Pharmacological analysis of the cholinergic input to the locust VPLI neuron from an extraocular photoreceptor system

1. The brain of the locust contains an extraocular photoreceptor (EOP), which provides the major synaptic excitation to the vasopressin-like immunoreactive (VPLI) interneuron of the suboesophageal ganglion. Although the precise location of the EOP remains unknown, its activity can be determined indirectly by intracellular recording from the VPLI neuron. The excitatory drive to the VPLI neuron occurs only in darkness and is absent in the light. 2. The EOP is preferentially sensitive to light of wavelength 494 +/- 7 (SD) nm (blue-green) and has an absorption spectrum characteristic of a rhodopsin-like photopigment. 3. In the presence of high divalent saline (20 mM Ca2+ and Mg2+), the VPLI neuron receives excitatory input in the light. This indicates that the excitatory input to the VPLI neuron is from a tonically active descending input, which normally is inhibited by the light-induced activation of the presynaptic EOP. 4. Stimulation of the connectives while recording the resultant excitatory postsynaptic potential (EPSP) evoked in VPLI shows that the descending input projects beyond the suboesophageal ganglion, extending as far as the metathoracic ganglion. 5. Pharmacological analysis shows that the descending input to the VPLI neuron is cholinergic: acetylcholine (ACh) strongly depolarizes the neuron and eserine, an ACh esterase inhibitor, markedly potentiates the synaptic excitation of the VPLI neuron. 6. Nicotinic and muscarinic receptor antagonists show that the excitation of VPLI consists of two pharmacologically discrete components. Nicotinic ACh receptors mediate a fast depolarization, whereas muscarinic ACh receptors evoke a more sustained depolarization. Accordingly, both a fast and slow depolarization can be evoked selectively in VPLI by direct application of either nicotine or muscarine. 7. Voltage-clamp analysis shows that the fast EPSP evoked current is similar to that produced by nicotine in that it decreases linearly with membrane depolarization. The current associated with the sustained depolarization is similar to that evoked by muscarine, increasing nonlinearly with membrane depolarization. 8. Activity of the descending input, or application of muscarine, lowers the spike-initiation threshold of the VPLI neuron, thereby increasing its excitability. 9. It is concluded that the presence of two ACh receptor subtypes act synergistically to allow continuous activity of the VPLI neuron for sustained periods (i.e., throughout the hours of darkness).