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R D Hawkins

Publications and source records attributed to R D Hawkins.

At least 73 records · Page 4Linked to original sources
PubMed1986

Effects of interstimulus interval and contingency on classical conditioning of the Aplysia siphon withdrawal reflex.

The siphon withdrawal reflex of Aplysia undergoes differential classical conditioning with cutaneous stimulation of the siphon or mantle shelf as the discriminative conditioned stimuli (CS+ and CS-) and shock to the tail as the unconditioned stimulus (US). The reflex has proved to be useful for analyzing the neural mechanisms of conditioning. To test the generality of this experimental system, we have begun to compare the properties of conditioning in Aplysia with those of conditioning in vertebrates. We first examined the effect of the interstimulus interval (ISI) by varying the time between presentation of the CS+ and the US in different groups of animals. Significant differential conditioning was obtained when the onset of the CS+ preceded the onset of the US by 0.5 sec, and marginal conditioning was obtained when the ISI was 1.0 sec. By contrast, no significant conditioning occurred when the CS+ preceded the US by 2, 5, or 10 sec, when the onsets of the stimuli were simultaneous, or when US onset preceded the CS+ by 0.5, 1.0, or 1.5 sec (backward conditioning). We next examined the effect of contingency by giving one group of animals normal differential conditioning, and a second group the same training but with additional USs inserted between the paired trials. Presentation of these additional USs reduced the degree to which the US was contingent on the CS+, but did not change the number of pairings. Animals receiving normal training again showed significant conditioning, whereas animals receiving additional USs showed no conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)

Aplysia↗
PubMed1985

Monosynaptic connections made by the sensory neurons of the gill- and siphon-withdrawal reflex in Aplysia participate in the storage of long-term memory for sensitization.

We have found that in the gill- and siphon- withdrawal reflex of Aplysia, the memory for short-term sensitization grades smoothly into long-term memory with increased amounts of sensitization training. One cellular locus for the storage of the memory underlying short-term sensitization is the set of monosynaptic connections between the siphon sensory cells and the gill and siphon motor neurons. We have now also found that these same monosynaptic connections participate in the storage of the memory underlying long-term Sensitization. We examined the amplitudes of the direct synaptic connections produced by siphon sensory neurons on the gill motor neuron L7 in nervous systems removed from control and from long-term sensitized animals 1 day after the end of long-term sensitization training. The connections were significantly larger in long-term sensitized animals than in control animals. The finding that long-term memory occurs at the same synaptic locus as the short-term memory should facilitate study of the cellular and molecular mechanisms involved in the conversion of short-term to long-term memory.

Journal Article↗
PubMed1985

Distribution of serotonin-immunoreactive cell bodies and processes in the abdominal ganglion of mature Aplysia.

Sensitization of the gill withdrawal reflex in Aplysia californica is an elementary form of learning, in part resulting from presynaptic facilitation of the LE mechanoreceptor neurons of the abdominal ganglion. It has previously been established that either application of serotonin or direct stimulation of a group of facilitatory neurons, the L29 cells of the abdominal ganglion, can simulate the effect of physiological stimulation in producing presynaptic facilitation. Because the evidence that serotonin serves as a facilitatory transmitter was indirect, we examined the distribution of serotonin-immunoreactive fibers and cell bodies in the abdominal ganglion in order to answer two questions: (1) do the sensory neurons receive serotonergic innervation and (2) are the L29 cells serotonergic? We observed two distinctive patterns of serotonergic innervation within the ganglion, sparse and dense. The sparse pattern is correlated with a serotonin-stimulated increase in cAMP in identified target cells, while the dense innervation is not. We found a sparse distribution of serotonin-immunoreactive fibers with varicosities close to both cell bodies and processes of identified LE sensory cells. It therefore is likely that the sensory neurons do receive serotonergic innervation. We also mapped the population of serotonergic neuronal cell bodies in the ganglion, and found five clusters of neurons. Cells in one of these clusters, the identified RB neurons, had previously been shown to synthesize serotonin from tryptophan and to contain the neurotransmitter in high concentration. Identified L29 facilitator cells marked by injection with Lucifer Yellow do not contain serotonin immunoreactivity and therefore evidently are not a source of serotonergic input onto sensory cells.

Animals↗
PubMed1984

Mapping midbrain sites for circling using current-frequency trade off data.

Electrical stimulation applied near the medial longitudinal fasciculus (MLF) produces ipsiversive circling. In this experiment we examined the spatial and temporal properties of this substrate in rats by determining the current required to produce a constant amount of circling at frequencies from 15 to 500 Hz. In general, when frequency was high, the required current was low and vice versa, but there were deviations from a reciprocal relationship. Frequencies above 200 Hz failed to reduce the current needed to reach criterion. Electrodes placed distant from the MLF required more current, and deviated from reciprocity in a systematic way. A geometric model of the overlap between substrate and stimulation field is proposed that allows estimation of spatial properties of the two regions: (1) the location of the center of the substrate for circling (0.7 mm lateral, 7.0 mm below dura in the coronal plane 1.0 mm posterior to ear bars); (2) the radius of the substrate (0.77 mm); and (3) the radius of excitation at each current. Current-frequency trade off data can provide an in vivo estimate of the behavioral effectiveness of a given electrode, and hence its placement relative to a behavioral substrate.

Animals↗
PubMed1984

A cellular mechanism of classical conditioning in Aplysia.

The defensive siphon and gill withdrawal of Aplysia is a simple reflex, mediated by a well-defined neural circuit, that exhibits sensitization in response to strong stimulation of the tail. The siphon withdrawal reflex also exhibits classical conditioning when a weak stimulus to the siphon or mantle shelf (the conditioned stimulus or CS) is paired with a shock to the tail (the unconditioned stimulus or US). Cellular studies indicate that the mechanism of this conditioning shares aspects of the mechanism of sensitization of the reflex: presynaptic facilitation due to a cAMP-mediated decrease in K+ current and consequent broadening of action potentials in the sensory neurones. Thus, tail shock (the US) produces greater facilitation of the monosynaptic EPSP from a sensory to a motor neurone if the shock is immediately preceded by spike activity in the sensory neurone than if it occurs without spike activity (sensitization), or if the shock and spike activity are presented in a specifically unpaired pattern. This activity-dependent amplification of facilitation involves a greater broadening of action potentials in paired than in unpaired sensory neurones and appears to be due to a greater depression of the same serotonin- and cAMP-sensitive K+ current involved in sensitization. These results indicate that a mechanism of classical conditioning of the withdrawal reflex is an elaboration of the mechanism underlying sensitization. By analogy, the mechanisms of higher-order features of learning, such as the effect of contingency, may be built from combinations of the molecular mechanisms of these simple forms of learning.

Animals↗
PubMed1984

A test of Hebb's postulate at identified synapses which mediate classical conditioning in Aplysia.

In 1949, D. O. Hebb proposed a novel mechanism for producing changes in the strength of synapses that could account for associative learning. According to Hebb , the strength of a synapse might increase when the use of that synapse contributes to the generation of action potentials in a postsynaptic neuron. Thus, an essential feature of this postulate is that action potentials must occur in both a postsynaptic cell and a presynaptic cell for associative synaptic changes to occur. We have directly tested Hebb 's postulate in Aplysia at identified synapses which are known to exhibit a temporally specific increase in efficacy during a cellular analogue of differential conditioning. We find that the mechanism postulated by Hebb is neither necessary nor sufficient to produce the associative change in synaptic strength that underlies conditioning in Aplysia. In contrast, impulse activity in the presynaptic cell must be paired with facilitatory input, supporting the hypothesis that the temporal specificity of classical conditioning in Aplysia can be accounted for by activity-dependent amplification of presynaptic facilitation.

Action Potentials↗
PubMed1983

Uptake of [3H]serotonin in the abdominal ganglion of Aplysia californica. Further studies on the morphological and biochemical basis of presynaptic facilitation.

Sensitization of the gill-withdrawal reflex in Aplysia california is mediated, in part, by a group of identified neurons, the L29 cells, which produce presynaptic facilitation of transmitter release from siphon sensory neurons. Physiological and pharmacological studies have provided indirect evidence that the L29 cells are serotonergic. In the present study we have used the specific uptake [3H]serotonin ([3H]5-HT) and electron-microscopic autoradiography in combination with horseradish peroxidase-labeling of identified neurons to characterize the fine structure of Aplysia serotonergic terminals and to examine more directly the transmitter biochemistry of the L29 neurons. Abdominal ganglia were incubated for 2 h in 10(-6) M [3H]5-HT and thick and thin plastic sections examined with the light and electron microscope. L29 varicosities, identified by labeling with HRP, were found to accumulate [3H]5-HT. In addition, [3H]-5-HT was localized to unidentified varicosities within the neuropil as well as to vesicle-filled terminals that formed axosomatic contacts in the cortical regions of the ganglion. The processes that accumulated [3H]5-HT contained conspicuous dense core vesicles identical in morphology to those previously described for L29. Some processes were found to make contact with HRP-labeled varicosities of sensory neurons. Comparison with results obtained from ganglia exposed to [3H]5-HT in the presence of either non-radioactive 5-HT or non-radioactive dopamine indicate that the uptake process is transmitter-specific. These studies provide additional evidence that the L29 cells are serotonergic and are consistent with the notion that aminergic neurons may be preferentially involved in modulatory synaptic actions.

Animals↗
PubMed1983

A cellular mechanism of classical conditioning in Aplysia: activity-dependent amplification of presynaptic facilitation.

A training procedure analogous to differential classical conditioning produces differential facilitation of excitatory postsynaptic potentials (EPSP's) in the neuronal circuit for the siphon withdrawal reflex in Aplysia. Thus, tail shock (the unconditioned stimulus) produces greater facilitation of the monosynaptic EPSP from a siphon sensory neuron to a siphon motor neuron if the shock is preceded by spike activity in the sensory neuron than if the shock and spike activity occur in a specifically unpaired pattern or if the shock occurs alone. Further experiments indicate that this activity-dependent amplification of facilitation is presynaptic in origin and involves a differential increase in spike duration and thus in Ca2+ influx in paired versus unpaired sensory neurons. The results of these cellular experiments are quantitatively similar to the results of behavioral experiments with the same protocol and parameters, suggesting that activity-dependent amplification of presynaptic facilitation may make a significant contribution to classical conditioning of the withdrawal reflex.

Action Potentials↗
PubMed1983

Differential classical conditioning of a defensive withdrawal reflex in Aplysia californica.

The defensive siphon and gill withdrawal reflex of Aplysia is a simple reflex mediated by a well-defined neural circuit. This reflex exhibits classical conditioning when a weak tactile stimulus to the siphon is used as a conditioned stimulus and a strong shock to the tail is used as an unconditioned stimulus. The siphon withdrawal component of this reflex can be differentially conditioned when stimuli applied to two different sites on the mantle skin (the mantle shelf and the siphon) are used as discriminative stimuli. The differential conditioning can be acquired in a single trial, is retained for more than 24 hours, and increases in strength with increased trials. Differential conditioning can also be produced within the field of innervation of a single cluster of sensory neurons (the LE cluster) since two separate sites on the siphon skin can serve as discriminative stimuli. The finding that two independent afferent inputs that activate a common set of interneurons and motor neurons can be differentially conditioned restricts the possible cellular loci involved in the associative learning.

Afferent Pathways↗
PubMed1983

Refractory periods of neurons mediating stimulation-elicited eating and brain stimulation reward: interval scale measurement and tests of a model of neural integration.

Paired-pulse stimulation methods have been used in many studies to measure the excitability properties of the directly stimulated neurons mediating stimulation-elicited behaviors. In this article those methods are evaluated quantitatively. The most frequently used method, in which rate of behavior is measured as a function of conditioning-test (C-T) interval, is shown to be inadequate since the results obtained depend on the frequency of stimulation. In order to account for this dependence, a physical model of brain stimulation which implies that the effects of frequency and C-T interval should be multiplicative is proposed. In separate experiments, conjoint and functional measurement methods were used to test this model and to measure the refractory periods of neurons mediating stimulation-elicited eating and brain stimulation reward. The results of both experiments strongly supported the multiplicative model for C-T intervals in the refractory period range (.4-2.0 msec) and indicated that the frequency threshold method (Yeomans, 1975, and the present Experiment 2) provides interval scale measurement of refractory period effects. The refractory period curves obtained for neurons mediating stimulation-elicited eating and brain stimulation reward were very similar, rising steeply at C-T intervals from .4 to 1.2 msec and more gradually at intervals from 1.2 msec to 2.0 msec. These curves were also similar to one previously obtained for bar-pressing self-stimulation (Yeomans, 1975). The implications of these similarities are discussed.

Animals↗
PubMed1979

Maintained contraction of the crayfish claw opener muscle in the absence of motor neuron activity.

In the whole crayfish (Astacus sp.), contraction of the dactyl opener muscle may persist for many minutes in the complete absence of action potentials in the excitor nerve or muscle. Such maintained contraction has not been observed in isolated claw preparations. Generally, maintained contraction resembles catch in that the muscle holds its previous position and does not recontract if stretched. In one animal, however, the muscle continued to slowly open in the absence of nerve activity. Maintained contraction occurred in 5 of 6 whole animal preparations; in those 5 animals it occurred on 49% of trials. We could find no difference in nerve activity or muscle potentials on trials when such contraction occurred and on those when it did not. It is proposed that in the whole animal maintained contraction may be controlled by a factor which is released into the blood supply when the animal is aroused.

Animals↗