Biomedical subjects
Publications and source records attributed to J H Gordon.
Although a permanent supersensitivity to dopamine agonists can be induced by lesions of the nigrostriatal dopamine tract, the ovariectomized rat is the first animal model of a permanent hypersensitivity to dopamine agonists in which the neurons survive. This permanent behavioral hypersensitivity to direct acting dopamine agonists is accompanied by an increase in D2 dopamine receptor density in the striatum.
Regulation of synaptic membrane adenylate cyclase is likely to involve interaction between neurotransmitter receptors, G-proteins and the adenylate cyclase catalytic unit as well as several other membrane proteins and lipids. Despite intensive study of this system, regulation of guanine nucleotide binding by the G-proteins which stimulate [Gs] or inhibit [Gi] adenylate cyclase has been examined only when those proteins have been purified and removed from the influence of the membrane environment. The hydrolysis-resistant photoaffinity GTP-analog, P3-(4-azidoanilido)-P1 5'-GTP (AAGTP) is able to bind specifically to the G-proteins in rat cerebral cortex synaptic membranes and, in this study, we have used this probe to examine the specificity and selectivity of guanine nucleotide binding to each G-protein without removing those proteins from the synaptic membrane. Marked differences were noted between guanine nucleotide binding data obtained with detergent-soluble G-proteins and data from this in situ approach. In these studies it was found that the affinity of the G-proteins binding AAGTP correlated well with the expression of adenylate cyclase activity, the affinity of both forms of Gs increasing under conditions favoring the stimulation of that enzyme.
Metoclopramide and sulpiride, two benzamide compounds, are equally potent in terms of their ability to block postsynaptic D2 dopamine receptors. However these compounds show a marked divergence in their ability to block dopamine autoreceptors, as metoclopramide is 20-25-fold more potent than sulpiride in blocking these receptors. When injected twice daily for 16 days, metoclopramide at a dose of 10 mg/kg/day will result in the development a postsynaptic dopamine receptor hypersensitivity (i.e., increased behavioral response to apomorphine upon cessation of the chronic treatment). An equivalent dose and treatment schedule with sulpiride has no apparent effect on dopamine receptor sensitivity. Because of the divergent pre- and postsynaptic potency of these two drugs it was possible to construct an autoreceptor "dose response curve" by varying the amount of these two drugs injected. Combinations of these two drugs were chosen so that the level or amount of postsynaptic dopamine receptor blockade was held constant while the amount of dopamine autoreceptor blockade was gradually increased. The results of this autoreceptor blockade "dose response curve" indicated that chronic autoreceptor blockade was involved in the increased dopamine receptor sensitivity that develops upon withdrawal from the neuroleptic drugs. These results suggest that the blockade of dopamine autoreceptors, and perhaps the resulting increase in autoreceptor sensitivity, is an integral component of the neuroleptic-induced dopamine receptor hypersensitivity.
The administration of estrogen to ovariectomized rats will result in a dopamine receptor hyposensitive phase at 24 hours followed within 48 hours by a hypersensitive phase. In an attempt to characterize further the molecular mechanism(s) which underlie this biphasic response, we studied the effects of estrogen and one of its metabolites (2-hydroxyestradiol) on striatal D-2 dopamine receptor agonist affinity states. Ovariectomized rats were treated daily with estradiol benzoate (EB; 10 micrograms/kg/day) for 3 days and sacrificed 24 hours after the last dose. Additional groups were treated with 2-hydroxyestradiol (2-OHE 2; 3 micrograms/kg/day), EB (100 micrograms/kg/day) or appropriate vehicle for 3 days and sacrificed 72 hours after the last injection. Animals sacrificed during the hyposensitive phase (i.e. 24 hours) displayed a significant decrease in the ratio of high/low agonist affinity states of the striatal D-2 receptor, while animals sacrificed during the EB- or 2-OHE 2-induced hypersensitive phase (i.e. 72 hours) showed no change in the ratio of high/low agonist affinity states.
The effects of the putative dopamine agonist, ciladopa hydrochloride (AY 27,110) a non-ergot compound, were investigated in animal models of dopaminergic activity to evaluate its possible role in the treatment of Parkinson's disease. Ciladopa induced stereotyped behavior in both rats and guinea pigs. Unlike apomorphine, however, ciladopa did not produce a maximum behavioral response, i.e. stereotyped gnawing. Pretreatment with haloperidol and sulpiride blocked the effects induced by ciladopa. Pretreatment with reserpine and alpha-methyl-p-tyrosine did not alter the behavioral effects of ciladopa. Ciladopa caused contralateral rotation in rats with unilateral lesions of the substantia nigra induced by 6-hydroxydopamine. Ciladopa induced vomiting in dogs. Small doses of ciladopa decreased locomotor activity in rats, an effect presumably mediated by presynaptic autoreceptors. The chronic injection of both subthreshold and suprathreshold doses of ciladopa failed to induce behavioral supersensitivity. Ciladopa binds to D-2 dopamine receptors in the mammalian caudate nucleus. These data indicate that ciladopa can cause stimulation of central dopaminergic receptors and that the drug is a partial dopamine agonist with direct-acting properties. Ciladopa differs from other available dopaminergic drugs and may possess therapeutic advantages for the treatment of Parkinson's disease.
In the present study, we have confirmed the existence of a biphasic response in striatal dopamine receptor sensitivity following the administration of estradiol benzoate (EB). This biphasic response consists of a hyposensitive phase 24 h after the last injection of EB, followed by a hypersensitive phase 72 h after the last injection of EB. In contrast to this, the administration of 2-hydroxyestradiol (2-OHE2), a catechol metabolite of estrogen, resulted in a striatal dopamine receptor hypersensitivity at both 24 and 72 h after the last injection of 2-OHE2. Studies on the in vivo metabolism of [3H]estradiol to its [3H]catechol metabolites indicated that the administration of piperonyl butoxide (PBO; a microsomal enzyme inhibitor) significantly decreased the level of [3H]catechol metabolites of [3H]estradiol in the striatum and in the medial basal hypothalamus. In addition, PBO administration resulted in about a 7-fold decrease in the ability of estradiol to induce a striatal dopamine receptor hypersensitivity. These data indicate that the biphasic response in striatal dopamine receptor sensitivity following estrogen, may be mediated by separate molecular mechanisms. The association of the hypersensitive phase with pharmacological doses and/or treatment paradigms, the development of a similar hypersensitivity following the administration of the 2-OHE2 metabolite of estrogen and the attenuation of the estrogen-induced striatal dopamine receptor hypersensitivity in PBO pretreated animals all suggest that this striatal dopamine receptor hypersensitivity may be mediated, at least in part, by the catecholestrogens.
Timed-pregnant Sprague-Dawley rats were injected intraperitonealy with Kepone dissolved in sesame oil on alternate days beginning on day 18 of gestation and extending through day 7 of lactation with doses of 10 or 5 mg/kg. At 7 days of age the neurologic development of the pups was assessed using the following tests: day of eye opening, occurrence of auditory startle, righting response, reflex suspension, tactile forelimb-placing response, negative geotaxis, and open-field activity. At 70 days of age, vaginal washes from female offspring were examined microscopically for evidence of cyclic activity. At 120 days of age, animals were sacrificed and the brains were examined histologically and the volume of the medial preoptic nucleus was determined. The results indicated that exposure of rat pups to Kepone via either placental and/or milk transfer can result in an acute, or transient, impairment of neurologic development and a possible permanent neuroendocrine impairment. The permanent neuroendocrine dysfunction may be similar to the androgenization phenomenon (i.e., female pups exposed to sex steroids during development) and thus may be related to the reported estrogenic properties of Kepone.
Glutamic acid decarboxylase (GAD) activity was studied in specific brain regions of a newly identified genetic (rat) model of human torsion dystonia. GAD activity was found to be significantly increased in the deep cerebellar nuclei of dystonic rats at 16, 20, and 24 days of age. GAD activity in the other regions examined (vermis, cerebellar hemispheres, caudate nucleus, and globus pallidus) did not differ from that of age-matched normal littermate controls. Diazepam treatment significantly reduced the frequency of dystonic movements in the mutant.
It has been reported that hypophysectomy (HYPOX) would antagonize the development of a neuroleptic-induced dopamine receptor hypersensitivity, and suggested that the neuroleptic-induced dopamine receptor hypersensitivity may be mediated by the neuroleptic-induced hyperprolactinemia. Conversely, we and others have reported on the ability of HYPOX animals to develop a neuroleptic-induced dopamine receptor hypersensitivity. The present study was undertaken to define the possible role(s) of prolactin in the modulation of striatal dopamine receptor sensitivity. The data from these studies indicate: that HYPOX alone will result in the development of a striatal dopamine receptor hypersensitivity; that the HYPOX-induced dopamine receptor hypersensitivity could be increased by the chronic administration and withdrawal of haloperidol; that administration of prolactin to HYPOX rats would partially antagonize the development of the neuroleptic-induced dopamine receptor hypersensitivity; and that the administration of prolactin alone had minimal effects on the apomorphine-induced behavior or neurochemistry of the HYPOX animals. These results suggest that the neuroleptics do not require the presence of a pituitary secretion (specifically, prolactin) to induce a striatal dopamine receptor hypersensitivity; however, they do indicate that a pituitary secretion, perhaps prolactin, may have the ability to modulate striatal dopamine sensitivity.
Seven days post-hypophysectomy female rats display a hyposensitivity to the locomotor effects of apomorphine and a hypersensitivity to the stereotypy effects of apomorphine, while at 28 days post-hypophysectomy they are hypersensitive to both the locomotor and stereotypy effects of apomorphine. The hyposensitivity to the locomotor effects, at 7 days post-hypophysectomy, was associated with a decrease in 3H-spiroperidol binding and an increase in tyrosine hydroxylase activity in the nucleus accumbens septi, whereas the hypersensitivity, at 28 days post-hypophysectomy, was associated with an increase in 3H-spiroperidol and a decrease in tyrosine hydroxylase activity in the n. accumbens septi. The increased apomorphine-induced stereotypy in both the 7 and 28 days post-hypophysectomized animals was related to an increased 3H-spiroperidol binding and a decreased tyrosine hydroxylase activity in the striatum. These behavioral and neurochemical data demonstrate that following hypophysectomy female rats will develop a transient decrease in dopamine receptor sensitivity in the n. accumbens septi, while the dopamine sensitivity in the striatum is increased. Thus the hypophysectomized female rat may prove to be a valuable model to study these two separate dopamine systems and their possible modulatory roles in the display of various behaviors.
The administration of pharmacologic doses of estrogen results in a biphasic response in striatal dopamine sensitivity, as measured by apomorphine-induced stereotypy. At 24 hr after the last dose of estradiol benzoate (EB) there is a suppression of apomorphine-induced stereotypy, which is followed by an increased sensitivity to apomorphine at 48 hr. The dopamine hyposensitivity is reflected postsynaptically by an increased KD (i.e., decreased affinity) for 3H-spiroperidol binding to striatal membranes, while the hypersensitive phase is reflected by an increased Bmax for 3H-spiroperidol binding to striatal membranes. Presynaptically during the hyposensitive phase the tyrosine hydroxylase displayed a decreased KM for the pterine cofactor. The decreased KM for the cofactor was retained in the hypersensitive animals, however the Vmax for tyrosine hydroxylase was decreased during the hypersensitive phase of the EB-induced changes in dopamine sensitivity. The presynaptic or autoreceptor sensitivity of the dopamine neurons projecting to the striatum was assessed by determining the apomorphine IC50 value for the inhibition of synaptosomal tyrosine hydroxylase activity. Utilizing this assay the animals that were hyposensitive to dopamine showed a normal presynaptic sensitivity, while those animals that had developed a hypersensitivity to dopamine following EB were also hypersensitive to dopamine presynaptically.
Administration of estrogen to rats during the period of withdrawal from chronic haloperidol attenuated the characteristic increase in apomorphine-induced stereotypy and the increase in (3H) spiroperidol binding. This apparent ability of estrogen to "down-regulate" brain dopamine receptors could lead to useful pharmacological treatments of tardive dyskinesia and possibly of other hyperdopaminergic states.
Hypophysectomized (HYPOX) or sham HYPOX (SHAM) and ovariectomized rats were injected with either estradiol benzoate (EB, 10 microgram/kg/day x 3) or sesame oil (0.25 ml/kg/day x 3). Twenty hours after the last dose of EB or oil all animals were injected with apomorphine and the resulting stereotypy scored. The HYPOX + oil group was more sensitive to the apomorphine than the SHAM + oil group (ED50 = 0.24 and 0.45 mg/kg, respectively). Treatment with EB resulted in a shift to the right of the dose response curves for both the HYPOX and SHAM groups. The ED50 for the HYPOX group was increased to 0.62 mg/kg, while the SHAM group increased to 0.93 mg/kg. The Bmax values for [3H]spiroperidol binding to striatal membranes were not significantly different for the HYPOX + EB, SHAM + EB, or SHAM + oil groups. However, the HYPOX + oil group was significantly increased (54%) at 7 days post-HYPOX. By 28 days post-HYPOX, the Bmax for the HYPOX animals had increased by 94% relative to SHAM animals. The HYPOX induced increase in dopamine sensitivity could be increased by chronic treatment and withdrawal of haloperidol. The haloperidol induced increase in apomorphine induced stereotypy and [3H]spiroperidol binding could be antagonized by EB treatment during the withdrawal phase of the haloperidol treatment. These data indicate that the pituitary has a modulating effect on striatal spiroperidol binding and apomorphine induced stereotypy, but that its presence is not required for estrogen to suppress the efficacy of dopamine or dopamine agonists.
The withdrawal from chronic haloperidol or estradiol benzoate (EB) treatment results in a behavioral supersensitivity to dopamine agonists in ovariectomized rats. On the other hand, the administration of EB during the withdrawal from haloperidol or the continuous treatment with EB will attenuate or prevent the development of a supersensitivity to dopamine agonists. The enhanced behavioral sensitivity to dopamine agonists is correlated with an increase in 3H-dopamine binding sites in striatal membranes. The administration of EB during the withdrawal from chronic haloperidol treatment or the continuous administration of EB decreases or prevents the proliferation of dopamine binding sites in the striatum that normally orccur upon withdrawal of these two substances. These results indicate that exogenous estrogens may modulate the number of dopamine receptors in the central nervous system and, as such, may decreasse the incidence and/or relieve the symptoms of tardive dyskinesia.
In the first experiment the role of gamma-aminobutyric acid (GABA) in the display of lordosis behavior was examined in septal-lesioned and sham-operated ovariectomized rats. Following estradiol benzoate (EB) priming, septal-lesioned rats were tested for lordosis behavior before and after bilateral infusion of picrotoxin or saline directly into the substantia nigra (SN). Sham animals were given the same behavioral tests but received intranigral infusion of either hydrazinopropionic acid (HPA) or saline. Picrotoxin, which blocks GABA receptors, was effective in suppressing the high levels of lordosis behavior seen in the EB-primed septal-lesioned female rat 30 min after infusion, but not at 120 min. Conversely, HPA, which elevates endogenous GABA levels, was effective in facilitating lordosis behavior in sham-operated rats treated with EB only. The lordosis quotient was moderately increased 30 min after HPA infusion, reached high levels at 120 min, and returned to low levels by 360 min post-infusion, demonstating the reversibility of the drug effect. Saline infusions in lesioned and sham-operated controls were without effect. In the second experiment sepal-lesioned and sham-operated rats were primed with EB and infused with the drugs as in the first experiment, but were sacrificed at the time the macimal behavioral effect had been observed in the first experiment. Tyrosine hydroxylase (TH) activity and dopamine (DA) and homovanillic acid (HVA) levels were measured. No effect on TH activity was found. However, sham-operated rats receiving HPA infusions had lower DA and HVA levels compared to those of saline-injected controls, and septal-lesioned rats receiving picrotoxin infusions had higher DA and HVA levels than those of lesioned saline-injected controls. Septal-lesioned saline-infused rats also showed decreased DA and HVA levels relative to sham-operated saline-infused animals. These results support the concept of a GABA inhibitory neuronal feedback system which modulates DA turnover and perhaps plays a critical role in the neural control of lordosis behavior.
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We have studied human thyroglobulin of extremely low iodine content obtained from a goitrous cretin who had no measurable peroxidase activity in his thyroid gland. Thyroglobulin from these patients is of interest because of the possibility that poorly iodinated thyroglobulin is particularly susceptible to dissociation and proteolysis. In the present study our data indicated that poorly iodinated thyroglobin isolated under conditions inhibiting proteolysis possessed properties similar to normal human thyroglobulin in its secondary, tertiary, and quaternary structures.