Partial Reversal of Phencyclidine-Induced Impairment of Prepulse Inhibition by Secretin

Partial Reversal of Phencyclidine-Induced Impairment of Prepulse Inhibition by Secretin Karyn M. Myers, Martin Goulet, James Rusche, Richard Boismenu, and Michael Davis Background: Secretin is a “gut-brain” peptide whose neural function is as yet poorly understood. Several clinical studies have reported modestly increased social interaction in autistic children following intravenous secretin administration. Very recently secretin also was administered to schizophrenic patients and found to increase social interaction in some individuals. Methods: In light of this finding, we assessed the ability of secretin to reverse phencyclidine- (PCP) induced impairment in prepulse inhibition (PPI), a leading animal model of sensorimotor gating deficits in schizophrenia. Results: Similar to atypical antipsychotics, secretin (1, 3, 10, 30, and 100 ␮g/kg) partially and dose-dependently reversed the PCP-induced deficit in PPI without significantly affecting baseline startle when administered intraperitoneally (IP) 10 minutes following IP administration of PCP (3 mg/kg). Conclusions: This finding may be relevant to observations of antipsychotic efficacy of secretin in schizophrenic patients as well as our previous report that systemically administered secretin is capable of modulating conditioned fear, even at quite low doses. Key Words: Antipsychotic, attention, autism, schizophrenia, sensorimotor gating, startle

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ecretin is a peptide whose role in digestion has been appreciated for some time (Lu and Owyang 1995; Mutt 1980; Ulrich et al 1993) and which has been discovered more recently to be localized to and functional within the brain as well. Both secretin (Itoh et al 1991; Kopin et al 1991) and its receptor (Ishihara et al 1991) have been cloned and are distributed throughout the brain (Charlton et al 1981, 1982; Fremeau et al 1983; Koves 2002; Nozaki et al 2002; Samson et al 1984; Tay et al 2004; Yung et al 2001). Intracerebroventricular (ICV.) administration of secretin modulates Fos expression at the level of the cortex, midbrain, and brainstem (Welch et al 2003), upregulates dopamine turnover and tyrosine hydroxylase activity in the hypothalamus, and inhibits prolactin release (Babu and Vijayan 1983; Fuxe et al 1979; Samson et al 1984). In vitro, secretin stimulates cAMP production in cultured brain cells and brain slice preparations (Fremeau et al 1986; Propst et al 1979; van Calker et al 1980), modulates the sensitivity of cerebellar Purkinje neurons to the neurotransmitter ␥-aminobutyric acid (GABA) (Yung et al 2001), and depolarizes nucleus tractus solitarius neurons (Yang et al 2004). Peripheral administration of secretin also induces central Fos expression, most notably within the central nucleus of the amygdala (CeA; Goulet et al 2003) and inhibits the expression of conditioned fear in rats as assessed with fear-potentiated startle (Myers et al 2004). Fear-potentiated startle is an amygdaladependent enhancement of the acoustic startle response when startle is elicited in the presence of a conditioned stimulus (CS) that previously was paired with footshock, relative to when startle is elicited in the presence of a neutral CS or in the absence of other explicit stimulation (Brown et al 1951; Davis and Astrachan 1978; Hitchcock and Davis 1986). Secretin is trans-

From the Department of Psychiatry and Behavioral Sciences (MD) and the Center for Behavioral Neuroscience (KMM, MD), Emory University, Atlanta, Georgia; Repligen Corporation (MG, JR, RB), Waltham, Massachusetts. Address reprint requests to Michael Davis, Ph.D., Center for Behavioral Neuroscience, Yerkes Neuroscience Building, Room 5200, 954 Gatewood Road NE, Atlanta, GA 30329; E-mail: [email protected]. Received August 5, 2004; revised December 15, 2004; revised March 9, 2005; accepted March 15, 2005.

0006-3223/05/$30.00 doi:10.1016/j.biopsych.2005.03.023

ported across the blood-brain barrier and taken up directly by neural tissue (Banks et al 2002) but may also influence brain function following systemic administration via its interaction with the area postrema, a circumventricular organ (Goulet et al 2003), as well as the vagus nerve (Yang et al 2004). The potential of secretin as a therapeutic tool for the treatment of autism has generated considerable interest. The apparent ability of secretin to improve social interaction in autism was discovered serendipitously by Horvath et al (1998), whose primary interest was in the evaluation of gastrointestinal problems occurring in a subset of autistic children. Observations of marked improvement in social interaction and communication in three children in that study prompted several additional studies whose outcomes have been mixed and include reports of modest or temporally limited benefits in some children (Chez et al 2000; Coniglio et al 2001; Kern et al 2002) as well as reports of no meaningful effect (Dunn-Geier et al 2000; Lightdale et al 2001; Owley et al 2001; Roberts et al 2001; Sandler et al 1999). Very recently secretin also has been tested with schizophrenic patients exhibiting many of the same social deficits as in autism, including reduced eye contact, social withdrawal, and communication deficiencies. This double-blind, placebo-controlled study of 22 treatment-refractory patients showed a statistically significant, transient improvement in clinical global impression (CGI) after a single intravenous dose of secretin (Sheitman et al 2004; see also Alamy et al 2004). The most common laboratory model of schizophrenia is based on the phenomenon of prepulse inhibition (PPI), which refers to a reduction in the amplitude of an acoustically elicited startle response when the onset of the startle-eliciting stimulus (“pulse”) is preceded 20-1000 msec by a weak “prepulse” stimulus of the same or a different sensory modality (for a review see Geyer et al 1990). Schizophrenic patients typically do not exhibit normal PPI, such that they startle to a similar extent when the pulse is presented in isolation as when it is preceded by a prepulse. Normal humans (Braff et al 2001) and nonhuman animals (Geyer et al 2001; Geyer and Ellenbroek 2003) similarly have reduced PPI after administration of certain classes of drugs, including dopaminergic (DA) agonists (e.g., D-amphetamine, apopmorphine) and N-methyl-D-aspartate (NMDA) antagonists (e.g., ketamine, phencyclidine). PPI deficits generally are reversible by antipsychotic drugs, although interestingly, as a general rule only atypical antipsychotics reverse deficits induced by NMDA antagonists, whereas both traditional and atypical antiBIOL PSYCHIATRY 2005;58:67–73 © 2005 Society of Biological Psychiatry

68 BIOL PSYCHIATRY 2005;58:67–73 psychotics reverse deficits induced by DA agonists (Geyer and Ellenbroek 2003). In light of the utility of secretin in improving social function in some schizophrenic patients, we examined whether secretin would exhibit measurable antipsychotic efficacy in a standard rodent PPI screen. Rats were administered PCP (3 mg/kg) or saline followed 10 minutes later by one of five doses of secretin (1, 3, 10, 30, or 100 ␮g/kg) or vehicle. Our decision to administer PCP/saline prior to secretin rather than the reverse was motivated by our desire to evaluate the clinical utility of secretin as a therapeutic agent for the treatment of schizophrenia. That is, we were interested in examining the potential of secretin to reverse a pre-existing impairment (i.e., by pretreatment with PCP) as opposed to its ability to block impairment induced subsequently (i.e., by administration of PCP post-secretin). Rats were placed in the chambers and five minutes later were presented with 110 dB startle stimuli (pulses) presented in isolation or preceded 50 ms by a light prepulse or 100 ms by an auditory prepulse of one of four intensities (62, 64, 68, or 76 dB). Because PPI changes in magnitude very little over multiple tests (cf. Gewirtz and Davis 1995), each rat was tested four times. Assignment of each rat to the PCP or saline condition alternated across the four tests, although each rat received the same dose of secretin or vehicle in every test.

Methods and Materials Animals A total of 24 male Sprague-Dawley rats (Charles River, portage, Michigan), weighing 300 – 400 g at the beginning of the experiment, was used. Animals were maintained on a 12:12 hour light-dark cycle (lights on at 0700) with food and water continuously available. All rats were housed in group cages of four rats each in a temperature controlled (24°C) animal colony. Apparatus Animals were trained and tested in 8 ⫻ 15 ⫻ 15-cm Plexiglas and wire-mesh cages. The cage floor consisted of four 6.0-mm diameter stainless steel bars spaced 18 mm apart. Each cage was suspended between compression springs within a steel frame and located within a custom-designed 90 ⫻ 70 ⫻ 70-cm ventilated sound attenuating chamber. Background noise (60 dB wideband) was provided by a General Radio Type 1390-B noise generator (Concord, Maryland) and delivered through high frequency speakers (Radio Shack Supertweeter; Tandy, Fort Worth, Texas) located 5 cm in front of each cage. Sound level measurements (sound pressure level) were made with a Bruel and Kjaer (Marlborough, Maryland) model 2235 sound level meter (A scale; random input) with the microphone (Type 4176) located 7 cm from the center of the speaker (approximating the distance of the rat’s ear from the speaker). Startle response were evoked by 50-msec, 110-dB white noise bursts (5 msec rise-decay) generated by a Macintosh G3 computer soundfile (0 –22 kHz), amplified by a Radio Shack amplifier (100 W; model MPA-200; Tandy), and delivered through the same speakers used to provide background noise. An accelerometer (model U321A02; PCB Piezotronics, Depew, New York) affixed to the bottom of each cage produced a voltage output proportional to the velocity of cage movement. This output was amplified (model 483B21; PCB Piezotronics) and digitized on a scale of 0-2500 U by an InstruNET device (model 100B; GW Instruments, Somerville, Massachusetts) interfaced to a Macintosh G3 computer. Startle amplitude was defined as the maximal www.sobp.org/journal

K.M. Myers et al peak-to-peak voltage that occurred during the first 200 msec after onset of the startle-eliciting stimulus. A 20-msec white noise burst (0 –20 kHz), varying in intensity (62, 64, 68, and 76-dB steps) was delivered through a 90-watt 3-way speaker (Radio Shack #12-1767) mounted on the rear interior face of the sound-attenuating chamber. A 300 lux, 20-msec flash of light provided by an 8-W fluorescent light bulb (rise-decay: 100 ␮sec), located approximately 5 cm behind and at a level of 5 cm above the grid floor of each startle cage, served as a visual prepulse stimulus. The presentation and sequencing of all stimuli were under the control of the Macintosh G3 computer using custom designed software (The Experimenter; Glassbeads, Inc., Newton, Connecticut). Drug Administration Phencyclidine (Sigma-Aldrich Co., St. Louis, Missouri) was freshly dissolved in phosphate-buffered saline (PBS) and injected intraperitoneally (IP) at a dose of 3 mg/kg and a volume of 1 ml/kg. Pilot studies indicated that this dose produced reliable disruption of prepulse inhibition without gross behavioral abnormalities. Secretin (Repligen Corp., Waltham, Massachusetts) (1, 3, 10, 30, or 100 ␮g/kg) was freshly dissolved in vehicle (10 mM trisodium citrate, pH 6.8; .004% polysorbate 80) and injected IP at a volume of 1 ml/kg prior to testing. The drug concentration range selected for this study was chosen to bracket the Cmax attained in humans when secretin is administered IV at .4 ␮g/kg. This dose, which is used in Repligen sponsored autism clinical trials, attains a Cmax of 3–5 ng/ml. In rats, a similar Cmax is observed at a dose of 40 ␮g/ml. Thus, the selected doses are considered to be relevant for understanding secretin’s clinical activity. Behavioral Procedures One rat was eliminated from the study because it exhibited negligible baseline startle in each of the four tests of prepulse inhibition. Matching. On each of 2 days, animals were placed into the test chambers and presented with 30 110-dB noise bursts at a 30-sec interstimulus interval (ISI). The mean startle amplitude across the 30 stimuli on the second day was used to match rats into groups whose mean baseline startle amplitudes did not differ statistically (see Results). Matching of groups in this manner maximizes the probability that modulation of startle by pharmacological agents (PCP or secretin) or prepulse stimuli is measured from the same baseline in all groups, reducing overall variability. Testing. The first of four testing sessions occurred five days after the second matching session. Half of the rats received injections of PCP (3 mg/kg) and half received saline, and 10 min later, rats of each group received a second injection of secretin (1, 3, 10, 30 or 100 ␮g/kg) or vehicle. Immediately following the second injection the rats were placed into the chambers where, after a 5-min period of no stimulation, the first test event was presented. The test session began with 10, 110-dB startle stimuli (“leaders”) presented at a 30-sec ISI. The purpose of these stimuli was to habituate the startle reflex to a stable baseline prior to the introduction of test trials, and the data from these trials were not considered further. Immediately thereafter the rats were presented with 60 additional startle stimuli (110-dB, 30-sec ISI), of which 10 were preceded by the visual prepulse and 40 were preceded by auditory prepulses (10 trials each with 62, 64, 68,

K.M. Myers et al and 76-dB noises). The remaining 10 startle stimuli were not preceded by prepulses and were included to assess baseline startle amplitude throughout the test session. The prepulse-topulse interval was 50-msec for the light and 100-msec for the auditory stimuli, and was chosen on the basis of previous research (Campeau and Davis 1995). Trial types were pseudorandomly arranged with the restriction that each of the 6 trial types occurred once within each successive block of 6 trials. Three additional test sessions, identical to the one just described, were conducted at 7–10 day intervals. Each animal received the same dose of secretin or vehicle in every test; however, the assignment of animals to PCP or saline conditions alternated. Thus, an animal that received PCP in test 1 received saline in test 2, PCP in test 3, and saline in test 4. This repeated crossover design was employed to minimize the number of animals while maximizing statistical power, and was considered possible in light of data indicating that prepulse inhibition does not change appreciably over repeated tests (cf. Gewirtz and Davis 1995). Data Analysis To examine possible baseline startle differences among the groups, the data from the pulse alone trials were analyzed with an analysis of variance (ANOVA) involving the following factors: Drug 1 (PCP, saline), a repeated measure; Exposure Number (first or second test with PCP or saline), a repeated measure; Drug 2 (vehicle and 1, 3, 10, 30, or 100 ␮g/kg secretin), a between-groups factor; and Order (PCP-sal-PCP-sal vs. sal-PCPsal-PCP), a between-groups factor. A more restricted analysis focusing on apparent modulation of baseline startle by secretin was conducted using the baseline startle amplitudes of only the animals receiving saline in any given test and an ANOVA involving the Drug 2, Exposure Number, and Order factors. To determine whether PCP disrupted PPI reliably, the data from the animals receiving vehicle (no secretin) were analyzed with an ANOVA with the following factors: Drug 1; Exposure Number; and Trial Type (baseline, light prepulse, 62 dB prepulse, 64 dB prepulse, 68 dB prepulse, 76 dB prepulse), a repeated measure. Animals receiving secretin were excluded from this analysis because secretin tended to reverse PCPinduced PPI disruption, and we were interested in documenting that this disruption occurred reliably in the absence of secretin treatment. For subsequent analyses of prepulse inhibition, mean startle amplitude on each of the 5 prepulse trial types (visual and 62, 64, 68, and 76 dB auditory prepulses) was expressed as a percentage of baseline startle, as follows: [(mean startle amplitude on prepulse trials – mean startle amplitude on pulse alone trials)/ mean startle amplitude on pulse alone trials] ⫻ 100. The transformed data were then analyzed with ANOVA with the same Drug 1, Drug 2, Exposure Number, Order, and Trial Type factors used in the analyses just described. A separate analysis using these same factors was applied to the data from the groups receiving vehicle and 30 ␮g/kg secretin. Finally, an analysis examining potential differences in the efficacy of secretin in the first versus the second half of the test sessions was conducted using all of these factors plus an additional repeated measure Block (1, 2).

Results The top panel of Figure 1 presents the data from each of the behavioral groups collapsed across the four test sessions. Inspection of the figure reveals several trends, each of which was

BIOL PSYCHIATRY 2005;58:67–73 69 subjected to statistical analysis as described below. First, PCP disrupted prepulse inhibition, as indicated by the fact that group PCP-veh did not exhibit appreciable change in startle magnitude on prepulse trials relative to pulse alone trials whereas group sal-veh showed substantial dampening of startle amplitude when the startle stimulus was preceded by a prepulse. Second, secretin had no systematic effect on baseline startle amplitudes on pulse alone trials but certain individual doses did suppress baseline startle considerably. Third and most importantly, secretin partially reversed the prepulse inhibition deficit in animals receiving PCP but had little effect on prepulse inhibition in animals receiving saline. The PCP-induced disruption of PPI was supported by statistical analyses of startle amplitudes of animals in the vehicle group. There was a significant main effect of Trial Type [F (5, 10) ⫽ 9.54; p ⬍ .01] and Drug 1 ⫻ Trial Type interaction [F (5, 10) ⫽ 7.31; p ⬍ .01], the latter of which indicates differential responding across the trial types in the saline and PCP conditions. Subsequent analyses revealed a significant simple main effect of Trial Type in the saline condition [F (5, 5) ⫽ 14.98; p ⬍ .01] but not in the PCP condition [F (5, 5) ⫽ 4.49; p ⬎ .05], supporting the observation that startle amplitudes were not suppressed on prepulse trials relative to baseline trials in animals exposed to PCP. In none of these analyses was the Drug 1 ⫻ Exposure Number interaction significant, indicating that PCP disrupted PPI equally in both PCP tests. The absence of a systematic effect of secretin upon baseline startle also was confirmed by statistical analysis. An ANOVA restricted to the data from the pulse alone trials revealed significant main effects of Drug 1 [F (1, 11) ⫽ 6.54; p ⬍ .05], indicating that baseline startle amplitudes were higher in the PCP condition than in the saline condition, and Exposure Number [F (1, 11) ⫽ 5.01, p ⬍ .05], indicating that baseline startle amplitudes were higher in the second exposure to either condition (saline or PCP) than in the first. No other main effects or interactions, including the main effect of Drug 2 or the Drug 1 ⫻ Drug 2 interaction, reached significance [Fs ⬍ 2.17; ps ⬎ .05]. On the other hand, there were relatively robust baseline effects of some doses of secretin considered individually. For example, animals receiving 30 ␮g/kg secretin exhibited significantly smaller baseline startle amplitudes in the saline condition than did animals in the vehicle group [t (12) ⫽ 2.53, p ⬍ .05]. For this reason, we attempted to minimize the contribution of group differences in baseline startle to apparent group differences in prepulse inhibition by transforming the data from the prepulse trials into a percent change score before proceeding with further statistical analysis (bottom panel of Figure 1). When expressed in this manner, the data clearly suggest a disruption of PPI by PCP and a partial reversal of that disruption by secretin. The main effect of Drug 1 was significant [F (1, 11) ⫽ 59.2; p ⬍ .01], indicating once again a dampening of PPI in the PCP condition relative to the saline condition. Critically, the Drug 1 x Drug 2 interaction also was significant [F (5, 11) ⫽ 5.58; p ⬍ .01], consistent with a reversal of PCP-induced PPI disruption. Further exploration of this interaction with ANOVAs restricted to the saline and PCP conditions revealed a significant main effect of Drug 2 in the PCP condition [F (5, 11) ⫽ 6.33; p ⬍ .01] but not in the saline condition [F (5, 11) ⫽ 2.52; p ⬎ .05], providing evidence for a dose-dependent reversal of the PCP-induced PPI disruption by secretin. Further corroborating this dose-dependent effect was a significant linear trend across secretin doses in the mean percent change scores (mean of all trial types) in the PCP condition [t (40) ⫽ ⫺2.32; p ⬍ .05], indicating that the www.sobp.org/journal

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K.M. Myers et al

Figure 1. Top panel: Mean startle amplitude on each of the six trial types in animals receiving saline or phencyclidine (PCP) followed 10 min later by one of five doses of secretin or vehicle (veh). The data are collapsed across four test sessions. Bottom panel: When the data from the prepulse trials are re-expressed as percent change from baseline, it is evident that secretin partially reversed PCP-induced disruption of PPI.

reversal was greater in groups receiving higher doses of secretin. The dose-dependency of secretin’s reversal of PCP-induced PPI disruption is highlighted in Figure 2. The greatest effect of secretin upon PCP-induced PPI disruption appeared to be with the 30 ␮g/kg dose, and we examined this group in more detail in a separate analysis. Figure 3 presents the data from this group when tested under the saline and PCP conditions together with the analogous data from the vehicle group for comparison. It is evident from the figure that 30 ␮g/kg secretin reversed the PCP-induced deficit in PPI, as there was very little difference in PPI magnitude in this group when tested under PCP and when tested under saline, whereas there was a large difference in PPI magnitude in the vehicle group under the same conditions. Consistent with these observations, there was a significant Drug 1 x Drug 2 interaction [F(1, 3) ⫽ 79.32, p ⬍ .01], and paired samples

Figure 2. Mean percent prepulse inhibition (PPI) collapsed across all trial types (visual and auditory prepulses) in animals receiving phencyclidine (PCP) followed by one of the indicated doses of secretin. Secretin dosedependently reversed PCP-induced impairment of PPI (linear trend p ⬍ .05).

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t-tests comparing responding in the saline and PCP conditions in each group indicated a significant difference in the vehicle group [t (2) ⫽ 4.48, p ⬍ .05] but not in the 30 ␮g/kg group [t (3) ⫽ .88, p ⬎ .05]. The figure also suggests a reduction of baseline PPI by 30 ␮g/kg secretin, and in fact an independent samples t-test revealed a significant difference between the sal-veh and sal-30 conditions [t (12) ⫽ ⫺2.48, p ⬍ .05]. That the reversal of PCP-induced PPI deficits by secretin persists despite a reduction of baseline PPI by secretin at this dose is intriguing, and suggests that the reversal effect may be quite robust. Because secretin was administered immediately before placing the animals into the chambers and five minutes before the

Figure 3. Mean percent prepulse inhibition (PPI) collapsed across all trial types (visual and auditory prepulses) in animals receiving saline (sal) or PCP followed by 30 ␮g/kg secretin (30) or vehicle (veh). Secretin significantly reversed PCP-induced impairment of PPI. *Statistically significant differences vs. vehicle. Statistically significant differences (p ⬍ .05).

K.M. Myers et al onset of the first test trial, it is possible that secretin exerted a more pronounced effect in the second half of the 40-min test session than in the first half, when more time had elapsed for CNS penetration. To examine this possibility we repeated the analysis of the percent change scores and included an additional repeated measure “Block.” There was no evidence for a change in the efficacy of secretin across the duration of the test session, as the main effect of Block, the Block ⫻ Drug 2 interaction, and the Block ⫻ Drug 1 ⫻ Drug 2 interaction all failed to reach significance [Fs ⬍ 3.63; ps ⬎ .05]. Hence, secretin was capable of reversing the PCP-induced PPI disruption relatively quickly following peripheral administration, perhaps via its interactions with the vagus nerve and neurons of area postrema located outside of the blood-brain barrier (Yang et al 2004).

Discussion Consistent with its apparent, modest antipsychotic effect in schizophrenic patients (Alamy et al 2004; Scheitman et al 2004), secretin exhibited efficacy similar to that seen with atypical antipsychotic compounds in a standard laboratory screen involving PCP-induced disruption of PPI. Thus, rats administered one of five doses of secretin 10 minutes following PCP injection exhibited significantly less disruption of PPI than did rats receiving vehicle. This finding also corroborates our previous report that peripherally administered secretin is capable of modulating behavior, even at relatively small doses (Myers et al 2004). Because both DA agonists and NMDA antagonists produce deficits in PPI through relatively distinct mechanisms, it would be interesting to determine if secretin is equally effective in reversing D-amphetamine-induced PPI deficits, for example, as it is in reversing PCP-induced deficits. In our hands, however, Damphetamine does not disrupt PPI reliably, despite markedly increasing baseline startle amplitude, indicating that this compound does penetrate the brain at behaviorally meaningful concentrations (data not shown). Secretin is not the only peptide to exert demonstrable antipsychotic effects in the rodent PPI test or to be implicated in the pathophysiology of schizophrenia. A large body of evidence has accrued suggesting that neurotensin agonists reverse PPI deficits induced by administration of compounds acting upon dopaminergic, glutamatergic, and serotonergic transmission (for a review see Kinkead et al 1999). More recently the cholecystokinin (CCK) agonist caerulein has been found to partially reverse amphetamine-induced impairment of PPI in rodents (Feifel et al 1999), perhaps consistent with its reported benefit in some clinical studies with schizophrenic patients (for a review see Nair et al 1986). Likewise, oxytocin dose-dependently reverses PPI deficits induced by dizocilpine and amphetamine but not apomorphine (Feifel and Reza 1999), and has produced favorable outcomes in clinical trials with schizophrenic patients (Bujanow 1974). Finally, the neuropeptides vasopressin and corticotropinreleasing factor (CRF) have been implicated in schizophrenia following observations of abnormal sensorimotor gating in transgenic mice expressing mutations of these systems (Dirks et al 2003; Feifel and Priebe 2001). The mechanisms by which these compounds might act to improve sensorimotor gating deficits and/or improve functioning in schizophrenic patients remain unknown. It has been proposed that oxytocin may act as a final common pathway mediating the effects of other neuropeptides, particularly CCK, based on evidence that CCK increases central oxytocin release (Neumann et al 1994). Interestingly, the atypical antipsychotic clozapine also

BIOL PSYCHIATRY 2005;58:67–73 71 increases central oxytocin tone (Uvnas-Moberg et al 1992), suggesting that the antipsychotic efficacy of this compound may be mediated in part by modulation of oxytocinergic transmission. It is possible that secretin might operate through oxytocin as an intermediary, although an interaction between these two neuropeptides has not been demonstrated. Future experiments involving co-administration of secretin and oxytocin antagonists following administration of PCP will be useful in exploring this possibility. Secretin also might interact with GABA to produce its effect. GABAergic systems have been implicated in both schizophrenia and sensorimotor gating (for a review see Wassef et al 2003). Very recently the GABAB receptor agonist baclofen has been reported to reverse the disruption of PPI produced by dizocilpine, an NMDA antagonist, but not by apomorphine (Bortolato et al 2004). This effect was prevented by pretreatment with the GABAB receptor antagonist SCH 50911, which by itself produced no effect on either PPI or dizocilpine-induced PPI disruption. It would be interesting to determine whether coadministration of SCH 50911 or other GABAB antagonists with secretin following PCP administration might partially or completely reverse the effect of secretin on PCP-induced disruption of PPI. Another tack one might take in exploring the mechanism of secretin’s behavioral effect is to consider putative loci within the brain where it might act. One possibility is the amygdala, which has been implicated in sensorimotor gating based on findings that lesions of (Wan and Swerdlow 1997) or infusions of the NMDA receptor antagonist AP5 into (Fendt et al 2000) the basolateral nucleus reduce PPI. Several pieces of evidence indicate that secretin influences amygdalar activation; for example, Fos expression is increased in the central nucleus following intracerebroventricular (Welch et al 2003) or intravenous secretin administration (Goulet et al 2003), and the expression of fearpotentiated startle, an amygdala-dependent behavior, is inhibited following IP secretin administration (Myers et al 2004). However, lesions of the central nucleus of the amygdala – the region seemingly targeted by secretin – have no effect on PPI (Hitchcock and Davis 1986), perhaps arguing against this region as a locus of secretin’s effect on sensorimotor gating deficits. Another potential target of secretin is the medial prefrontal cortex (mPFC), which has been implicated in NMDA antagonistinduced PPI disruption based on the finding that ibotenic acid lesions of mPFC prevent dizocilpine-induced but not apomorphine-induced disruption of PPI (Schwabe and Koch 2004). Consistent with this hypothesis, Fos expression is increased in the mPFC following intracerebroventricular administration of secretin (Welch et al 2003). However, systemic administration of a dose of secretin that more closely approximated the dose range used in the present study (i.e., 40 ␮g/kg) did not affect mPFC Fos expression measurably (Goulet et al 2003), suggesting that mPFC activation may not have been a significant factor in our experiment. In conclusion, the present study indicates that secretin exhibits efficacy in a classic laboratory screen of antipsychotic compounds, possibly supporting the hypothesis that dysfunction of secretinergic neurotransmission is a factor in schizophrenia.

This work was supported by the Repligen Corporation. Martin Goulet, James Rusche, and Richard Boismenu are employed by and own shares of Repligen Corporation, which is developing secretin for clinical use. www.sobp.org/journal

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