As a
hormone in the hypothalamic-pituitary-adrenocortical (HPA) axis
corticotropin-releasing factor (CRF) mediates stress responses. CRF can also act as a
neuromodulator of synaptic transmission outside the HPA axis. A major site of extrahypothalamic expression of CRF and its
G-protein-coupled receptors is the amygdala, a key player in affect-related disorders such as anxiety. The laterocapsular division of the central nucleus of the amygdala (CeLC) is important for the modulation of
pain affect. This study determined the effects of CRF1 and
CRF2 receptor antagonists in CeLC neurons in an
arthritis pain model. Extracellular single-unit recordings were made from CeLC neurons in anesthetized adult rats. All neurons responded more strongly to noxious than to innocuous mechanical stimulation (compression) of peripheral tissues, including the knee. Evoked responses and background activity were measured before and during the development of a
kaolin/
carrageenan-induced knee joint
arthritis. Drugs were administered into the CeLC by microdialysis before and/or after
arthritis induction. All CeLC neurons showed increased responses to mechanical stimuli ("sensitization") 5-6 h postinduction of
arthritis. A selective
CRF1 receptor antagonist (
NBI27914; 1-100 microM, concentration in microdialysis probe; 15 min) inhibited evoked responses and background activity in
arthritis (n = 9) but had no effect under normal conditions before
arthritis (n = 9). In contrast, a selective
CRF2 receptor antagonist (
Astressin-2B; 1-100 microM, 15 min) had no effect in
arthritis (n = 7) but increased the neurons' responses under normal conditions (n = 8). These data suggest that CRF1 receptors in the amygdala contribute to
pain-related sensitization, whereas the normally inhibitory function of CRF2 receptors is lost in the
arthritis pain model.