3. Chronaxies obtained with wire electrodes were compared with those from stimulation of the same fibre through saline-filled micropipettes (2-12 jsm tip diameter). Rheobase values with the micropipettes ranged from 1-6 to 20 1uA, indicating a close proximity of the pipette to the axon. For these axons, chronaxies from metal wire electrodes ranged from 0-12 to 2-4 ms and for micropipettes from 0-04 to 0-65 ms. In almost all cases, chronaxies for micropipette stimulation were lower than those for metal wire electrodes. Furthermore, with micropipettes chronaxies were independent of conduction velocity.4. The results are shown to be related to differences in time constant of the activated region of axon and charge requirements of threshold activation. The two stimulating conditions, i.e. micro-electrodes compared with wire electrodes, are analogous to the theoretical point stimulated cable and uniformly polarized membrane cases.5. The results are discussed in relation to the possibility of determination of fibre type from stimulation characteristics. A distinction between chronaxies ofmyelinated and non-myelinated fibres can be made using wire electrodes of 250 jam diameter, but not with micro-stimulation, as with micropipettes (2-12 jm diameter).

SUMMARY1. Iontophoretic application of acetylcholine to neurones in the medulla and pons of the unanaesthetized decerebrate cat excited 35-5 % and inhibited 22 %. This was not a representative sample of lower brain stem neurones, since most penetrations were made medially. Some cholinoceptive neurones were in the medial reticular formation.2. The time course of the response to different doses has been described. 8. It is concluded that the excitatory responses have both nicotinic and muscarinic properties but the inhibitory responses are only muscarinic. Since the pharmacology of these two types of response is different, it is concluded that the inhibitory action is not due to an indirect effect from excitation of a neighbouring inhibitory neurone.

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The primary mechanism of activation of intracellular prohormones seems to involve proteolytic cleavage at sequences of consecutive basic residues. Thus, all the known biologically active peptides derived from the prohormone of corticotropin and beta-endorphin appear to be excised initially by enzymes with this specificity. The C-terminal peptide, beta-endorphin (1-31), is generated by cleavage at a lysyl arginine sequence and an additional cleavage can give rise to the related peptides, beta-endorphin (1-27) and beta-endorphin (1-26). These derivatives of beta-endorphin are released by an endopeptidase that appears to catalyse cleavage on the carboxyl side of paired lysine residues, followed by the action of a carboxypeptidase B-like enzyme (Fig. 1). The beta-endorphin fragments, beta-endorphin (1-27) and beta-endorphin (1-26), have been isolated from porcine and bovine pituitary but the C-terminal dipeptide, glycyl glutamine, has not been reported previously. Here we describe the isolation of glycyl glutamine from porcine pituitary and present evidence for its presence in sheep brain stem. When applied ionophoretically to brain stem neurones in the rat, the dipeptide exhibited an inhibitory action on cell firing.

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