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In the studies described in this thesis the ability of muscarinic and ?1-adrenoceptor, as well as depolarising stimuli to initiate phosphoinositide metabolism in various regions of rat brain were examined. Furthermore, the ability of these stimuli to initiate phosphoinositide hydrolysis in developing brain was observed. Both muscarinic and ?1-adrenoceptor-induced phosphoinositide hydrolysis have marked regional distributions in rat brain. This regional distribution of functional responsiveness seems to correlate reasonably well with measurements of known receptor density. It is also clear that there is no variability in the coupling of both of these receptors in rat brain, thus implying a relationship between the functional responses and receptor occupancy. Pirenzepine appears to be able to differentiate between muscarinic receptor-induced phosphoinositide responses in the hindbrain from those in the forebrain regions. Both elevated K+ and veratrine can initiate phosphoinositide hydrolysis in rat brain. The regional responses to elevated K+ seem, at least, in part to be due to transmitter release, although a role for voltage-sensitive Ca++ channels in such responses is indicated by the effects produced by dihydropyridine Ca++ channel antagonists and activators. Muscarinic and ?1-adrenoceptors show different developmental patterns of phosphoinositide responsiveness. The ?1-adrenoceptor seems to be more efficiently coupled during the first two weeks of postnatal development whereas the muscarinic receptor shows no variability in coupling. Instead, carbachol produces supramaximal responses in young rats. Lithium also potentiates [3H]-InsP1 and [3H]-InsP2 accumulations more in young rats. Moreover lithium produces a time-dependent inhibition of [3H]-InsP3 and [3H]-InsP4 in both young and adult rats. Physostigmine produces no enhancement of the response to elevated K+ in young rats. Furthermore, brain slices from young rats seem to be more sensitive to the Ca++ channel activator BAY-K8644. The significance of these results are discussed in the text.

Intracellular recordings were made from neurones of the rat locus coeruleus (lc) contained within a brain slice maintained <i>in vitro</i>. When applied to the slice in known concentrations, K opioid receptor agonists produced a concentration-dependent, naloxone-reversible depression of the electrically evoked excitatory post-synaptic potential (epsp). This effect of K agonists was observed in the absence of changes in the membrane potential or input resistance of the post-synaptic cell. Similarly, the K agonists had no effect on the tetrodotoxin-resistant action potential waveform. Naloxone antagonised the response to U50488 with an apparent dissociation equilibrium constant (K<SUB>d)</SUB> of 28 nM, consistent with the response being mediated via K opioid receptors. In contrast, u opioid receptor agonists caused a membrane hyperpolarisation concomitant with a fall in neuronal input resistance, and depressed the tetrodotoxin-resistant action potential. These effects were concentration-dependent and antagonised by naloxone; the hyperpolarising action of [D-Ala<SUP>2</SUP> , NMePhe<SUP>4</SUP> , Gly-ol<SUP>5</SUP> ] enkephalin (DAGO) was antagonised by naloxone with a K<SUB>d</SUB> of 1.5 nM. These findings are in agreement with previous reports that u receptor activation increases a potassium conductance in lc neurones. The epsp was depressed, but not abolished, by the excitatory amino acid antagonists, 2-amino-5-phosphonovaleric acid (2APV) and kynurenic acid, suggesting that the epsp was at least partly mediated by an excitatory amino acid. U50488 did not depress the depolarisation produced by local application of L-glutamic acid. In addition to the epsp, a noradrenergic inhibitory post-synaptic potential (ipsp) could be evoked in lc neurones. U50488 depressed the ipsp, but this effect was not reversed by naloxone and therefore not mediated via opioid receptors. U50488 had no effect on the all or nothing depolarising potential which could be evoked in a proportion of lc neurones. The effect of U50488 on the epsp was reduced when experiments were performed in the presence of agents - either barium, quinine or 4-aminopyridine - which block potassium conductances. An <i>in vitro</i> autoradiographic study of <SUP>3</SUP> H bremazocine binding within the lc revealed that the majority of binding was displaced by a combination of unlabelled DAGO and [D-Ser<SUP>2</SUP> ] Leu enkephalin Threonine (DLSET) and so represented u sites. A significant proportion however, was displaceable by unlabelled U50488 and thus represented K binding sites. It is concluded that K opioid receptors are situated pre-synaptically within the lc and when activated depress excitatory synaptic transmission.

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