Electrical activity of caudal neurosecretory neurons in seawater and freshwater-adapted Platichthys flesus, in vivo
Faculty of Life Sciences, The University of Manchester, Manchester M13 9PT, UK
* Author for correspondence (e-mail: cathy.mccrohan{at}manchester.ac.uk)
Accepted 8 November 2004
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Summary |
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Key words: flounder, Platichthys flesus, neurosecretion, Dahlgren cell, osmoregulation, electrical bursting activity
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Introduction |
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Electrophysiological recording from an isolated CNSS preparation,
comprising spinal cord and urophysis in vitro, indicated the presence
of two types of Dahlgren cell (Hubbard et
al., 1996a; Brierley et al.,
2003
). Type 1 (T1) cells are spontaneously active with activity
patterns ranging from tonic, through phasic, to the generation of
characteristic bursting, reminiscent of that reported for mammalian
vasopressin neurons (Leng et al.,
1999
). Type 2 (T2) cells are silent and relatively inexcitable;
they exhibit strong spike frequency accommodation and fire only a single
action potential in response to a long duration depolarising stimulus.
Otherwise, membrane properties of the two cell types are similar, apart from a
significantly smaller spike afterhyperpolarisation (AHP) in T2 compared with
T1 cells (Brierley et al.,
2003
). Variations in firing patterns are likely to be related to
differential neuropeptide secretion from the urophysis and we might therefore
expect to see differences in electrical activity of Dahlgren cells under
different osmoregulatory conditions.
We have previously compared spontaneous firing patterns and membrane
properties of Dahlgren cells in isolated (in vitro) CNSS taken from
flounder fully adapted to either SW or FW
(Brierley et al., 2003). This
revealed few differences, apart from apparently less robust bursting activity
in FW-adapted cells. However, this preparation lacks descending modulatory
input, which may carry information about the fish's internal and external
osmotic environment. The role of such descending input is suggested by
pharmacological investigations, which identified a range of potential
neuromodulators in this system, including noradrenaline, serotonin and
acetylcholine (Hubbard et al.,
1996b
,
1997
;
Brierley et al., 2003
).
The aim of this study was initially to determine whether electrophysiology and activity of Dahlgren cells in vivo are comparable with those previously reported in vitro. We then aimed to test the hypotheses that: first, SWFW adaptation is associated with differences in electrical activity patterns of Dahlgren cells (and hence peptide secretion) in vivo; and, second, activity in the CNSS is modulated by descending inputs from the brain. We first compared intrinsic membrane properties and firing patterns of Dahlgren cells between SW- and FW-adapted preparations in vivo, and between in vivo and in vitro preparations. We then carried out initial studies to examine effects of manipulation of central pathways, including those potentially involved in signalling changes in osmolarity of the external medium.
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Materials and methods |
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Fish were anaesthetised as detailed under UK Home Office licensing
procedures. They were immersed in MS222 (ethyl-m-aminobenzoate methane
sulphonate, 100 mg l1) or Benzocaine (ethyl-p-aminobenzoate,
50 mg l1) (Sigma, UK). After 45 min fish became
sedated, were weighed and then given an intraperitoneal injection of Saffan
(alphaxalone/alphadalone acetate mixture, 36 mg kg1 body
weight, Schering-Plough Animal Health, UK). This produces long term (48
h), stable anaesthesia in teleosts
(Oswald, 1978). During
induction of surgical anaesthesia, the regular opercular rhythm associated
with gill ventilation gradually disappeared. The fish was placed on a
platform, covered in damp paper towel and immobilised to stabilise the caudal
region for surgery and recording. The gills were ventilated by continuous
perfusion of water (SW or FW, 1000 ml kg1 body weight
min1) through the buccal cavity and over the gills
via a centrifugal pump (Type 1250, Eheim Ltd, Germany). Water was
recirculated through a feeder tank (20 l volume), where it was aerated and
cooled to 78°C. During surgery and recording, the general
physiological state of the fish was continuously monitored by checking the
normal red colour of the gills and by monitoring the electrocardiogram (ECG)
using small metal electrodes on the skin surface. Heart rate was typically
3050 beats min1. Fish were maintained under stable
anaesthesia for up to 8 h.
A 2.53.0 cm incision was made adjacent to the lateral line in the caudal region and the muscle overlying the spinal column retracted. Bone covering preterminal spinal cord segments 25 was drilled or clipped away and the meningeal sheath surrounding the cord removed using fine forceps. Accumulation of tissue fluid within the surgical site caused blockage of intracellular microelectrodes, so a Ringer perfusion/extraction system was established. Ringer was designed to match plasma ion levels for flounder adapted to SW or FW. Seawater Ringer composition was (in mmol l1): K2HPO4 1.0, KCL 0.5, NaCl 155, NaHCO3 10, CaCl2 2.12, MgSO4 1.0, D-glucose 5.56, adjusted to pH 7.7 (osmolality 315 mOsmol kg1 H2O). Freshwater Ringer had a NaCl concentration of 145 mmol l1 and osmolality 280 mOsmol kg1 H2O. Ringer was cooled to 34°C and then perfused (1.0 ml min1) over the exposed spinal cord via a peristaltic pump (Type 302S, Watson Marlowe Ltd, UK). This maintained the viability of the tissue where local blood vessels were disrupted when the meningeal sheath was removed for intracellular recording. The temperature at the recording site, monitored using a small thermocouple and electronic thermometer, was 1012°C and Ringer was not recirculated.
Intracellular recording
Intracellular recordings were made from Dahlgren cells using glass
microelectrodes pulled on a micropipette puller (Model 750, David Kopf
Instruments, CA, USA) and filled with 3 mol l1 potassium
acetate (resistance 50100 M). The electrode was connected to an
Axoclamp 2A amplifier (Axon Instruments, CA, USA) and data captured (sampling
rate 5 kHz) via a CED 1401 converter (Cambridge Electronic Design,
UK). Data were stored and subsequently analysed using CED Signal (v1.72) or
Spike2 (v2.02) software. Following cell penetration, the electrode was bridge
balanced using 500 ms 0.3 nA current pulses at 0.3 Hz. Only cells that
maintained a stable resting potential more negative than 50 mV and could
generate overshooting action potentials were considered viable.
Extracellular recording
Recordings were made from preterminal segments 23 using a glass
suction electrode (diameter 300600 µm)
(Brierley et al., 2003).
Signals were recorded differentially and amplified using a Neurolog
(Digitimer, UK) AC NL104 amplifier with NL 100K probe (headstage), filtered
(NL125, 5 Hz and 1.25 kHz cut off and a 50 Hz notch filter), and digitised
through a CED micro 1401. Spike2 (v4.09) software was used for data capture
(sampling rate 8 kHz) and off-line analysis. This software enables separation
of activity of individual units, by generating an action potential template
for each unit and scanning the recording for similar waveforms. Action
potentials from different units are then displayed as event marks on separate
traces.
Acute salinity exchange
Separate 20 l tanks of SW and FW were cooled to 78°C and water
pumped through appropriately adjusted flow-meters to a manually operated
two-way valve. Seawater or FW could then be selected as required to supply the
fish mouthpiece. Water draining from the gill covers was returned to the
appropriate tank for cooling and recirculation. The system was designed to
minimise mechanical disruption of the recordings during switching.
Nerve stimulation
The branchial branches of the glossopharyngeal and vagal cranial nerves
were exposed by dissection near to their roots in the brainstem medulla and
supported in a liquid paraffin pool on a pair of 0.2 mm diameter, stainless
steel hook electrodes for stimulation. Electrically isolated constant voltage
stimuli were produced by a S88C stimulator and SIU5 isolator (Grass Instrument
Co., USA) and consisted of 1 strains of 0.2 ms duration pulses at 20 Hz, with
voltages in the range 1040 V.
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Results |
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For comparison with previous work on isolated CNSS
(Hubbard et al., 1996a;
Brierley et al., 2001
,
2003
), the membrane properties
of T1 and T2 cells in vivo were analysed further.
Table 1A presents membrane and
action potential parameters for T1 cells from fully SW- and FW-adapted fish.
There were no significant differences between the two adaptation states for
any of these, suggesting no difference in underlying membrane conductances.
Although relatively few T2 cells were encountered, similar data are presented
for T2 cells (Table 1B). Again,
there were no obvious differences in membrane parameters between SW- and
FW-adapted fish. As expected from previous studies in vitro
(Hubbard et al., 1996a
;
Brierley et al., 2003
),
significant differences were found both in action potential threshold and in
AHP duration of T2 compared with T1 cells (SW-adapted)
(Table 1).
|
As reported by Brierley et al.
(2001,
2003
) for Dahlgren cells in
vitro, spontaneous activity in T1 cells in vivo varied between
tonic, phasic and bursting activity patterns
(Fig. 1B). Activity patterns
were classified as defined by Brierley et al.
(2003
): `bursting', comprising
discrete >20 s bursts of action potentials separated by periods of >20 s
quiescence; `phasic', showing irregular activity including periods of high
(13 Hz) and low (<1 Hz) frequency firing; and `tonic', consisting of
continuous activity
1 Hz with few or no silent periods. There was no
obvious difference in the range of activity patterns of cells recorded
intracellularly from SW- and FW-adapted fish. Burst parameters were calculated
for bursting T1 cells (Fig. 1C)
to highlight differences between SW- and FW-adapted states
(Table 2A). The trend was
towards longer burst duration and cycle period in FW compared with SW fish.
However, these differences were not statistically significant, possibly owing
to substantial variation between cells. The only parameter that showed a
difference was the deceleration phase (time from peak firing rate to the end
of a burst), which was significantly longer in cells in FW-adapted fish,
suggesting less tight control over burst termination than in SW cells.
|
Table 2B presents comparable
data, taken from Brierley et al.
(2001; M. J. Brierley,
unpublished), for bursting T1 cells recorded intracellularly in the in
vitro (isolated) CNSS preparation. In contrast to cells recorded in
vivo, there was a trend towards shorter bursts in cells from FW-compared
with SW-adapted preparations, which was reflected in significantly fewer
spikes per burst. Statistical comparison between burst parameters of cells
recorded in vivo and in vitro revealed significant
differences (Table 2). In
SW-adapted fish, burst duration and cycle period were significantly longer
(P<0.01; Student's t-test) and the number of action
potentials per burst significantly greater (P<0.05) in
vitro than in vivo; maximum and mean firing frequency within
bursts did not differ. In FW fish, bursts were similar between the two
preparations; there were no significant differences between burst parameters
recorded in vitro compared with in vivo, apart from a lower
(P<0.05) mean firing frequency within bursts in vitro
(Table 2B).
Extracellular recording
Spike2 analysis allowed separation of activity of up to 14 Dahlgren cells
from each fish, yielding a total of 56 cells from five SW-adapted and 69 cells
from five FW-adapted fish. Dahlgren cells were clearly identified by their
long duration action potentials (Brierley
et al., 2003). Action potential duration at half spike amplitude
was 2.5±0.5 ms for SW and 2.5±0.4 ms for FW Dahlgren cells.
Cells with extracellular spike duration less than 1.8 ms were not included in
the analysis. These were probably
neurons with markedly briefer spikes
(<1 ms; Brierley et al.,
2003
). Action potential amplitude was 0.29±0.14 mV for SW
and 0.31±0.12 mV for FW Dahlgren cells.
Fig. 2 shows a recording from a
SW-adapted fish, showing total activity and that of 10 individual units with
different activity patterns.
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To identify any differences in spontaneous activity patterns of Dahlgren
cells between SW- and FW-adapted fish, extracellular recordings were taken for
2000 s and then analysed. Each unit was classified for its firing pattern:
tonic, phasic, bursting, as defined by Brierley et al.
(2003). (Silent cells, by
definition, were not recorded.) The total number of spikes generated (per 1000
s) was used as a measure of overall activity. The degree of `patternedness' of
cells' firing activity was assessed by sorting the spike data into 50 s time
bins and measuring the maximum and minimum number of spikes per bin.
Table 3 summarises these data
for all units from SW and FW adapted fish. Cells were almost all classified as
either bursting (28% SW; 16% FW) or phasically active (72% SW; 82% FW), with
the majority the latter. Furthermore, the proportion of cells classified as
bursting in SW-adapted CNSS (28%) was significantly (P<0.05;
2-test) greater than in FW-adapted (16%) fish. There was no
significant difference in any other measured parameter between the two
adaptation states. Overall activity was highly variable between cells, ranging
from less than 20 to more than 2000 spikes per 1000 s. However, the mean of
these rates did not differ between SW and FW. The maximum firing frequency was
around 5 Hz, as reported in vitro
(Brierley et al., 2001
).
Minimum and maximum firing rates per 50 s time bin were similar for the two
adaptation states.
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Flounder were exposed to acute change in salinity of water perfusing the
gills during continuous extracellular recording from the CNSS. Three SW- and
four FW-adapted flounder were recorded for 2000 s and then the gill perfusion
was changed to FW and SW, respectively, followed by further recording for 2000
s. Firing activity was compared pre- and post-water change. No overall changes
in total firing activity, maximum or minimum firing rates of Dahlgren cells
were observed. However, some cells did change their firing pattern, including
a number that were either recruited (silent in control) or whose activity was
abolished (became silent) following water change.
Table 4 shows the proportion of
cells showing the different firing patterns. It was possible to include silent
cells in this analysis, but only those that showed a change in activity (from
silent to active or vice versa) following water change. A change from
SW to FW led to a significant (P<0.05; 2-test)
increase in the number of silent cells, from 9 to 23%. Conversely, the FW to
SW switch led to an apparent reduction in silent cells, though this was not
significant. The proportion of cells generating other activity patterns was
unchanged following the salinity switch.
|
Visual inspection of recordings suggested that firing activity of different units within a preparation was often correlated (e.g. Fig. 3). In order to quantify this, a Correlation Index (CI) was calculated for CNSS of each individual fish both before (2000 s) and after (2000 s) the switch in gill perfusion (Table 5). For each unit, spikes were sorted into 20 s time bins. Cross correlation was used to compare each unit with all the others for each time bin and assessed for significance using Pearson correlation (two-tailed; significance assumed at P<0.05). The CI was defined as the proportion of the total comparisons for each CNSS that showed significance. Correlation Index ranged from 0.06 to 0.94 and was significantly (P<0.05, unpaired) greater in SW compared with FW fish. Furthermore, a switch from SW to FW led to a significant reduction in CI (P<0.05, paired), whereas the switch from FW to SW caused a significant increase in CI (P<0.05, paired).
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In 17 fish (nine SW-, eight FW-adapted), at the end of an experiment, the
spinal cord was severed one or two segments anterior to the recording site. In
12 cases (five SW, seven FW), this resulted in a marked increase in the firing
activity of Dahlgren cells (Fig.
4). A similar response was obtained from one fish (SW-adapted) in
which the cord was cut much higher up, at its junction with the posterior
hindbrain. The response to cutting the cord was characterised by an initial
burst of irregular large amplitude (12 mV) spikes lasting for 10
s. This was followed by a prolonged period (up to 20 min) of increased overall
activity, mainly owing to recruitment of previous silent units, many of which
were bursting (Fig. 4).
Preliminary experiments were also carried out to identify any responses in the
CNSS following stimulation of input pathways to the brain. Electrical
stimulation of the branchial branches of the glossopharyngeal and/or vagal
nerves (1 strain, 20 Hz, 0.2 ms, 40 V) led to a long lasting (up to 400 s)
change in Dahlgren cell firing, comprising an overall reduction in activity
(Fig. 5A). Tactile (mild pinch)
stimulation of lips and fins also induced marked changes in activity in some
preparations, lasting up to 500 s and consisting of an initial period of
inhibition (
150 s) followed by a gradual return to the control activity
level, and sometimes including recruitment of previously silent units
(Fig. 5B,C). These results
imply a strong descending influence over Dahlgren cell activity.
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Discussion |
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One interesting difference between in vivo and in vitro
multiunit data was a lower proportion of cells exhibiting bursting or tonic,
as opposed to phasic, activity patterns in the former. Brierley et al.
(2003) reported 6065%
bursting, 20% phasic and 1520% tonically active cells recorded
extracellularly in vitro. By contrast, in this study 7282% of
cells were phasic with only 1628% bursting and 02% tonic.
Presumably this was again due to the absence of descending modulatory input in
the isolated in vitro CNSS preparation, supporting the view that
bursting activity is an intrinsic emergent property of the CNSS and that
extrinsic (descending) modulation disrupts this pattern, having the overall
effect of making Dahlgren cells fire more irregularly (phasic activity).
When comparisons were made between T1 cells recorded from chronically SW-
vs FW-adapted fish in vivo, there were no differences in
resting membrane properties or action potential generation. However, bursting
cells in FW-adapted fish showed an apparently longer burst duration, which was
due to a significantly longer deceleration phase. A similar finding was
reported in vitro where bursts in FW-adapted cells were more variable
in duration than their SW counterparts
(Brierley et al., 2003),
suggesting less tight control over burst duration and termination in the FW
state.
Although firing activity was similar in T1 Dahlgren cells from chronically SW- and FW-adapted fish, the proportion of cells showing different activity patterns was not. Significantly more cells were bursting in SW-adapted fish, again supporting the view that bursting is less robust in the FW-adapted state. Similarly the Correlation Index was greater in SW preparations and showed a significant reduction following acute transfer to FW, with a corresponding increase for the reverse transfer (FW to SW). The acute switch to FW in chronically SW-adapted fish also led to a significant increase in the number of silent Dahlgren cells (i.e. de-recruitment).
Bursting activity in Dahlgren cells depends on intrinsic cellular
mechanisms including Ca-mediated depolarising after potentials (DAP) and
post-burst after depolarisation (ADP), which maintain repetitive firing and
underlie prolonged burst generation respectively (Brierley et al.,
2001,
2003
,
2004
). However, the significant
CIs found here also suggest co-ordination of cellular firing within the
population. We have no evidence that Dahlgren cells are directly synaptically
linked (Winter et al., 2000
),
so that co-ordination within the population is most likely mediated either by
patterned descending input or by rhythm generation intrinsic to the CNSS,
involving either a presynaptic interneuronal network or release of an
intrinsic neuromodulator. With respect to the latter, nitric oxide (NO) has
been implicated as a local neuromodulator in this system. Dahlgren cells in
another teleost, Oreochromis niloticus, express nitric oxide synthase
(Cioni et al., 1997
) and in
flounder we have shown that they are excited by application of a NO donor
(Brierley et al., 2002
). Thus,
it is possible that Dahlgren cell activity is coordinated by local positive
feedback, in a similar way to the synchronised high frequency bursts of
oxytocin neurons (Russell et al.,
2003
). The functional relevance of coordinated activity between
Dahlgren cells is probably related to the regulation of peptide secretion.
Bursts of spikes in neuroendocrine cells are known to enhance peptide
secretion, when compared with tonic firing
(Cazalis et al., 1985
). The
degree of synchronisation of the timing of these bursts in a neuron population
(i.e. CI) could potentially allow for a continuum of secretory pattern,
ranging from continuous low level secretion (no synchrony) through to highly
pulsatile release. Similarly, multiunit recordings from populations of
immortalised GnRH neurons (Nunemaker et
al., 2001
) revealed episodes of increased firing activity within
the population, which are due to co-incident activity in subpopulations of
neurons, and which appear to underlie episodic (pulsatile) GnRH secretion. It
was suggested that variations in the number of units contributing to such
episodes could account for changes in GnRH pulse amplitude in this system
(Nunemaker et al., 2001
).
The response to cutting the spinal cord was excitatory, suggesting the presence of a descending continuous tonic inhibitory drive to the CNSS. The initial brief excitatory burst (10 s) was probably due to activation of excitatory fibres as they were cut. The longer term response included recruitment of units and enhanced bursting activity, again supporting the hypothesis that bursting activity is intrinsic to the CNSS and may be disrupted by descending pathways. Five out of nine SW-adapted and seven out of eight FW-adapted fish responded in this way to cutting the cord. Electrical stimulation of branchial nerve branches was intended to simulate input to the brain stem from the branchial arches, where osmoreceptors might be expected to be located on the gills. The subsequent inhibition of Dahlgren cell activity further supports the idea of net descending inhibition to the CNSS, which was followed by recovery to the original activity level. Tactile stimulation of lips and fin delivers input to the brain stem via the trigeminal nerve, and produced a similar response to branchial nerve stimulation, as well as a possible rebound effect. These findings provide preliminary evidence for descending modulation of the CNSS in response to activation of sensory nerves with potential relevance for the maintenance of osmotic homeostasis. It is likely that both electrical and tactile stimulus regimes led to brainstem arousal and hence descending inhibition.
The Dahlgren cells appear to be subject to a balance of inhibitory and
excitatory input, both intrinsic and extrinsic to the CNSS. For example, we
have already demonstrated profound inhibition via adrenergic
receptors (Hubbard et al.,
1996b) as well both nicotinic and muscarinic cholinergic
modulation (Brierley et al.,
2003
). Similarly, firing patterns in mammalian hypothalamic
oxytocin neurons are regulated via a range of excitatory (e.g.
noradrenergic) and inhibitory (opioid) inputs
(Russell et al., 2003
). These
cells receive input from the brain stem, which mediates vagal and spinal
feedback from cervix and uterus during parturition. Such comparisons further
emphasise the parallels that may be drawn between hypothalamic and CNSS
magnocellular neuroendocrine systems.
The physiological role of changes in Dahlgren cell firing patterns, in
terms of neuroendocrine peptide secretion and target tissue action, remains to
be established. However, our earlier observations of altered circulating and
urophysial peptide levels (Arnold-Reed et
al., 1991; Bond et al.,
2002
), and their reported actions on gill
(Marshall and Bern, 1979
),
bladder (Loretz and Bern,
1981
) and gut (Loretz et al.,
1985
) transport, are consistent with a predominant function of
urotensins to support fish survival in a hypertonic SW environment. The
electrophysiological data reported here are in agreement with this picture in
so far as SW fish showed higher proportions of bursting cells, with a more
robust bursting pattern, and an apparently greater correlation between firing
activities of Dahlgren cells in SW compared with FW CNSS. The sensitivity of
Dahlgren cell firing patterns to higher centre mediated responses to external
sensory input is likely to be of importance, not only for sensing potential
challenges of variation in water salinity, but also in enabling a role for the
CNSS in modulating cortisol responses to specific stressors
(Winter et al., 2000
).
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Acknowledgments |
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References |
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Arnold-Reed, D. E., Balment, R. J., McCrohan, C. R. and Hackney, C. M. (1991). The caudal neurosecretory system of Platichthys flesus: general morphology and responses to altered salinity. Comp. Biochem. Physiol. A 99,137 -143.[CrossRef]
Bern, H. A., Pearson, D., Larson, B. A. and Nishioka, R. S. (1985). Neurohormones from fish tails: the caudal neurosecretory system. I. `Urophysiology' and the caudal neurosecretory system of fishes. Rec. Prog. Horm. Res. 41,533 -552.[Medline]
Bond, H., Winter, M. J., Warne, J. M., McCrohan, C. R. and Balment, R. J. (2002). Plasma concentrations of arginine vasotocin and urotensin II are reduced following transfer of the euryhaline flounder (Platichthys flesus) from seawater to fresh water. Gen. Comp. Endocrinol. 125,113 -120.[CrossRef][Medline]
Brierley, M. J., Ashworth, A. J., Banks, J. R., Balment, R. J.
and McCrohan, C. R. (2001). Bursting properties of caudal
neurosecretory cells in the flounder Platichthys flesus. J. Exp.
Biol. 204,2733
-2739.
Brierley, M., Woodburn, M., Craven, T., Banks, J., Cioni, C., Bordieri, L., Balment, R. and McCrohan, C. R. (2002). Modulation of electrical activity patterns in teleost osmoregulatory neurons. Proc 21st Conf. European Comparative Endocrinologists (ed. R. Keller, H. Dircksen, D. Sedlmeier and H. Vaudry), pp.347 -352. Bonn, Germany: Monduzzi Editore.
Brierley, M. J., Ashworth, A., Craven, T. P., Woodburn, M.,
Banks, J. R., Lu, W., Riccardi, D., Balment, R. J. and McCrohan, C. R.
(2003). Electrical activity of caudal neurosecretory neurons in
seawater and freshwater adapted flounder: responses to cholinergic agonists.
J. Exp. Biol. 206,4011
-4020.
Brierley, M. J., Bauer, C. S., Lu, W., Riccardi, D., Balment, R. J. and McCrohan, C. R. (2004). Voltage- and Ca2+-dependent burst generation in neuroendocrine Dahlgren cells in the teleost Platichthys flesus. J. Neuroendocrinol. 16,832 -841.[CrossRef][Medline]
Cazalis, M., Dayanithi, G. and Nordmann, J. J. (1985). The role of patterned burst and interburst interval on the excitation-coupling mechanism in the isolated rat neural lobe. J. Physiol. 369,45 -60.[Abstract]
Cioni, C., Greco, A., Pepe, A., de Vito, L. and Colasanti, M. (1997). Nitric oxide synthase in the caudal neurosecretory system of the teleost Oreochromis niloticus. Neurosci. Lett. 238,57 -60.[CrossRef][Medline]
Hubbard, P. C., McCrohan, C. R., Banks, J. R. and Balment, R. J. (1996a). Electrophysiological characterization of cells of the caudal neurosecretory system in the teleost, Platichthys flesus.Comp. Biochem. Physiol. 115A,293 -301.
Hubbard, P. C., Balment, R. J. and McCrohan, C. R. (1996b). Adrenergic receptor activation hyperpolarizes the caudal neurosecretory cells of the flounder Platichthys flesus. J. Neuroendocrinol. 8,153 -159.[Medline]
Hubbard, P. C., Balment, R. J. and McCrohan, C. R. (1997). Inhibitory effect of 5-hydroxytryptamine receptor activation on caudal neurosecretory cells of the flounder, Platichthys flesus. J. Neuroendocrinol. 9, 561-566.
Lederis, K., Fryer, J. N. and Yulis, C. R. (1985). The fish neuropeptide urotensin I: its physiology and pharmacology. Peptides 6, 353-361.[CrossRef][Medline]
Leng, G., Brown, C. H. and Russell, J. A. (1999). Physiological pathways regulating the activity of magnocellular neurosecretory cells. Prog. Neurobiol. 57,625 -655.[CrossRef][Medline]
Loretz, C. A. and Bern, H. A. (1981). Stimulation of sodium transport across the teleost urinary bladder by urotensin II. Gen. Comp. Endocrinol. 43,325 -330.[Medline]
Loretz, C. A., Howard, M. E. and Siegel, A. J. (1985). Ion transport in the goby intestine: cellular mechanism of urotensin II stimulation. Am. J. Physiol. 249,G285 -G293.
Marshall, N. S. and Bern, N. A. (1979). Teleostean urophysis: urotensin II and ion transport across the isolated skin of a marine teleost. Science 204,519 -521.[Medline]
Nunemaker, C. S., DeFazio, R. A., Geusz, M. E., Herzog, E. D.,
Pitts, G. R. and Moenter, S. M. (2001). Long-term recordings
of networks of immortalized GnRH neurons reveal episodic patterns of
electrical activity. J. Neurophysiol.
86, 86-93.
Oswald, R. L. (1978). Injection anaesthesia for experimental studies in fish. Comp. Biochem. Physiol. C 60,19 -26.[CrossRef][Medline]
Russell, J. A., Leng, G. and Douglas, A. J. (2003). The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy. Front. Neuroendocrinol. 24,27 -61.[CrossRef][Medline]
Winter, M. J., Hubbard, P. H., McCrohan, C. R. and Balment, R. J. (1999). A new radioimmunoassay for measurement of plasma urotensin II in the euryhaline flounder Platichthys flesus. Gen. Comp. Endocrinol. 114,249 -256.[CrossRef][Medline]
Winter, M. J., Ashworth, A., Bond, H., Brierley, M. J., McCrohan, C. R. and Balment, R. J. (2000). The caudal neurosecretory system: control and function of a novel neuroendocrine system in fish. Biochem. Cell Biol. 78,193 -203.[CrossRef][Medline]