence seizures. After selective destruction of dopaminergic neurons within the substantia nigra with 6-hydroxydopamine (5 micrograms/side) or hemisection of the ascending nigral output, bilateral intranigral injection of muscimol (2 ng/side) still significantly suppressed generalized non-convulsive seizures. Bilateral lesioning of the ventromedial nucleus of the thalamus did not abolish the antiepileptic effects of intranigral muscimol (2 ng/side) and the GABA antagonist, picrotoxin, when given into this thalamic nucleus (10 ng/side) also failed to induce suppression of spike and wave discharges. The antiepileptic effects of intranigral injection of muscimol (2 ng/side) was reversed by bilateral electrolytic lesions of the superior colliculus. Blockade of the GABAergic transmission at this level with picrotoxin (40 ng/side) significantly suppressed generalized non-convulsive seizures. Finally, excitation of collicular cell bodies with low doses of kainic acid (4 and 8 ng/side) also resulted in a suppression of spike and wave discharges. These results demonstrate that the GABAergic nigrocollicular pathway is critical for the inhibitory control of the substantia nigra over generalized non-convulsive seizures. The data further suggest that antiepileptic effects observed following potentiation of GABAergic transmission in the substantia nigra result from a disinhibition of collicular cell bodies.

Genetic mapping and analysis of somatostatin expression in Snell dwarf mice

Mice homozygous for the gene dwarf (dw) have elevated levels of somatostatin (SS) in extra-hypothalamic brain regions. By in situ hybridization, increased levels of SS mRNA were observed in regions shown previously to contain higher levels of the SS peptide. Thus, the rate of transcription and/or the stability of SS mRNA are affected by the dw mutation. Since both dw and the gene encoding SS, Smst, are located on mouse chromosome 16, two backcrosses segregating dw and Smst were used to determine whether dw is an allele of Smst. In one backcross, an inbred strain derived from the subspecies Mus musculus molossinus was used to provide a high degree of DNA sequence polymorphism. The gene order and map distances determined on this backcross were: (centromere) - Prm-1 - 7 - Igl-1 - 3 - Smst - 29 - dw - 15 - Sod-1 - 4 - Ets-2, demonstrating clearly that Smst and dw are distinct genes. Additional evidence against a primary role for SS excess in the pathogenesis of dw/dw mice was obtained by injecting normal newborn mice with a potent SS analog (cyclo II). In contrast to the pattern of cell loss observed in the dwarf anterior pituitary, the pituitaries of injected mice were indistinguishable from normal controls, further suggesting that the Smst locus is not the primary site of dw gene action.

Effects of hormone therapy on the central cholinergic neurotransmission of the Snell dwarf mouse

Effects of growth and thyroid hormone therapy on central cholinergic neurotransmission has been followed in the cholinergic-deficient Snell dwarf mouse. Growth hormone and thyroxine can reverse the neurotransmission impairment even in adulthood. Furthermore, it appears that in the dw/dw mouse, hormone deficiency becomes determinant only after the critical period of neurogenesis, in a late postnatal developmental stage. At first sight, these observations show that the disturbed cholinergic neurotransmission of the mutant might be linked to the pituitary and thyroxine deficiency that is characteristic of this mouse. Hormone therapy selectively stimulates cholinergic activity in cholinergic-deficient structures, where it reactivates presynaptic markers. The differential responsiveness of the neuroanatomical and neurochemical targets could be linked to different regulatory hormone effects or to timing in respect to sensitivity to hormones during development.