Tail Pinch Induces Fos Immunoreactivity Within Several Regions of the Male Rat Brain: Effects of Age

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Abstract

SMITH, W. J., J. STEWART AND J. G. PFAUS. Tail pinch induces Fos immunoreactivity within several regions of the male rat brain: Effects of age. PHYSIOL BEHAV 61(5) 717–723, 1997.—Brief, intermittent stressors, such as low-level foot shock or tail pinch, induce a general excitement and autonomic arousal in rats that increases their sensitivity to external incentives. Such stimulation can facilitate a variety of behaviors, including feeding, aggression, sexual activity, parental behavior, and drug taking if the appropriate stimuli exist in the environment. However, the ability of tail pinch to induce general arousal and incentive motivation appears to diminish with age. Here we report on the ability of tail pinch to induce Fos immunoreactivity within several brain regions as a function of age. Young (2–3 months) and middle-aged (12–13 months) male rats were administered either five tail pinches (one every 2 min), one tail pinch, or zero (sham) tail pinches (n = 4 per stimulation condition). Rats were sacrificed 75 min following the onset of stimulation, and their brains were prepared for immunocytochemical detection of Fos protein. Fos immunoreactivity was induced by one and five tail pinches in several brain regions, including the anterior medial preoptic area (mPOA), paraventricular nucleus of the hypothalamus (PVN), paraventricular nucleus of the thalamus (PV-Thal), medial amygdala (MEA), basolateral amygdala (BLA), lateral habenula (LHab), and ventral tegmental area (VTA), of young rats compared with those that received zero tail pinches. In contrast to young rats, middle-aged rats had significantly less Fos induced by one and five tail pinches in the mPOA, PVN, MEA, BLA, and VTA, but an equivalent amount induced in the LHab. Fos immunoreactivity was not found within the medial prefrontal cortex, nucleus accumbens, striatum, lateral septum, or locus coeruleus in either young or old rats. Tail pinch appears to activate regions of the brain known to be involved in behavioral responses to both incentive cues and stressors. The lower level of cellular reactivity to tail pinch in middle-aged rats suggests a diminished neural responsiveness to incentives and stressors.

Section snippets

Animals and Stimulation

Male Long-Evans rats were obtained from Charles River Canada, St. Constant, Québec. Twelve young (2–3 months) and 12 middle-aged (12–13 months) males were used as subjects. The young males received at least 10 tests of sexual behavior prior to the experiment and were all sexually active. The middle-aged males had been used extensively as studs prior to the experiment but had ceased to show stable copulatory activity in the month preceding the experiment. Rats were housed in groups of two in

Results

All rats responded to the tail pinches by vocalization and turning towards the haemostatic forcep. After receiving one or two tail pinches, the rats in the five-pinch group would often vocalize as soon as their tail was touched by the forcep, and would attempt to escape immediately following a tail pinch. Some of these rats vocalized when picked up in their home cages prior to transportation to the stimulation chamber.

Fos immunoreactivity was obvious in several regions of the brain, most

Discussion

The present study demonstrates a significant effect of both age and number of stimulations in neural responsivity to tail pinch. A progressive and significant increase in Fos was observed in the PVN, PV-Thal, and VTA with greater amounts of tail pinch. The mPOA, MEA, and BLA also showed a progressive increase in Fos with greater amounts of tail pinch; however, this effect was significant only between zero and five pinches. An age effect was observed in the mPOA, PVN, PV-Thal, BLA, and VTA, such

Acknowledgements

This research was supported by grants from the Natural Sciences and Engineering Council of Canada (OGP-0138878) and Fonds pour la Formation de Chercheurs et l'Aide à la recherche du Québec (960-CE-103). We thank Drs. Yavin Shaham and Charles V. Mobbs for useful discussions.

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