BEHAVIORAL BIOLOGY 17, 119-122 (1976), Abstract No. 5235

BRIEF REPORT

The Influence of Stimulus Pairing and of the Shuttle-Shock Contingency in the Performance of Shuttle Responses

to a Buzzer by Weanling Rats I

IV.~N IZQUIERDO and ESPER A. CAVALHEIRO

Departamento de Fisiologia, Escola Paulista de Medicina, Rua Botucatu 862, 04023 S~o Paulo, Brazil

Weanling (20-day old) and adult Wistar rats were submitted to four different training procedures in a shuttle-box. Each of the training pro- cedures involved 50 5-sec buzzers and a variable number of 1.5 mA footshocks. Performances of shuttle responses to the buzzer was similar in both age groups both in a "no pairing-no contingency" situation (shocks at random intervals from buzzers irrespective of the performance of shuttle responses or not), and in a classical conditioning situation (shocks paired to buzzers on every trial, also irrespective of whether or not there was shuttling to the buzzer). However, the incidence of shuttle responses to the buzzer was much lower in the weanling group in two other tests in which there was a contingency of shocks upon shuttle responses: one in which the buzzer-shock interval was randomly variable (i.e., no "pairing"), and another one in which the two stimuli were paired (a typical two-way avoidance paradigm). It is concluded that 20-day old rats are unable to cope with a shuttle-shock instrumental contingency, while being as able as adult rats to develop shuttle responses through a drive process or through a "pairing" (classical) process.

Rats normally do not perform shuttle responses to a buzzer (SBs) in a

shuttle-box unless they also receive footshocks in the same session (Izquierdo,

1975a). In this case, SBs occur even if the footshocks are presented without a

definite temporal relation to the buzzers and without any contingency upon

responses (Izquierdo, 1974a, b, 1975a, 1976a, b), which suggests that at least

part of this behavior is attributable to the induction o f a peculiar drive state

by the shocks (Anisman and Waller, 1973) and not necessarily to association

mechanisms. However, if there is a constant temporal relation between the

two stimuli (which here shall be defined as "pairing") and/or an SB-shock

instrumental contingency, SB performance becomes much higher (Izquierdo,

1975a, 1976a, b; Izquierdo and Cavalheiro, 1976). The exception are 20- or

21-day old rats, who respond much less than adults in a typical two-way

1Supported by a grant (I. I.) and a fellowship (E. A. C.) from FAPESP, Brazil.

119

Copyright ©1976 by Academic Press, Inc. All rights of reproduction in any form reserved.

120 IZQUIERDO AND CAVALHEIRO

avoidance paradigm using paired buzzers and shocks, even though their SB performance in a "no pairing-no contingency" buzzer-shock test is similar to that of adults (Izquierdo, Salzano, Thomd, and Thaddeu, 1975). This raises the question of whether weanling rats lack the capacity to cope with "pairing" or with "contingency" association mechanisms. The present experi- ment is an attempt to answer this question.

Wistar rats were used. They were weaned at the age of 30 days. Before weaning, six littermates were kept with their mother in a 25 × 25 × 15 cm wood box, and after weaning six to eight animals of the same sex were housed together in a 60 × 25 × 15 cm box. Forty-eight 20-day old and 48 adult rats of both sexes were tested. Each age group was subdivided into subgroups of 12 animals. Each subgroup was submitted to one of four different behavioral tests in a shuttle-box. The shuttle-box was 50 × 25 × 25 cm, made of wood painted grey except for the front wall which was of glass. A buzzer lay on the outer side of the lid at the midline. The floor of the shuttle-box consisted of 2-ram bronze bars spaced 7 mm apart. A 25 × 0.5 × 0.5 cm piece of wood separated the two sides of the grid.

In each of the four behavioral tests, 50 buzzers of 5-sec duration were presented at intervals which varied randomly between 5 and 40 sec. Orienting (lateral or upward head movements with pricking of the ears and with or without sniffing and rearing) and shuttle responses to the buzzer (SBs) were counted. In addition to the buzzers, in each experimental situation 1.5 mA footshocks were delivered. All animals of all groups responded to all shocks with an escape reaction within less than 1 sec. There was no detectable difference in the latency or quality of this response between any test or age group. The number and mode of presentation of the shocks varied with the test. In test A, 25 shocks were interspersed among the 50 buzzers at buzzer-shock intervals which also were varied at random between 5 and 40 sec. Since the buzzer-shock interval was not constant and there was no contingency of the shocks upon responses, this clearly was a "no pairing-no contingency" situation. In test B, there were 50 shocks, one after every buzzer, irrespective of whether there was or not an SB to that buzzer. Therefore, this test was a classical conditioning situation (Katzev and Mills, 1974) in which both stimuli were contiguous ("pairing") and in which, by definition, the SB-shock instrumental "contingency" was absent. In test C, the buzzers were followed at a randomly variable interval (5 to 40 sec) by shocks, but each shock was contingent upon the omission of an SB in the immediate- ly preceding buzzer. This corresponds to a two-way avoidance procedure without stimulus pairing (in the sense defined above, Izquierdo, 1976a, b; Izquierdo and Cavalheiro, 1976). In test D, each of the 50 buzzers was immediately followed by a shock (pairing) unless there was an SB (contin- gency); this was, therefore, a typical two-way avoidance paradigm with pairing and contingency.

SHUTTLING IN WEANLING RATS 121

TABLE 1

Performance of Orienting and Shuttle Responses to a Buzzer by 20-Day Old (Weanling) and Adult Rats in Four Different Behavioral Tests

in a Shuttle-Box involving Buzzers and Shocks as Stimuli a

Weanling Adult Experimental responses responses

paradigm % %

Orientingresponses A 95.7±0.9 79.6±2.5 b B 96.8±0.9 96.7±1.3 C 93.3±0.6 98.4±0.6 D 99.7±0.2 96.7±0.9

Shuttle responses A 9.7 ± 2.6 7.8 ± 1.7 B 21.8 ± 1.4 c 22.7 ± 1.4 c C 9.7 ± 1.0 d 29.3 ± 1.4 c,e D 21.0 ± 2.5 b 43.3 ± 3.9 b

aTwelve animals per group. Data expressed as means + SE. Significance levels estimated by a Duncan multiple range test.

bDifferent from all other groups at a 0.005 level. CDifferent from foth weanling and adult A groups at 0.005 level. dDifferent from weanling B and D groups at 0.005 level. eDifferent from adult B group at 0.05 level.

Results appear in Table 1. As was reported previously (Izquierdo e t al.,

1975) adult rats made less orienting responses than weanlings in the "no

pairing-no contingency" test A. All other differences in performance of orienting responses to the buzzer between tests or groups were not significant.

With regard to SB performance, the two age groups behaved similarly in tests A and B (no pairing-no contingency and classical conditioning, respec-

tively), whereas the weanling group was clearly inferior in the two avoidance situations, C and D. In fact, performance of the 20-day old group in test C (avoidance without "pairing") did not differ significantly from that in test A

(no pairing-no contingency). Therefore, weanling rats were unable to acquire an SB performance level

similar to that of adults through contingency processes, while they were seemingly equally able to do so through either a classical (pairing) procedure or a no pairing-no contingency paradigm. The difference in performance

between both age groups in tests C and D can not be attributed to a simple sensory or motor deficit of the weanlings, since at least in the two other tests the performance of both age groups was similar and, in all tests, the incidence, quality, and latency of orienting and escape responses were similar in both age groups (with the exception of orienting in test A mentioned above). At the

122 IZQUIERDO AND CAVALHEIRO

age of 20 days a number of neurobiological mechanisms are immature in the rat: catecholamine metabolism (Keller, Bartholini, and Pletscher, 1973), the dento-hippocampal connection (Altman, Brurmer, and Bayer, 1973), and possibly others. Since at least those two variables (catecholamine metabolism and integrity of the dento-hippocampal connection) seem to be important for learning (Izquierdo, 1975b), the observed behavioral difference might, in principle, be attributed to any of the two. The present observations do not allow for any definite conclusion regarding possible neurobiological

mechanisms.

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