Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, Texas 77225
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ABSTRACT |
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Crow, Terry and Lian-Ming Tian. Monosynaptic Connections Between Identified A and B Photoreceptors and Interneurons in Hermissenda: Evidence for Labeled-Lines. J. Neurophysiol. 84: 367-375, 2000. The cellular and synaptic organization of the eye of the nudibranch mollusk Hermissenda is well-documented. The five photoreceptors within each eye are mutally inhibitory and can be classified into two types: A and B based on electrophysiological and anatomical criteria. Two of the three type B and two type A photoreceptors can be further identified according to their medial or lateral positions within each eye. In addition to reciprocal synaptic connections between photoreceptors, photoreceptors also project to second-order neurons in the cerebropleural ganglion. The second-order neurons receive convergent synaptic input from two additional sensory pathways; however, it has not been previously established if lateral A, lateral B, or medial B photoreceptors converge onto the same second-order neurons. To determine the specific synaptic organization of these components of the visual system, we have examined monosynaptic connections between identified lateral and medial type A and B photoreceptors and second-order cerebropleural (CP) interneurons. We found that monosynaptic connections between identified lateral A and lateral and medial B photoreceptors and CP interneurons follow a labeled-line principle. Illumination of the eyes or extrinsic depolarizing current applied to identified photoreceptors evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different CP interneurons. The PSPs in CP interneurons followed one-for-one spikes in the photoreceptors and could be elicited in artificial seawater solutions containing high divalent cations. Identified photoreceptors projected to more than one CP interneuron and expressed both excitatory and inhibitory connections with the different CP interneurons. In examples where a monosynaptic connection between a lateral B photoreceptor and a CP interneuron was identified, lateral A, medial A, or medial B photoreceptors did not project to the same CP interneuron. Moreover, when connections between medial B and CP interneurons were identified, lateral A, medial A, and lateral B connections were not found to project to the same CP interneuron. Similar results were obtained for a lateral A and CP interneuron connection. These results indicate that divergent labeled-lines exist between specific photoreceptors and second-order CP interneurons and potential convergence of synaptic input from primary and secondary elements of the visual system must occur at sites that are postsynaptic to the CP interneurons.
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INTRODUCTION |
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Morphological and
electrophysiologcal studies of the eyes of Hermissenda
have resulted in a basic understanding of the cellular and synaptic
organization of the peripheral components of the visual system
(Alkon 1973, 1975
; Alkon and
Fuortes 1972
; Crow et al. 1979
; Dennis
1967
). The eyes are discrete structures that lie on the dorsal
surface of the cerebropleural ganglion, and each contains two type A
and three type B photoreceptors that can be distinguished using
anatomical and electrophysiological criteria (Alkon
1973
; Alkon and Fuortes 1972
). Type B
photoreceptors have been shown to excite and inhibit two classes of
second-order neurons that surround the terminal photoreceptor processes
within the cerebropleural ganglion (Akaike and Alkon
1980
; Goh and Alkon 1984
). The second-order
neurons that are inhibited by B photoreceptors also received inhibition
from both the hair cells of the statocyst and chemosensory pathways.
The second type of second-order neuron that is excited by B
photoreceptors is also depolarized by activation of either statocyst
hair cells or chemosensory pathways (Akaike and Alkon
1980
). Convergence from different sensory modalities onto
cerebropleural (CP) interneurons is thus well-documented; however, with
the exception of the medial A connection with interneurons, it is not
known whether projections from other identified A and B photoreceptors
converge onto the same interneurons or express divergence to different
aggregates of interneurons. This is an issue of functional importance
since the visual system of Hermissenda is one site of
intrinsic modifications in cellular excitability and synaptic efficacy
produced by Pavlovian conditioning (for reviews see Alkon
1989
; Crow 1988
; Sahley and Crow
1998
). Both enhanced excitability and synaptic enhancement have
been detected in identified photoreceptors within the eyes of
conditioned animals (Frysztak and Crow 1994
,
1997
). Membrane conductances of identified type A and B
photoreceptors have been characterized (Acosta-Urquidi and Crow
1985
; Alkon 1989
; Yamoah and Crow
1994
, 1996
; Yamoah et al. 1998
),
and specific conductances in identified photoreceptors have been shown
to change with conditioning (Alkon et al. 1982
, 1985
; Farley and Han 1997
). Taken
collectively, studies of neural correlates of conditioning have shown
that differences in the expression of cellular modifications are found
between identified photoreceptors, indicating that not all cells have
the same potential for supporting plasticity (Alkon et al.
1985
; Crow 1985
; Frysztak and Crow
1993
, 1994
, 1997
). Moreover, it
is not known whether differences in conditioning correlates observed at
the level of the primary sensory neurons are maintained in the
second-order neurons of the visual system and in other components of
the pathway supporting the conditioned stimulus (CS).
In this report we provide evidence that monosynaptic connections between identified type A and B photoreceptors are to different aggregates of CP interneurons. In examples where a monosynaptic connection between a lateral type B photoreceptor and an interneuron was identified, lateral type A, medial type A, and B photoreceptors did not project to the same interneuron. Moreover, when connections between lateral type A or medial type B photoreceptors and CP interneurons were identified, other identified photoreceptor synaptic connections to the same interneuron were not detected. These results provide evidence for labeled-lines between identified type A and B photoreceptors and second-order interneurons in the CNS of Hermissenda.
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METHODS |
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Animals
Adult Hermissenda crassicornis were used in the experiments. The animals were obtained from Sea Life Supply, Sand City, CA and maintained in closed artificial seawater (ASW) aquaria at 14 ± 1°C on a 12-h light-dark cycle. Animals were fed small pieces of scallop daily. All electrophysiological procedures were conducted during the light phase of the light/dark cycle.
Intracellular recordings
Intracellular recordings from identified medial or lateral type
A or type B photoreceptors and CP interneurons were collected from
isolated nervous systems. Anatomical and electrophysiological criteria
were used to identify lateral or medial type A and B photoreceptors
within the eyes as described previously (Alkon and Fuortes
1972; Frysztak and Crow 1994
). We did not record
from the intermediate type B photoreceptor since it cannot be
unequivocally identified based on single electrode impalements. The CP
interneurons were localized to a region of the cerebropleural ganglion
as noted in a previous publication (Akaike and Alkon
1980
). The isolated nervous systems were incubated in a
protease solution (Sigma Type VIII; 0.67 mg/ml, 5 min) and rinsed with
ASW prior to the surgical desheathing of a small area of the
cerebropleural ganglion to expose the cell bodies of CP interneurons.
Cerebropleural interneurons were identified based on soma size, cell
layer and location in the cerebropleural ganglion, electrophysiological
responses to light and extrinsic current stimulation, and synaptic
input from identified type A and B photoreceptors.
The desheathed circumesophageal nervous systems were pinned to a
silicone elastomer (Sylgard; Dow Chemical) stage in a recording chamber
filled with ASW of the following composition (mM): 460 NaCl, 10 KCl, 10 CaCl2, and 55 MgCl2,
buffered with 10 mM HEPES and brought to pH 7.46 with dilute NaOH.
Experiments were also conducted in high divalent cation ASW (30 mM
CaCl2 and 165 mM MgCl2)
that raised action potential threshold, thus suppressing spontaneous
activity and reducing polysynaptic activation of CP neurons. The ASW in
the recording chamber was monitored by a thermistor and held at 15 ± 0.5°C. Illumination of the eyes was provided by a tungsten halogen
incandescent lamp attached to a fiber optic bundle mounted underneath
the recording chamber. Maximum light intensity was attenuated with
neutral density filters expressed in negative log units. Identified
pairs of A or B photoreceptors and CP interneurons were impaled with
microelectrodes filled with 4m KAc and connected to the two headstages
of an Axoclamp 2A (Axon Instruments, Foster City, CA). Electrode
resistances varied between 60 and 90 M. Standard intracellular
recording and stimulation techniques were employed.
Electrophysiological data were collected on both videotape (Vetter
Instruments) and a Gould chart recorder. Single spikes in identified A
and B photoreceptors elicited by brief extrinsic current pulses and
trains of action potentials elicited by current steps were applied in
the dark through a bridge circuit. Depolarizing generator potentials in
identified photoreceptors were evoked by light steps of different
intensities following appropriate periods of dark adaptation. Evidence
for monosynaptic connections between photoreceptors and CP interneurons
was provided by postsynaptic potentials (PSPs) with relatively constant
latencies and a one-for-one relationship between photoreceptor action
potentials and PSPs in both normal ASW and in ASW solutions containing
high divalent cations (3 times Ca2+ and 3 times
Mg2+). Additional supporting evidence for
monosynaptic connections was provided by the linear regression analysis
that examined the statistical relationship between light intensity and
complex PSP amplitude in CP interneurons or between light intensity and
spike and unitary PSP frequency in CP interneurons. Previous work has also shown that Co2+-ASW eliminated PSPs recorded
from CP interneurons (Goh and Alkon 1984
). Consistent
with this proposal for a chemical synapse is our observation that
extrinsic current injections into CP interneurons did not produce
voltage changes in identified photoreceptors, and electrotonic
potentials below spike threshold produced by depolarization or
hyperpolarization of identified photoreceptors did not result in
detectable voltage changes in the CP interneurons. Taken together,
these criteria indicate that the synaptic connections between
identified photoreceptors and CP interneurons are chemical and monosynaptic.
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RESULTS |
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CP interneuron excitation
In a preliminary study (data not shown) and a previous report
(Goh and Alkon 1984), it was shown that Lucifer
yellow-labeled CP interneurons did not send processes into pedal or
cerebropleural nerves. In addition, we observed that stimulation of
pedal nerves with extrinsic current through suction electrodes did not
elicit antidromic spikes in CP interneurons. Collectively, these
results are consistent with the identification of CP neurons as
interneurons, a designation that is used in this report. Simultaneous
intracellular recordings from identified type A or B photoreceptors and
CP interneurons revealed that illumination of the eye elicited
excitatory and inhibitory responses from different aggregates of CP
cells. In the example shown in Fig.
1A, a light step (
1.0)
elicited a depolarizing generator potential and increased spike
frequency in a lateral B photoreceptor and a depolarization and
increased spike frequency in the recording from the CP interneuron
(Fig. 1A2). The maximum discharge of the CP interneuron
corresponded temporally to the peak of the depolarizing generator
potential shown in Fig. 1A1. In the example shown in Fig.
1B1 from a different preparation, the CP interneuron was
hyperpolarized below threshold for spike generation. Before the
presentation of the light step spontaneous spikes in the lateral B
photoreceptor elicited simultaneous excitatory PSPs (EPSPs) in the CP
interneuron with a one-for-one relationship. The light step elicited a
10- to 15-mV depolarization of the CP interneuron exceeding the
spike-generating threshold at a peak potential that corresponded in
time to the peak amplitude of the generator potential recorded from the
lateral B photoreceptor (see Fig. 1B2). During the
long-lasting depolarization of the photoreceptor following the light
step and the subsequent return to baseline activity, EPSPs recorded
from the CP interneuron followed one-for-one spikes recorded from the
lateral B photoreceptor. Since action potentials recorded from the CP
interneurons that were not hyperpolarized often masked the complex EPSP
elicited by light, we measured either the number of unitary EPSPs or
number of action potentials in CP interneurons during the first 5 s of illumination of the photoreceptors. The results of the linear regression analysis showed a significant correlation between light intensity and the number of spikes or EPSPs recorded from the CP
interneurons (r = 0.93; P < 0.02, n = 5). The synaptic connections between the lateral B
photoreceptor and CP interneurons were further examined by
hyperpolarizing both neurons to block spontaneous spike activity. In
the example shown in Fig. 2A1,
a single spike elicited by an extrinsic current pulse in the lateral B
photoreceptor evoked a unitary EPSP in the CP interneuron (Fig.
2A2). EPSPs recorded from CP interneurons followed spikes
elicited by either light or extrinsic current with a relatively short
and constant latency. In the example shown in Fig. 2B1, a
2-s depolarizing extrinsic current step applied to the lateral B
photoreceptor elicited EPSPs in the CP interneuron (Fig.
2B2) that followed one-for-one the spikes in the B
photoreceptor. These results are consistent with a direct or
monosynaptic connection between the lateral B photoreceptor and the CP
interneuron. To provide additional evidence for a monosynaptic
connection, simultaneous recordings were collected from preparations
bathed in ASW containing high divalent cations (3 times
Ca2+ and Mg2+). As shown in
Fig. 2C, in the high divalent cation solution a single spike
elicited from a lateral B photoreceptor evoked a unitary EPSP in the CP
interneuron (Fig. 2C2). In another example from a
preparation maintained in a high divalent cation solution, a 1-s
current step elicited several spikes from the B photoreceptor (Fig.
2D1) and EPSPs in the CP interneuron that followed the
spikes one-for-one (see Fig. 2D2).
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CP interneuron inhibition
An example of a simultaneous recording from a lateral B
photoreceptor and CP interneuron showing an inhibitory response to light is shown in Fig. 3A. The
CP interneuron exhibited a 12-mV hyperpolarization with a peak
amplitude that corresponded in-time to the peak of the depolarizing
generator potential recorded from the lateral B photoreceptor (Fig.
3A1). The CP interneuron in this example was spontaneously
active and exhibited an inhibition of spike activity during the period
of illumination of the B photoreceptor. Occasionally, B photoreceptors
will exhibit brief periods of membrane oscillation following the
termination of light. As shown in Fig. 3A2, the CP
interneuron was inhibited during the excitatory phase of the
oscillation and resumed spontaneous activity during the hyperpolarizing
phase of the B photoreceptor oscillation (Fig. 3A1). As
shown previously for the excitatory connections between B
photoreceptors and CP interneurons, IPSPs recorded from the CP
interneuron followed one-for-one with spikes in the lateral B
photoreceptor (see Fig. 3A). Consistent with the results for excitation, the regression analysis revealed a linear relationship and
significant correlation between light intensity and the amplitude of
the complex inhibitory PSP (IPSP) recorded in the CP interneuron (r = 0.97; P < 0.005, n = 5). Illumination of the eyes with unattenuated light (maximum intensity) resulted in a complex IPSP recorded from CP
interneurons with the largest amplitude (mean, 14.3 mV). Light
attenuated 4.0 log units resulted in a complex IPSP recorded from the
CP interneurons with the smallest amplitude (mean, 8.0 mV).
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In neurons hyperpolarized below the threshold for spontaneous spike generation, single spikes generated in lateral B photoreceptors elicited by extrinsic current evoked unitary IPSPs in CP interneurons as shown in the example in Fig. 3B2. In response to an extrinsic depolarizing 2-s current step, CP interneurons exhibited unitary IPSPs that followed B photoreceptor spikes one-for-one (see Fig. 3C2). Synaptic inhibition from lateral B photoreceptors is sufficient to block spontaneous activity in CP interneurons as shown in Fig. 3D. A series of 2-s depolarizing current steps applied to the lateral B photoreceptor produced brief hyperpolarization and complete inhibition of spontaneous firing of the CP interneuron as shown in Fig. 3D2. The results of experiments employing illumination of the lateral B photoreceptors and stimulation with extrinsic current suggests that the inhibitory connection between the lateral B photoreceptor and CP interneuron is monosynaptic. This was examined further with preparations exposed to ASW containing high divalent cations (3 times Ca2+ and Mg2+). As shown in Fig. 3E a single spike elicited by a brief current pulse in the lateral B photoreceptor evoked a unitary IPSP recorded from the CP interneuron in high Ca2+-Mg2+ ASW (Fig. 3E2). The PSPs elicited by single spikes exhibited a relatively short and constant latency. These results provide additional evidence that the inhibitory connection between the lateral B photoreceptor and CP interneuron is monosynaptic. Simultaneous recordings from a total of 54 pairs of identified photoreceptors and CP interneurons revealed that 29 exhibited excitatory synaptic connections and 25 pairs expressed inhibitory photoreceptor-CP interneuron connections. In addition, recordings from 114 CP interneurons revealed that 62 cells exhibited excitatory responses to light stimulation and 52 cells inhibitory responses. Taken collectively, these results show that there are two populations of CP interneurons, one of which is excited by identified photoreceptor input and the other inhibited by monosynaptic input from identified photoreceptors.
Specificity of synaptic connections
We determined whether identified A and B photoreceptors projected to different populations of CP interneurons or, alternatively, whether there is a convergence of synaptic input onto the same CP interneuron from different identified photoreceptors. This was examined by first establishing a synaptic connection between an identified A or B photoreceptor and CP interneuron, and then testing for potential connections with other identified A or B photoreceptors. The results are summarized in the data shown in Table 1. The primary connection that was examined was between the lateral type B photoreceptor and CP interneurons. Of the 18 cases where the connection between the lateral B photoreceptor and CP interneuron was established and other potential connections were tested, there were no examples where the CP interneuron also received synaptic input from either the lateral type A, medial type B, or medial type A photoreceptor. An example of an inhibitory connection between a lateral B photoreceptor and CP interneuron is shown in Fig. 4A2. In this preparation, the same CP interneuron did not exhibit a PSP following the elicitation of a single spike in the medial B (Fig. 4B2), medial A (Fig. 4C2) or lateral A photoreceptor (Fig. 4D2). Consistent with this observation, an examination of excitatory connections between lateral B photoreceptors and CP interneurons did not reveal connections from other identified photoreceptors. Figure 5A shows an example for a lateral B photoreceptor-CP interneuron excitatory connection (Fig. 5A2) without input to the same CP inteneuron from a medial B (Fig. 5B2) or a lateral A photoreceptor (Fig. 5C2). We also studied in a few examples the specificity of connections between lateral A and medial B photoreceptors and CP interneurons. As shown in the example where a connection between a lateral A photoreceptor and CP interneuron was established (Fig. 6A2), connections tested between a medial A (Fig. 6B2) and lateral B photoreceptor and CP interneuron (Fig. 6C2) were not observed. An examination of the group data (n = 21), where potential multiple connections could be examined in the same experiment, did not reveal one case of multiple photoreceptor convergence, i.e., only one identified photoreceptor projected to the same CP interneuron (see Table 1). To demonstrate that photoreceptors that did not exhibit connections with CP interneurons were healthy neurons and could support synaptic transmission, we examined connections between these photoreceptors and other identified photoreceptors, since the photoreceptors are mutually inhibitory. As shown in Fig. 7A2, a single spike elicited from a lateral B photoreceptor evoked an EPSP recorded in a CP interneuron. The same CP interneuron did not receive an input from a lateral A photoreceptor (Fig. 7B2). However, stimulation of the lateral A photoreceptor elicited an IPSP recorded from the lateral B photoreceptor (Fig. 7C2). Taken collectively, these results indicate that the connections between lateral type B photoreceptors and CP interneurons follow a labeled-line principle, and recordings from other identified A and B photoreceptors are consistent with this type of organization.
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Divergent projections to CP interneurons
It is well-documented that the different identified photoreceptors
are mutually inhibitory (Alkon and Fuortes 1972;
Crow et al. 1979
). Therefore an identified photoreceptor
can exhibit a dual synaptic function, producing an EPSP in a CP
interneuron and expressing an inhibitory connection to another
photoreceptor as shown in Fig. 7 and proposed by Akaike and
Alkon (1980)
. However, dual synaptic projections to different
CP interneurons has not been previously established. After identifying
a synaptic connection between a lateral B photoreceptor and CP
interneuron, we examined other CP interneurons for a potential
connection with the same lateral B photoreceptor. In all examples where
this was examined (n = 8), the lateral B photoreceptor
projected to two or more CP interneurons. The connections between a
single lateral B photoreceptor and multiple CP interneurons ranged
between two and four. An example of an excitatory and inhibitory
synaptic connection with two different CP interneurons is shown in Fig.
8. The single spike in the lateral B
photoreceptor elicited an EPSP in the CP interneuron (Fig.
8A2) and an IPSP in a different CP interneuron (Fig.
8B2). In another preparation, an example of multiple
connections from a lateral B photoreceptor to CP interneurons recorded
in a high divalent cation solution is shown in Fig. 8, C2
and D2. A single spike in the lateral B photoreceptor
elicited an EPSP in one CP interneuron (Fig. 8C2) and an
IPSP in a different CP interneuron (Figs. 8D2). The group
data showed that approximately half of the connections were excitatory
(10/19) and half were inhibitory (9/19), which is similar to the
percentage of excitatory and inhibitory connections found with the
total sample.
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DISCUSSION |
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Synaptic organization of the visual system
The synaptic organization of the Hermissenda eye is
well-documented (Alkon 1975; Alkon and Fuortes
1975
). The type A and B photoreceptors are mutually inhibitory,
exhibiting an example of lateral inhibition. The mutually inhibitory
synaptic connections between photoreceptors are monosynaptic based on
both electrophysiological and morphological criteria (Alkon and
Fuortes 1972
; Crow et al. 1979
). In addition to
inhibition from neighboring photoreceptors, both statocyst hair cells
and chemosensory synaptic input to the photoreceptors has been shown to
be inhibitory (Alkon 1973
; Alkon et al.
1978
). However, at the level of second-order visual neurons, different aggregates of cells are excited and inhibited by activation of statocyst hair cells and chemosensory neurons (Akaike and
Alkon 1980
). While the photoreceptors within the eyes are
mutually inhibitory, there are differences in the strength of the
synaptic connections between identified type A and B photoreceptors.
Stimulation of the medial B photoreceptor with an extrinsic current
step produced strong inhibition of spontaneous firing of the medial A
photoreceptor in contrast to the modest effect of both lateral B
inhibition of lateral A spontaneous activity (Frysztak and Crow
1993
) and lateral A inhibition of lateral B photoreceptors (see
Fig. 7). Previous work has shown that the intermediate and medial B
photoreceptors produced stronger inhibition of medial type A
photoreceptors than lateral B photoreceptors (Goh and Alkon
1984
).
Labeled-lines
While there is convergence from different sensory systems onto the
same second-order neurons (Akaike and Alkon 1980), the issue of convergent or divergent synaptic input to second-order visual
neurons from all the identified photoreceptors has not been previously
addressed in Hermissenda. In the initial report of the
identification of second-order visual neurons it was proposed, but not
demonstrated, that interneurons may receive synaptic input from more
than one photoreceptor (Akaike and Alkon 1980
). In their study, a connection between an unidentified type B photoreceptor and
second-order neuron was established by both extrinsic current stimulation and illumination of the eye. It was proposed in one example, using light stimulation of the eye, that a small IPSP recorded
in the B photoreceptor corresponded to a small EPSP in the CP
interneuron, and thus both PSPs could be produced by a common
presynaptic source (photoreceptor). However, they did not present
recordings from the putative presynaptic source (photoreceptor) that
would account for either the IPSPs or EPSPs. Indeed, most early studies
that examined the electrophysiology of type A and B photoreceptors did
not appreciate the functional significance of further classification of
both types into lateral and medial photoreceptors with the exception of
Goh and Alkon (1984)
. They showed that stimulation of
medial type A photoreceptors elicited PSPs in CP interneurons that
projected to a putative motor neuron, and neither lateral type A nor
unidentified type B photoreceptors produced PSPs in the same
interneurons (Goh and Alkon 1984
). In this report we
have evidence that identified lateral type B photoreceptors have
monosynaptic connections to multiple CP interneurons that do not
receive synaptic input from other identified A and B photoreceptors. Consistent with this observation is the finding that lateral A photoreceptors project to CP interneurons that do not receive synaptic
input from either medial A, lateral B, or medial B photoreceptors. The
few examples of medial B connections to CP interneurons in this study
are in agreement with our observations of lateral B and lateral A
photoreceptors. Thus our results indicate that identified A and B
photoreceptors project to different aggregates of second-order visual
interneurons, consistent with a labeled-line organizational principle.
Divergence of photoreceptor synaptic connections
Studies of relatively simple nervous systems have provided the
opportunity to examine the synaptic actions mediated by different branches of a single neuron to different postsynaptic target neurons (for representative examples see Blitz and Nusbaum 1999;
Kandel et al. 1967
). In a number of examples in these
systems, the same neurotransmitter produces both inhibition and
excitation in different postsynaptic targets (Kandel et al.
1967
; Strumwasser 1962
; Tauc and
Gerschenfeld 1961
). In Aplysia, action potentials in
interneuron L10 produces a synchronous EPSP in follower cells R15 and
an IPSP in follower cell L3 (for review see Kandel and Gardner
1972
). Previous work in Hermissenda reported two
different synaptic actions of different branches of a B photoreceptor
(Akaike and Alkon 1980
); an action potential in a type B
photoreceptor evoked an EPSP in a central visual neuron and an IPSP in
neighboring photoreceptors. The dual synaptic actions of photoreceptor
branches is likely mediated by acetylcholine, the only transmitter
identified in the eyes (Heldman et al. 1979
). In this
paper we show that the divergence of synaptic input from photoreceptors
to second-order interneurons is more extensive. A type B photoreceptor
has been shown to project to as many as four CP interneurons in
addition to the multiple inhibitory synaptic connections to different
photoreceptors within the Hermissenda eye (see Figs. 7 and
8). This finding can explain the observation of Goh and Alkon
(1984)
, that PSPs recorded from pedal neuron MN1 during
stimulation of the medial A photoreceptor and CP interneuron occurred
with greater frequency than the number of spikes in the CP interneuron,
suggesting the contribution of other interneurons. The analysis of the
synaptic actions mediated by different branches of a single identified
photoreceptor has revealed that approximately half are excitatory
connections to CP interneurons, and half are inhibitory.
Functional implications of divergent photoreceptor projections
Cellular neurophysiological studies of the visual system of
conditioned Hermissenda have identified several examples of
modifications in excitability extrinsic to both type A and B
photoreceptors (Alkon et al. 1982, 1985
;
Crow 1985
; Crow and Alkon 1980
;
Farley and Alkon 1982
; Farley et al.
1990
), and changes in synaptic efficacy between identified
photoreceptor synapses (Frysztak and Crow 1994
, 1997
; Gandhi and Matzel 1999
). There is
now considerable evidence that specific identified photoreceptors
express different types of cellular correlates following conditioning.
Although not specifically identified as medial B photoreceptors,
Alkon et al. (1985)
proposed that medial B
photoreceptors exhibited an increase in the amplitude of light-elicited
generator potentials (West et al. 1982
) and an increase
in light-elicited spike frequency following conditioning (Farley
and Alkon 1982
). Moreover, in conditioned animals, lateral B
photoreceptors express a decreased excitability to the CS,
which may be due to both intrinsic conductance changes and enhancement of synaptic inhibition from neighboring photoreceptors (Crow
1985
). In addition, lateral A photoreceptors of conditioned
animals exhibit an increase in excitability to both the CS and
extrinsic current, while medial A photoreceptors do not express
enhanced excitability but do show enhancement of the medial B to medial
A synaptic connection (Frysztak and Crow 1993
,
1994
, 1997
). A previous examination of a
partial neural circuit from photoreceptors to a putative motor neuron
focused on medial type A synaptic projections to CP interneurons (Goh and Alkon 1984
). This study showed that stimulation
of medial A photoreceptors with light or extrinsic current produced
excitation of CP interneurons and depolarization of a putative motor
neuron (MN1). It was further proposed that type B inhibition of the
medial A photoreceptor during light stimulation would result in a
decrease in excitatory synaptic input to MN1, which potentially could
affect the orientation of animals toward a light stimulus. While
lateral type A and unidentified type B photoreceptors did not elicit
PSPs in the CP interneurons that projected to MN1 (Goh and Alkon
1984
), projections from other CP interneurons to MN1 were not
examined. In addition, projections from other identified photoreceptors to CP interneurons and putative motor neurons were also not
established. Moreover, the synaptic input to MN1 from other CP
interneurons or different aggregates of CP interneurons has not been
thoroughly investigated.
We previously reported that the lateral type A photoreceptor is the
only A photoreceptor type that encodes for the pairing specificity of
the CS and unconditioned stimulus (US) (Frysztak and
Crow 1993). Therefore the CP interneurons that receive synaptic input from the lateral A photoreceptors would have information concerning pairing specificity. However, neural correlates of conditioning detected in putative motor neurons are not consistent with
the hypothesis that type A photoreceptor activity is the only critical
event in phototactic suppression (Hodgson and Crow 1992
). Studies have shown that CS-elicited activity recorded
from pedal neurons or multiunit activity recorded from pedal nerves is
reduced below the frequency of spontaneous activity recorded in the
dark (Crow 1981
; Hodgson and Crow 1991
,
1992
; Richards and Farley 1987
). Because
type A photoreceptors do not discharge action potentials spontaneously
in the dark, their activity in response to light cannot be less than
their activity in the dark. Taken collectively, the evidence suggests
that both identified type B and type A photoreceptors may contribute to
alterations in the activity of target neurons contributing to the
circuitry controlling muco-ciliary locomotion.
The divergence of identified photoreceptor projections to specific interneuronal aggregates may provide for the opportunity to maintain the differential expression of correlates of conditioning in the different A and B photoreceptors at the level of second-order neurons. Such an organization would allow plasticity intrinsic to some elements of the visual system to be processed at postsynaptic targets separate and parallel from elements of the visual system responsible for normal light intensity discriminations. We are currently investigating the postsynaptic targets of the CP interneurons to determine whether this divergent organization is maintained at the level of motor neurons supporting foot contraction and/or muco-ciliary locomotion.
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ACKNOWLEDGMENTS |
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This work was supported by National Institute of Mental Health Grants MH-58698 and MH-01363 to T. Crow.
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FOOTNOTES |
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Address for reprint requests: T. Crow, Dept. of Neurobiology and Anatomy, University of Texas Medical School, PO Box 20708, Houston, TX 77225 (E-mail: terry.crow{at}uth.tmc.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 22 November 1999; accepted in final form 4 April 2000.
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