Topography of Fos-Immunoreactive and NADPH-d-Reactive Neurons in the Limbic Structures of the Basal Forebrain and in the Hypothalamus during Realization of Motivated Operant Movements in Rats

Abstract

We estimated in rats the expression of early gene c-fos (marker of neuronal activation) and NADPH-diaphorase activity (NO-synthase marker) in the limbic structures of the basal forebrain and in the hypothalamus. Estimations were performed in the norm, in the state of starvation, and after realization of long-lasting (repeated 4 to 12 times per minute for 30 min) motivated stereotyped food-procuring forelimb movements. In food-deprived animals, a significantly greater (Р < 0.05), as compared with the control, number of Fos-immunoreactive (Fos-ir) and NADPH-diaphorase-reactive (NADPH-dr) neurons was observed in limbic structures, namely in the medial septum (MS), nuclei of the vertical and horizontal branches of the diagonal fascia (VDB and НDB), magnocellular preoptic nucleus (MCPO), complex of the substantia innominata−basal nucleus of Meynert of the pallidum, SI-GP(B), as well as in the laterodorsal tegmental nucleus (LDTg), medial part of the pallidum (MGP), paraventricular and lateral nuclei of the hypothalamus (Pa and LH), and islands of Calleja (ICj and ICjM). In the limbic structures and pontine nuclei of rats of the experimental group (that performed operant movements), greater mean densities of labeled neurons were found in the succession LDTg < SI < MCPO < GP(B) < MS < VDB < HDB. The maximum mean density of Fos-ir neurons (13.8 ± 0.9 labeled nuclei within 200 × 200 μm2 area) was found in the HDB. In the hypothalamic nuclei of starving rats, c-fos expression was two times higher than that in the control. After realization of operant movements, the intensity of expression in the LH was somewhat smaller, while in the Ра it was higher. The maximum density of NADPH-dr neurons was observed in the Pa (303.4 ± 18.7 cells), in the ICj and ICjM (287 ± 11.6 and 260 ± 8.7 neurons, respectively), and in the MGP (93 ± 6.7 labeled cells). When analyzing the distribution of labeled neurons in experimental rats, we found high densities of double-labeled cells (Fos + NADPH-d positivity) in the Pa, MGP, ICj, and ICjM. Such specificity of changes in the c-fos expression and NADPH-d reactivity in the hypothalamus correlates, perhaps, with the formation of motivation signals related to a delay in food accessibility and supply of food. Modifications of neuronal activity in limbic structures reflect involvement of the latter in the formation of motor programs for food-procuring movements and their realization.