Ann D. Crocker

The effects of selective dopamine receptor agonists and antagonists on body temperature in rats

Body temperature was measured at repeated time intervals following the administration of various dopamine agonists and antagonists. The D-1 and D-2 receptor agonist, apomorphine, produced dose-dependent hypothermia. This effect was inhibited by the D-2 receptor antagonist, spiroperidol. Stimulation of D-2 receptor by LY171555 produced dose-dependent hypothermia, which was attenuated by pretreatment with spiroperidol and not altered by the D-1 receptor antagonist SCH23390. The D-1 receptor agonist, SK&F38393 had no effect on body temperature. SCH23390 administered alone produced initial hyperthermia and subsequent hypothermia. When administered with apomorphine, SCH23390 both attenuated and potentiated the hypothermic response, depending on the dose and time of administration of each drug. The results suggest that dopamine receptor agonists induce hypothermia by stimulation of the D-2 receptor subtype.

Selective breeding for differences in cholinergic function: Pre- and postsynaptic mechanisms involved in sensitivity to the anticholinesterase, DFP

To determine the contribution of presynaptic cholinergic mechanisms to the increased sensitivity of a genetically selected line of Sprague-Dawley rats (Flinders S-line) to the anticholinesterase, diisopropyl fluorophosphate (DFP), rats were sacrificed by focused microwave irradiation of the head 1 min after a pulse injection of deuterium-labeled choline into the tail vein. The S-line rats exhibited higher concentrations of labeled acetylcholine (ACh) in the cortex than the rats bred for resistance to DFP (Flinders R-line). To determine the contribution of postsynaptic cholinergic mechanisms the concentration of brain muscarinic ACh receptors (mAChR) was determined. The S-line rats exhibited higher concentrations of striatal and hippocampal mAChR than the R-line rats. Thus, both pre- and postsynaptic cholinergic mechanisms may contribute to the increased sensitivity to DFP but their relative importance varies with brain region: increased ACh synthesis in the cortex and increased concentrations of mAChR in the striatum and hippocampus.

Selective breeding for increased cholinergic function: increased serotonergic sensitivity.

The effects of the serotonergic antagonist cyproheptadine and the agonist 1(m-chlorophenyl) piperazine (mCPP) on core body temperature, locomotor activity and operant responding for a water reward were determined in two lines of Sprague-Dawley rats selectively bred for differences in sensitivity to the anticholinesterase, diisopropyl fluorophosphate (DFP). Both cyproheptadine and mCPP induced a dose-dependent hypothermia that was significantly greater in the line of rat more sensitive to DFP (the Flinders Sensitive Line--FSL). On the other hand, the mild stimulant effects of cyproheptadine on operant responding and locomotor activity were similar in the two lines, whereas the marked inhibitory effects of mCPP on these two measures were significantly greater in the FSL rats. This study also confirmed that the FSL rats were significantly more sensitive to the hypothermic effects of oxotremorine, a muscarinic agonist, and showed that pretreatment with cyproheptadine reduced the hypothermic effects of oxotremorine to a similar extent in the two lines. These findings indicate that rats selectively bred for increased cholinergic function (FSL) also differ in their sensitivity to serotonergic agonists and antagonists, thereby extending the evidence for cholinergic-serotonergic interactions in the rat.

Adrenocortical hormone status affects responses to dopamine receptor agonists

The effects of altered adrenocortical hormone status were investigated on hypothermic and behavioural responses elicited following systemically administered apomorphine or LY171555. Hormonal status was modified by surgical adrenalectomy, followed by subsequent replacement therapy with corticosterone, and by chronic corticosterone treatment of intact rats, followed by its withdrawal. The incidence of stereotyped sniffing produced by both apomorphine and LY171555 was increased in the adrenalectomized group and decreased following replacement therapy and in intact rats treated with chronic corticosterone, compared with sham-operated animals and saline-treated controls, respectively. Withdrawal of chronic corticosterone treatment in intact rats reversed the effects of the chronic treatment on dopamine-mediated responses. Similar changes were observed in hypothermic responses to the two dopamine agonists. Striatal D-1 and D-2 dopamine receptor concentration and affinity were unaffected by adrenal hormone modification suggesting that corticosterone may act at a site distal to the dopamine receptor to bring about the observed changes in dopamine-mediated behavioural responsiveness.

An animal model of extrapyramidal side effects induced by antipsychotic drugs: relationship with D2 dopamine receptor occupancy.

1. Muscle rigidity was assessed quantitatively and objectively as increases in electromyographic (EMG) activity (muscle rigidity) in the hindlimb muscles of the rat following subcutaneous administration of haloperidol, fluphenazine and thioridazine. 2. Behavioural changes were assessed as increases in the catalepsy score, defined as the time taken for an animal to move off an inclined grid. 3. Increased tonic EMG activity, or the presence of catalepsy was related to the level of occupancy of dopamine D2 receptors in the striatum and substantia nigra of the brain, measured using ex vivo quantitative autoradiography. 4. Increases in tonic EMG activity and the induction of catalepsy were associated with >80% occupancy of striatal and nigral D2 receptors by fluphenazine, while haloperidol increased tonic EMG activity at D2 occupancies of >57%. 5. Thioridazine at doses ranging from 1-15 mg/kg failed to increase EMG activity and occupied <61% of striatal D2 receptors. 6. Overall the findings support the hypothesis that muscle rigidity is observed when a threshold level of D2 receptors in the striatum and substantia nigra are occupied by antipsychotic drugs. 7. This conclusion is consistent with the results of positron emission tomography (PET) studies in humans, and those from our past studies in rats using raclopride, chlorpromazine and clozapine, in which a threshold of approximately 70% striatal and nigral D2 receptor occupancy has been demonstrated.

Effects of intrahippocampal injections of the cholinergic neurotoxin AF64A on open-field activity and avoidance learning in the rat

The effects of direct intrahippocampal administration of the cholinergic neurotoxin, AF64A, were investigated in male rats. Bilateral injections of AF64A (5 nmole/2 microliters) produced a significant decrease in choline acetyltransferase (CAT) activity in the dorsal hippocampus (25%) and overlying frontoparietal cortex (30%) but no changes in the striatum. Rats lesioned with AF64A exhibited increased levels of open-field activity, which was most marked at 1 week after the lesion; however, the rates of intrasession habituation were similar in lesioned and control rats. Lesioned rats also displayed deficits in acquisition and retention of a passive avoidance task and less dramatic deficits in acquisition of two-way shuttle box avoidance. These findings indicate that lesioning of cholinergic terminals in the hippocampus and/or cerebral cortex with AF64A leads to long-term deficits in learning and memory as well as increases in open-field activity.

Enhanced benzodiazepine responsiveness in rats with increased cholinergic function

The effects of diazepam and muscimol on locomotor activity were examined in Flinders Sensitive Line (FSL) rats, derived by selective breeding methods from randomly bred Sprague-Dawley (RB) rats for increased behavioural and physiological sensitivity to the anticholinesterase, diisopropylfluorophosphate (DFP). Previous reports of increased behavioural sensitivity to oxotremorine, associated with increased striatal and hippocampal muscarinic receptor concentrations, were confirmed in FSL rats compared to RB rats. The FSL rats were more sensitive to the locomotor suppressant effects of diazepam and muscimol compared to RB. Binding experiments with [3H]-diazepam showed that FSL rats had an increased benzodiazepine receptor concentration in the striatum and hippocampus compared to Flinders Resistant Line rats (FRL). FRL did not differ significantly from RB in diazepam-induced changes in locomotor activity or the concentration of benzodiazepine receptors. No significant differences in the affinity of benzodiazepine receptors was detected between the three rat lines in the brain regions investigated. Thus FSL rats showed an increased behavioural sensitivity to both diazepam and muscimol which was associated with a greater concentration of benzodiazepine receptors in the striatum and hippocampus compared to RB and FRL rats.

Hypothyroidism leads to increased dopamine receptor sensitivity and concentration

Rats treated with iodine-131 were confirmed to be hypothyroid by their reduced baseline core body temperatures, reduced serum thyroxine concentrations and elevated serum thyroid stimulating hormone concentrations. When hypothyroid rats were compared to euthyroid controls they were more sensitive to the effects of apomorphine (1.0 mumol/kg) on stereotypy, operant responding and body temperature and showed a smaller reduction in locomotor activity after injection of haloperidol (0.25 mumol/kg). Receptor binding studies on striatal homogenates indicated that hypothyroid rats had increased concentrations of D2 dopamine receptors but there was no change in the affinity. It is concluded that hypothyroidism increases dopamine receptor sensitivity by increasing receptor concentration.

A fixed interval momentary sampling method for assessing on-going behaviours induced by dopamine receptor agonists

1. An observational recording procedure for the objective measurement of unconditioned behaviour is described. 2. During an observation period an auditory cue (e.g. the click of a metronome) spaced at regular intervals (e.g. every 10 seconds) signals the observer to rate the presence or absence of target behaviours at the precise instant of the cue. 3. Practical and methodological considerations of the use of this technique are discussed. 4. This method has been used to compare behaviours elicited by the dopamine agonists apomorphine, LY-171555 and SK&F-38393. 5. These behaviours were also assessed using a stereotypy rating scale and these results are contrasted with those obtained using the time-sample method.

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.