GABA inhibition of luminescence from lantern shark (Etmopterus spinax) photophores.

Photogenic organs (photophores) of the velvet belly lantern shark (Etmopterus spinax) are under hormonal control, since melatonin (MT) and prolactin (PRL) trigger luminescence while α-melanocyte-stimulating hormone (α-MSH) prevents this light to be emitted. A recent study supported, however, the presence of numerous nerve fibres in the photogenic tissue of this shark. Immunohistochemical and pharmacological results collected in this work support these nerve fibres to be inhibitory GABAergic nerves since (i) GABA immunoreactivity was detected inside the photogenic tissue, where previous labelling detected the nerve fibre structures and (ii) GABA was able to inhibit MT and PRL-induced luminescence, which was on the other hand increased by the GABA(A) antagonist bicuculline (BICU). In addition, we also demonstrated that BICU can induce light per se by provoking pigment retraction in the pigmented cells composing the iris-like structure of the photophore, attaining, however, only about 10% of hormonally induced luminescence intensity at 10(-3)mol L(-1). This strongly supports that a GABA inhibitory tonus controls photophore "aperture" in the photogenic tissue of E. spinax but also that MT and PRL have more than one target cell type in the photophores.

Nitric oxide in the control of luminescence from lantern shark (Etmopterus spinax) photophores.

Photophores (photogenic organs) of the lantern shark Etmopterus spinax are under hormonal control, with prolactin (PRL) and melatonin (MT) triggering the light emission. Differential sensitivity to these hormones in adult individuals suggests, however, that the luminescence of this shark is controlled by an additional mechanism. In this study, different techniques were used to investigate a potential modulator of E. spinax luminescence - nitric oxide (NO). NO synthase (NOS)-like immunoreactivity (IR) was found in the photocytes (photogenic cells) of the photophores. In addition, acetylated tubulin IR also supported the presence of nerves running through the photogenic tissue and innervating different structural elements of the photophores: photocytes, pigmented cells from the iris-like structure and lens cells. Pharmacological experiments confirmed a modulatory action of NO on the hormonally induced luminescence: NO donors sodium nitroprusside (SNP) and hydroxylamine decreased the time to reach the maximum amplitude (TL(max)) of MT-induced luminescence while these substances decreased the maximum amplitude of PRL-induced luminescence (and also the TL(max) in the case of SNP). The small impact of the NOS inhibitor l-NAME on hormonally induced luminescence suggests that NO is only produced on demand. The cGMP analogue 8BrcGMP mimicked the effects of NO donors suggesting that the effects of NO are mediated by cGMP.

Involvement of contractile elements in control of bioluminescence in Northern krill, Meganyctiphanes norvegica (M. Sars).

Inside the light organs of the bioluminescent (light-producing) crustacean Meganyctiphanes norvegica (krill), numerous capillaries drain haemolymph into the light-producing structure (lantern). We have investigated the arrangement and function of filamentous material found around the opening of the capillaries. These have been suggested to work as sphincters, controlling the haemolymph (i.e. oxygen) supply to the lantern and thereby the production of light. Electron microscopy shows that the filamentous material consists of thick and thin muscle filaments arranged in perpendicular blocks around the opening of each capillary. The actin probe rhodamine phalloidin has revealed that one component is filamentous actin. Clusters of vesicle-dense nerve profiles surround the cells containing filamentous material and antibodies against 5-hydroxytryptamine (5-HT) reveal that 5-HT containing nerves lead to the filamentous area. When exposed to the muscle-relaxing substances papaverine and verapamil, krill respond with luminescence, suggesting that the sphincter structures are functionally involved in the control of light production. Treatment with the muscle-contracting drugs Bay K8544 and thapsigargin gives no light response. Thus, 5-HT stimulates light production in krill; however, a combination of 5-HT and the muscle-relaxing drugs or Bay K8544 potentiates the effect of 5-HT. Thapsigargin quenches the response to 5-HT. Our results corroborate speculations of earlier authors who have suggested that the sphincter structures are of a muscular nature and important in controlling light production in krill. However, other parameters in addition to the oxygen supply to the lantern are involved in controlling bioluminescence in the light organs of M. norvegica.

Serotonin and nitric oxide interaction in the control of bioluminescence in northern krill, Meganyctiphanes norvegica (M. Sars).

The role of nitric oxide (NO) in the control of bioluminescence (light production) in the crustacean Meganyctiphanes norvegica (krill) was investigated using pharmacological and immunohistochemical methods. All nitrergic drugs tested failed to induce bioluminescence per se but modulated light production stimulated by 5-hydroxytryptamine (5-HT). NO donors [sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP)] injected in live specimens significantly reduced light production stimulated by 5-HT, whereas inhibition of the enzyme NO synthase (NOS) with l-NAME (N(G)-nitro-l-arginine methyl ester) resulted in an enhancement of the 5-HT response. The effects of NO do not seem to be mediated via production of cGMP as injections of a cGMP analogue (8-Bromoguanosine 3',5'-cyclic monophosphate) gave inconclusive effects on the 5-HT-stimulated light response. Inhibition of cGMP production with ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) did not affect the light response. Moreover, a few individuals showed a considerably higher response to 5-HT in April and June compared with specimens collected in the autumn and winter. Furthermore, both NOS-like and 5-HT-like materials were detected by immunohistochemistry inside the light organs. NOS-like immunoreactivity was primarily observed in structures associated with vessels inside the light organs, whereas 5-HT-like material was abundant in nerve fibres throughout the whole light organ. The results suggest that NO has a modulatory role at several levels in the control of light production in M. norvegica and that NO and 5-HT interact in this regulation.

Nitric oxide in control of luminescence from hatchetfish (Argyropelecus hemigymnus) photophores.

Nitric oxide synthase-like immunoreactivity (NOS-LI IR) was detected by immunohistochemistry in ventral light organs of the mesopelagic fish, Argyropelecus hemigymnus. Strong NOS-LI IR was present in nerve fibres and in other cells central for production or modulation of light: immunoreactive fibres surrounded the photophores, and were also present in the filter area. Filter cells, particularly in the outer layers, showed strong IR throughout the cytoplasm. Pharmacological studies suggested that nitric oxide (NO) modulates adrenaline-stimulated light emission, and that the modulation is correlated to the ability of the light organ to respond to adrenaline. Adrenaline is known to produce two different types of light response in isolated photophores from Argyropelecus: a slow, long-lasting, high intensity response, or a fast and weak response of short duration. Incubation of photophores in the NO donors sodium nitroprusside or S-nitroso-N-acetylpenicillamine prior to adrenaline stimulation reduced the intensity of the strong and long-lasting type of response, but had little or even a potentiating effect on the weakly responding photophores. Hydroxylamine, which is converted to NO if catalase activity is present in the tissue, reduced the duration and the intensity of the adrenaline response in all tested organs. The NOS-inhibitor L-thiocitrulline potentiated the adrenaline response in the weakly responding organs; the weaker the adrenaline effect, the stronger the potentiation caused by L-thiocitrulline. The strongly responding organs were instead inhibited by L-thiocitrulline. The results suggest that NO has an important role in the control of light emission from Argyropelecus hemigymnus photophores. The cGMP analogue dibutyryl cGMP, the guanylate cyclase inhibitor ODQ and the phosphodiesterase inhibitor pentoxiphylline had no effect, indicating that the NO effect does not involve cGMP.