We have identified the processes of mechanoreceptor sensory neurons by intracellular injection of horseradish peroxidase in order to study the structure of synapses which exhibit profound, behaviourally-relevant plasticity. These synapses are located at small, varicose expansions along or at the end of the fine, microtubule-containing neurites, and they are crowded with vesicles some of which are associated with the varicosity membrane at regions of membrane specialization morphologically equivalent to active zones described in other species. These active zones occur between pre- and postsynaptic processes at two varieties of apposition: a conventional flat one, and a more elaborate indented one. At indented appositions, the presynaptic varicosity is invaginated by a thin (less than 0.25 micrometer diameter) spine of variable length. The active zones of indented synapses have approximately twice the vesicle frequency of flat synapses, suggesting that indented synapses are more effective. Sensory neuron terminals are relatively uniform in their structure, having similar concentrations of vesicles and numbers of active zones, and the majority of the processes postsynaptic to them are less than 0.5 micrometer in diameter. These regularities, and the presence of two strikingly-different types of synaptic apposition, flat and indented, should facilitate structural comparisons of neurons from naive and behaviourally-modified animals. The possible dynamic interconversion of indented and flat appositions at the synaptic terminals of sensory neurons and its behavioural relevance are discussed.

SUMMARY1. The earliest components of the developing innervation of the rabbit intestine to be detected in this study were the cholinergic excitatory and the intrinsic inhibitory innervation. These developed simultaneously in the rabbit at 17 days of gestation. Both were also present in the mouse by the 16th day of gestation. Responsiveness of rabbit tissue to exogenous acetylcholine appeared together with the advent of a functional cholinergic innervation. Since excitatory responses were potentiated by eserine, the tissue was probably able to inactivate acetylcholine through hydrolysis mediated by cholinesterase. Early relaxant responses resisted blockade by adrenergic neurone blocking agents and by antagonists active at a-and /J-adrenoceptors.2. The development of the adrenergic innervation lagged far behind that of the other two components. Specific uptake of noradrenaline in the rabbit was detected for the first time at the 21st day of gestation and stores of noradrenaline could not be detected histochemically until 26-28 days. However, relaxant responses to stimulation of the perivascular sympathetic supply, such as characterize adult tissues, had not yet developed by the time of birth. Relaxation in response to perivascular stimulation could be seen 30 days after birth.3. Morphologic studies indicated that the longitudinal layer of smooth muscle was very primitive when an effective innervation was established.

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Incorporation of L-[ 3 H]fucose into glycoproteins was studied in R2, the giant neuron in the abdominal ganglion of Aplysia . [ 3 H]fucose injected directly into the cell body of R2 was readily incorporated into glycoproteins which, as shown by autoradiography, were confined almost entirely to the injected neuron . Within 4 h after injection, 67% of the radioactivity in R2 had been incorporated into glycoproteins ; at least 95% of these could be sedimented by centrifugation at 105,000 g, suggesting that they are associated with membranes . Extraction of the particulate fraction with sodium dodecyl sulfate (SDS), followed by gel filtration