In four experiments, the effects of augmenting or blocking dopamine receptor activity on response suppression learning of Colburn X Colburn chicks were determined. In each experiment, 4-day-old chicks were trained to key peck for heat reward and then tested for response suppression learning by using either a response-contingent punishment or an extinction-punishment task. Before response suppression testing, different groups of chicks were injected ip with apomorphine (1.0, 2.0, or 4.0 mg/kg) either alone or after pretreatment with haloperidol (0.5 or 1.0 mg/kg). Regardless of the response suppression task used, chicks injected with apomorphine had difficulty inhibiting their responding; whereas, chicks injected with haloperidol, either alone or before apomorphine treatment, responded on fewer trials than saline-treated chicks. During extinction testing, 4-day-old chicks given only apomorphine showed the typical suppressive effect of punishment on responding rather than the paradoxical punishment-induced increase in responding found in normal 1-day-old chicks. These results indicate that activation of dopamine receptors retards response suppression learning of the 4-day-old chick, but functional changes in central dopaminergic mechanisms are not primarily responsible for the normal age-dependent improvement in response suppression learning of the young chick.

This report is one in a series of studies investigating the effects of exposure to environmental complexity and training (ECT) upon brain biochemistry and anatomy. The data reported here replicate and extend two generalizations from previous studies: First, there is a monotonic relation between complexity of environment and terminal cerebral measures. Second, the cerebral effects of an early period in an impoverished environment can be reversed, in part at least, by a succeeding period in an enriched environment.

The effect of the number of prepunishment acquisition trials on the age dependency of passive avoidance (PA) learning of the Vantress X Arbor Acre chick was determined in both key-peck and runway tests In nine experiments, 1-and 4-day-old chicks were first trained to respond for heat reward, and then, following a variable number of reinforced acquisition trials, the chicks' responses were punished with aversive wing shocks. The major finding of these experiments was that the age dependency of PA learning of the young chick is related specifically to the number of reinforced training trialb given prior to PA testing. When a large number of prepunishment acquisition trials were given, 1-day-old chicks learned as quickly as 4-day-old chicks to withhold responding when punished. However, when only a few acquisition trials preceded PA testing, 1-day-old chicks showed significantly less response suppression than 4-day-old chicks These acquisition effects indicate that the age-dependent changes in PA learning of the chick are not solely due to developmental changes in general inhibitory ability. Rather, these PA results suggest that the 1-day-old chick, compared with the 4-day-old chick, is deficient in learning, or detecting changes in, stimulus-and/or response-reinforcement contingencies.

Plasma proteins are only somewhat larger than the intercellular spaces of the cerebral microvessels that constitute the blood-brain barrier or of the choroid plexus villi that elaborate cerebrospinal fluid (CSF). We hypothesized that the integrity of these barriers in anesthetized rabbits might be compromised during head-down tilt (HDT). Plasma protein and osmolality, hematocrit, and CSF protein concentration were compared in rabbits exposed to 1 h of HDT (n = 20) and prone rabbits (n = 10). In addition, the concentration of trypan blue dye, injected intravenously at the end of HDT or the prone position, was measured in brain homogenate. Finally, arterial blood pressure was measured via a catheterized carotid artery. HDT disrupted the barrier between blood and CSF, as indicated by a significantly (P < 0.01) greater brain trypan blue concentration in the HDT rabbits [172.2 +/- 14.4 (SD) micrograms/g dry wt] than in the prone rabbits (29.8 +/- 4.4 micrograms/g dry wt). Moreover CSF protein 5 min after HDT onset was significantly increased compared with control in HDT rabbits (54.6 +/- 1.9 vs. 81.4 +/- 5.2 mg/dl; n = 8) but not in prone rabbits (55.6 +/- 2.7 vs. 57.2 +/- 5.0 mg/dl; n = 6). Changes in the plasma protein-to-hematocrit ratio in the HDT animals, but not in the prone animals, were also compatible with a loss of fluid from the vascular compartment.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged training of six strains of rats has been shown to lead to a significant decrease in the specific cholinesterase (ChE) activity of the sensory cortex, as well as to a significant increase in specific ChE activity of the subcortex (Krech, Rosenzweig & Bennett, 1960;Rosenzweig, Krech, Bennett, & Diamond, 1962). When the cerebral ChE changes were expressed in terms of a cortical-subcortical ratio, this CS ratio provided the most consistent and most significant measure of ChE changes in the rat brain. Krech et al. (1960) used three degrees of environmental complexity and training: isolated controls (1C), social-living controls (SC), and animals that in addition to social living were given daily "free play" experience and approximately 38 days of formal maze testing (ECT). The more complex the environment (i.e., from 1C to SC to ECT), the lower was the cortical-subcortical ratio of specific ChE activity. These brain ChE changes were observed after approximately 80 days of exposure to the differential treatments. The animals were sacrificed for biochemical analysis immediately after the complex training program. Rosenzweig et al. (1962) have also found that the ECT animals, compared with their 1C littermates, showed significantly greater weight of cerebral cortex and significantly greater total ChE activity in the subcortex and in the whole brain.The present study was designed to investigate three questions which the design of the previous experiments could not answer: (a)