Recovery of function: implications for theories of brain function

Serial lesion effect in rat medial frontal cortex as a function of age

This experiment examined the potential for behavioral recovery in juvenile, adult, and senescent rats following serial lesions of medial frontal cortex. The subjects were trained on spatial delayed alternation in a T-maze under conditions designed to enhance the probability of a serial lesion effect. All subjects were given extensive handling and adaptation to the maze, interoperative training, and long interoperative and postoperative intervals. There were several major behavioral findings: (a) the aged intact subjects were not impaired in their ability to learn spatial delayed alternation, (b) one-stage bilateral lesions of frontal cortex produced equivalent deficits on spatial delayed alternation at all ages, (c) subjects in all of the age categories demonstrated a serial lesion effect, but (d) the 150 day and 570 day serial lesions groups demonstrated significantly better performances than the 35 day serial lesions group on several measures of performance.

Structure and function in neurobiology: a conceptual framework and the localization of functions

A framework of concepts on structure and function of the nervous system is presented, in which we attempt to formulate the intended meanings of some ambiguous statements on structure and function in neurobiology in a set of strictly defined concepts. Function. The word "function" as generally used has different meanings. Two meanings of the question "What is the function of B?" are particularly relevant for this subject: (1) "What does B do?" and (2) "Why did B evolve?" It is shown that question 1 must be answered before question 2. Function of brain region S. The question "What does brain region S do?" has a meaning identical to the question "What is the relation between inputs and outputs of S, at specified conditions of S?" The answer to this question is the I/O-function. The I/)-function can be described at different levels, for instance at the molecular or cellular level. When neuroscientists ask "What is the function of brain region S?", the intended meaning appears to be either "What is the extra-CNS I/O-function of S?," or "Why did S evolve?" The "extra-CNS I/O-function of CNS subsystem S" is "the I/O-function of S in which input and output elements are outside the CNS". In many cases, neuroscientists want to know the "behavioral I/O-function of CNS subsystem S," i.e., the "I/O-function of S in which input and output elements are outside the organism"; they want to know "what is represented by the inputs of S outside the organism, and what are the effects of the outputs of S outside the organism." Loosely speaking, when a neuroscientist wants to know "the function of brain region S," he wants to know "the meaning of its neural messages, and their behavioral effects." Some examples of generally accepted behavioral I/O-functions of clearly sensory and motor parts of the CNS are presented. Localization of function. The following conclusions are drawn for "localization of function F." (1) "Function F" must be specified as "I/O-function F' " referring to input and output elements outside the CNS. (2) "Localization of I/O-function F' " is a kind of shorthand for "identification of the neural activity representing the states of these input and output elements." (3) If neurons exist whose activity only (or mainly) represents this input ("pontifical cells"), and/or only (or mainly) generates this output ("command neurons") (or, in other words, whose behavioral I/O-function is F'), it might be said metaphorically that "function F is localized in these neurons." In most cases, the existence of such neurons is still on open question.

Reversible hyperphagia and obesity following intracerebral microinjections of colchicine into the ventromedial hypothalamus of the rat

Colchicine, a drug which produces a reversible inhibition of intraaxonal transport and synaptic transmission, was used as a reversible neural blocker to investigate the role of the ventromedial hypothalamus (VMH) in the control of ingestive behavior and body weight regulation. Male Sprague-Dawley rats received intracranial microinjections of colchicine into the VMH. Volume and concentration of the colchicine solution were varied to assess specificity of action and dose-response relationship. When colchicine (2 and 4 microgram) was microinjected bilaterally into the VMH, there was a dose-dependent increase in food and water intakes and body weight gain which lasted several days. The acute period of hyperphagia was followed by a marked depression in feeding which persisted until body weight was lowered to control levels. This suppression of feeding appeared to be a consequence of the preceding period of hyperphagia and obesity, since colchicine-treated rats which were pair-fed with controls to prevent obesity continued to maintain normal food intake and body weight gain when later fed ad libitum. The results of this study confirm the importance of the VMH in the long term regulation of feeding, and indicate that reversible neuronal blocking with colchicine is a useful technique for investigating the neural substrates of feeding and other behaviors.

Time-dependent changes in commissural field potentials in the dentate gyrus following lesions of the entorhinal cortex in adult rats

Previous neuroanatomical work has shown that lesions of the entorhinal cortex in adult rats cause the commissural projections to spread from their normally restricted locus in the inner molecular layer approximately 40-50 mum into the outer molecular layer (that is, into the zone deafferented by the lesion). In the present study we measured the effects of the entorhinal lesion on the distribution of short-latency potentials elicited by commissural stimulation in the molecular layer. Studies with animals tested at various times after the lesion and with a preparation that permitted recording from the same rat at several post-lesion intervals both indicated that the commissural response spread 100-150 mum towards the deafferented outer molecular layer, while the maximum response spread 50-100 mum. These effects were first detectable by 9 days after the lesion and were fully developed by 15 days post-lesion. These findings suggest that the growth of the commissural system seen after entorhinal lesions results in the rapid formation of functional terminals and are discussed in relationship to the behavioral consequences of brain lesions.