Environment and Behavior: Neurochemical Effects of Different Diets in the Calf Brain

Abstract

Calves reared for the production of white veal are subjected to stressful events due to the type of liquid diet they receive. Stress responses are mediated by three main stress-responsive cerebral regions: the prefrontal cortex, the paraventricular nucleus of the hypothalamus, and the nucleus of the solitary tract of the brainstem. In the present study, we have investigated the effects of different diets on these brain regions of ruminants using immunohistochemical methods. In this study, 15 calves were used and kept in group housing systems of five calves each. They were fed with three different diets: a control diet, a milk diet, and a weaned diet. Brain sections were immunostained to evaluate the distribution of neuronal nitric oxide synthase and myelin oligodendrocyte glycoprotein immunoreactivity in the prefrontal cortex; the expression of oxytocin in the paraventricular nucleus; and the presence of c-Fos in the A2 group of the nucleus of the solitary tract. The main results obtained indicate that in weaned diet group the oxytocin activity is lower than in control diet and milk diet groups. In addition, weaning appears to stimulate myelination in the prefrontal cortex. In summary, this study supports the importance of maintaining a nutritional lifestyle similar to that occurring in natural conditions.

Keywords: calf; diets; nucleus of the solitary tract; paraventricular nucleus; prefrontal cortex.

Figure 1

Brightfield photomicrographs of coronal sections (A–C) and histograms (D–G) showing morphological types and morphometric analysis of oxytocin-immunoreactive neurons in the paraventricular nucleus. Note that neurons show a polygonal (A), fusiform (B), and spheroidal (C) morphology (arrowheads). The mean perikaryal area is significantly larger in the milk diet group than in the control and wean diet groups (D). In particular, the milk diet group exhibits the largest areas of polygonal (E) and spheroidal (G) neurons. Scale bar = 20 µm in (C) (applies to (A–C)).

Figure 2

Brightfield photomicrographs of coronal sections (A–C) and histograms (D–G) showing the distribution of oxytocin immunoreactivity in the paraventricular nucleus of the control diet group (A), milk diet group (B), and wean diet group (C). The density (number of somata/mm²) of oxytocin-immunoreactive (IR) neurons does not vary significantly in the different groups (D–G). Note that the intensity of immunostaining is lower in wean diet group than in the control and milk diet groups (see also Table 1). Scale bar = 500 µm in (C) (applies to (A–C)).

Figure 3

Brightfield photomicrographs of coronal sections (A–C) and histogram (D) showing the distribution of myelin oligodendrocyte glycoprotein (MOG) immunoreactivity in the prefrontal cortex of the control diet group (A), milk diet group (B), and wean diet group (C). Note that the percentage of the image covered by MOG immunoreactivity (D) is significantly higher in wean diet group than in control diet group. Scale bar = 20 µm in (C) (applies to (A–C)).

Figure 4

Brightfield photomicrographs of coronal sections (A–C) and histogram (D) showing the distribution of neuronal nitric oxide synthase (nNOS) immunoreactivity in the prefrontal cortex of the control diet group (A), milk diet group (B), and wean diet group (C). Comparison among the three groups shows no significant difference in neuronal densities (number of somata/mm²; A–D). In every group, nitrergic cells are non-pyramidal neurons with angular-(E,F) or fusiform-(G) shaped cell body. Scale bars = 200 µm in (C) (applies to (A–C)); 20 µm in (G) (applies to (E–G)).

Figure 5

Brightfield photomicrographs of coronal sections (A–F) and histograms (G) showing the distribution of c-Fos immunoreactivity in the nucleus of the solitary tract (bounded by the dotted line) (A2 group) of the control diet group (A,D), milk diet group (B,E), and wean diet group (C,F). The density (number of somata/mm²) of c-Fos-immunoreactive neurons is very similar in the different groups (arrowheads). Scale bars = 500 µm in (C) (applies to (A–C)); 20 µm in (F) (applies to (D–F)). Abbreviations: TS, tractus solitarius.

Figure 6

Oxytocin as a regulator of stress response: schematic representation. High level of stressors arising from the gastrointestinal tract increase oxytocin production in magnocellular and parvocellular neurons of the paraventricular nucleus. Magnocellular neurons modulate the adrenocorticotropic hormone (ACTH) activity through the release of oxytocin into portal circulation, whereas parvocellular neurons regulate the adrenal cortex activity via autonomic outflow.