In vivo and in vitro aspects of imidazoline-2(1/2)sites within the central nervous system

The in vivo and in vitro characteristics of the I2 binding site were probed using the technique of drug discrimination and receptor autoradiography. Data presented in this thesis indicates the I2 ligand 2-BFI generates a cue in drug discrimination. Further studies indicated agmatine, a proposed endogenous imidazoline ligand, and a number of imidazoline and imidazole analogues of 2-BFI substitute significantly for 2-BFI. In addition to specific I2 ligands the administration of NRl's (noradrenaline reuptake inhibitors), the sympathomimetic d-amphetamine, the α1-adrenoceptor agonist methoxamine, but not the β1 agonist dobutamine or the β2 agonist salbutamol, gave rise to significant levels of substitution for the 2-BFI cue. The administration of the α1-adrenoceptor antagonist WB4101, prior to 2- BFI itself significantly reduced levels of 2-BFI appropriate responding. Administration of the reversible MAO-A inhibitors moclobemide and Ro41-1049, but not the reversible MAO-B inhibitors lazabemide and Ro16-6491, gave rise to potent dose dependent levels of substitution for the 2-BFI cue. Further studies indicated the administration of a number of β-carbolines and the structurally related indole alkaloid ibogaine also gave rise to dose dependent significant levels of substitution. Due to the relationship of indole alkaloids to serotonin the 5-HT releaser fenfluramine and a number of SSRI's (selective serotonin reuptake inhibitor) were also administered and these compounds gave rise to significant partial (20-80% responses to the 2-BFI lever) levels of substitution. The autoradiographical studies reported here indicate [3H]2-BFI labels I2 sites within the rat arcuate nucleus, area postrema, pineal gland, interpeduncular nucleus and subfornical organ. Subsequent experiments confirmed that the drug discrimination dosing schedule significantly increases levels of [3H]2-BFI 12 binding within two of these nuclei. However, levels of [3H]2-BFI specific binding were significantly reduced within four of these nuclei after chronic treatment with the irreversible MAO inhibitors deprenyl and tranylcypromine but not pargyline, which only reduced levels significantly in two. Further autoradiographical studies indicated that the distribution of [3H]2-BFI within the C57/B mouse compares favourably to that within the rat. Comparison of these levels of binding to those from transgenic mice who over-express MAO-B indicates two possibly distinct populations of [3H]2-BFI 12 sites exist in mouse brain. The data presented here indicates the 2-BFI cue is associated with the selective activation of α1-adrenoceptors and possibly 5-HT receptors. 2-BFI trained rats recognise reversible MAO-A but not MAO-B inhibitors. However, data within this thesis indicates the autoradiographical distribution of I2 sites bears a closer resemblance to that of MAO-B not MAO-A and further studies using transgenic mice that over-express MAO-B suggests a non-MAO-B I2 site exists in mouse brain.

Tetrahydrobiopterin metabolism, neurotransmitters and behaviour in the rat

Various neurotoxins were investigated to assess their suitability for developing an animal model to study partial brain BH4 deficiency, neurotransmitters and behavioural alterations. Acute dosing with lead, diethylstilboestrol (DES), amphetamine and scopolamine produced no significant changes in rat brain BH4 metabolism though total biopterins in the liver were significantly reduced by lead and DES. Acute starvation of adult rats decreased brain biopterins. This loss of biopterins may be due to enhanced oxidative catabolism of the active cofactor caused by glutathione depletion. Dietary administration of a BH4 biosynthesis inhibitor, DAHP, consistently decreased brain total biopterins in weaner rats but did not alter the levels of DA, NA, 5-HT or metabolites. However the DAHP diet also induced a marked reduction in food intake. Rats subjected to an equivalent degree of food restriction without inhibitor showed significant but less severe reductions in brain biopterins and again no effect on transmitter levels. DAHP produced a significant decrease in locomotor activity and rearing. This could not be ascribed to reduction in food intake as animals subjected to just dietary restriction showed an increase in these activities. As gross brain levels of DA, NA and 5-HT were unaltered by DAHP the behavioural changes associated with the induced deficiency in brain total biopterins might not have been mediated through the action of these compounds. Although localised changes in neurotransmitter levels may have been obscured by gross analysis it is also possible that the behaviour changes were mediated by a role of BH4 not yet elucidated. Long-term administration of a high aluminium low calcium diet to mice produced no effect on gross brain total biopterins, catecholamines, serotonin or choline acetyltransferase activity though significant behavioural changes were observed.

The pharmacology of centrally acting drugs in animals of altered thyroid status

The pharmacological effects of a number of centrally acting drugs have been compared in euthyroid mice and mice made hyperthyroid by pretreatment with sodium-1-thyroxine. The potencies of two barbiturates, pentobarbitone and thiopentone - as indicated by the duration of their hypnotic actions and their acute toxicities - are increased in hyperthyroid mice. An acutely active uncoupler of phosphorylative oxidation is 2, 4-dinitrophenol, an agent which proved to be a potent hypnotic when administered intracerebrally. An attempt has been made to relate the mechanism of action of the barbiturates to the uncoupling effects of thyroxine and 2, 4-dinitrophenol. The pharmacological effects of chlorpromazine, reserpine and amphetamine-like drugs have also been studied in hyperthyroid mice. After pretreatment with thyroxine, mice show a reduced tendency to become hypothermic after chlorpromazine or reserpine; in fact, under suitable laboratory conditions these agents produce a hyperthermic effect. Yet their known depressant effects upon locomotor activity were not substantially altered. Thus it appeared that depression of locomotor activity and hypothermia are not necessarily correlated, an observation at variance with previously held opinion. These results have been discussed in the light of our knowledge of the role of the thyroid gland in thermoregulation. The actions of tremorine and its metabolite, oxotremorine, have also been examined. Hyperthyroid animals are less susceptible to both the hypothermia and tremor produced by these agents. An attempt is made to explain these observations, in view of the known mechanism of action of oxotremorine and the tremorgenic actions that thyroxine may have. A number of experimental methods have been used to study the anti-nociceptive (analgesic) effects of drugs in euthyroid and hyperthyroid mice. The sites and mechanisms of action of these drugs and the known actions of thyroxine have been discussed.