Plus-maze following the microinjection of muscimol into the dorsal periaqueductal gray of the rat

Performance no labirinto em cruz elevado após microinjeção de muscimol na substância cinzenta periaquedutal dorsal do rato

LETTERS TO THE EDITORS

Plus-maze following the microinjection of muscimol into the dorsal periaqueductal gray of the rat

Performance no labirinto em cruz elevado após microinjeção de muscimol na substância cinzenta periaquedutal dorsal do rato

Dear Editor,

Some experimental findings have indicated that the rat dorsal periaqueductal gray (DPAG) is part of a longitudinally organized neural system responsible for behavioral and vegetative manifestations of anxiety-like and aversive/defensive behaviors as well as active and passive emotional coping strategies towards threatening stimuli.¹ DPAG-evoked defense reactions might also provide some insights about panic attacks in humans. Systemically injected diazepam or intra-DPAG injection of midazolam promoted anxiolytic effects when animals were tested in the elevated plus-maze (EPM),² a behaviorally and pharmacologically validated animal model for the study of anxiety.³ The inhibitory neurotransmitter g-aminobutyric acid (GABA) and both GABA_A and GABA_B receptors are found in the DPAG. Here we report discrepant results found when we looked for the behavioral effects of microinjections of different doses of muscimol, a GABA_A receptor agonist, into the DPAG of rats tested in the EPM compared to the reported anxiolytic effects of midazolam.

We studied 3-month old male Wistar rats locally bred under standard laboratory conditions and according to international laws for ethical care. The EPM and its use were identical to previous descriptions.² Rats were submitted to unilateral stereotaxic surgery (Figure 1) and were microinjected into the DPAG with saline (0.3 µl, n = 10), muscimol at the doses of 20 pg (n = 8), 50 pg (n = 12) and 150 pg (n = 8) or midazolam at the doses of 0.1mg (n = 5) or 35.3 µg (n = 7). Other experimental groups served for testing the experimental procedure itself and we studied rats injected i.p. with saline and midazolam and tested in the EPM (n = 5-8, respectively) and a 'non-target' group (n = 15), i.e., rats that received saline or muscimol microinjections in the vicinity of the DPAG. Moreover, in another experiment, muscimol decreased maternal aggressive behavior when microinjected into the paraventricular hypothalamic nucleus (data not shown).

Interestingly, results showed that DPAG microinjections of muscimol or midazolam had no effects on EPM performance. Neither the percentage of open arm entries nor the time spent in open arms, which are considered two indexes of anxiety,³ were affected by muscimol when we compared both the saline and the 'non-target' groups. Intra-DPAG muscimol did not promote an overall sedative action as evaluated by the total number of arm entries and the entries in closed arms (Figure 2). On the other hand, systemically injected midazolam promoted its expected anxiolitic actions compared to saline.

Therefore, DPAG GABA_A receptors appear not to affect rat EPM performance. Caution is recommended when interpreting previously reported intra-DPAG midazolam anxiolytic actions² due to its intrinsic chemical characteristics (i.e., its solubility occurs in low pH solutions) and possible diffusion to other areas that can also affect this behavior, such as the nearby dorsal raphe.^2,4 It is noteworthy that midazolam effects cannot be completely antagonized by bicuculline, a GABA receptor antagonist. Indeed, benzodiazepines can interact with other neurotransmitters within the DPAG and, compared to muscimol, can exert different actions on heterogeneous GABAA receptor subtypes.⁵ The well-known complexity of the neurochemical basis of anxiety is supported by the findings reported above.

Rosa M M de Almeida, Márcia Giovenardi,

Adriana Damra, Rosana M Frey,

Alberto A Rasia-Filho

Laboratory of Neurosciences, Universidade do Vale do Rio

dos Sinos (UNISINOS), São Leopoldo (RS), Brazil

References

1. Keay KA, Bandler R. Parallel circuits mediating distinct emotional coping reactions to different types of stress. Neurosci Biobehav Rev. 2001;25(7-8):669-78.

2. Russo AS, Guimarães FS, de Aguiar JC, Graeff FG. Role of benzodiazepine receptors located in the dorsal periaqueductal grey of rats in anxiety. Psychopharmacology (Berl). 1993;110(1-2):198-202.

3. Fernandes C, González MI, Wilson CA, File SE. Factor analysis shows that female rat behaviour is characterized primarily by activity, male rats are driven by sex and anxiety. Pharmacol Biochem Behav. 1999;64(4):731-8.

4. Menard J, Treit D. Effects of centrally administered anxiolytic compounds in animal models of anxiety. Neurosci Biobehav Rev. 1999;23(4):591-613.

5. Mehta AK, Ticku MK. An update on GABAA receptors. Brain Res Brain Res Rev. 1999;29(3-2):196-217.

Financing: Universidade do Vale do Rio dos Sinos (UNISINOS), Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Conflict of interests: None

Publication Dates

• Publication in this collection
  24 Mar 2006
• Date of issue
  Mar 2006