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The Journal of Neuroscience, September 15, 2001, 21(18):7063-7068
Secretin Facilitates GABA Transmission in the Cerebellum
Wing-Ho
Yung1,
Po-Sing
Leung1,
Samuel S. M.
Ng2,
Jie
Zhang1,
Savio C. Y.
Chan1, and
Billy K. C.
Chow2
1 Department of Physiology, The Chinese University of
Hong Kong, Shatin, Hong Kong, and 2 Department of Zoology,
University of Hong Kong, Pokfulam, Hong Kong
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ABSTRACT |
Secretin was the first hormone discovered in human history, and
yet, its function as a neuropeptide has been overlooked in the past.
The recent discovery of the potential use of secretin in treating
autistic patients, together with the conflicting reports on its
effectiveness, urges an in-depth investigation of this issue. We show
here that in the rat cerebellar cortex, mRNAs encoding secretin are
localized in the Purkinje cells, whereas those of its receptor are
found in both Purkinje cells and GABAergic interneurons. Immunoreactivity for secretin is localized in the soma and dendrites of
Purkinje cells. In addition, secretin facilitates evoked, spontaneous, and miniature IPSCs recorded from Purkinje cells. We propose
that secretin is released from the somatodendritic region of Purkinje cells and serves as a retrograde messenger modulating GABAergic afferent activity.
Key words:
secretin; cerebellum; Purkinje cells; GABA; inhibitory
postsynaptic currents; autism
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INTRODUCTION |
Secretin was first discovered as a
gastrointestinal hormone by Bayliss and Starling (1902) . They showed
that an injection of the upper intestinal mucosal extract into an
anesthetized dog resulted in the stimulation of pancreatic secretion
and hepatic bile flow. To date, the essential role of secretin in
regulating secretion of bicarbonate, electrolytes, and volume from
pancreatic ductule epithelial cells are firmly established (Daniel,
1991). Other actions of secretin in the digestive system include
inhibition of gastric emptying and acid output (Raybould and Holzer,
1993; Jin et al., 1994), stimulation of hepatic bile flow (McGill et al., 1994), and promotion of pancreatic growth (Petersen et al., 1978;
Solomon et al., 1978). However, there is still no convincing evidence
to indicate the neuroendocrine function of secretin in the CNS,
although there is some evidence to suggest that secretin is present in
the brain and is neuroactive. For example, secretin-like bioactivity
and immunoreactivity were found in mammalian brain extracts (Mutt et
al., 1979; O'Donohue et al., 1981). High-affinity binding sites for
secretin have also been detected (Fremeau et al., 1983). Furthermore,
secretin increases cAMP concentration in a variety of brain areas (van
Calker et al., 1980; Fremeau et al., 1986). However, up to the present,
neither has Northern blot analysis detected secretin receptor
transcripts in any part of the human brain tested, namely amygdala,
caudate nucleus, corpus callosum, hippocampus, substantia nigra,
subthalamic nucleus, and thalamus (Chow, 1995), nor has there been any
direct electrophysiological evidence to indicate that secretin can
modulate the excitability or function of central neurons.
Recently, the question of whether secretin is neuroactive has received
a renewed attention. The reported improvement in the behavior of
autistic children receiving secretin injections supports this idea
(Horvath et al., 1998). However, there are other conflicting reports on
the effectiveness of secretin in treating autism (Sandler et al., 1999;
Chez et al., 2000). There is also evidence to suggest a correlation
between autism and abnormalities in the cerebellum (Courchesne, 1997;
Riva and Giorgi, 2000). These findings thus urge an in-depth
investigation on the neuroendocrine roles of secretin, particularly the
relevant neuronal pathways and mechanisms involved in exerting its
actions. In the present study, we aim to answer these questions by
examining the expression of secretin and its receptor in the rat
cerebellum as well as the in vitro electrophysiological
effects of secretin on central neurons.
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MATERIALS AND METHODS |
Northern blot. Poly(A)+
RNA was prepared from the Sprague Dawley (SDw) rat cerebellum.
Five micrograms of the Poly(A)+ RNA were
electrophoresed in a 1% agarose gel with 0.8% formaldehyde and
transblotted onto a Hybond N+ membrane
(Amersham Pharmacia Biotech, Arlington Heights, IL). Northern blot
hybridization was performed using
32P-labeled cDNA encoding rat secretin
(nucleotides 1-453) (Kopin et al., 1990) or the N-terminal domain of
the secretin receptor (nucleotides 213-639) (Ishihara et al., 1991).
The partial cDNAs were produced by RT-PCR using specific primers.
In situ hybridization histochemistry. The rat secretin and
secretin receptor partial cDNAs were used to generate
digoxigenin-labeled sense and antisense riboprobes using
T3 and T7 polymerases,
respectively. Cerebella from SDw rats (n = 8; aged
14-16 d) were fixed with freshly prepared 4% paraformaldehyde at
4°C for 1 hr. The fixed cerebella were embedded in paraffin, and
sagittal sections (5 µm) were cut. The procedures for in
situ hybridization were described previously (Leung et al., 1999).
Briefly, sections were first digested with proteinase K, rinsed in 0.1 M triethanolamine, and acetylated in 0.25%
acetic anhydride for 20 min at room temperature. Prehybridization was
performed by incubating sections with 50% formamide containing 4×
SSC for 30 min at 42°C. Hybridization was performed overnight
at 42°C in a humidified chamber with antisense or sense riboprobe (20 ng/µl) containing 50% formamide, 4× SSC, 0.25 mg/ml yeast tRNA,
0.25 mg/ml salmon DNA, 100 mg/ml dextran sulfate, and 1× Denhardt's
solution. Posthybridized sections were then washed with 2× SSC
containing 50% formamide at 50°C. Excess probe was removed by
digestion with RNase A (40 µg/ml) for 30 min at 37°C. Sections were
finally washed sequentially with 2×, 1×, and 0.5× SSC at 37°C.
Hybridized probes were detected with anti-digoxigenin antibody
conjugated to alkaline phosphatase (1:500) and visualized by
nitroblue-tetrazolium-chloride/5-bromo-4-chloro-indolyl-phosphate detection kit according to the manufacturer's instructions (Roche Diagnostics, Indianapolis, IN). The sections were counterstained with
methyl green, dehydrated in a graded ethanol series, and mounted with
Permount (Fisher Scientific, Tustin, CA).
Immunohistochemistry. Cerebella from SDw rats were fixed
with 4% paraformaldehyde and embedded in paraffin. Immunocytochemical staining was performed by Vectastain ABC Elite kit (Vector
Laboratories, Burlingame, CA) with minor modifications. Briefly,
sagittal sections (5 µm) of the cerebellum were deparaffinized by
xylene, rehydrated in a graded series of ethanol, and treated with
0.3% H2O2 in methanol for
30 min. After washing in PBS for 10 min, the sections were incubated
with 5% normal goat serum for 1 hr. Excess serum was drained off, and
the sections were incubated either with rabbit anti-porcine secretin
(1:1000) (Anawa, Wangen, Switzerland), mouse anti-parvalbumin (1:500),
or mouse anti-glial acidic fibrillary protein (GFAP; 1:2000) (Chemicon,
Temecula, CA) for 24 hr at 4°C. After rinsing thoroughly in PBS, the
sections were treated with the corresponding anti-rabbit or anti-mouse
biotinylated secondary antibody (1:200) for 1 hr, followed by
incubation with the avidin-biotin-horseradish peroxidase complex
reagent for 1 hr. The presence of the immunoreactive cells in the
cerebellum was detected by 0.05% 3,3'-diaminobenzidine tetrahydrochloride in 0.1 M Tris buffer. The
sections were counterstained briefly with hematoxylin or methyl green,
dehydrated, and mounted. To test the specificity of the immunostaining,
the following controls were performed: (1) omission of the primary or
secondary antibodies or (2) liquid phase preabsorption of the secretin
antiserum with 0.1 mM porcine secretin (Peninsula
Laboratories, Belmont, CA) for 24 hr at 4°C.
Electrophysiological recordings. Somatic recordings were
performed from Purkinje cells in parasagittal cerebellar slices, prepared from 14- to 16-d-old SDw rats, by a conventional patch-clamp amplifier (List Electronics, Darmstadt, Germany). Visualization of the
neurons near the top surface of the cerebellar slice was aided by
infrared videomicroscopy and differential interference contrast optics.
Pipettes were pulled from borosilicate glasses (Sutter Instrument,
Novato, CA) and had resistances from 2.2-5 M when containing the
following internal solution (in mM): 140 KCl, 2 Na2-ATP, 2 MgCl2, 10 HEPES,
and 1 EGTA. GTP (0.4 mM) was freshly added to the
internal solution before experimentation. The artificial CSF
(ACSF) contains, in mM: 2 KCl, 120 NaCl, 2 MgSO4, 1.2 KH2PO4, 26 NaHCO3, 2.5 CaCl2, and 11 glucose. During an experiment, the slices were continuously superfused
with the ACSF at a rate of 1.5-2 ml/min. The temperature of the ACSF
in the recording chamber was maintained at 34 ± 1°C by a
heat-exchanger. Series resistance was checked periodically and
typically had values of ~15 M. The series resistance was not
compensated, but the recording was abolished when its value changed
significantly (>15%) during an experiment. The cells were clamped at
 75 mV. Spontaneous and TTX-resistant miniature inward synaptic
currents were largely abolished by bicuculline methiodide (10 µM), indicating that they were
GABAA receptor-mediated. To evoke inhibitory
synaptic currents, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 µM) and (±)-2-amino-5-phosphonopentanoic acid
(APV; 50 µM) were added to the ACSF, and
currents were passed from an ACSF-containing pipette in the molecular
layer located 50-200 µm from the recording neuron. To evoke EPSCs,
bicuculline (10 µM) was added in the ACSF.
EPSCs originating from the parallel fiber were identified based on
their property of paired-pulse facilitation. Data were captured on-line
or off-line by the Digidata-pClamp package (Axon Instruments, Foster
City, CA) or CED Patch and Voltage Clamp Software Package (Cambridge
Electronics Design, Cambridge, UK). Automatic detection and analysis of
the spontaneous and miniature IPSCs were performed using home-grown
software. Numerical data are expressed as mean ± SEM. Comparison
of the amplitude distribution of IPSCs before and after drug treatment
was made by applying the Kolmogorov-Smirnov test; otherwise
statistical tests were based on Student's paired or unpaired
t tests, as appropriate. Drugs were obtained from the
following sources: rat secretin, forskolin (Calbiochem, La Jolla, CA);
CNQX, APV, SQ22536, 3-isobutyl-1-methylxanthine (IBMX), bicuculline
methiodide (Research Biochemicals, Natick, MA); vasoactive intestinal
polypeptide (VIP), pituitary adenylyl cyclase-activating polypeptide
(PACAP), Tris-GTP (Sigma, St. Louis, MO); and TTX (Alomone Labs,
Jerusalem, Israel).
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RESULTS |
By RT-PCR, a conspicuous expression of both secretin and
secretin receptors was found in the rat cerebellum (data not shown). To
confirm their expression in the cerebellum, we performed Northern blot
analysis using partial cDNA probes corresponding to rat secretin (nucleotides 1-453) (Kopin et al., 1990) and the secretin receptor (nucleotides 213-639) (Ishihara et al., 1991). Positive hybridization signals for both the ligand and the receptor were detected in the rat
cerebellum (Fig. 1A).
To determine the cellular distributions of these transcripts, in
situ hybridization studies were performed. It was found that
Purkinje cells strongly expressed secretin and secretin receptor mRNAs
(Fig. 1B,C). To confirm the
presence of secretin peptide in cerebellar Purkinje cells,
immunohistochemical staining was performed. In agreement with the data
obtained from in situ hybridization, within the cerebellar
cortex, secretin immunoreactivity was found only in Purkinje cells.
Both the soma and the dendrites of these cells were positively
immunostained (Fig. 1D). In the in situ
hybridization, on closer examination, some cells in the molecular layer
close to the Purkinje cells were also labeled, albeit more weakly, with
the secretin receptor probe. These cells were found mainly in the lower
half of the molecular layer, suggesting that they were basket cells. To
confirm this, we compared consecutive sections of the in
situ hybridization staining with those of immunostaining using
antibodies against parvalbumin and GFAP. Parvalbumin is a marker for
GABA cells in the cerebellar cortex (Kosaka et al., 1993), whereas GFAP
is a marker for glia, including Bergmann glial cells, in the cerebellum (Reichenbach et al., 1995). Figure 2
shows the results of these stainings on consecutive 5 µm sections of
the cerebellum. Many of the small-sized cells that were stained
positively in in situ hybridization (Fig.
2B) were also immunopositive for parvalbumin (Fig.
2A). On the other hand, GFAP-immunopositive elements
had a very different pattern of distribution (Fig. 2C).
These data suggest that basket cells in the cerebellar cortex also
express secretin receptors. On the whole, the expression of secretin
and its receptor in discrete neuronal types in the brain strongly imply
that they serve specific neural functions. The data also suggest that,
in the cerebellar cortex, secretin, synthesized and released from the
Purkinje cells, is targeted onto basket cells and Purkinje cells
themselves.
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Figure 1.
Expression of secretin and secretin receptors in
the rat cerebellum. A, In Northern blot analysis,
positive signals for secretin and its receptor were detected in the
cerebellum. B, In in situ hybridization,
prominent secretin mRNA signals were found in Purkinje cells when using
the antisense (left) probe but not the sense
(right) probe. C, Purkinje cells also
expressed secretin receptor mRNA. Left, Antisense probe;
right, sense probe. D, Immunoreactivity
for secretin was confined to the soma and dendrites of Purkinje cells
(left) and was absent in consecutive control sections
(right) incubated with antiserum preabsorbed with
secretin. Scale bars: B, C, 100 µm;
D, 40 µm. GL, Granule cell layer;
ML, molecular layer; P, Purkinje
cells.
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Figure 2.
Coexpression of parvalbumin and secretin receptor
mRNA in cells in the cerebellar cortex. A,
B, In these consecutive sections, parvalbumin
immunoreactivity and in situ hybridization signals were
colocalized in Purkinje cells and some neurons in the molecular layer.
The arrows indicate two corresponding neurons in the two
sections. C, In another consecutive section,
GFAP-immunopositive cell bodies were mainly confined in the granule
cell layer and did not colocalize with cells labeled in the in
situ hybridization. Scale bar, 20 µm. GL,
Granule cell layer; ML, molecular layer;
P, Purkinje cell layer.
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To elucidate the electrophysiological effects of secretin, we performed
whole-cell patch-clamp recordings from Purkinje cells in rat cerebellar
slices and examined the action of secretin on the membrane current.
Secretin (3-300 nM) did not cause any observable change in
the holding current or membrane excitability of Purkinje cells. We next
tested the effect of secretin on IPSCs originating from interneurons
and EPSCs originating from parallel fibers in the molecular
layer (details in Materials and Methods). A brief (3-5 min) exposure
to secretin (3-300 nM) resulted in a clear increase in the
amplitude and frequency of the spontaneous and evoked IPSCs (Fig.
3A). These actions of secretin
had an onset latency of 1-2 min in our system and were long-lasting,
typically requiring >30 min for complete recovery. In seven cells
tested, 30 nM secretin increased the amplitude of
the evoked IPSCs to 180 ± 23% (p < 0.001) and that of the spontaneous IPSCs to 162 ± 12%
(p < 0.001). The action of secretin was
specific to IPSCs because the EPSCs evoked by stimulating the parallel
fibers were not affected by secretin (n = 4) (Fig.
3B). The potentiation on the amplitude of the evoked IPSCs
was also accompanied by a consistent reduction in the paired-pulse
ratio (Fig. 3C). In seven cells, the mean paired-pulse ratio
decreased from 0.86 ± 0.04 to 0.82 ± 0.04 (p < 0.05) (Fig. 3C).
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Figure 3.
Secretin facilitates GABAergic, but not
glutamatergic, synaptic transmission in the rat cerebellum.
A, Raw traces showing that evoked and spontaneous IPSCs
were augmented by bath application of 30 nM of secretin.
Paired stimuli (arrows), separated by a 50 msec
interval, were given every 4 sec. Four overlaying traces are plotted in
each case. B, Top traces on the
left, which were averaged from 50 evoked IPSCs, revealed
that the amplitude and waveform of parallel fiber (PF)-PSC were
unaffected by secretin, in sharp contrast to the evoked IPSC shown in
the bottom trace. Mean data of the effect of 30 nM secretin on PF-PSCs and evoked IPSC are shown on the
right. C, On the left, the
normalized IPSCs from the cell shown in A and
B revealed a reduction of paired-pulse ratio
(IPSC2/IPSC1).
Numbers in the parentheses denote the
sample size. *p < 0.05 (paired t
test); ***p < 0.001 (t test).
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The increase in the frequency of the spontaneous IPSCs suggests that
secretin may increase the firing of presynaptic neurons. However, this
possibility was excluded in basket cells because secretin did not cause
any change in whole-cell currents of these cells (n = 3) or their firing rate in cell-attached mode (n = 3).
Nevertheless, the reduction of the paired-pulse ratio of the evoked
IPSCs indicates that the effect of secretin has a presynaptic locus of
action (Dobrunz and Stevens, 1997). Because a change in the frequency
of action potential-independent synaptic currents is a well established
indicator of the involvement of a presynaptic mechanism (Van der Kloot,
1991), we examined the effects of secretin on tetrodotoxin
(TTX)-resistant, or miniature, IPSCs. Secretin consistently increased
the frequency of the miniature IPSCs (Fig. 4A). The effect on the
amplitude was, however, more variable. For example, at 30 nM, secretin increased the IPSC amplitude in four
of seven cells tested (p < 0.05;
Kolmogorov-Smirnov test). Such an increase could be attributed to an
increased proportion of larger events. In the rest of the cells, there
was no significant change in the amplitude. On average, there was a
10.3 ± 5.4% (n = 7) increase in the mean
amplitude of the miniature IPSCs. The effects of various concentrations
of secretin on these parameters are summarized in Figure
4B. A clear dose-dependent relationship was apparent
only in the frequency but not the amplitude. These findings indicate
that secretin increases the probability of vesicular release from
presynaptic terminals. This process may contribute to the observed
potentiation on the amplitudes of the evoked and spontaneous IPSCs by
secretin. It should be noted that a change in the mean amplitude of the
miniature IPSCs is not necessarily inconsistent with a presynaptic
mechanism, as has been demonstrated by the presynaptic effect of
NMDA in the same synapse (Glitsch and Marty, 1999). For example,
secretin may lead to the selective facilitation of a subpopulation of
terminals that are associated with larger IPSC amplitudes.
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Figure 4.
Effect of secretin on miniature IPSCs and
postsynaptic GABAA receptor sensitivity of Purkinje cells.
A, A typical experiment showing that secretin increased
the frequency of GABAA-mediated miniature IPSCs. Secretin
was added to the superfusion solution for 4 min. Two sets of traces at
the time points indicated are shown on a fast time-base. In this cell,
recovery from the effect of secretin took 35 min. B,
Concentration-response curve of the effects of secretin on the
frequency (left) and amplitude (right) of
the miniature IPSCs. A clear dose-dependent effect was found in the
frequency only. Numbers in the
parentheses denote the number of cells tested.
C, The postsynaptic sensitivity of GABAA
receptor of a Purkinje cell in the absence (left) or
presence (right) of 30 nM secretin was
compared. Small volumes (1 µl) of 0.1 mM GABA were
applied directly to the bath at a fixed distance from the recorded
cell.
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The in situ hybridization data indicated that both Purkinje
cells and basket cells express secretin receptors. To address whether
secretin may increase the amplitudes of IPSCs by increasing the
sensitivity of postsynaptic GABAA receptors on
Purkinje cells, we compared the inward current induced by exogenously
applied GABA in the absence or presence of secretin. In all cells
tested, the peak inward current was not augmented by secretin; instead, an average of 21.8 ± 9.4% (n = 6) reduction in
the amplitude was found (Fig. 4C). The response partially
recovered when secretin was removed. At this stage, we are not certain
of the nature of this inhibition. Nevertheless, this finding did not
support the notion that secretin has a positive modulatory effect on
GABAA receptors. To test if the facilitatory
effects are specific to secretin, VIP and PACAP, two structurally and
pharmacologically related neuropeptides, were used, but these peptides
did not produce any effect (10 nM;
n = 3 each). In addition, the minimal doses of secretin
needed to produce facilitation are at nanomolar concentrations (1-3
nM) and the EC50 value was
determined to be at 14 nM, indicating the
involvement of secretin receptors only (Rawlings and Hezareh, 1996; Di
Paolo et al., 1999). Taken together, these observations support the
idea that secretin acts via secretin receptors expressed on basket cell terminals.
The mechanism by which secretin facilitates GABA release was examined
in another series of experiments, which were focused on the miniature
IPSCs. We first examined the possibility that secretin facilitates
vesicular release by augmenting calcium influx (Tiaho and Nerbonne,
1996). As illustrated in Figure
5A, the effect of secretin was
not sensitive to 100 µM
Cd2+, a broad-spectrum blocker of
voltage-dependent Ca2+ channels. These
data indicate that the miniature IPSCs are facilitated by a
Ca2+ influx-independent mechanism. Because
it is well established that secretin increases cAMP concentration in
non-neuronal target tissues (McGill et al., 1994; Ulrich et al., 1998)
and there is evidence that it has the same action in neuronal tissues
(van Calker et al., 1980; Fremeau et al., 1986), the involvement of cAMP as the second messenger was examined. The adenylyl cyclase activator forskolin (10 µM, n = 4) and the cAMP phosphodiesterase inhibitor IBMX (100 µM; n = 3) both mimicked the
effects of secretin on the miniature IPSC frequency (data not shown).
These data were in agreement with previous studies (Llano and
Gerschenfeld, 1993; Mitoma and Konishi, 1999). Furthermore, 20 min of
preincubation of 100 µM of SQ22536, an
inhibitor of the adenylyl cyclase (Lippe and Ardizzone, 1991; Kondo and
Marty, 1997), significantly reduced the effect of secretin on the
miniature IPSC frequency (p < 0.01; n = 6) (Fig. 5A). SQ22536 itself did not
have a clear effect on the mIPSCs. These data, which are summarized in
Figure 5B, indicate that increased concentration of
intracellular cAMP, but not influx of
Ca2+, underlies the facilitatory effect of
secretin on the mIPSCs. The result, however, does not exclude the
possibility that potentiation of the amplitudes of the evoked and
spontaneous IPSCs involves augmentation of calcium influx via
modulation of voltage-dependent Ca2+
channels or modulation of presynaptic K+
conductances.
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Figure 5.
Mechanism of secretin-induced potentiation of
miniature IPSCs in Purkinje cells. A, The general
calcium channel blocker Cd2+ at 100 µM
did not attenuate the facilitatory effect of secretin on miniature IPSC
frequency (top panel). In contrast, SQ22536, a
specific inhibitor of adenylyl cyclase, significantly reduced the
effect of 30 nM secretin (bottom
panel). B, Histogram summarizing the
effects of Cd2+ and SQ22536 on miniature IPSC
frequency. Results are expressed as percentage changes over the control
value before addition of the drugs. Numbers in the
parentheses denote the number of cells tested in each
case; **p < 0.01.
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DISCUSSION |
In this study, we found that secretin and its receptor are
expressed in specific neuronal populations of the cerebellar cortex. These data are in line with previous studies showing that secretin immunoreactivity was found in the cerebellum (O'Donohue et al., 1981),
which also has the highest secretin binding density in the rat brain
(Fremeau et al., 1983). Furthermore, we showed that secretin
selectively facilitates GABAergic inputs onto Purkinje cells via a
presynaptic and cAMP-dependent mechanism. It is likely that the source
of secretin is Purkinje cells, which express secretin mRNA and
secretin, and that the peptide acts on its own receptors located on
basket cell terminals. These novel actions of secretin, which are
distinct from those on the endocrine system (Daniel, 1991; Nussdorfer
et al., 2000), strongly support the hypothesis that secretin serves as
a neuropeptide in the rat brain. Although secretin was the first
hormone discovered in human history, there is still no convincing
evidence to indicate that secretin, like other related members in the
same peptide family including PACAP, VIP, growth hormone releasing
factor (GRF), glucagon, and glucagon like peptide-1 (GLP-1), is also a
neuropeptide. Information provided by the present report represents a
new piece of the jigsaw puzzle in seeking a complete picture of this
family of brain-gut peptides, in terms of their physiology and function.
What is the physiological role of secretin in the cerebellum? One
hypothesis that can be formulated is that after depolarization, Purkinje cells release secretin to stabilize themselves by facilitating the inhibitory inputs from the basket cells. The presence of secretin immunoreactivity in the soma and dendrites of Purkinje cells suggests that secretin is released in the somatodendritic region. Analogous somatodendritic release has been postulated for other peptides (Huang
and Neher, 1996) and amino acid transmitters (Glitsch et al., 1996;
Zilberter et al., 1999). In this context, secretin may serve as a
retrograde messenger and produce effects that are opposite to those of
glutamate, another retrograde messenger postulated in the
interneuron-Purkinje cell synapse (Glitsch et al., 1996).
It has been suggested that autism is a genetic disorder (Philippe et
al., 1999). Current research links autism to biological or neurological
differences in the brain, for instance, abnormalities in the structure
of the brain. In fact, abnormalities in the cerebellum, such as the
size and number of Purkinje cells, have long been suspected in the
etiology of autism (Courchesne, 1997; Riva and Giorgi, 2000). More
recently, the involvement of Purkinje cells in this disease has been
supported by a rat model of autism in which the cerebellar Purkinje
cells of the virally infected brain are selectively destroyed
(Pletnikov et al., 1999). Our results therefore provide a link for the
speculative relationship between secretin, cerebellum, and autism, as
well as an explanation for the potential use of secretin as a drug to
treat this disease (Horvath et al., 1998). Furthermore, in view of our
observation that secretin and its receptor are also expressed in other
brain regions, it is expected that the electrophysiological effects of
secretin are not confined to the cerebellum.
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FOOTNOTES |
Received April 13, 2001; revised June 18, 2001; accepted June 26, 2001.
This work has been supported by the Research Grant Council of the Hong
Kong Special Administrative Region (China) (Grants HKU416/96M
and 7181/99M to B.K.C.C.). We acknowledge Kenny K. W. Ho, David
Lam, T. P. Wong, and Ken Yung for their help and Chris H. K. Cheng for reviewing this manuscript.
Correspondence should be addressed to Dr. Billy K. C. Chow,
Department of Zoology, Kadoorie Biological Sciences Building, University of Hong Kong, Pokfulam Road, Hong Kong. E-mail:
bkcc{at}hkusua.hku.hk.
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