J Comp Physiol A (1991) 168:605-617 dourrml of N~wrnL Phy~logy A ~o~ 9 Springer-Verlag 1991 The vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria. I. Anatomy K.S.J. Thompson 1, N.M. Tyrer z, S.T. May 1, and J.P. Bacon 1'* 1 School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK z Department of Biochemistry and Applied MolecularBiology,UMIST, ManchesterM60 1QD, UK Accepted February 14, 1991 Summary. Antiserum to arginine-vasopressin has been used to characterise the pair of vasopressin-like immuno- reactive (VPLI) neurons in the locust. These neurons have cell bodies in the suboesophageal ganglion, each with a bifurcating dorsal lateral axon which gives rise to predominantly dorsal neuropilar branching in every ganglion of the ventral nerve cord. There are extensive beaded fibre plexuses in most peripheral nerves of thor- acic and abdominal ganglia, but in the brain, the periph- eral plexuses are reduced while neuropilar branching is more extensive, although it generally remains superficial. An array of fibres runs centripetally through the lamina- medulla chiasma in the optic lobes. Lucifer Yellow or cobalt intracellular staining of single VPLI cells in the adult suboesophageal ganglion shows that all immuno- reactive processes emanate from these two neurons, but an additional midline arborisation (that was only par- tially revealed by immunostaining) was also observed. Intracellularly staining VPLI cells in smaller larval in- stars, which permits dye to reach the thoracic ganglia, confirms that there is no similar region of poorly-im- munoreactive midline arborisation in these ganglia. It has been previously suggested that the immunoreactive superficial fibres and peripheral plexuses in ventral cord ganglia serve a neurohaemal function, releasing the lo- cust vasopressin-like diuretic hormone, F2. We suggest that the other major region of VPLI arborisation, the poorly immunoreactive midline fibres in the suboeso- phageal ganglion, could be a region where VPLI cells receive synaptic input. The function of the centripetal array of fibres within the optic lobe is still unclear. Abbreviations: A VP arginine vasopressin; DIT dorsal intermediate tract; FLRF Phe-Leu-Arg-Phe; FMRF-arnide Phe-Met-Arg-Phe- amide; LDT lateral dorsal tract; LVP lysine vasopressin; MDT median dorsal tract; MVT median ventral tract; SEM scanning electron microscopy; SOG suboesophageal ganglion; VIT ventral intermediatetract; VNC ventral nerve cord; VPLIvasopressin-like immunoreactive * To whom offprint requests should be sent Key words: Locust - Neuroanatomy - Immunohisto- chemistry - Vasopressin-like peptides Introduction Antibodies generated against neurotransmitters or en- zymes involved in transmitter synthesis generally label a vast tangle of fibres in both the invertebrate and verte- brate CNS. It is rare that the morphology of individual cells can be deciphered within this tangle unless some characteristic feature of cell body location or axon tra- jectory enables them to be subsequently stained with intracellular dyes (e.g. the locust TCD neurons staining with anti-GABA (Tyrer et al. 1988)). Some antibodies, on the other hand, stain only a few cells within the CNS; the morphology of individual cells may then be distin- guished by immunohistology alone (e.g. serotonergic neurons in the locust CNS (Taghert and Goodman 1984; Taghert et al. 1984)). Antibodies to arginine-vasopressin (AVP), or to the peptide Phe-Leu-Arg-Phe (FLRF) fall into this latter category for the locust because they usually stain only the same two neurons in the whole nervous system (R6my and Girardie 1980; Evans and Cournil 1990). In Locusta migratoria these neurons have immunoreac- tive cell bodies, 55 lam in diameter, which lie on the ven- tral midline of the suboesophageal ganglion (SOG) (R6my et al. 1979). Immunoreactive fibres, presumed to arise from these cells, extend throughout the CNS, anter- iorly into the brain and posteriorly into all thoracic and abdominal ganglia. In addition, plexuses of immunore- active fibres are seen in nearly all peripheral nerve roots of the brain and ventral nerve cord (VNC) (R6my and Girardie 1980). Ultrastructural investigations show numerous 200 nm diameter neurosecretory vesicles in these cells. In peripheral nerves, vesicle-containing neurosecretory fibres are said to lie immediately under the nerve sheath. Perineurial cells often show a reduction in cytoplasm