Available online at www.sciencedirect.com
Behavioural Brain Research 186 (2008) 91–97
Neonatal tactile stimulation reverses the effect of neonatal isolation on
open-ﬁeld and anxiety-like behavior, and pain sensitivity
in male and female adult Sprague–Dawley rats
A. Imanaka, S. Morinobu
, S. Toki, S. Yamamoto, A. Matsuki, T. Kozuru, S. Yamawaki
Department of Psychiatry and Neurosciences, Division of Frontier Medical Science, Programs for Biomedical Research,
Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, 734-8551 Hiroshima, Japan
Received 19 March 2007; received in revised form 23 July 2007; accepted 28 July 2007
Available online 6 August 2007
It is well known that early life events induce long-lasting psychophysiological and psychobiological inﬂuences in later life. In rodent studies,
environmental enrichment after weaning prevents the adulthood behavioral and emotional disturbances in response to early adversities. We
compared the behavioral effect of neonatal isolation (NI) with the effect of NI accompanied by tactile stimulation (NTS) to determine whether
NTS could reverse or prevent the effects of NI on the adulthood behavioral and emotional responses to environmental stimuli. In addition, we also
examined the sex difference of the NTS effect. Measurements of body weights, an open-ﬁeld locomotor test, an elevated plus maze test, a hot-plate
test, and a contextual fear-conditioning test were performed on postnatal day 60. As compared with rats subjected to NI, rats subjected to NTS
showed signiﬁcantly higher activity and exploration in the open-ﬁeld locomotor test, lower anxiety-like behavior in the elevated plus maze test,
and signiﬁcantly prolonged latencies in the hot-plate test, and this effect was equal among males and females. In the contextual fear-conditioning
test, whereas NTS signiﬁcantly reduced the enhanced freezing time due to NI in females, no signiﬁcant difference in the freezing time between
NI and NTS was found in males. These ﬁndings indicate that adequate tactile stimulation in early life plays an important role in the prevention of
disturbances in the behavioral and emotional responses to environmental stimuli in adulthood induced by early adverse experiences.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Neonatal isolation (NI); Neonatal tactile stimulation (NTS); Rat; Open-ﬁeld locomotor test; Elevated plus maze test; Hot-plate test; Contextual fear-
It is well known that early experiences induce long-lasting
psychophysiological and psychobiological changes in later life.
Numerous studies have demonstrated that early adverse expe-
riences such as maternal separation (MS) or neonatal isolation
(NI), which interrupt dam–pup interactions, affect the devel-
opment of the central nervous system and subsequently lead
to enhanced susceptibility to stress in adulthood, both behav-
iorally and endocrinologically [8,10,12,14,17,37]. NI and MS
differ with respect to isolation of individual offspring. With most
MS procedures, only the dams are removed to separate cages,
Abbreviations: NI, neonatal isolation; NTS, neonatal tactile stimulation
Corresponding author. Tel.: +81 82 257 5205; fax: +81 82 257 5209.
E-mail address: firstname.lastname@example.org (S. Morinobu).
while the pups remain in their home cage. With NI procedures,
pups are isolated and placed individually into containers sepa-
rately from their dams and littermates. In contrast, the protective
or therapeutic effects of early intervention on the development
of stress vulnerability during the interruption of the dam–pup
relationship has not been as thoroughly examined. For exam-
ple, brief handling of neonatal rats during maternal separation
was reported to induce resistance of the hypothalamo-pituitary-
adrenal (HPA) axis to stress in adult rats . Several studies
have demonstrated that adult rats subjected to neonatal handling
exhibited less anxiety-like behaviors in the elevated plus maze
as compared with nonhandled rats [19,27].
Another type of neonatal handling, neonatal tactile stimula-
tion (NTS), also has a distinct effect on the development of stress
reactivity. Rats subjected to NTS show increased curiosity and
problem-solving ability, and exhibit less emotionality in stress-
ful situations . NTS prevents the rise of serum corticosterone
0166-4328/$ – see front matter © 2007 Elsevier B.V. All rights reserved.