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Uptake of 203Hg2+ in the olfactory system in pike
Toxicology Letters ELSEVIER
Toxicology Letters 84 (1996) 107-l 12
Upta.ke of 203Hg2+in the olfactory system in pike Kathleen Borg-Neczak, Hans Tjiilve* Department of Phamzacology and Toxicology, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences, Biomedicum. Box 573, S-751 23 Vppsaa, Sweden
Received 16 August 1994;revision received 19 September 1995;accepted 11 October 1995
Abetract
Inorganic mercury ( zo3Hg2+)was applied to the olfactory chambers or was given i.v. to pike (Esox lucius) and the uptake of the metal in the olfactory system and the brain was examined by autoradiography and gamma spectrometry. Application of 203Hg2”in the olfactory chambers resulted in an accumulation of the metal in the olfactory nerves and the anterior parts of the olfactory bulbs of the brain. The levels of *03Hg2+in other brain areas, such as the telencephalon, the optic tecti and the cerebellum, remained low. Application of 203Hg2+in only one olfactory chamber resulted in an uptake of the metal only in the ipsilateral olfactory nerve and olfactory bulb. Intravenous injection of the zo3Hg2+resulted in a labelling of the olfactory system and the brain, which was much lower than of the blood. These results indicate that the U)3Hg2+is taken up in the olfactory neurones from the olfactory receptor cells in the olfactory rosettes and is transported to the terminal parts of the olfactory neurones in the olfactory bulbs. The uptake of mercury as well as some other metals in the olfactory system may result in noxious effects and this may be an important component in the toxicology of metals in fish. Keywords: Mercury; Olfactory system; Pike
1. Introduclioll The olfactory neurones are bipolar primary sensot-y neurones that anatomically interconnect the
environment with the brain. In fish the olfactory receptor cells are continuously exposed to the various inorganic and organic materials which are present in the water. Autoradiographic studies at our department hLave shown that cadmium ( iWCd2+), manganese ( 54Mn2+) and inorganic mercury (203Hg2+) are taken up from the water l
Corresponding author.
into the olfactory system in brown trout (Sulmo truttu) [l-3]. Using pike as experimental animals we have shown that cadmium and manganese are taken up in the olfactory receptor cells in the olfactory rosettes and then move along the axons to the olfactory bulbs [4,5]. In the present study we have examined the uptake of 203Hg2+ in the olfactory system of the pike. 203Hg2+ was applied to the olfactory chambers and the localization of the metal in the olfactory nerves and the brain was examined by autoradiography and gamma spectrometry. In addition, to examine if 203Hg2+is taken up in the
0378-4274/95/$09.50 0 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0378-4274(95)036:22-R
108
K. Borg-Neczak.
H. ljdve
/ Toxicology Letters 84 (19%)
107-I I2
Fig. 1. (A) Detail of a whole-body autoradiogram (frontal section) of a pike killed 120 h after application of 27 pg *03Hg2+in each olfactory chamber. (B) The corresponding tissue-section. Left olfactory nerve
Left olfactory bulb
Right olfactory nenre Right olfactory bulb Telencephalon
Tectum
Cerebeltum
Fig. 2. (A) Detail of a whole-body autoradiogram (frontal section) of a pike killed 120 h after application of 27 fig 203Hg2+in the left olfactory chamber. No 203Hg2+was applied to the right olfactory chamber. (B) The corresponding tissue-section.
K. Borg-Need,
H. Tjiilve/ Toxicology Letters 84 (1996) 107-112
olfactory system and the brain from the blood pike were given 203Hg2+:i.v. and the localization of the metal in these tissues was then examined by autoradiography. 2. MateriaIs and mt:thods 2. I. Chemicals 203HgC12 (qxc. activity 1.85 mCi/mg Hg’+), dissolved in 0.1 M HCI, was obtained from the Radiochemical Centre (Amersham, England). Before use, the 203Hg2+solution was evaporated by a stream of N2 gas. Physiological saline was then added to obtain a concentration of 1 &i/p1 (0.54 &pl). Other chemicals used in the study were of analytical grade and obtained from regular commercial sources.
2.2. Animals Pike (Esox Zucius),caught by net fishing in Lake Mllaren (Sweden), were obtained from a local fisherman. They were kept in a corf (wooden box) for less than 1 day after capture and then transported to the laboratory. They weighed between 1.7 and 2.8 kg and were exclusively female. The pike were kept in 200-l all-glass aquaria in artificial aerated freshwater, pH 7.0, containing 0.15 g KHC03, 1.42 g CaCOs, 0.16 g MgG, 4 ml 1 M HCl and 6.5 ml 0.5 M H2S04 per 100 1 double deionized water. Before the experiments the pike were allowed to get acclimatized for 1 day at 10.0 f 0.5”C, which was the temperature used in the study. 2.3. Experiments In a first experiment 203Hg2+was applied to the olfactory chambers d pike. The application was performed with a micropipette, as described previously [S]. The 203Hg2+ solution (50 ~1; 50 pCi; 27 pg 203Hg2+)was applied to each naris. The nares were closed with latex seal [4] and the pike were put back into the aquaria. Two pike were killed after 120 h and used for autoradiography with tape-sections according to Ullberg et al. [6]. In lone of these pike 203Hg2+had
109
been applied to both nares. In the other pike the isotope had been applied only to the left naris. Additional pike, with isotope applied to both nares, were killed after 12, 24, 48 and 72 h, and the uptake of the metal in the brain was determined by gamma spectrometry in an Intertechnique CG 4000 Gamma Counter, as described previously [4]. In a second experiment 2 pike were injected i.v. in the vena cava caudalis with 203Hg2+ (100 &i/kg; 54 &kg). The pike were killed after 15 and 72 h and used for autoradiography.
3. Results
Autoradiography of the pike exposed to 203Hg2+for 120 h via both nares showed a labelling along the olfactory nerves. At the nerve endings there was a labelling of the superficial zones of the olfactory bulbs, giving the labelled areas a crescent-like appearance (Fig. 1). The posterior parts of the olfactory bulbs were devoid of radioactivity and the same applied to the other parts of the central nervous system, such as the telencephalon, the optic tecti and the cerebellum (Fig. 1). Autoradiography of the pike exposed to 203Hg2+for 120 h in only the left naris showed a strong labelling of the olfactory nerve and the anterior part of the olfactory bulb at the same side, whereas on the right side the nerve and bulb were unlabelled (Fig. 2). The quantitative determinations of the radioactivity in the central nervous system showed low ‘03Hg2+ levels in all the examined parts of the brain 12 h after the application of the metal in the nares (Fig. 3). At later intervals there was an increasing labelling of the olfactory bulbs, whereas in other parts of the brain the radioactivity remained low (Fig. 3). Autoradiography of the pike given 203Hg2+i.v. showed a high radioactivity in the blood. The labelling of the olfactory rosettes, the olfactory nerves, the olfactory bulbs of the brain as well as other parts of the brain and the spinal cord was much lower than in the blood (Fig. 4). In these pike the liver was the tissue which accumulated the highest amount of radioactivity.
110
K. Borg-Neczak, H. Tjdve / Toxicology Letters 84 (19%)
107-I 12
4. Didon --o-
Olf. bulb
-
Telencephalan
-
Tse”rn
-
Cerebellum
Fig. 3. Concentration of zoSHg2+as a function of time in some brain areas of pike killed at different intervals after application of 27 cg m3Hg2+ in each olfactory chamber. The data represent measurementsin single pike (except for the cerebellum the figures are the mean values from the left and right sides).
OlktolynenreollactorybulbTekneephaknTectum
The results of the present study have shown that ‘03Hg2+applied to the olfactory chambers of pike is taken up in the olfactory nerves and the olfactory bulbs of the brain. Our autoradiography in the pike given mercury i.v. shows that the bloodbrain barrier will limit the penetration of the metal into the central nervous system, and the labelling of the olfactory nerves and the olfactory rosettes also remained low in these animals. These results provide strong evidence that the labelling of the olfactory nerves and the olfactory bulbs of the brain after application of the metal in the olfactory chambers does not emanate from the blood. This
Bloodmd~lunsphalcord~(lu#
Fig. 4. (A) Detail of a whole-body autoradiogram (sag&al section) of a pike given *03Hg2+i.v. and killed after 72 h. (B) The corresponding tissue-section.
K. Borg-Nerd.
H. @Se/Toxicology
possibility is further excluded in the experiment in which mercury was applied only to one naris, resulting in an uptake of the metal only in the ipsilateral olfactory nerve and olfactory bulb. The autoradiography showed that the accumulation of mercury in the olfactory bulbs was confined to the anterior parts. This result concords with the observation in our previous study with cadmium [4]. These data indicate that mercury and cadmium are accumulated in the terminal parts of the olfactory axons, but are unable to pass the synapses to the dendrites of the mitral cells or other secondary neurones in the olfactory bulbs. In contrast, our study with %Mn2+ showed a labelling of the entire olfactory bulb and also of the telencephalon in the pike [5]. Thus, manganese appears to have the ability to migrate via secondary neurones into the telencephalon. The accumulation of mercury in the olfactory neurones implies th,at the metal is taken up in the olfactory receptor ~~11s.A localization of mercury in the olfactory charmbers of pike exposed to the metal via the water has been shown by Biickstrijm [7]. The mechanism for the uptake of the mercury in the olfactory rece:ptor cells is not known. It has been proposed that 203Hg2+may pass biological membranes as an uncharged HgC12 complex [8] and an uptake in the receptor cells in this way is one possibility. An alternative mechanism is an uptake via sodium or calcium channels, as has been proposed as a mode of entry of 203Hg2+in motor nerve terminals in frog muscle [9]. Entry of mercury into receptor cells by an endocytotic mechanism is another possibility. The mercury may then bind to surface protein(s) at the receptor cells followed by entdocytotic internalization. Endocytotic mechanisms are known to be involved in the uptake of various macromolecules into nerve terminals [lo]. Witlhin the axons the mercury is likely to be transported bound to some tissue constituent(s). 203Hg2+has a strong affinity for sulfhydryl groups of proteins [l l] and it is possible that the metal is transported in the olfactory neurones bound to such sites. The movement of mercury from the olfactory receptor cells in the olfactory epithelium to the axonal terminals in the olfactory bulbs in the fish im-
Letters 84 (1996) 107-112
111
plies that the metal is transported in an anterograde direction. Studies in mammals have shown that mercury also can be transported retrogradely. Thus, i.m. or i.v. injections of mercury in mice and rats have been shown to result in an accumulation of the metal in the motor neurones in the spinal cord and the brain stem, apparently due to a leakage of the metal from bloodvessels in muscles followed by uptake into motor nerve terminals [12,13]. There are reports that other metals, such as cadmium and lead, may also undergo retrograde axonal transport in mammals v41. There is evidence for the uptake of metals such as gold [ 151, aluminium [ 161 and cadmium [ 171 in the olfactory system in mammals. Thus, the olfactory route may be a pathway for metals into the central nervous system. It has been shown that 203Hg2+and other metals may inhibit or alter olfaction and olfactionrelated behaviour in fish [ 18-201. Olfaction plays an important role in many biological functions in fish [21]. Thus, effects of metals on the olfactory sense related to accumulation in the olfactory system may be an important component in the toxicology of metals in fish. Acknowledgements This study was supported by the Swedish Work Environment Fund. References VI TjHlve, H., Gottofrey, J. and BjGrklund, I. (1986) Tissue disposition of lWCd2+ in the brown trout (S&no rrurra) studied by autoradiography and impulse counting. Toxicol. Environ. Chem. 12, 31-45. PI Rouleau, C., TjBlve, H., Gottofrey, J. and Pelletier, 8. (1995) Uptake, distribution, and elimination of %in(II) in the brown trout (Saltno frutra). Environ. Toxicol. Chem. 14,483-490. 131Borg, K., Gottofroy, J. and TjPlve, H. (1988). Bioaccummulation and tissue-disposition of cadmium, mercury and nickel in the brown trout (S&no rrutra) studied by wholebody autoradiography and impulse counting. In: F. Simon (Ed.). Veterinary Pharmacology, Toxicology and Therapy in Food Producing Animals, European Association for Veterinary Pharmacology and Toxi-
112
[4]
(51
[6]
[7]
[8]
[9]
[lo]
(111
[12]
K. Borg-Necmk.
H. QZlve / Toxicology Letters 84 (19%)
cology, 4th Congress, Budapest, Hungary. ISBN 963 7149007, Q. 295. Gottofrey, J. and Tjlilve, H. (1991) Axonal transport of cadmium in the olfactory nerve of the pike. Pharmacol. Toxicol. 69, 242-252. Tjiilve, H., Mejare, C. and Borg-Neczak, K. (1995) Up take and transport of manganese in primary and secondary olfactory neurones in pike. Pharmacol. Toxicol. 77, 23-31. Ullberg, S., Larsson, B. and Tjglve, H. (1982) Autoradiography. In: H.J. Glenn and L.G. Colombetti (Eds.), Biologic AQQbXtiOnS of Radiotracers, CRC Press, Inc., Boca Raton, Fla., pp. 55-108. Biickstriim, J. (1969) Distribution studies of mercuric pesticides in quail and some fresh-water fishes. Acta Pharm. Toxicol. 27, SuppI. 3, 9, I-103. Gutknecht, J. (1981) Inorganic mercury (Hg”) transQOIT through lipid bilayer membranes. J. Membr. Biol. 61, 61-66. Miyamoto, M.T. (1983) Hg2+ causes neurotoxicity at an intracellular site following entry through Na and Ca channels. Brain Res. 267, 375-379. Yamamoto, T., Iwasaki, Y., Kouno, H., Iizuka, H. and Zhao, J.-X. (1987) Retrograde transport and differential accumulation of serum proteins in motor neurons. Implication for motor neuron diseases. Neurology 37, 843-846. Berlin, M. (1986) Mercury. In: L. F&erg, G.F. Nordberg and V.B. Vouk @Is.), Handbook on the Toxicology of Metals, Vol. II, 2nd Ed., Elsevier Science Publishers B.V., Amsterdam-New York-Oxford, pp. 387-445. Arvidson, B. (1992) Inorganic mercury is transported
107-112
from muscular nerve terminals to spinal and brainstem motorneurons. Muscle Nerv. 15, 1089-1094. I131 Schibning, J.D. (1993) Retrograde axonal transport of mercury in rat sciatic nerve. Toxicol. Appl. Pharmacol. 121,43-49. 1141 Arvidson, B. (1994) A review of axonal transport of metals. Toxicology 88, I-14. [I51 De Lorenzo, A.J.D. (1970) The olfactory neuron and the blood-brain barrier. In: G.E.W. Wolstenholme and J. Knight @is.), Taste and Smell in Vertebrates, J. & A. Churchill, London, pp. 151- 176. 1161 Perl, D.P. and Good, P.F. (1987) Uptake of aluminum into central nervous system along nasal-olfactory pathways. Lancet 1, 1028. 1171 Evans, J. and Hastings, L. (1992) Accumulation of Cd(I1) in the CNS depending on the route of administration: intraperitoneal, intratracheal or intranasal. Fundam. Appl. Toxicol. 19,275-278. WI Hara, T.J., Law, Y.M.C. and Macdonald, S. (1976) Effects of mercury and copper on the olfactory response in rainbow trout, Solmo gairdneri. J. Fish. Res. Board Can. 33, 1568-1573. I191 Baatrup, E., Dting, K.B. and Winberg, S. (1990)Differential effects of mercurial compounds on the electroolfactogram (EOG) of salmon (Salmo salar L.). Ecotoxicol. Environ. Saf. 20, 269-276. WI Rehnberg, B.C. and Schreck, C.B. (1986) Acute metal toxicology of olfaction in coho salmon: behavior, recep tots, and odor-metal complexation. Bull. Environ. Contam. Toxicol. 36, 579-586. WI Hara, T.J. (1994) Olfaction and gustation in fish: an overview. Acta Physiol. Stand. 152, 207-217.
Toxicology Letters 84 (1996) 107-l 12
Upta.ke of 203Hg2+in the olfactory system in pike Kathleen Borg-Neczak, Hans Tjiilve* Department of Phamzacology and Toxicology, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences, Biomedicum. Box 573, S-751 23 Vppsaa, Sweden
Received 16 August 1994;revision received 19 September 1995;accepted 11 October 1995
Abetract
Inorganic mercury ( zo3Hg2+)was applied to the olfactory chambers or was given i.v. to pike (Esox lucius) and the uptake of the metal in the olfactory system and the brain was examined by autoradiography and gamma spectrometry. Application of 203Hg2”in the olfactory chambers resulted in an accumulation of the metal in the olfactory nerves and the anterior parts of the olfactory bulbs of the brain. The levels of *03Hg2+in other brain areas, such as the telencephalon, the optic tecti and the cerebellum, remained low. Application of 203Hg2+in only one olfactory chamber resulted in an uptake of the metal only in the ipsilateral olfactory nerve and olfactory bulb. Intravenous injection of the zo3Hg2+resulted in a labelling of the olfactory system and the brain, which was much lower than of the blood. These results indicate that the U)3Hg2+is taken up in the olfactory neurones from the olfactory receptor cells in the olfactory rosettes and is transported to the terminal parts of the olfactory neurones in the olfactory bulbs. The uptake of mercury as well as some other metals in the olfactory system may result in noxious effects and this may be an important component in the toxicology of metals in fish. Keywords: Mercury; Olfactory system; Pike
1. Introduclioll The olfactory neurones are bipolar primary sensot-y neurones that anatomically interconnect the
environment with the brain. In fish the olfactory receptor cells are continuously exposed to the various inorganic and organic materials which are present in the water. Autoradiographic studies at our department hLave shown that cadmium ( iWCd2+), manganese ( 54Mn2+) and inorganic mercury (203Hg2+) are taken up from the water l
Corresponding author.
into the olfactory system in brown trout (Sulmo truttu) [l-3]. Using pike as experimental animals we have shown that cadmium and manganese are taken up in the olfactory receptor cells in the olfactory rosettes and then move along the axons to the olfactory bulbs [4,5]. In the present study we have examined the uptake of 203Hg2+ in the olfactory system of the pike. 203Hg2+ was applied to the olfactory chambers and the localization of the metal in the olfactory nerves and the brain was examined by autoradiography and gamma spectrometry. In addition, to examine if 203Hg2+is taken up in the
0378-4274/95/$09.50 0 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0378-4274(95)036:22-R
108
K. Borg-Neczak.
H. ljdve
/ Toxicology Letters 84 (19%)
107-I I2
Fig. 1. (A) Detail of a whole-body autoradiogram (frontal section) of a pike killed 120 h after application of 27 pg *03Hg2+in each olfactory chamber. (B) The corresponding tissue-section. Left olfactory nerve
Left olfactory bulb
Right olfactory nenre Right olfactory bulb Telencephalon
Tectum
Cerebeltum
Fig. 2. (A) Detail of a whole-body autoradiogram (frontal section) of a pike killed 120 h after application of 27 fig 203Hg2+in the left olfactory chamber. No 203Hg2+was applied to the right olfactory chamber. (B) The corresponding tissue-section.
K. Borg-Need,
H. Tjiilve/ Toxicology Letters 84 (1996) 107-112
olfactory system and the brain from the blood pike were given 203Hg2+:i.v. and the localization of the metal in these tissues was then examined by autoradiography. 2. MateriaIs and mt:thods 2. I. Chemicals 203HgC12 (qxc. activity 1.85 mCi/mg Hg’+), dissolved in 0.1 M HCI, was obtained from the Radiochemical Centre (Amersham, England). Before use, the 203Hg2+solution was evaporated by a stream of N2 gas. Physiological saline was then added to obtain a concentration of 1 &i/p1 (0.54 &pl). Other chemicals used in the study were of analytical grade and obtained from regular commercial sources.
2.2. Animals Pike (Esox Zucius),caught by net fishing in Lake Mllaren (Sweden), were obtained from a local fisherman. They were kept in a corf (wooden box) for less than 1 day after capture and then transported to the laboratory. They weighed between 1.7 and 2.8 kg and were exclusively female. The pike were kept in 200-l all-glass aquaria in artificial aerated freshwater, pH 7.0, containing 0.15 g KHC03, 1.42 g CaCOs, 0.16 g MgG, 4 ml 1 M HCl and 6.5 ml 0.5 M H2S04 per 100 1 double deionized water. Before the experiments the pike were allowed to get acclimatized for 1 day at 10.0 f 0.5”C, which was the temperature used in the study. 2.3. Experiments In a first experiment 203Hg2+was applied to the olfactory chambers d pike. The application was performed with a micropipette, as described previously [S]. The 203Hg2+ solution (50 ~1; 50 pCi; 27 pg 203Hg2+)was applied to each naris. The nares were closed with latex seal [4] and the pike were put back into the aquaria. Two pike were killed after 120 h and used for autoradiography with tape-sections according to Ullberg et al. [6]. In lone of these pike 203Hg2+had
109
been applied to both nares. In the other pike the isotope had been applied only to the left naris. Additional pike, with isotope applied to both nares, were killed after 12, 24, 48 and 72 h, and the uptake of the metal in the brain was determined by gamma spectrometry in an Intertechnique CG 4000 Gamma Counter, as described previously [4]. In a second experiment 2 pike were injected i.v. in the vena cava caudalis with 203Hg2+ (100 &i/kg; 54 &kg). The pike were killed after 15 and 72 h and used for autoradiography.
3. Results
Autoradiography of the pike exposed to 203Hg2+for 120 h via both nares showed a labelling along the olfactory nerves. At the nerve endings there was a labelling of the superficial zones of the olfactory bulbs, giving the labelled areas a crescent-like appearance (Fig. 1). The posterior parts of the olfactory bulbs were devoid of radioactivity and the same applied to the other parts of the central nervous system, such as the telencephalon, the optic tecti and the cerebellum (Fig. 1). Autoradiography of the pike exposed to 203Hg2+for 120 h in only the left naris showed a strong labelling of the olfactory nerve and the anterior part of the olfactory bulb at the same side, whereas on the right side the nerve and bulb were unlabelled (Fig. 2). The quantitative determinations of the radioactivity in the central nervous system showed low ‘03Hg2+ levels in all the examined parts of the brain 12 h after the application of the metal in the nares (Fig. 3). At later intervals there was an increasing labelling of the olfactory bulbs, whereas in other parts of the brain the radioactivity remained low (Fig. 3). Autoradiography of the pike given 203Hg2+i.v. showed a high radioactivity in the blood. The labelling of the olfactory rosettes, the olfactory nerves, the olfactory bulbs of the brain as well as other parts of the brain and the spinal cord was much lower than in the blood (Fig. 4). In these pike the liver was the tissue which accumulated the highest amount of radioactivity.
110
K. Borg-Neczak, H. Tjdve / Toxicology Letters 84 (19%)
107-I 12
4. Didon --o-
Olf. bulb
-
Telencephalan
-
Tse”rn
-
Cerebellum
Fig. 3. Concentration of zoSHg2+as a function of time in some brain areas of pike killed at different intervals after application of 27 cg m3Hg2+ in each olfactory chamber. The data represent measurementsin single pike (except for the cerebellum the figures are the mean values from the left and right sides).
OlktolynenreollactorybulbTekneephaknTectum
The results of the present study have shown that ‘03Hg2+applied to the olfactory chambers of pike is taken up in the olfactory nerves and the olfactory bulbs of the brain. Our autoradiography in the pike given mercury i.v. shows that the bloodbrain barrier will limit the penetration of the metal into the central nervous system, and the labelling of the olfactory nerves and the olfactory rosettes also remained low in these animals. These results provide strong evidence that the labelling of the olfactory nerves and the olfactory bulbs of the brain after application of the metal in the olfactory chambers does not emanate from the blood. This
Bloodmd~lunsphalcord~(lu#
Fig. 4. (A) Detail of a whole-body autoradiogram (sag&al section) of a pike given *03Hg2+i.v. and killed after 72 h. (B) The corresponding tissue-section.
K. Borg-Nerd.
H. @Se/Toxicology
possibility is further excluded in the experiment in which mercury was applied only to one naris, resulting in an uptake of the metal only in the ipsilateral olfactory nerve and olfactory bulb. The autoradiography showed that the accumulation of mercury in the olfactory bulbs was confined to the anterior parts. This result concords with the observation in our previous study with cadmium [4]. These data indicate that mercury and cadmium are accumulated in the terminal parts of the olfactory axons, but are unable to pass the synapses to the dendrites of the mitral cells or other secondary neurones in the olfactory bulbs. In contrast, our study with %Mn2+ showed a labelling of the entire olfactory bulb and also of the telencephalon in the pike [5]. Thus, manganese appears to have the ability to migrate via secondary neurones into the telencephalon. The accumulation of mercury in the olfactory neurones implies th,at the metal is taken up in the olfactory receptor ~~11s.A localization of mercury in the olfactory charmbers of pike exposed to the metal via the water has been shown by Biickstrijm [7]. The mechanism for the uptake of the mercury in the olfactory rece:ptor cells is not known. It has been proposed that 203Hg2+may pass biological membranes as an uncharged HgC12 complex [8] and an uptake in the receptor cells in this way is one possibility. An alternative mechanism is an uptake via sodium or calcium channels, as has been proposed as a mode of entry of 203Hg2+in motor nerve terminals in frog muscle [9]. Entry of mercury into receptor cells by an endocytotic mechanism is another possibility. The mercury may then bind to surface protein(s) at the receptor cells followed by entdocytotic internalization. Endocytotic mechanisms are known to be involved in the uptake of various macromolecules into nerve terminals [lo]. Witlhin the axons the mercury is likely to be transported bound to some tissue constituent(s). 203Hg2+has a strong affinity for sulfhydryl groups of proteins [l l] and it is possible that the metal is transported in the olfactory neurones bound to such sites. The movement of mercury from the olfactory receptor cells in the olfactory epithelium to the axonal terminals in the olfactory bulbs in the fish im-
Letters 84 (1996) 107-112
111
plies that the metal is transported in an anterograde direction. Studies in mammals have shown that mercury also can be transported retrogradely. Thus, i.m. or i.v. injections of mercury in mice and rats have been shown to result in an accumulation of the metal in the motor neurones in the spinal cord and the brain stem, apparently due to a leakage of the metal from bloodvessels in muscles followed by uptake into motor nerve terminals [12,13]. There are reports that other metals, such as cadmium and lead, may also undergo retrograde axonal transport in mammals v41. There is evidence for the uptake of metals such as gold [ 151, aluminium [ 161 and cadmium [ 171 in the olfactory system in mammals. Thus, the olfactory route may be a pathway for metals into the central nervous system. It has been shown that 203Hg2+and other metals may inhibit or alter olfaction and olfactionrelated behaviour in fish [ 18-201. Olfaction plays an important role in many biological functions in fish [21]. Thus, effects of metals on the olfactory sense related to accumulation in the olfactory system may be an important component in the toxicology of metals in fish. Acknowledgements This study was supported by the Swedish Work Environment Fund. References VI TjHlve, H., Gottofrey, J. and BjGrklund, I. (1986) Tissue disposition of lWCd2+ in the brown trout (S&no rrurra) studied by autoradiography and impulse counting. Toxicol. Environ. Chem. 12, 31-45. PI Rouleau, C., TjBlve, H., Gottofrey, J. and Pelletier, 8. (1995) Uptake, distribution, and elimination of %in(II) in the brown trout (Saltno frutra). Environ. Toxicol. Chem. 14,483-490. 131Borg, K., Gottofroy, J. and TjPlve, H. (1988). Bioaccummulation and tissue-disposition of cadmium, mercury and nickel in the brown trout (S&no rrutra) studied by wholebody autoradiography and impulse counting. In: F. Simon (Ed.). Veterinary Pharmacology, Toxicology and Therapy in Food Producing Animals, European Association for Veterinary Pharmacology and Toxi-
112
[4]
(51
[6]
[7]
[8]
[9]
[lo]
(111
[12]
K. Borg-Necmk.
H. QZlve / Toxicology Letters 84 (19%)
cology, 4th Congress, Budapest, Hungary. ISBN 963 7149007, Q. 295. Gottofrey, J. and Tjlilve, H. (1991) Axonal transport of cadmium in the olfactory nerve of the pike. Pharmacol. Toxicol. 69, 242-252. Tjiilve, H., Mejare, C. and Borg-Neczak, K. (1995) Up take and transport of manganese in primary and secondary olfactory neurones in pike. Pharmacol. Toxicol. 77, 23-31. Ullberg, S., Larsson, B. and Tjglve, H. (1982) Autoradiography. In: H.J. Glenn and L.G. Colombetti (Eds.), Biologic AQQbXtiOnS of Radiotracers, CRC Press, Inc., Boca Raton, Fla., pp. 55-108. Biickstriim, J. (1969) Distribution studies of mercuric pesticides in quail and some fresh-water fishes. Acta Pharm. Toxicol. 27, SuppI. 3, 9, I-103. Gutknecht, J. (1981) Inorganic mercury (Hg”) transQOIT through lipid bilayer membranes. J. Membr. Biol. 61, 61-66. Miyamoto, M.T. (1983) Hg2+ causes neurotoxicity at an intracellular site following entry through Na and Ca channels. Brain Res. 267, 375-379. Yamamoto, T., Iwasaki, Y., Kouno, H., Iizuka, H. and Zhao, J.-X. (1987) Retrograde transport and differential accumulation of serum proteins in motor neurons. Implication for motor neuron diseases. Neurology 37, 843-846. Berlin, M. (1986) Mercury. In: L. F&erg, G.F. Nordberg and V.B. Vouk @Is.), Handbook on the Toxicology of Metals, Vol. II, 2nd Ed., Elsevier Science Publishers B.V., Amsterdam-New York-Oxford, pp. 387-445. Arvidson, B. (1992) Inorganic mercury is transported
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