DIFFERENTIAL REGULATION OF A HOMOLOGOUS PEPTIDERGIC NEURON IN GRASSHOPPERS - EVOLUTION OF A NEURAL CIRCUIT

The vasopressin-like immunoreactive (VPLI) neurons of grasshoppers hav e paired cell bodies in the suboesophageal ganglion and both anterior and posterior running axons. In non-oedipodine grasshopper species (e. g. Schistocerca gregaria), most of their arborisations are distributed in dorsal and lateral neuropil, while in oedipodine species (e.g. Loc usta migratoria), the neurons have additional extensive axonal project ions in both the optic lobes and proximal portions of the ganglionic p eripheral nerves. This study demonstrates that these morphological dif ferences correlate with their physiology. In L. migratoria, VPLI neuro n activity is regulated primarily via a spontaneously active interneur on which descends from the brain. This descending interneuron is inhib ited by a light-activated brain extraocular photoreceptor. Regulation of VPLI neuron activity by an extraocular photoreceptor is also seen i n the other oedipodine grasshopper investigated. In the four non-oedip odines examined (from two subfamilies), we find no extraocular photore ceptor regulation of VPLI neuron activity. Despite this, VPLI neuron i n S. gregaria does appear to be driven by a descending interneuron hom ologous to that in L. migratoria. The descending interneuron in both s pecies receives similar mechanosensory input and excites the VPLI neur on via cholinergic synapses. Histamine injection into the medial proto cerebrum of both species causes strong inhibition of the descending in terneuron. The evolution of the neural circuitry, by which an extraocu lar photoreceptor comes to regulate the descending interneuron in oedi podine species, is discussed.