Graded potentials and action potentials in the large ocellar interneurons of the bee

Abstract

The response characteristics to light stimuli of first order ocellar interneurons in the bee (Apis mellifera carnica) were analysed with intracellular recordings. The neurons were identified using intracellular dye injection (Lucifer yellow). The responses of L-(large) neurons can vary. They range from transient graded hyperpolarizations in response to light-On and -Off, (sometimes accompanied by tonic hyperpolarization during the light stimulus), to spontaneous tonic action potentials which are inhibited by light. Some neurons show both hyperpolarizations and action potentials. The varying responses to light stimuli are dependent only on the state of the L-neuron and are not correlated systematically with particular types of L-neurons. The experiments demonstrate that the action potentials are generated within the L-neurons and are not conducted antidromically from other neurons. The amplitude and frequency of the action potentials depend on the membrane potential of the L-neuron. With light stimulation, or application of hyperpolarizing current, spikes can be generated within some of the L-neurons. Presumably non-spiking L-neurons are refractory due to maintained depolarization. They have membrane potentials of about −40 mV. Action potentials in L-neurons were recorded only with high resistance microelectrodes. The vitality of the animal can not be correlated with the occurrence of spikes. Functional significance of spontaneous tonic spike discharges in L-neurons is discussed in the context of behavioural experiments in the bee. It is possible, that a given L-neuron uses either spiking or non-spiking signal transmission depending on the behavioural situation. Although very fine microcapillary electrodes were used, no successful recordings or stainings of S-(small) neurons in the ocellar nerve were achieved. Therefore, results of non-identified ocellar interneurons which have been attributed to S-neurons should be critically reviewed.