Amnesia produced by pre-training infusion of serotonin into the substantia nigra

Enhanced training protects memory against amnesia produced by concurrent inactivation of amygdala and striatum, amygdala and substantia nigra, or striatum and substantia nigra

Memory is markedly impaired when normal activity of any of a number of cerebral structures is disturbed after a learning experience. A growing body of evidence indicates, however, that such interference with neuronal function becomes negligible when the learning experience is significantly enhanced. We now report on the effects of enhanced training on retention after temporary inactivation of cerebral nuclei known to be involved in memory, namely the substantia nigra (SN), striatum (STR), and amygdala (AMY). When training was conducted with a relatively low intensity of footshock (1.0 mA), post-training infusion of lidocaine into the SN, STR, or AMY produced a marked memory deficit. Increasing the aversive stimulation to 2.0 mA protected memory from the amnesic effect of intranigral lidocaine, but there was still a deficit after its infusion into the STR and AMY. Administration of lidocaine into each of these nuclei, in the groups that had been trained with 3.0 mA, was completely ineffective in producing alterations in memory consolidation. Simultaneous infusion of lidocaine into STR + SN, AMY + SN, or AMY + STR was also ineffective in altering memory formation when the highest footshock intensity was used for training. To our knowledge, this is the first demonstration that an enhanced learning experience guards against memory deficits after simultaneous temporary interruption of neural activity of brain nuclei heretofore thought to be necessary for memory formation. These findings support the proposition that brain structures involved in memory processing are functionally connected in series during memory consolidation and that, after an enhanced learning experience, these structures become functionally connected in parallel.

Functional disconnection of the substantia nigra pars compacta from the pedunculopontine nucleus impairs learning of a conditioned avoidance task

The pedunculopontine tegmental nucleus (PPTg) targets nuclei in the basal ganglia, including the substantia nigra pars compacta (SNc), in which neuronal loss occurs in Parkinson's disease, a condition in which patients show cognitive as well as motor disturbances. Partial loss and functional abnormalities of neurons in the PPTg are also associated with Parkinson's disease. We hypothesized that the interaction of PPTg and SNc might be important for cognitive impairments and so investigated whether disrupting the connections between the PPTg and SNc impaired learning of a conditioned avoidance response (CAR) by male Wistar rats. The following groups were tested: PPTg unilateral; SNc unilateral; PPTg-SNc ipsilateral (ipsilateral lesions in PPTg and SNc); PPTg-SNc contralateral (contralateral lesions in PPTg and SNc); sham lesions (of each type). SNc lesions were made with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine HCl (MPTP, 0.6micromol); PPTg lesions with ibotenate (24nmol). After recovery, all rats underwent 50-trial sessions of 2-way active avoidance conditioning for 3 consecutive days. Rats with unilateral lesions in PPTg or SNc learnt this, however rats with contralateral (but not ipsilateral) combined lesions in both structures presented no sign of learning. This effect was not likely to be due to sensorimotor impairment because lesions did not affect reaction time to the tone or footshock during conditioning. However, an increased number of non-responses were observed in the rats with contralateral lesions. The results support the hypothesis that a functional interaction between PPTg and SNc is needed for CAR learning and performance.

Learning processing in the basal ganglia: a mosaic of broken mirrors

In the present review we propose a model to explain the role of the basal ganglia in sensorimotor and cognitive functions based on a growing body of behavioural, anatomical, physiological, and neurochemical evidence accumulated over the last decades. This model proposes that the body and its surrounding environment are represented in the striatum in a fragmented and repeated way, like a mosaic consisting of the fragmented images of broken mirrors. Each fragment forms a functional unit representing articulated parts of the body with motion properties, objects of the environment which the subject can approach or manipulate, and locations the subject can move to. These units integrate the sensory properties and movements related to them. The repeated and widespread distribution of such units amplifies the combinatorial power of the associations among them. These associations depend on the phasic release of dopamine in the striatum triggered by the saliency of stimuli and will be reinforced by the rewarding consequences of the actions related to them. Dopamine permits synaptic plasticity in the corticostriatal synapses. The striatal units encoding the same stimulus/action send convergent projections to the internal segment of the globus pallidus (GPi) and to the substantia nigra pars reticulata (SNr) that stimulate or hold the action through a thalamus-frontal cortex pathway. According to this model, this is how the basal ganglia select actions based on environmental stimuli and store adaptive associations as nondeclarative memories such as motor skills, habits, and memories formed by Pavlovian and instrumental conditioning.