- Email: [email protected]
Telencephalic ablation results in decreased startle response in goldfish
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –16 5
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / b r a i n r e s
Short Communication
Telencephalic ablation results in decreased startle response in goldfish Lyndsey E. Collinsa , Robert F. Waldecka,b,⁎ a
Neuroscience Program, University of Scranton, Scranton, PA 18510, USA Department of Biology, University of Scranton, Scranton, PA 18510, USA
b
A R T I C LE I N FO
AB S T R A C T
Article history:
Our investigation concerns the connection between the telencephalon and the startle
Accepted 29 June 2006
response, mediated by reticulospinal neurons. Before surgery fish respond to the startle
Available online 31 July 2006
stimulus in 95% of the trials and 66% of the time with complete full turns. Following telencephalon removal fish respond in only 50% of the trials but make complete full turns
Keywords:
only 7% of the time. There is no significant change found in control fish. This suggests a
Mauthner cell
modulatory role of the telencephalon in regards to startle behavior.
Limbic system
© 2006 Elsevier B.V. All rights reserved.
Vibratory stimuli. C-start response Startle plasticity Reticulospinal formation
In higher order vertebrates the limbic system, a “motivating center”, is believed to play a key role in fear conditioning and arousal (Davis, 1992). Neocortex's involvement in such behavioral shaping is presumed, although not easily examined. The goldfish telencephalon, lacking neocortex, offers a unique comparison with the limbic system (Hainsworth et al., 1967) and a conserved pattern of basic organization between fish and mammals is suggested by the functional and anatomical homology between goldfish and higher vertebrate telencephalic structures (Portavella et al., 2004; Echteller and Saidel, 1981; Villani et al., 1996). Also, conditioned avoidance learning is influenced by discrete lesions of areas of the goldfish telencephalon (Portavella et al., 2002, 2003, 2004). Telencephalon ablation results in a loss of initiative suggesting involvement in arousal and enabling motivational cues necessary for normal avoidance learning (Aronson and Kaplan, 1968; Flood et al., 1976; Overmier and Curnow, 1969). Goldfish exhibit a robust startle response. Following vibratory stimulation the animal's body typically forms a Cshape (Eaton et al., 1981). This startle response is well
described and has been extensively studied (Eaton et al., 2001; Korn and Faber, 1996; Waldeck et al., 2000; Zottoli and Freemer, 2003) in terms of the brainstem escape network (Eaton and Emberly, 1991; Svoboda and Fetcho, 1996), however, the role of other upstream neuroanatomical structures, such as the telencephalon, in the modulation of this behavior are not known. Our results suggest that the telencephalon plays a role in setting the “gain” of the brainstem neural escape network and thus regulates both the likelihood of a response and its execution. Eighteen goldfish (Carassius auratus) were used and all of the procedures were in accordance with approved guidelines established by IACUC. Startle responses were elicited using pressure of the impact of a ball on the water, as in previous studies (Eaton and Emberly, 1991). This startle stimulus has vibratory and mechanical components and no attempt was made to prevent the fish from seeing it. Fish were placed in a large tank (51.5 cm × 25.5 cm) and responses were recorded with a Panasonic VHS-C with 26× zoom video camera (30 frames/s) suspended above the tank. A 5 cm diameter ball
⁎ Corresponding author. University of Scranton, Department of Biology, Loyola 106, Scranton, PA 18510, USA. E-mail address: [email protected] (R.F. Waldeck). 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.06.092
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –1 65
(39 g) was released from a height of 20 cm above the water surface and directed to hit in front of the fish to elicit startle responses. Fish were tested 3 days before surgery and 3 days after and on each day six trials were done. Each trial consisted of a ball drop. The inter-trial interval was 30 s. On the fourth day the fish was anesthetized in 0.45 g/L ethyl-m-aminobenzoate until breathing ceased. The fish was then transferred to the operating chamber where water containing 0.15 g/L MS-222 was run continually over the gills. A portion of the skull was removed, exposing the telencephalon, which was removed completely with a scalpel. The exposed brain area was then covered as previously described (Zottoli and Freemer, 2003). The telencephalon can be completely ablated and easily removed without damaging more caudal structures, such as the optic tectum (Fig. 1). In five animals, the telencephalon was exposed, but not ablated. These sham operated control animals were sealed and tested in the same way as the test animals. By stimulating fish at rest, when their bodies were straight, response identification was easily seen. For each trial a frame-by-frame analysis of the video-tape determined the angle of response. Robust responses made measurement of the exact angle unnecessary. Trials were categorized as in previous studies (Zottoli and Freemer, 2003), greater than 90° with the use of whole body was classified as a complete full turn, a turn less than 90° with trunk movement only was
Fig. 1 – Representative goldfish brains following no ablation (top) and complete ablation of the telencephalon (bottom). Rostral is right. Following ablation only olfactory tubercle and olfactory nerves are observed. Figures were taken at similar magnification.
163
classified as a partial response, responses that included only a head turn, eye or jaw movement, or a slight move backward from the stimulus was classified as non-turn, and no response at all was classified as no response. Types of responses and frequencies per fish were calculated. Occasionally a fish moved out of camera range and, therefore, a trial was not counted. Descriptive group statistics (mean and standard deviation, SD) were found for both control and experimental fish pre- and post-surgery. The paired samples t-test was used to determine whether there was a difference in response frequency between control fish pre- and post-surgery or between experimental fish pre- and post-surgery. Levene's test for equality of variances was used to examine the spread of the distributions for experimental and control fish pre- and post-surgery. Finally, the independent samples t-test (α = 0.05) was used to examine whether there was a significant difference in response frequency between the control and experimental groups pre- or post-surgery. The average percentage of response frequency for each fish was entered into the analysis and yielded five values for the control fish and thirteen values for the experimental fish, pre- and post-surgery. Prior to telencephalon removal all animals responded to the startle stimulus with a fast startle response. This response consisted of an initial C-shaped bend of the head and tail around the center of mass of the fish followed by a tail flip. On average, pre-surgery animals responded to the stimulus in almost every trial. In sham operated fish (n = 5) there was no significant change in startle response frequency following surgery. (t = 1.000, significance two tailed = 0.374; d.f. 4). The majority of responses were complete full turns away from the stimulus (Fig. 2A). Although there was a 14% diminishment of complete turn responses following the sham surgery, it was not a significant change ( t = 2.139, significance two tailed = 0.099; d.f. 4). The sham surgery also only minimally altered the distribution of type of responses (Fig. 2B). In contrast, following ablation, twelve of the thirteen fish exhibited a significant decrease in response frequency to the stimulus (t = 6.261, significance two tailed = 0.000; d.f. 12). On average they responded to the startle stimulus in only 50% of the trials, compared to 95% of the trials before ablation (Fig. 2C, D). One of the thirteen fish showed no change in overall response frequency. The frequency of complete full turn responses to the stimulus was significantly decreased from 66% of the trials before ablation to 7% following ablation (t = 9.917. significance two tailed = 0.000, d.f. 12). The experimental animals, unlike the control animals, displayed a dramatically altered distribution of types of responses. Experimental fish were more sluggish swimmers and two of them observed for 3 months, showed no recovery. No significant difference was found between the presurgery frequency of responses for experimental and control groups for overall responses (t(18) = 0.277, p = 0.785; d.f. 16) or for frequency of complete full turn responses (t(18) = 0.237, p = 0.816; d.f. 16). In sharp contrast, there was a significant difference (t(18) = −3.668, p = 0.002; d.f. 16) between the postsurgery frequency of responses for experimental and control groups. For post-surgery frequency of complete full turn responses Levene's test indicated that equal variances could not be assumed for the distribution (F(18) = 5.051, p = 0.039).
164
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –16 5
Fig. 2 – The average frequency of the types of responses following sham operation (control) and telencephalon ablations (experimental). In control animals frequency and type of response pre-surgery (A) and post-surgery (B) remain similar. In experimental fish frequency and type of response pre-surgery (C) is dramatically different than those post-surgery (D). Full = complete full turn responses.
Thus, the results of the t-test were adjusted accordingly using the SPSS statistical software package The post-surgery comparison of the experimental and control fish was the only group of data that required this adjustment, because Levene's demonstrated that equal variances could be assumed for all other data sets. A significant difference remained between experimental and control fish for the frequency of complete full-turn response, even after this adjustment (t(18) = −4.712, p = 0.005; d.f. 5.112). The startle response of the goldfish is a main reference for the study of vertebrate sensorimotor processing (Korn and Faber, 1996). The response is controlled by a well described brainstem escape network (Eaton and Emberly, 1991; Faber et al., 1989; Fetcho, 1991; Zottoli, 1977) and includes two groups of reticulospinal neurons (Eaton et al., 2001; O'Malley et al., 1996). The first group (includes the M-cell) determines the extent of the initial angular deviation and the second group determines the onset and direction of the second stage of the response (Eaton et al., 2001). Both groups together presumably lead to propulsive force and turning flexibility of the complete response. The M-cell receives sensory input from various visual, auditory, mechanical, or vibratory sources (Eaton and
Hackett, 1984; Zottoli and Faber, 2000). One of these typically will suffice to trigger the response. The reticulospinal neurons terminate in widespread areas of the spinal cord. Although much is known of the brainstem escape network, little is known of the possible regulation of the network from rostral centers. The telencephalon is a good candidate for study because there appears homology with mammalian limbic structures, such as the hippocampus and amygdala (Flood et al., 1976). These studies point to the medial telencephalic pallia (MP) as important for emotional memory and the lateral telencephalic pallia (LP) for spatial, relational, or temporal memory. These differential effects are similar to those produced by lesions of the amygdala and hippocampus in mammals (Portavella et al., 2004). N-methylD-aspartate (NMDA) receptors are concentrated in the goldfish telencephalon (Xiaojuan et al., 2003). These receptors are generally involved in various forms of plasticity of the nervous system. Our data support a connection between the telencephalon and the escape network. Following complete ablation of the telencephalon there is a significant decrease in the probability of triggering the startle behavior and the responses are much
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –1 65
weaker. Because sham operated animals show little change, surgery alone does not explain the differences in behavior observed. The ablation did not remove the ability to swim. Previous studies on the telencephalon suggest it might regulate lower brain mechanisms (Aronson and Kaplan, 1968; Overmier and Papini, 1986). More recent work suggests a connection with avoidance learning and the motivational centers of the goldfish (Portavella et al., 2002, 2003, 2004). Our results support the notion that the brainstem escape network may be “aroused” by the activities of the telencephalon. When this area is ablated the brainstem area loses its “gain” control and the resultant behavior is modified; fish are less likely to respond to a stimulus or, if they do respond they are less likely to have a complete response. Given the low temporal resolution of the video recording it wasn't possible to use kinematic characterizations of the behavior to distinguish between the two parallel reticulospinal systems. The lower likelihood of responses following ablation may be due to the telencephalon's affect on group 1 neurons related to triggering the startle. The weaker responses found, those less than 90°, however, may be due to isolated influences on the group 2 neurons. In the future, with use of a high-speed camera, this can be explored to determine if the telencephalon is influencing one of the systems more than the other.
Acknowledgments The authors wish to thank Dr. J. Timothy Cannon for photographic assistance, Dr. Thomas P. Hogan for statistical assistance, and Melissa Reynolds and Katie German for assistance in fish maintenance.
REFERENCES
Aronson, L.R., Kaplan, H., 1968. Function of the teleostean forebrain. In: Ingle, D. (Ed.), The Central Nervous System and Fish Behavior. University of Chicago Press, pp. 107–125. Davis, M., 1992. The role of the amygdala in fear and anxiety. Ann. Rev. Neurosci. 15, 353–375. Eaton, R.C., Emberly, D.S., 1991. How stimulus direction determines the trajectory of the Mauthner-initiated escape response in a teleost fish. J. Exp. Biol. 161, 469–487. Eaton, R.C., Hackett, J.T., 1984. The role of the Mauthner cell in fast-starts involving escape in teleost fishes. In: Eaton, R. (Ed.), Neural Mechanisms of Startle Behavior. Plenum Press, New York, pp. 213–265. Eaton, R.C., Lavender, W.A., Wieland, C.M., 1981. Identification of Mauthner initiated response patterns in goldfish: evidence from simultaneous cinematography and electrophysiology. J. Comp. Physiol., A 144, 521–531. Eaton, R.C., Lee, R.K., Foreman, M.B., 2001. The Mauthner cell and
165
other identified neurons of the brainstem escape network of fish. Prog. Neurobiol. 63, 467–485. Echteller, S.M., Saidel, W.M., 1981. Forebrain connections in the goldfish support telencephalic homologies with land vertebrates. Science 212, 683–694. Faber, D.S., Fetcho, J.R., Korn, H., 1989. Neuronal networks underlying the escape response in goldfish: general implications for motor control. Ann. N. Y. Acad. Sci. 563, 11–33. Fetcho, J.R., 1991. The spinal network of the Mauthner cell. Brain Behav. Evol. 37, 298–316. Flood, N.C., Overmier, J.B., Savage, G.E., 1976. Teleost telencephalon and learning: an interpretive review of data and hypotheses. Physiol. Behav. 16, 783–798. Hainsworth, F.R., Overmier, J.B., Snowdon, C.T., 1967. Specific permanent deficits in instrumental avoidance responding following forebrain ablation in the goldfish. J. Comp. Physiol. Psychol. 63, 111–116. Korn, H., Faber, D.S., 1996. Escape behavior: brainstem and spinal cord circuitry and function. Curr. Opin. Neurobiol. 6, 826–832. O'Malley, D.M., Kao, Y.-H., Fetcho, J.R., 1996. Imaging the functional organization of zebrafish hindbrain segments during escape behaviors. Neuron 17, 1145–1155. Overmier, J.B., Curnow, P.G., 1969. Classical conditioning, pseudoconditioning and sensitization in “normal” and forebrain-less goldfish. J. Comp. Physiol. Psychol. 68, 193–198. Overmier, J.B., Papini, M.R., 1986. Factors modulating the effects of teleost telencephalon ablation on retention, relearning, and extinction of instrumental avoidance behavior. Behav. Neurosci. 100, 190–199. Portavella, M., Vargas, J.P., Torres, B., Salas, C., 2002. The effects of telencephalic pallial lesions on spatial, temporal and emotional learning in goldfish. Brain Res. Bull. 57, 397–399. Portavella, M., Salas, C., Vargas, J.P., Papini, M.R., 2003. Involvement of the telencephalon in spaced-trial avoidance learning in the goldfish (Carassius auratus). Physiol. Behav. 80, 49–56. Portavella, M., Torres, B., Salas, C., 2004. Avoidance response in goldfish: emotional and temporal involvement of medial and lateral telencephalic pallium. J. Neurosci. 24, 2335–2342. Svoboda, K.R., Fetcho, J.R., 1996. Interactions between the neural networks for escape and swimming in goldfish. J. Neurosci. 16, 843–852. Villani, L., Zironi, I., Guarnieri, T., 1996. Telencephalo-habenulo-interpeduncular connections in the goldfish: a diI study. Brain Behav. Evol. 48, 205–212. Waldeck, R.F., Pereda, A., Faber, D.S., 2000. Properties and plasticity of paired-pulse depression at a central synapse. J. Neurosci. 20, 5312–5320. Xiaojuan, X., Bazner, J., Qi, M., Johnson, E., Freidhoff, R., 2003. The role of telencephalic NMDA receptors in avoidance learning in goldfish (Carassius auratus). Behav. Neurosci. 117, 548–554. Zottoli, S.J., 1977. Correlation of the startle reflex and Mauthner cell auditory responses in unrestrained goldfish. J. Exp. Biol. 66, 65–81. Zottoli, S.J., Faber, D.S., 2000. The Mauthner cell what has it taught us? Neuroscientist 6, 25–37. Zottoli, S.J., Freemer, M., 2003. Recovery of C-starts, equilibrium and targeted feeding after whole spinal cord crush in the adult goldfish Carassius auratus. J. Exp. Biol. 206, 3015–3029.
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / b r a i n r e s
Short Communication
Telencephalic ablation results in decreased startle response in goldfish Lyndsey E. Collinsa , Robert F. Waldecka,b,⁎ a
Neuroscience Program, University of Scranton, Scranton, PA 18510, USA Department of Biology, University of Scranton, Scranton, PA 18510, USA
b
A R T I C LE I N FO
AB S T R A C T
Article history:
Our investigation concerns the connection between the telencephalon and the startle
Accepted 29 June 2006
response, mediated by reticulospinal neurons. Before surgery fish respond to the startle
Available online 31 July 2006
stimulus in 95% of the trials and 66% of the time with complete full turns. Following telencephalon removal fish respond in only 50% of the trials but make complete full turns
Keywords:
only 7% of the time. There is no significant change found in control fish. This suggests a
Mauthner cell
modulatory role of the telencephalon in regards to startle behavior.
Limbic system
© 2006 Elsevier B.V. All rights reserved.
Vibratory stimuli. C-start response Startle plasticity Reticulospinal formation
In higher order vertebrates the limbic system, a “motivating center”, is believed to play a key role in fear conditioning and arousal (Davis, 1992). Neocortex's involvement in such behavioral shaping is presumed, although not easily examined. The goldfish telencephalon, lacking neocortex, offers a unique comparison with the limbic system (Hainsworth et al., 1967) and a conserved pattern of basic organization between fish and mammals is suggested by the functional and anatomical homology between goldfish and higher vertebrate telencephalic structures (Portavella et al., 2004; Echteller and Saidel, 1981; Villani et al., 1996). Also, conditioned avoidance learning is influenced by discrete lesions of areas of the goldfish telencephalon (Portavella et al., 2002, 2003, 2004). Telencephalon ablation results in a loss of initiative suggesting involvement in arousal and enabling motivational cues necessary for normal avoidance learning (Aronson and Kaplan, 1968; Flood et al., 1976; Overmier and Curnow, 1969). Goldfish exhibit a robust startle response. Following vibratory stimulation the animal's body typically forms a Cshape (Eaton et al., 1981). This startle response is well
described and has been extensively studied (Eaton et al., 2001; Korn and Faber, 1996; Waldeck et al., 2000; Zottoli and Freemer, 2003) in terms of the brainstem escape network (Eaton and Emberly, 1991; Svoboda and Fetcho, 1996), however, the role of other upstream neuroanatomical structures, such as the telencephalon, in the modulation of this behavior are not known. Our results suggest that the telencephalon plays a role in setting the “gain” of the brainstem neural escape network and thus regulates both the likelihood of a response and its execution. Eighteen goldfish (Carassius auratus) were used and all of the procedures were in accordance with approved guidelines established by IACUC. Startle responses were elicited using pressure of the impact of a ball on the water, as in previous studies (Eaton and Emberly, 1991). This startle stimulus has vibratory and mechanical components and no attempt was made to prevent the fish from seeing it. Fish were placed in a large tank (51.5 cm × 25.5 cm) and responses were recorded with a Panasonic VHS-C with 26× zoom video camera (30 frames/s) suspended above the tank. A 5 cm diameter ball
⁎ Corresponding author. University of Scranton, Department of Biology, Loyola 106, Scranton, PA 18510, USA. E-mail address: [email protected] (R.F. Waldeck). 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.06.092
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –1 65
(39 g) was released from a height of 20 cm above the water surface and directed to hit in front of the fish to elicit startle responses. Fish were tested 3 days before surgery and 3 days after and on each day six trials were done. Each trial consisted of a ball drop. The inter-trial interval was 30 s. On the fourth day the fish was anesthetized in 0.45 g/L ethyl-m-aminobenzoate until breathing ceased. The fish was then transferred to the operating chamber where water containing 0.15 g/L MS-222 was run continually over the gills. A portion of the skull was removed, exposing the telencephalon, which was removed completely with a scalpel. The exposed brain area was then covered as previously described (Zottoli and Freemer, 2003). The telencephalon can be completely ablated and easily removed without damaging more caudal structures, such as the optic tectum (Fig. 1). In five animals, the telencephalon was exposed, but not ablated. These sham operated control animals were sealed and tested in the same way as the test animals. By stimulating fish at rest, when their bodies were straight, response identification was easily seen. For each trial a frame-by-frame analysis of the video-tape determined the angle of response. Robust responses made measurement of the exact angle unnecessary. Trials were categorized as in previous studies (Zottoli and Freemer, 2003), greater than 90° with the use of whole body was classified as a complete full turn, a turn less than 90° with trunk movement only was
Fig. 1 – Representative goldfish brains following no ablation (top) and complete ablation of the telencephalon (bottom). Rostral is right. Following ablation only olfactory tubercle and olfactory nerves are observed. Figures were taken at similar magnification.
163
classified as a partial response, responses that included only a head turn, eye or jaw movement, or a slight move backward from the stimulus was classified as non-turn, and no response at all was classified as no response. Types of responses and frequencies per fish were calculated. Occasionally a fish moved out of camera range and, therefore, a trial was not counted. Descriptive group statistics (mean and standard deviation, SD) were found for both control and experimental fish pre- and post-surgery. The paired samples t-test was used to determine whether there was a difference in response frequency between control fish pre- and post-surgery or between experimental fish pre- and post-surgery. Levene's test for equality of variances was used to examine the spread of the distributions for experimental and control fish pre- and post-surgery. Finally, the independent samples t-test (α = 0.05) was used to examine whether there was a significant difference in response frequency between the control and experimental groups pre- or post-surgery. The average percentage of response frequency for each fish was entered into the analysis and yielded five values for the control fish and thirteen values for the experimental fish, pre- and post-surgery. Prior to telencephalon removal all animals responded to the startle stimulus with a fast startle response. This response consisted of an initial C-shaped bend of the head and tail around the center of mass of the fish followed by a tail flip. On average, pre-surgery animals responded to the stimulus in almost every trial. In sham operated fish (n = 5) there was no significant change in startle response frequency following surgery. (t = 1.000, significance two tailed = 0.374; d.f. 4). The majority of responses were complete full turns away from the stimulus (Fig. 2A). Although there was a 14% diminishment of complete turn responses following the sham surgery, it was not a significant change ( t = 2.139, significance two tailed = 0.099; d.f. 4). The sham surgery also only minimally altered the distribution of type of responses (Fig. 2B). In contrast, following ablation, twelve of the thirteen fish exhibited a significant decrease in response frequency to the stimulus (t = 6.261, significance two tailed = 0.000; d.f. 12). On average they responded to the startle stimulus in only 50% of the trials, compared to 95% of the trials before ablation (Fig. 2C, D). One of the thirteen fish showed no change in overall response frequency. The frequency of complete full turn responses to the stimulus was significantly decreased from 66% of the trials before ablation to 7% following ablation (t = 9.917. significance two tailed = 0.000, d.f. 12). The experimental animals, unlike the control animals, displayed a dramatically altered distribution of types of responses. Experimental fish were more sluggish swimmers and two of them observed for 3 months, showed no recovery. No significant difference was found between the presurgery frequency of responses for experimental and control groups for overall responses (t(18) = 0.277, p = 0.785; d.f. 16) or for frequency of complete full turn responses (t(18) = 0.237, p = 0.816; d.f. 16). In sharp contrast, there was a significant difference (t(18) = −3.668, p = 0.002; d.f. 16) between the postsurgery frequency of responses for experimental and control groups. For post-surgery frequency of complete full turn responses Levene's test indicated that equal variances could not be assumed for the distribution (F(18) = 5.051, p = 0.039).
164
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –16 5
Fig. 2 – The average frequency of the types of responses following sham operation (control) and telencephalon ablations (experimental). In control animals frequency and type of response pre-surgery (A) and post-surgery (B) remain similar. In experimental fish frequency and type of response pre-surgery (C) is dramatically different than those post-surgery (D). Full = complete full turn responses.
Thus, the results of the t-test were adjusted accordingly using the SPSS statistical software package The post-surgery comparison of the experimental and control fish was the only group of data that required this adjustment, because Levene's demonstrated that equal variances could be assumed for all other data sets. A significant difference remained between experimental and control fish for the frequency of complete full-turn response, even after this adjustment (t(18) = −4.712, p = 0.005; d.f. 5.112). The startle response of the goldfish is a main reference for the study of vertebrate sensorimotor processing (Korn and Faber, 1996). The response is controlled by a well described brainstem escape network (Eaton and Emberly, 1991; Faber et al., 1989; Fetcho, 1991; Zottoli, 1977) and includes two groups of reticulospinal neurons (Eaton et al., 2001; O'Malley et al., 1996). The first group (includes the M-cell) determines the extent of the initial angular deviation and the second group determines the onset and direction of the second stage of the response (Eaton et al., 2001). Both groups together presumably lead to propulsive force and turning flexibility of the complete response. The M-cell receives sensory input from various visual, auditory, mechanical, or vibratory sources (Eaton and
Hackett, 1984; Zottoli and Faber, 2000). One of these typically will suffice to trigger the response. The reticulospinal neurons terminate in widespread areas of the spinal cord. Although much is known of the brainstem escape network, little is known of the possible regulation of the network from rostral centers. The telencephalon is a good candidate for study because there appears homology with mammalian limbic structures, such as the hippocampus and amygdala (Flood et al., 1976). These studies point to the medial telencephalic pallia (MP) as important for emotional memory and the lateral telencephalic pallia (LP) for spatial, relational, or temporal memory. These differential effects are similar to those produced by lesions of the amygdala and hippocampus in mammals (Portavella et al., 2004). N-methylD-aspartate (NMDA) receptors are concentrated in the goldfish telencephalon (Xiaojuan et al., 2003). These receptors are generally involved in various forms of plasticity of the nervous system. Our data support a connection between the telencephalon and the escape network. Following complete ablation of the telencephalon there is a significant decrease in the probability of triggering the startle behavior and the responses are much
BR A I N R ES E A RC H 1 1 1 1 ( 2 00 6 ) 1 6 2 –1 65
weaker. Because sham operated animals show little change, surgery alone does not explain the differences in behavior observed. The ablation did not remove the ability to swim. Previous studies on the telencephalon suggest it might regulate lower brain mechanisms (Aronson and Kaplan, 1968; Overmier and Papini, 1986). More recent work suggests a connection with avoidance learning and the motivational centers of the goldfish (Portavella et al., 2002, 2003, 2004). Our results support the notion that the brainstem escape network may be “aroused” by the activities of the telencephalon. When this area is ablated the brainstem area loses its “gain” control and the resultant behavior is modified; fish are less likely to respond to a stimulus or, if they do respond they are less likely to have a complete response. Given the low temporal resolution of the video recording it wasn't possible to use kinematic characterizations of the behavior to distinguish between the two parallel reticulospinal systems. The lower likelihood of responses following ablation may be due to the telencephalon's affect on group 1 neurons related to triggering the startle. The weaker responses found, those less than 90°, however, may be due to isolated influences on the group 2 neurons. In the future, with use of a high-speed camera, this can be explored to determine if the telencephalon is influencing one of the systems more than the other.
Acknowledgments The authors wish to thank Dr. J. Timothy Cannon for photographic assistance, Dr. Thomas P. Hogan for statistical assistance, and Melissa Reynolds and Katie German for assistance in fish maintenance.
REFERENCES
Aronson, L.R., Kaplan, H., 1968. Function of the teleostean forebrain. In: Ingle, D. (Ed.), The Central Nervous System and Fish Behavior. University of Chicago Press, pp. 107–125. Davis, M., 1992. The role of the amygdala in fear and anxiety. Ann. Rev. Neurosci. 15, 353–375. Eaton, R.C., Emberly, D.S., 1991. How stimulus direction determines the trajectory of the Mauthner-initiated escape response in a teleost fish. J. Exp. Biol. 161, 469–487. Eaton, R.C., Hackett, J.T., 1984. The role of the Mauthner cell in fast-starts involving escape in teleost fishes. In: Eaton, R. (Ed.), Neural Mechanisms of Startle Behavior. Plenum Press, New York, pp. 213–265. Eaton, R.C., Lavender, W.A., Wieland, C.M., 1981. Identification of Mauthner initiated response patterns in goldfish: evidence from simultaneous cinematography and electrophysiology. J. Comp. Physiol., A 144, 521–531. Eaton, R.C., Lee, R.K., Foreman, M.B., 2001. The Mauthner cell and
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