Neural and endocrine effects on classical conditioning: A comparison of ACTH and hippocampectomy

Abstract

The effects of bilateral hippocampal lesions and systemic elevation of ACTH were compared using the classically conditioned rabbit nictitating membrane response. Animals with hippocampal lesions and unoperated animals that received ACTH (5I.U. /kg) exhibited a facilitated rate of acquisition and decreased conditioned response onset latency relative to normal control groups or animals with cortical lesions. The administration of ACTH did not alter performance in animals with hippocampal damage, indicating that the effects of these treatments are not additive. The performance of a second group of animals that was treated with a smaller dosage of ACTH (2.5 I.U./kg) was no different from that of any other group, suggesting a dose-dependent effect of the hormone. The selective replication of hippocampal deficits ascribed to a loss of response modulation suggests that ACTH may influence learning by stimulating hippocampal neurons.

Eyeblink classical conditioning: Hippocampal formation is for neutral stimulus associations as cerebellum is for association–response.

“…Studies of eyeblink classical conditioning using lesion and recording techniques in the hippocampal formation have suggested that the hippocampal formation plays a very different role from the cerebellum in even the basic 250-ms delay paradigm. In contrast to the effects of lesions of the interpositus nucleus of the cerebellum, animals in which the dorsal hippocampal formation had been removed showed no deficits in the acquisition of CRs in the delay paradigm whether the CS preceded the US by 250 ms (Akase, Alkon, & Disterhoft, 1989; Port, Mikhail, & Patterson, 1986; Schmaltz & Theios, 1972), fewer than 250 ms (Port, Mikhail, Kline, & Patterson, 1985; Port, Mikhail, & Patterson, 1985), or more than 250 ms (Berger & Orr, 1983; Christiansen & Schmajuk, 1992; Orr & Berger, 1985; Port, Mikhail, & Patterson, 1985; Solomon, Solomon, Vander Schaaf, & Perry, 1983; Woodruff-Pak, Li, Hinchliffe, & Port, 1997). An increased rate of CR acquisition in rabbits with hippocampal lesions has been reported in several studies (Port, Mikhail, Kline, et al, 1985; Port, Mikhail, & Patterson, 1985; Schmaltz & Theios, 1972).…”

Temporal patterns of the rabbit's nictitating membrane response to compound and component stimuli under mixed CS–US intervals.

“…However, to reproduce multiple-peaked CRs, theoretical workers have so far found it necessary to use multiple elements and a more complex representation of the CS (Desmond, Blazis, Moore, & Berthier, 1986). In analytic terms, the production of CRs with multiple peaks each coinciding with a locus of the US requires two types of temporal integration (see Port, Mikhail, Kline, & Patterson, 1985; Port, Mikhail, & Patterson, 1985; Port & Patterson, 1984). First, the nervous system must develop a relatively elaborate “stimulus map,” which is an encoding of the temporal relations between the CSs and the USs.…”

Systemic elevation of ACTH and hippocampal activity during classical conditioning of the rabbit nictitating membrane response

“…The previous experiment (Port, Mikhail, Kline, & Patterson, 1985) had indicated that this dose effectively facilitated learning when a short lSI was utilized. The sham-injected and unpaired groups both received injections of the vehicle.…”

“…The notion that th~ hippocampus modulates the timing of CRs dependent upon the stimulus configuration indicates a purposive yet nonessential role in associative learning for the hippocampal model. In a recent study (Port, Mikhail, Kline, & Patterson, 1985), we compared the effects of ACTH injection with those of bilateral hippocampal lesions during short (150-msec) delay conditioning of the rabbit NM response. Both manipulations facilitated the rate of acquisition and decreased the onset latency of CRs.…”