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Biomedical subjects

A D Crocker

Publications and source records attributed to A D Crocker.

At least 19 recordsLinked to original sources
PubMed1999

Raclopride and chlorpromazine, but not clozapine, increase muscle rigidity in the rat: relationship with D2 dopamine receptor occupancy.

The aim of the present study was to investigate the relationship between effects on muscle tone and D2 receptor occupancy of two typical antipsychotic drugs, raclopride and chlorpromazine, and the atypical drug, clozapine. Increased muscle tone (i.e., muscle rigidity), was measured as increases in tonic electromyographic (EMG) activity of the antagonistic muscles of the rat hind limb. D2 dopamine receptor occupancy was assessed in the striatum and substantia nigra, areas involved in the regulation of muscle tone. Raclopride and chlorpromazine produced dose-dependent increases in EMG activity associated with D2 occupancy of 68%-80% in the striatum and 67%-76% in the nigra. No significant increases in EMG were observed with clozapine which showed low D2 occupancy. The results are consistent with those from human studies showing extrapyramidal side effects were associated with striatal D2 occupancy of > 70%.

Animals↗
PubMed1998

The effects of an irreversible dopamine receptor antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), on the regulation of muscle tone in the rat: the role of the substantia nigra.

Recent evidence has questioned the view that the increased muscle tone of Parkinson's disease results solely from reduced release of dopamine in the striatum, by emphasising the important role of the substantia nigra. The aim of the current study was to compare the effects on muscle tone of inactivating D1 and D2 dopamine receptors throughout the brain with those seen following their inactivation only in the substantia nigra. Inactivation of dopamine receptors by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline injected either intraperitoneally, or bilaterally into the substantia nigra, resulted in similar increases in muscle tone, measured as changes in tonic electromyographic (EMG) activity. The magnitude and onset of EMG increases was related to the level of dopamine receptor inactivation. The results are consistent with the hypothesis that nigral dopamine mechanisms play a key role in the maintenance of muscle tone.

Animals↗
PubMed1995

Expression of stereotyped behaviour requires stimulation of nigral D1 dopamine receptors.

Dopamine receptors in the pars reticulata of the substantia nigra were inactivated following bilateral injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Loss of D1, but not D2, receptors was associated with a significant decrease in the incidence of stereotyped head-down sniffing elicited by the mixed D1/D2 receptor agonist, apomorphine. The results support the hypothesis that D1 receptors in the substantia nigra are necessary for the expression of dopamine mediated stereotyped behaviour.

Animals↗
PubMed1995

Dopamine receptors in the substantia nigra are involved in the regulation of muscle tone.

The aim of the present study was to localize the dopamine receptors involved in the regulation of muscle tone. A strategy was used whereby the effects on muscle tone of injecting the irreversible dopamine receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in discrete brain regions were assessed. Increases in muscle tone were measured as changes in electromyographic activity of the gastrocnemius and tibialis muscles of conscious, unrestrained rats. No increases in muscle tone were found after injections of EEDQ into the anterior and posterior striatum, which produced marked reductions in dopamine receptor concentration. The effects of muscle tone of injecting EEDQ into the substantia nigra pars reticulata were also assessed. Large increases in muscle tone were observed associated with inactivation of either D1 or D2 dopamine receptors in the substantia nigra. The increased muscle tone was not reduced by subcutaneous administration of apomorphine, despite the presence of a normal population of striatal dopamine receptors. These findings provide evidence that dopamine receptors in the substantia nigra play an important role in the regulation of muscle tone. Further, they challenge the hypothesis that the muscle rigidity of Parkinson disease results primarily from loss of striatal dopamine receptor stimulation.

Animals↗
PubMed1995

A new view of the role of dopamine receptors in the regulation of muscle tone.

1. Clinical and experimental studies support the view that nigrostriatal dopaminergic neurons are involved in the maintenance of muscle tone. 2. The effects of inactivating dopamine receptors in the rat substantia nigra on muscle tone, assessed as tonic electromyographic (EMG) activity, was investigated. 3. Dopamine receptors were inactivated by injections of the irreversible dopamine receptor antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) into the pars reticulata of the substantia nigra. 4. Significant increases in EMG activity were observed from 2 to 24 h after EEDQ injection and were associated with a loss of nigral dopamine receptors. 5. Increases in EMG activity were found only when both D1 and D2 receptors were inactivated. 6. Inactivation of alpha-adrenoceptors, 5HT2c and muscarinic receptors had no effect on EMG activity. 7. Injections of apomorphine did not reduce the increased EMG activity associated with nigral dopamine receptor loss. 8. The results are consistent with the hypothesis that dopamine receptors in the substantia nigra of the rat play an important role in the regulation of muscle tone.

Animals↗
PubMed1994

Management of full thickness burns to lactating breasts.

A 19-year-old female sustained 8 per cent full thickness petrol flame burns to the neck, chest, breasts, upper abdomen and portions of the right upper limb. The patient had been breast-feeding until the time of the injury. Following assessment of the risks of potential complications such as engorgement, hyperprolactinaemia, mastitis, milk fistulae, glandular loss, scarring and nipple distortion, surgical tangential excision was delayed until bromocriptine produced cessation of lactation and breast involution occurred. This case report demonstrates that consideration of the potential problems in burns to the lactating breasts and prophylactic management can lead to successful healing and probable restoration of function.

Adult↗
PubMed1993

Quantitative electromyographic changes following modification of central dopaminergic transmission.

Muscle tone was assessed by measurement of quantitative electromyographic (EMG) activity recorded from electrodes chronically implanted in the gastrocnemius and anterior tibialis muscles of conscious, unrestrained rats. Following treatment with reserpine or denervation with 6-hydroxydopamine a significant increase in EMG activity of both muscles was observed, confirming the utility of the method for studying the effects of changes in central dopaminergic transmission on muscle tone.

Animals↗
PubMed1993

Dopamine agonist-mediated inhibition of acetylcholine release in rat striatum is modified by thyroid hormone status.

K(+)-evoked acetyl[3H]choline ([3H]ACh) release was inhibited in a concentration-dependent manner by apomorphine and the D2 agonist quinpirole in striatal slices prepared from euthyroid and hypothyroid rats. However, there was a significant increase in the maximum inhibition observed with both agonists in the hypothyroid compared with the euthyroid group, which paralleled the increased D2 agonist sensitivity reported for stereotyped behavior. The D2 antagonist raclopride decreased, and the D1 antagonist SCH 23390 increased, the inhibition of [3H]ACh release by apomorphine, confirming an inhibitory role for D2 receptors and an opposing role for D1 receptors. Because there is no difference in D1 or D2 receptor concentration between the euthyroid and hypothyroid groups, it is suggested that thyroid hormone modulation of D2 receptor sensitivity affects a receptor-mediated event. Following intrastriatal injection of pertussis toxin (PTX), apomorphine no longer inhibited [3H]ACh release. In fact, increased [3H]-ACh release was observed, an effect reduced by SCH 23390, providing evidence that D1 receptors enhance [3H]-ACh release, and confirming that a PTX-sensitive G protein mediates the D2 response. As it has been reported that thyroid hormones modulate G protein expression, this mechanism may underlie their effect on dopamine agonist-mediated inhibition of ACh.

Acetylcholine↗
PubMed1992

Localisation of striatal muscarinic receptors involved in dopamine receptor-mediated behavioural responses.

Stereotyped sniffing responses to the direct dopamine agonist, apomorphine, were assessed following intrastriatal injection of the alkylating derivative of oxotremorine, BR 401. Subsequently, the localisation and extent of muscarinic receptor alkylation after injection into various striatal sites were assessed by quantitative autoradiography. The results of these experiments provide evidence that a regional subset of striatal muscarinic receptors is involved in apomorphine-induced stereotyped sniffing. In addition, these receptors are localised in a similar area to that previously shown to contain the dopamine D2 receptors responsible for mediating apomorphine-induced sniffing. Thus, striatal muscarinic receptors involved in dopamine-agonist induced behaviours share a close anatomical association with the dopamine receptors at which the agonist acts.

Animals↗
PubMed1992

Selective breeding for increased cholinergic function: biometrical genetic analysis of muscarinic responses.

Biometric genetic analyses of behavioral and physiologic responses known to be related to muscarinic cholinergic receptors (hypothermia, hypoactivity, inhibited avoidance, and reduced responding for water) were studied in genetic crosses and backcrosses of the Flinders sensitive line (FSL) and Flinders resistant line (FRL) of rats. The FSL rats were more sensitive to the direct muscarinic agonists, arecoline and oxotremorine, and to the indirect agonist, physostigmine, than any other group. The next most sensitive group was the F1 x FSL backcross, followed by the F2, F1, F1 x FRL backcross, and the FRL, in that order. These differences between the genetic groups could be accounted for completely by either solely additive or additive plus dominance genetic factors. When dominance genetic factors contributed to the differences among groups (6 out of 15), the F1 responded like the FRL rats. The variance of the responses measured made it impossible to obtain reliable estimates of the number of genes involved in many instances; when such estimates were possible, several genes (greater than or equal to 3) appeared to be involved. We conclude that muscarinic sensitivity in rats is under genetic control, with the greatest contribution coming from additive genetic factors. Because the FSL rat appears to be a genetic animal model of depression, the finding of several genes influencing muscarinic responses may help account for the difficulties investigators have had in locating a single major gene or biological marker for human depressive disorders.

Analysis of Variance↗
PubMed1991

Dopamine sensitivity in rats selectively bred for increases in cholinergic function.

Because of the extensive literature demonstrating an interaction between cholinergic and dopaminergic systems, the Flinders Sensitive (FSL) and Flinders Resistant (FRL) Lines of rats, selectively bred for differences in cholinergic function, were tested for differences in dopamine sensitivity. Large differences in sensitivity to dopamine agonists were detected, but the direction depended upon the function: The FSL rats were supersensitive to the hypothermic effects of dopamine agonists, but were subsensitive to the stereotypy-inducing effects. Measurement of dopamine receptors by either standard binding techniques or autoradiography failed to demonstrate any receptor differences in the FSL and FRL rats. Behavioural studies with dopamine antagonists were less clear-cut, but suggested that the FSL rats might be more sensitive to their catalepsy-inducing effects. These findings indicate that the changes in dopamine sensitivity which accompany cholinergic supersensitivity are function-dependent, but are not associated with parallel changes in dopamine receptor concentration.

Animals↗
PubMed1990

Effects of inactivation of D1 dopamine receptors on stereotypic and thermic responses to quinpirole (LY 171555).

The purpose of the present study was to investigate reports that the expression of dopamine agonist-induced behaviours is dependent upon the ratio of D1 to D2 receptor activation. Selective inactivation of the D1 dopamine receptor was achieved using the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) while D2 receptors were maintained at control levels by the use of a D2 antagonist, raclopride. Stereotypic and hypothermic responses to quinpirole (LY 171555) were assessed and related to striatal D1 and D2 receptor concentrations. Results showed that the incidence of stereotyped sniffing behaviour induced by LY 171555 was reduced in parallel with D1 receptor loss and sniffing behaviour was abolished at low D1 receptor (less than 40% of control) concentrations. Hypothermic responses to LY 171555 were unaffected by D1 receptor loss. These findings suggest that activation of D1 receptors is a critical component of stereotypic, but not hypothermic, responses to LY 171555, the magnitude of the sniffing response being positively associated with D1 receptor concentration.

Animals↗
PubMed1990

Pretreatment with an irreversible muscarinic agonist affects responses to apomorphine.

The main aim of the present study was to investigate if responses to the direct dopamine agonist, apomorphine, could be modified by changes in the activity of cholinergic neurones. A novel approach was adopted in which these responses were assessed following reduction of muscarinic receptor concentration (mAChR) in the brain (assessed from [3H] QNB binding) by the alkylating derivative of oxotremorine, N-[4-(2-chloroethylmethylamino)]-2-pyrrolidone (BM 123). Stereotyped responses elicited by apomorphine were significantly reduced when QNB binding was 12% and 50% of control values. No changes in [3H] spiperone binding were found. There was significant hypothermia in the group with 12% QNB binding sites which was significantly increased by apomorphine. Body temperature returned to normal when QNB binding was 50% of control values. There was a significant decrease in activity when QNB sites were reduced to 12% of normal and vertical activity was still significantly reduced at 50% QNB binding, though horizontal activity was not then different from controls. These data are consistent with the hypothesis that changes in the function of mAChR modify responses elicited by dopamine receptor stimulation in both the striatum and other brain regions.

Animals↗
PubMed1990

The hypothyroid rat as a model of increased sensitivity to dopamine receptor agonists.

Control and hypothyroid rats were challenged with a range of doses (0.5-4 mumol/kg) of either the nonselective dopamine agonist, apomorphine, or the selective D2 receptor agonist. LY 171555, and their stereotyped head-down sniffing (SHDS) responses measured. The dose-response curves for both agonists were shifted to the left in the hypothyroid rats compared to water-treated controls. Increasing doses of the selective D2 antagonist, raclopride, caused a parallel shift to the right in the LY 171555-induced SHDS dose-response curve. Schild analysis revealed a decreased sensitivity to raclopride in the hypothyroid animals. The selective D1 antagonist SCH 23390 was observed to decrease the maximal response elicited by LY 171555 in a dose-dependent manner and the hypothyroid rats were more sensitive to this effect. It was concluded that hypothyroid rats showed an apparent increased sensitivity to D2 receptor agonists and a decreased sensitivity to D2 antagonists. In addition, the facilitation effect of the D1 receptor on the D2 receptor appeared less tightly coupled in the hypothyroid rats.

Animals↗
PubMed1989

Effects of adrenalectomy on responses mediated by dopamine D-1 and D-2 receptors.

The effects of surgical adrenalectomy were investigated on behavioural responses produced by the selective D-1 agonist, SK&F 38393, alone, and in combination with the D-2 agonist, quinpirole (LY171555). Further, stereotyped responses to apomorphine and LY171555 were assessed following treatment with either the D-1 or the D-2 antagonists, SCH 23390 and raclopride, respectively. There was no difference between sham-adrenalectomized (sham) and adrenalectomized (ADX) groups in responses to SK&F 38393. Although concomitant stimulation of both receptor subtypes increased the incidence of stereotyped sniffing behaviour, there was no difference in the magnitude of this effect between the sham and ADX groups. Raclopride reduced LY171555-induced sniffing and hypothermia less in ADX rats than in sham controls, which was consistent with the hypothesis that adrenocortical hormones affect D-2 receptor responsiveness. SCH 23390 had a greater inhibitory effect on LY171555 responses, but a smaller effect on apomorphine responses in the ADX group compared with their sham controls. It is concluded that the amplified D-2-stimulated response observed in ADX rats may be more dependent on tonic D-1 receptor activation than the control D-2 response of shams.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗
PubMed1989

Localization of striatal dopamine receptor function by central injection of an irreversible receptor antagonist.

The stereotypic head-down sniffing response to systemically administered apomorphine (0.65 mumol/kg) was assessed in rats 48 h after the bilateral injection of 0.2-0.5 microliters of the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (60 micrograms/microliters) into the caudate-putamen and nucleus accumbens. This response was significantly attenuated in animals that had received injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into the anterior/ventral part of the caudate-putamen but not in those that received injections into regions more dorsal/posterior. Animals were killed after apomorphine challenge and the region of dopamine D1 or D2 receptor reduction due to N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline mapped and quantified. This analysis revealed that the dopamine receptors involved in the apomorphine-induced stereotyped head-down sniffing response were located in a discrete region of the ventrolateral caudate-putamen and the dorsolateral nucleus accumbens. Animals that were pretreated with the selective dopamine D2 receptor antagonist raclopride (0 20 mumol/kg, i.p.) 20 min prior to central injection of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into this area showed a dose-dependent protection of the stereotyped sniffing response to systemic apomorphine 48 h later. This combination of techniques constitutes a novel way to investigate striatal function and the results obtained support the concept of a functional subdivision of both the caudate-putamen and the nucleus accumbens.

Animals↗