Pigment-dispersing factor in the AME

Best characterized among the AME-associated neurons are the PDF-ir neurons with somata in the lamina (PDFLAs) and the medulla (PDFMEs). PDFLAs can be found at a ventral (vPDFLAs) and a dorsal position of the lamina (dPDFLAs). Both groups contain 50-70 neurons (Homberg et al. 1991; Stengl and Homberg 1994; Petri et al. 1995; Wei et al. 2010). Around twelve PDF-ir somata are located anterior to the AME, which were named anterior PDF-ir medulla neurons (aPDFMEs, Fig. 12, Petri et al. 1995). According to their soma-size, the intensity of the PDF-immunoreactivity, the DCV content, and the coexpression of other neuropeptides these neurons can be classified into faintly labeled small, stronger labeled medium-sized, and intensely labeled large aPDFMEs (Reischig and Stengl 2003b; Soehler et al. 2011). Each subgroup contains about four neurons. While the large and medium-sized aPDFMEs belong to the VNe group of AME-neurons, which were shown to form output pathways to different targets in the protocerebrum and coupling pathways to the contralateral optic lobe, the small aPDFMEs belong to the DFVNe group, which are local neurons of the AME. Because of their weak PDHLI the small aPDFMEs have not been investigated in detail to date (Reischig and Stengl 2003b). Additionally, about two large and two small PDF-ir somata can be found at a more posterior position, which were termed posterior PDFMEs (pPDFMEs, Fig. 12, Petri et al. 1995). Since large and medium-sized aPDFMEs could not be clearly distinguished from each other, the discrimination was based on additional FMRFamide- and orcokinin-immunoreactivity (Soehler et al. 2011). According to this rule, all large PDFMEs, found at anterior or posterior position, do not coexpress FMRF- and orcokinin-immunoreactivity and the sum of these neurons was shown to be always six. These neurons were suggested to be a group with a common developmental origin, from which some can move to a posterior position during development. Alternatively, all six large somata can be found at anterior position (Soehler et al. 2011).

PDF-ir arborizations in the AME were found mainly in the interglomerular and shell neuropil and always contained large and medium-sized DCVs (Reischig and Stengl 1996). The large PDFMEs were shown to contain medium-sized DCVs and have synapses with fibers in the anterior and shell neuropil of the AME (Reischig and Stengl 2003b). In contrast to large PDFMEs, medium-sized aPDFMEs contain large DCVs and innervate mainly the interglomerular neuropil but also the anterior neuropil of the AME. Some of the PDFMEs were shown to coexpress different neuropeptides: All small and medium-sized aPDFMEs and the small pPDFMEs always coexpress FMRFamide-immunoreactivity. All medium-sized aPDFMEs and a subgroup of the small aPDFMEs, but not the pPDFMEs, also coexpress orcokinin- and baratin-like immunoreactivity (Soehler et al. 2011). One neuron of the medium-sized aPDFMEs additionally shows MIP-immunoreactivity and thus could possibly coexpress five neuropeptides: baratin, FMRFamide, MIP, orcokinin and PDF (Soehler et al.

2011; Schulze et al. 2012). Remarkably, the medium-sized aPDFMEs were the first neurons reported to express more than two neuropeptides in a single neuron (Soehler et al. 2011). While the somata showed immunolabeling for PDF, FMRFamide and orcokinin, double-labeling in fibers and terminals for PDF and orcokinin was never observed and just a few termination sites but no commissural fibers showed double-labeling for PDF and FMRFamide. It was suggested that these neurons employ

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peptide sorting and that the differential release of neuropeptide mixtures could be used to phase-control specific physiological processes daytime dependently (Soehler et al. 2011).

Fig. 12. Neurons associated with the accessory medulla of R. maderae. A. Three-dimensional model of the accessory medulla (AME) with associated neurons, redrawn and modified after (Reischig and Stengl 2003b). The distal tract (DT) is suggested to transmit light input into the AME B. Scheme of the localization of PDF-ir medulla neurons (PDFMEs), modified from (Soehler et al. 2011). The number of the grey circles does not reflect the correct sizes of the groups. Three medium-sized and the largest anterior PDFMEs, together with another VNe, are suggested to form a coupling pathway to the contralateral AME. For further details see text. Abbreviations: ANe: anterior neurons, di: distal, do: dorsal, DFVNe: distal frontoventral neurons, MFVNe: medial frontoventral neurons, MNe: medial neurons, VNe: ventral neurons, VPNe:

ventroposterior neurons. Scale bar: 50 µm.

The branching pattern of the PDFLAs appears to be restricted to the optic lobes, where the lamina, the distal medulla and the AME are innervated (Stengl 1994; Petri et al. 1995; Reischig et al. 2004;

Wei et al. 2010). In the optic lobe the projections of the PDFMEs connect the AME to the medulla and the lamina via the anterior fiber fan and/or the median layer fiber system of the medulla (Fig. 13, Fig. 14). The fibers of the anterior fiber fan were shown to innervate the distalmost layer of the medulla and the proximal layer of the lamina. On the other hand the PDFMEs project to different targets in the protocerebrum and the contralateral optic lobe via the anterior optic commissure and the posterior optic commissure. These fibers leave the optic lobe via the lobula valley tract, which runs from the proximal end of the medulla to the proximal end of the lobula. There the fibers bifurcate into the AOC and the posterior optic tract, which leads into the POC (Stengl 1994; Stengl and Homberg 1994; Reischig and Stengl 2002, 2003b; Wei et al. 2010). Several branching sites, termed plexi (p1 - p5) and anterior fiber plexus (AFP), and two meeting points of fiber bundles, termed arborization areas (a1 and a2), can be detected (Fig. 13, Fig. 14). The fibers of the AOC innervate the SLP and SMP, branch in the plexi p3 - p5, and show the arborization areas a1 and a2.

The fibers running through the POC extensively branch in p1, located within the LOVT, and p2 and innervate the POTU. Remarkably, two plexi, p1 and AFP, were shown to connect several PDF-ir branching sites: Fibers originating from p1 directly interconnect the AME with the plexi p2, p3, and AFP and with the SLP, ILP, and VLP. Fibers originating from the AFP, which is located laterally to the mushroom body, interconnect the AME with all plexi, the arborization area a1 and the SLP. Thus,

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these plexi connect the AOC with the POC, allowing for exchange of information between fibers running through both commissures (Wei et al. 2010). Additionally, PDF-ir ramifications from an unknown soma were detected posterior to the lobus glomerulatus in the tritocerebrum (Fig. 13, Fig. 14).

Fig. 13. Branching pattern of the PDF expressing neurons in the brain of R. maderae. PDF-ir neurons were implemented in a three-dimensional model of R. maderae's brain. The brain is shown in frontal view. The PDF-ir neurons innervate the optic lobes and different areas of the protocerebrum. Additionally, the antennal lobe (AL) in the deutocerebrum and the lobus glomerulatus (LG) in the tritocerebrum are innervated. For further information see text. Abbreviations: AME:

accessory medulla, AOC/POC: anterior/posterior optic commissure, CA: calyx, CBL/CBU: lower/upper division of the central body, LA: lamina, LO: lobula, MB: mushroom body, ME: medulla, ML: median lobe, PB: protocerebral bridge, PED:

pedunculus, VL: ventral lobe. Scale bar: 300 µm. Modified after (Wei et al. 2010).

Fig. 14. Arborization sites of the PDF-expressing neurons. The PDF-immunoreactive medulla neurons (PDFMEs) arborize in all optic lobe neuropils and in different arborization sites (a1 and a2) and plexi (AFP and p1 - p5) in the protocerebrum. For further information see text. Abbreviations: AFP: anterior fiber plexus, AL: antennal lobe, AME: accessory medulla, AN:

antennal nerve, aPDFME: anterior PDFME, AOC/POC: anterior/posterior optic commissure, CX: central complex, ILP:

inferior lateral protocerebrum, LA: lamina, LO: lobula, MB: mushroom body, ME: medulla, ON: optic nerves, SLP: superior lateral protocerebrum, SMP: superior median protocerebrum. Modified after (Wei et al. 2010).

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Indicated by varicosities, PDF was suggested to be released in the lamina, the AME, all five plexi, the arborization areas a1 and a2, the POTU, SLP, and SMP. Interestingly, the distances between arborization sites of PDF-ir fibers in the AME, all five plexi, both arborization areas, and the POTU were shown to be not significantly different from each other or integer multiples from each other. It was suggested, that the relationship between the distances could contribute to different delay lines, possibly enabling postsynaptic neurons to measure phase differences (Wei et al. 2010).

Two of the three presumptive AMAE coupling tracts (tract 4 and 7) resembled the characteristic branching pattern of the PDFMEs (Reischig and Stengl 2002). Indeed, the combination of the tracer backfill/injection strategy with immunocytochemistry revealed that four aPDFMEs form a direct coupling pathway for both AMAE (Reischig et al. 2004; Soehler et al. 2011). Further immunocytochemical double- and triple-labeling studies demonstrated that orcokinin and FMRFamide also play a role in coupling, and that PDF, orcokinin, and FMRFamide are partially colocalized in the coupling neurons (Hofer and Homberg 2006a; Soehler et al. 2011). Four aPDFMEs (the largest and three medium-sized somata), belonging to the VNe cells, were shown to form the MC I group. The largest aPDFME did not coexpress orcokinin or FMRFamide and was shown to project via the POC and the AOC to the contralateral AME and to most if not all midbrain targets in both brain hemispheres. In contrast, the three medium-sized aPDFMEs coexpress PDF, orcokinin, FMRFamide, and baratin. These cells appear to project solely via the AOC and innervate at least the contralateral AME, the POTU and the dorsal part of the SLP (Soehler et al. 2011).

PDF was also detected via mass spectrometry in brains of R. maderae. The peptide sequence could be identified except for one amino acid, whose identity was leucine or isoleucine (Hamasaka et al.

2005). Recently, the identity of isoleucin⁴ was confirmed in a transcriptome analysis (personal communication with Achim Werckenthin, University of Kassel). Thus, the sequence of Rhyparobia-PDF is identical to Meimuna opalifera-, A. domesticus-, and G. bimaculatus-Rhyparobia-PDF (Tab. 3).

Tab. 3. PDF sequences of different insect species (modified from (Hamasaka et al. 2005)) Diptera

Anopheles gambiae NSELINSLLSLPKTMNDAa (Matsushima et al. 2003) Drosophila melanogaster NSELINSLLSLPKNMNDAa (Park and Hall 1998) Musca domestica NSELINSLLSLPKSMNDAa (Matsushima et al. 2004) Phormia regina NSELINSLLSLPKNMNDAa (Matsushima et al. 2003) Hemiptera

Meimuna opalifera NSEIINSLLGLPKVLNDAa (Sato et al. 2002) Orthopteromorpha

Acheta domesticus NSEIINSLLGLPKVLNDAa (Rao and Riehm 1988)

Carausius morosus NSELINSLLALPKVLNDAa (mentioned in Hamasaka et al. 2005) Gryllus bimaculatus NSEIINSLLGLPKVLNDAa (Singaravel et al. 2003)

Periplaneta americana NSELINSLLGLPKVLNDAa (mentioned in Hamasaka et al. 2005)

Rhyparobia maderae NSEIINSLLGLPKVLNDAa (Hamasaka et al. 2005), personal communication with Dr. Achim Werckenthin (University of Kassel) Romalea microptera NSEIINSLLGLPKLLNDAa (Rao et al. 1987)

The variable amino acids at positions 4, 10, 14, and 15 are color-coded.

To date it is still not fully understood, whether PDF is only a non-photic input and also an output signal of the circadian clock, or whether it also plays a role in the light entrainment pathway. For

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injections of synthetic Arg¹³-A. domesticus PDF into the vicinity of the AME, a monophasic all-delay PRC with maximum phase-delays at the late subjective day was obtained (Petri and Stengl 1997).

Because the PRC was not light-like, it was suggested that PDF functions as non-photic input signal (Petri and Stengl 1997) and as coupling signal from the contralateral AME (Reischig and Stengl 2002;

Reischig et al. 2004). However, more recent injections of P. americana-PDF into the complex eye resulted in a biphasic, light-like PRC suggesting that PDF could function in light entrainment (Schendzielorz et al. 2014). The non-photic function of PDF was also supported by intracellular recordings, in which AME-neurons, which resembled parts of the PDF-ir branching pattern, were shown to be insensitive to light during the daytime (Loesel and Homberg 2001). However, there is evidence for light-sensitivity of PDF-ir neurons: When cockroaches were raised in non-24 h periods (T22 = LD 11:11, T26 = LD 13:13) or in different photoperiods (LD 6:18 or LD 18:6) the number and the branching pattern of PDFMEs was affected (Wei and Stengl 2011). The medium-sized aPDFMEs were shown to be most light-responsive in these experiments: The number of somata increased with increasing period length and increasing photoperiod, the number of PDF-ir fibers in the AOC increased with increasing period length, and the length of the fibers in the AOC and POC increased with longer photoperiods. Therefore, it was hypothesized that the medium-sized aPDFMEs could have longer endogenous periods, enabling them to couple better to longer exogenous periods, and that these neurons are activated by light and inhibited by darkness only at ZT 11-13 (Wei and Stengl 2011).

In another cockroach species, B. germanica, pdf expression was knocked down via RNAi with an astonishing efficiency: Starting from the second day after injection of double-stranded pdf RNA, pdf mRNA levels decreased and the animals lost their locomotor activity rhythm in DD as well as LD conditions and also decreased their amount of activity, demonstrating the importance of PDF signaling in this insect species (Lee et al. 2009).