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Environmental stressors seem to play a role in exacerbation of symptoms of interstitial cystitis (IC), both in cats and in human beings. These observations suggest a role for the sympathetic nervous system in the pathophysiology of IC. To begin to assess the underlying role in IC of the pontine nucleus locus coeruleus (LC), the most important source of norepinephrine in the central nervous system, we compared the intensity of tyrosine hydroxylase immunoreactivity (THIR) in sections of LC obtained from cats with IC and from healthy cats. Cats with IC were studied during quiescent periods in an attempt to avoid the risk of flare-induced activation of the LC.Six cats diagnosed with IC and six healthy cats were studied. Cats with IC were monitored to ensure that no behavioral or urinary signs attributable to IC had been observed for at least two weeks prior to the study. Cats were euthanized and perfused with 4% paraformaldehyde, after which brainstem tissues were collected. Coronal sections (10 micro m.) of LC were prepared and examined for THIR.THIR in total LC, parabrachial nucleus and LC complex was significantly greater (p <0.05) in samples from cats with IC than from healthy cats.The increased THIR in the LC of cats with IC provides additional evidence for increased sympathetic nervous system activity in patients with IC, even during periods of absence of clinical signs.